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Investigations of Structural, Elastic, Electronic and Thermodynamic Properties of X2TiAl Alloys: A Computational Study
Abstract
First-principle calculations have been adopted in order to reveal and deeply understand the structural, electronic, elastic, thermodynamic, and vibrational properties of full-Heusler X 2 TiAl (X = Au, Ru, and Zr) alloys in the L 2 1 phase. The fundamental physical properties such as bulk modulus, its pressure derivative, anisotropy factor, shear modulus, Poisson’s ratio, Cauchy pressure, elastic constants, heat capacity, thermal expansion coefficient, and Young’s modulus are obtained and compared with the literature. Debye temperature and Grüneisen parameter are also evaluated over the temperature range of 0–1500 K. Electronic band structures and their partial density of states and phonon dispersion curves are presented for all alloys and used to interpret the electronic and mechanical properties and stabilities of alloys in the L 2 1 phase.
... Poisson oranı için kritik değer 0,26'dır [14]. Cauchy basıncında (Cp=C12-C44) [19] ise pozitif/negatifliğine bakılır. Eğer pozitif ise sünek olduğu söylenir. ...
... Bundan dolayı, Young modülünün düzlemler boyunca değişimi Şekil 3.a'da verilmiştir. Ni2ZnAl alaşımın anizotrpisi üzerine yapılmış herhangi bir çalışmaya rastlanılmadığından benzer yapıdaki X2Tial (X=Au, Ru, Zr) alaşımlarının anizotropisi [19] ile kıyaslandığında, bu yapılar ve teorik hesaplamalardan elde edilen veriler ile tutarlıdır. Yapılan hesaplamalar ve Şekil 3'de verilen görseller dikkate alındığında, Ni2ZnAl alaşımının anizotrop özellik göstereceği söylenebilir. ...
In this study, ground state properties of Ni2ZnAl alloy in L21 phase from Heusler family were optimized. The calculated parameters are in harmony with the available literature data. Elastic constants were calculated using optimized parameters. The calculated elastic constants were found to meet the Born mechanical stability criteria. By using these constants, some mechanical and thermodynamic properties of the material such as elastic modulus, Vicker hardness, anisotropic nature, melting temperature were investigated in detail. Calculations showed that the Ni2ZnAl alloy is ductile, soft, and anisotropic. As such, it is a candidate material for applications that do not require hardness. The free energy, vibrational energy, entropy, and heat capacity of the Ni2ZnAl alloy were investigated using a semi- harmonic approach in the range of 0-800 K. All the total energy calculations were performed using the open-source Quantum Espresso software and ab-initio pseudopotential method based on the density functional theory (DFT) scheme within a generalized gradient approximation (GGA). According to the data obtained because of the study, Ni2ZnAl alloy is a potential candidate for industrial use.
... Herein, the mechanical qualities of the investigated compounds have been assessed through elastic constants (C ij ) and other derived parameters. C ij constants are computationally determined for RbXH 3 (X=Cr and Zr) using stress-strain method [49] and are displayed in Table 1. ...
In this work, we explore the physical properties of RbXH3 (X=Cr, Zr) perovskite hydrides for solid‐state hydrogen storage. The structural, mechanical, electronic, optical, and hydrogen storage properties were theoretically investigated using density functional theory and CASTEP software. The selected candidates were fully relaxed and optimized in the cubic phase space group Pm‐3 m. The structural phase stability was verified by means of thermodynamic, dynamic and mechanical stabilities. Mechanical analyses based on Poisson's ratio (ν), G/B ratio, and Cauchy pressure show that RbCrH3 and RbZrH3 exhibit brittle behavior with preference of ionic bonding. The electronic structures unveil half‐metallicity in RbCrH3 compound and metallic‐like behavior in RbZrH3. Furthermore, optical calculations were also conducted to gain additional insights into the physical properties of RbXH3 compounds. The gravimetric hydrogen storage (Cwt %) capacities have been calculated as 2.09 wt % and 1.64 wt % for RbCrH3 and RbZrH3, respectively. The hydrogen desorption temperatures have been obtained as 545.11 K and 548.15 K for RbCrH3 and RbZrH3, respectively. Our calculation propose RbCrH3 hydride as potential material for hydrogen storage application.
... In addition, we can derive the magnitude of brittleness and toughness of both mineral phases under two calculations by Poisson's ratio v and Pugh (K/G) criterion. [37][38][39] When v > 0.26, the material exhibits ductility which increases along with v; conversely, low v translates to more brittleness. The Pugh criterion is also used to predict the brittleness/ductility of the material. ...
There are works have reported the crystal structures and mechanical properties of ferrite cement (C4AF) at the atomic scale with deviation owing to the omission of the Coulomb interaction effect (Ueff) between 3d electrons of Fe in C4AF. In this work, the DFT+U method was used to evaluate its effect on their electronic structures and mechanical properties of C4AF with two different phases I2mb (C4AF‐I) and Pnma (C4AF‐P). The FeO bonds of the two phases are all weaker and display Ueff due to the presence of Fe ions. The mechanical properties of C4AF calculated using DFT+U method significantly differ from those obtained without considering Ueff, in which the former shows lower inferior mechanical properties than the latter. This work presents a comparative study the effect of Coulomb interaction to the internal electronic structures and mechanical properties, which will pave the way for designing high hydration reaction cement and high toughness materials.
... The Poisson's ratio is the measurement of covalency. The Poisson's ratio is 0.1 for covalently bonded compounds, 0.25 for ionic solids, and 0.3 for metallic bonded compounds [46,47]. It is understood from Poisson's ratios (0.29-0.31) that PdVSi and PdVSn alloys have ionic characters, and PdVGe and PdVSb alloys have metallic characters. ...
The half-metallic and pressure-induced elastic properties of PdVSi, PdVGe, PdVSn, and PdVSb were investigated. First, FM phases were obtained energetically as the most stable. The semiconducting characters seemed in spin-down states. The band gaps were calculated as 0.807 eV, 0.676 eV, 0.481 eV and 0.370 for PdVSi, PdVGe, PdVSn, and PdVSb, respectively. Since the spin polarization value of PdVSb alloy is less than 100% in GGA method, it is nearly half-metallic ferromagnetic, while other alloys are true half-metallic ferromagnetic. However, in HSE method, the forbidden and half-metallic band gaps were also calculated for PdVSb alloy as 0.375 eV and 0.134 eV, respectively. HSE method has a direct effect on the band gap values. PdVSi, PdVGe, and PdVSn, alloys have 1.00 μB/f.u. total magnetic moments, while PdVSb alloy has 2.00 μB/f.u. total magnetic moment. According to elastic results, all alloys are elastically stable and ductile materials. Debye temperatures were obtained as 371.53 K, 317.71 K, 311.78 K, and 263.13 K at 0 GPa pressure. PdVSi, PdVGe, PdVSn, and PdVSb alloys have been obtained as very remarkable materials with their structural, electronic, magnetic and pressure-induced elastic properties.
... The C 44 is another chief stiffness parameter which directly provides the information about the hardness of materials [38]. Through the investigation of the values of C 44 we have observed that Pt 3 Ti has the highest value of C 44 demonstrating that Pt 3 Ti is the hardest one compared to all others materials listed in Table 2.The Cauchy pressure defined as C P = C 12 -C 44 indicates the efficient bonding of metals and compounds where the positive value of C P denotes the metallic bond nature and negative value of C P refers the covalent bond nature of a material [39]. Since this is the noble work on elastic constants of Pt 3 X (X = Ti, Cu) it is not possible to compare the observed results at this moment. ...
The several physical features including structural, mechanical, electronic, optical and thermal properties of Pt3Ti and Pt3Cu have investigated here hypothetically for the first time. The possible results of Pt3Ti and Pt3Cu have been compared to other pt-based compounds for reliability of this work. The DFT simulation depends on ab-initio manner through CASTEP regulation is utilized for all of the investigation. The structural properties of these materials are in better agreement with the test data already available. The calculated elastic constants demonstrate mechanical stability for all the compounds. The computed Poisson's as well as Pugh's ratios demonstrate the behavior of ductility of these phases, with Pt3Cu is being far more ductile over Pt3Ti. The large Young’s modulus and hardness value of Pt3Ti ensured that Pt3Ti is harder material than Pt3Cu. The high machinability index of Pt3Cu ensured that this phase has high damage-tolerant capability and useful application in industry. The anisotropic factor and universal index of anisotropy ensure the elastic nature of anisotropic for all of these phases. The study of density of states (both TDOS and PDOS) as well as electronic band structures evidences the metallic behavior for these materials. The optical features of compounds Pt3Ti and Pt3Cu have also been investigated through CASTEP code for the first time and recommended that these phases have huge application in solar cell to reduce extra solar heat. The calculated Debye and melting temperature ensured that Pt3Ti is more thermally conductive and can be used in high temperature structural materials. The very low minimum thermal conductivity ensured that Pt3Cu can be used as a better Thermal Barrier Coating (TBC) material than Pt3Ti.
... By examining the Poisson's ratio of LiSrH3, it may be anticipated that covalent bonding is dominant for LiSrH3. with cohesive and binding energies of material`s atoms [18,19]. Higher bulk modulus means higher resistivity against volume change [20]. ...
Increasing catastrophic climate events, energy needs, human population lead to look for clean, cheap and environmentally friendly energy production methods and sources. International agreements have been made to lower carbon emissions and support carbon free way of energy productions. Hydrogen technology is suggested as one excellent way of accomplishing these aims. Hydrogen is an excellent energy carrier with almost zero carbon emission and high efficiency. There are four steps to make hydrogen energy ready for usage: production, transportation, storage and converting it to electricity. Each step has its own obstacles to be overcome. Among the storage methods, solid state storage of hydrogen is very promising since it allows us to store hydrogen in high content and safely. Thus, there are intense ongoing research on this area. Therefore, this study adopts a well proved, time and cost saving method density functional theory to search and evaluate mechanical and electronic properties of LiSrH3 perovskite hydride with space group 𝑃𝑚3̅𝑚 (221) for hydrogen storage purposes. Two critical parameters gravimetric hydrogen density and hydrogen desorption temperature along with elastic constants, Poisson’s ratio, Shear, bulk, Young modulus are collected and discussed. The mechanical evaluation is demonstrated that LiSrH3 is a mechanically stable material, however, elastic constant evaluation is showed that it is a brittle material which can be an obstacle when handling the material. The electronic band structure is also obtained which demonstrated an indirect band gap of 3.65 eV.
... The greater the hardness of the material, the smaller the Poisson's ratio; On the contrary, the softer the material, the greater the Poisson's ratio, which is fully proved by the data of Poisson's ratio and hardness in Table 5. According to Pugh's empirical rule [33][34][35], if the value of B/G is less than 1.75, the material has ductility, otherwise it does not. Obviously, the Pugh ratio of complete Ti 5 Sn 3 obtained from Table 5 is 2.24, which is ductile, and the B/G of Sn vacancy Ti 5 Sn 3 is 4.04, indicating that the Sn vacancy can improve the ductility of the material. ...
Titanium alloy is widely used in biomedical materials. Ti-Sn alloy is a new type β titanium alloy with no toxicity. In this paper, the mechanical and electronic properties of Ti5Sn3 with vacancy defects have been studied by using first-principles method. The vacancy formation energy, vacancy formation enthalpy, elastic constant, elastic modulus, hardness and electronic structure of perfect Ti5Sn3 and Ti5Sn3 with different vacancies were also calculated and discussed. The results show that Ti5Sn3 is more likely to form vacancies at VTi2. In addition, the bulk deformation resistance of Ti5Sn3 is weakened by the vacancy, and the shear resistance, stiffness and hardness of Ti5Sn3 are increased by the Ti vacancy, but the brittleness of Ti5Sn3 is increased. On the contrary, the presence of Sn vacancy decreases the shear resistance, stiffness and hardness of Ti5Sn3, and increases the toughness of Ti5Sn3. By analyzing the change of electronic structure, it is found that removing the Ti atom at the VTi2 position can improve the interaction between atoms, while Sn vacancy can weaken the interaction.
... 28 A solid's structural, electronic, and optical characteristics can be accurately predicted by employing this method. 29 Based on a generalized gradient approximation (GGA), the exchange−correlation functional is implemented in terms of the Perdew−Burke−Ernzerhof (PBE) 30 and modified Becke Johnson (mBJ) potential. 31 We also tested the hybrid functional through the Heyd−Scuseria−Ernzerhof (HSE) scheme for band gap comparison. ...
Over the past few years, metal halide perovskite solar cells have made significant advances. Currently, the single-junction perovskite solar cells reach a conversion efficiency of 25.7%. Perovskite solar cells with a wide band gap can also be used as top absorber layers in multi-junction tandem solar cells. We examined the dynamical and thermal stability, electronic structure, and optical features of In2PtX6 (X = Cl, Br, and I) perovskites, utilizing first-principle calculations. The stability is predicted using phonon dispersion spectrum and ab initio molecular dynamics simulation and also through the convex hull approach. The lattice constants and the optimized volume show an increasing trend with changing halide ions. The band structures computed for In2PtCl6, In2PtBr6, and In2PtI6 indicate their semiconducting nature with band gap values of 2.06, 2.01, and 1.35 eV, respectively. Halogens p and Pt d orbitals, respectively, play a prominent role in the formation of states around valence band maximum and conduction band minimum. The compounds, namely, In2PtBr6 and In2PtI6, exhibit high dielectric constants and small carrier effective masses. Furthermore, we found that In2PtI6 reveals a maximum theoretical efficiency owing to its optimum band gap and high optical absorption and is comparable to MAPbI3 in the studied range. Our results suggest that In2PtX6 (X = Cl, Br, and I) are suitable materials for single-junction and top absorber layers in tandem solar cells.
... Bulk modulus is a degree of resistivity towards volume change which is executed by an applied pressure. This can provide substantial information about average bound strength because of having a strong correlation with cohesive and binding energies of material's atoms [51,52]. Higher bulk modulus means higher resistivity against volume change [53]. ...
Titanium tetra hydride is considered for hydrogen storage purposes. Firstly, formation energy, hydrogen desorption temperature and gravimetric hydrogen density of TiH4 is computed. Secondly, an ab initio constant pressure molecular dynamic simulation under pressure is performed to reveal behaviour of TiH4 for the first time. The result exhibits two phase transitions successively. C2/m phase of TiH4 transforms into C2/c phase at 40 GPa simulation pressure. Then, elastic constants of phases are determined to examine mechanical stability of phases. Based on the evolution of elastic constants, it is found that C2/m phase fulfils Born stability criteria for a monoclinic structure, indicating that C2/m phase is mechanically stable whereas C2/c phase is not mechanically stable. Additionally, several critical parameters which are important for hydrogen storage such as brittleness and ductility, Young and Shear modulus are obtained and analysed. In addition, electronic structures of phases are calculated and evaluated. Finally, dynamic stability from phonon dispersion curves is examined. C2/m phase is also found to be dynamically stable.
... The mechanical stability of crystals are analysed using elastic constants. Elastic constants define the ratio of stress to strain and provide information about existence of different forces and also bonding [31]. For an orthorhombic symmetry, nine elastic constants defined as C 11 , C 22 , C 33 , C 44 , C 55 , C 66 , C 12 , C 13 , C 23 [29,32]. ...
Hydrogen storage is a critical step for commercialisation of hydrogen consumed energy production. Among other storage methods, solid state storage of hydrogen attracts much attention and requires extensive research. This study rationally and systematically designs novel solid state hydrides; Li2CaH4 (GHD is obtained as −6.95 wt %) and Li2SrH4 (GHD is obtained as −3.83 wt %) using computational method. As a first step, we suggest and predict crystal structures of solid state Li2CaH4 and Li2SrH4 hydrides and look for synthesizability. Then, the mechanical stabilities of hydrides are identified using elastic constants. Both hydrides fulfil the well-known Born stability criteria, indicating that both Li2CaH4 and Li2SrH4 are mechanically stable materials. Several critical parameters, bulk modulus, shear modulus, Cauchy pressures, anisotropy factors of hydrides and bonding characteristics are obtained and evaluated. Furthermore, electronic and optical band structures of hydrides are computed. Both Li2CaH4 and Li2SrH4 have indirect bands gaps as 0.96 eV (Г-U) and 1.10 eV (Г-R). Thus, both materials are electronically semiconducting. Also, Bader charge analysis of hydrides have been carried out. Charge density distribution suggests an ionic-like (or polarized covalent) bonding interaction between the atoms.
... The elastic parameters of materials are crucial for understanding mechanical properties of compounds such as stability, anisotropic behaviour, internal forces, ductility and brittleness [28]. The elastic constants of a material (C ij ) defines its response towards an externally applied force. ...
In this study, new, lightweight perovskite type hydrides; MgNiH3 (GHD is calculated as ~3.51 wt%) and MgCuH3 (GHD is calculated as ~3.32 wt%) are investigated. Hydrogen storage, structural, elastic, mechanical, electronic and thermodynamic behaviour of these hydrides are investigated using first principle calculations as a tool. Our elastic and mechanical analysis revealed that these hydrides are mechanically stable and have a ductile nature which is a necessary assessment required for handling materials for transportation. Furthermore, electronic band structures of hydrides indicate metallic characteristics for both hydrides. Many unknown thermodynamic properties of these ternary hydrides are revealed and discussed.
... Table 1 also presents Cauchy pressures of RhMnX (X=Sb and Sn) compounds which give information about ductility/brittleness and bonding characteristics of them [28,29]. The negative Cauchy pressure is an indication of brittleness, directional bonding features and non-metallic behaviour whereas a positive Cauchy pressure means that the material is ductile, metallic and have a non-directional bonding characteristic [30][31][32]. Based on the evaluations of Cauchy pressure of these compounds, it can be noticed that both compounds are ductile, metallic and have non-directional bonding in C1 b phase. ...
This study employs first principles method to run an investigation on structural, electronic, elastic, vibrational, thermodynamic and magnetic properties of RhMnX (X = Sb and Sn) half-Heusler compounds in C1 b phase. The elastic constants (C 11, C 12 and C 44) for RhMnX (X = Sb and Sn) half-Heusler compounds in C1b phase are computed using the energy-strain method. Directional changes of Young and Shear modulus, Poisson's ratios and compressibility's of RhMnX (X = Sb and Sn) compounds have been analysed. Evaluations of mechanical stabilities via obtained elastic constants are exhibited stable natures for these compounds. Moreover, the compounds are ductile based on the Pugh's criteria. Electronic band structures and their partial density of states are presented for both compounds and used to interpret the electronic band profiles of these compounds in the C1b phase. Specifically, phonon dispersion curves of these compounds have been derived for the first time. Full phonon properties of these compounds suggest that both compounds are dynamically stable. Besides, the change in specific heat capacity and entropy of RhMnX (X = Sb and Sn) compounds have been also computed between 0–500 K.
... The Young modulus and elastic anisotropy are also important physical parameters. As Young modulus of a solid increases it gets much stiffer [37]. Therefore, Ir3Nb is much stiffer than that of Ir3Hf. ...
The mechanical, electronic and vibrational properties of Ir-based refractory superalloys (Ir3Hf and Ir3Nb) in the L12 structure were studied in the framework of ab initio calculations. The obtained equilibrium lattice constants and bulk modulus were reported and compared with the existing data. The elastic constants of alloys were determined using energy strain method. The results were utilised to evaluate mechanical stability of alloys in the crystal structure of L12. Both alloys were found to be mechanically stable based on the Pugh’s criteria. Subsequently, electronic band structures and partial and total densities of states have been obtained for Ir3Hf and Ir3Nb. The band structures of alloys demonstrated metallic behaviour whilst the conductivity was mostly governed by Ir 5d states. Moreover, phonon distribution curves of both alloys were obtained by employing the linear response technique within the density functional theory. Both alloys are found to be dynamically stable based on phonon modes evaluation.
... To be used as catalyst in PECs, spinel oxide should be nontoxic and have suitable band gaps [7,8]. Electronic properties of materials are very important since they give insight into the conducting, non-conducting and semiconducting behavior of the material [9][10][11]. Thus it is crucial to obtain the band gaps of the studied materials accurately. ...
... The Young modulus and elastic anisotropy are also important physical parameters. As Young modulus of a solid increases it gets much stiffer [43]. If it is around unity (A=1), the solid is isotropic. ...
A comprehensive study has been carried out in order to reveal physical properties of antiperovskite compound GeNCa3 in the cubic structure. This study presents extensive properties including mechanical, electronic, vibrational and thermodynamical by means of generalised approximation approach within the density functional theory. The equilibrium lattice constant and bulk modulus of the compound are obtained via energy-volume data. The mechanical stability evaluation is conducted based on Born’s criteria using elastic constants. Subsequently, electronic band structures and partial and total densities of states are computed for antiperovskite GeNCa3. The electronic band structure of the compound reveals a metallic character with highest contribution to the conductivity Ge 4p and Ca 3d states. Moreover, phonon distribution curve is obtained by employing the linear response technique. The results indicate a dynamically stable compound. Finally, thermodynamic properties such as entropy and specific heat value and the results are presented.
This manuscript reports the computational study of three new perovskite-type hydrides for solid-state hydrogen storage technology. Herein, Density Functional Theory (DFT) has been implemented within CASTEP code to probe the structural, electronic, mechanical, optical and hydrogen storage properties of MgXH3
(X = Al, Sc and Zr). Thermodynamic stability assessments reveal that all materials exhibit negative formation energy (
E
), guaranteeing their energetic stability and suitability for experimental synthesis. Band structure and density of states plots highlight metallic properties in MgXH3
hydrides. The mechanical stability of these perovskites was ensured by the compliance of their elastic constants with Born’s stability criteria. Bulk modulus, shear modulus, Poisson’s ratio, Cauchy pressure, etc. are also processed from elastic constants. Pugh coefficient and Cauchy pressure unveil ductile behavior in MgAlH3
and MgZrH3
, and brittleness in MgScH3
. Optical properties revealed that MgXH3
compounds have maximum absorption and conductivity in the ultraviolet region. The gravimetric hydrogen storage capacities were theoretically determined as 5.274, 4.015, and 2.487 wt%, for MgAlH3
, MgScH3
, and MgZrH3
respectively. This research work suggests that MgXH3
(X = Al, Sc and Zr) can be used as promising materials for hydrogen storage applications.
In this manuscript, the structural, electronic, magnetic, elastic and thermodynamic properties of Mn2Cr1−xVxSi (x = 0, 0.25, 0.5, 0.75 and 1) alloys are studied by employing the Quantum Espresso code in the framework of DFT simulations. We have used Perdew–Burke–Ernzerhof's (PBE) generalized gradient approach (GGA) for all the calculations. The optimized lattice constant, bulk modulus and elastic constants of these alloys are predicted and evaluated, and then, shear modulus, Young modulus, Poisson’s ratio, Vickers hardness, elastic anisotropy, Debye temperature and melting temperatures are obtained using elastic constants. The brittle/ductile nature and isotropic/anisotropic behaviors of the materials are investigated. By analyzing the B/G ratio, we conclude that all materials except Mn2VSi are ductile in nature. According to the results of the calculations on the electronic properties of the alloys, it is clearly seen that they have metallic character due to the overlap between the conduction and the valence band at the Fermi level for the spin-up and spin-down states. Finally, thermodynamic properties of Mn2Cr1−xVxSi (x = 0, 0.25, 0.5, 0.75 and 1) alloys have been calculated using the quasi-harmonic Debye model by applying stress to the optimized crystal structures. The results indicate that these alloys are promising candidates for high-temperature applications.
Bu çalışmada Yoğunluk Fonksiyoneli Teorisi (DFT)’ne dayanan Quantum-Espresso yazılımı kullanılarak kübik L21 yapıda Ni2TiAl bileşiğinin yapısal, elastik ve termodinamik özellikleri incelenmiştir. Bileşiğin mekanik kararlı olduğu görüldükten sonra elastik modülü, anizotropisi, Vicker sertliği ve erime sıcaklıkları hesaplanmıştır. Hesaplamalar ve analizler sonucunda bileşiğin 1000K üzeri erime sıcaklığına sahip olmasından dolayı yüksek sıcaklık uygulamaları için aday malzeme olabileceği düşünülmektedir. 10 GPa altı Vicker sertliğine sahip olmasından dolayı yumuşak malzeme sınıfına girmektedir. Anizotropi analizlerinden malzemenin anizotrop olduğu görüldü.
The goal of this research is to predict the influence of temperature, pressure and Sb concentrations on the thermodynamic properties of \(\hbox {YP}_{1-x}\hbox {Sb}_{x}\) alloys, where x ranges from 0 to 1 in steps of 0.25, using density functional theory, comprised in the ABINIT package and Gibbs2 program. The calculations are carried out with GGA methods, which yield exchange and correlation terms. Also, the thermodynamic stability of the alloys under consideration was investigated by computing the miscibility critical temperature. Debye’s quasi-harmonic model, as implemented in the Gibbs2 code, was applied to calculate the thermodynamic properties of the \(\hbox {YP}_{1-x}\hbox {Sb}_{x}\) alloys. These results constitute our initial theoretical predictions.
Due to growing interest to predict and design new potential Heusler alloys by using theoretical calculations and highly functional software, research on Heusler alloys has taken great attention. From this point of view, this study considers investigation of X2MgAl (X = Sc, Ti and Y) alloys by adopting first principles calculations for the first time. A thorough investigation has been carried out to reveal these alloys’ mechanical, electronic, vibrational and thermodynamic properties. It is seen that all alloys have negative formation energies as − 0.278 eV/atom for Sc2MgAl, − 0.058 eV/atom for Ti2MgAl and − 0.304 eV/atom for Y2MgAl which indicates synthesisability and thermodynamic stability. Mechanical stability investigations based on the elastic constants of alloys have revealed that all alloys are mechanically stable. The electronic band structures of alloys demonstrate that X2MgAl (X = Sc, Ti and Y) alloys are metallic since there is no energy gap near the Fermi level. Cauchy’s pressures of alloys are found as − 17.791 GPa for Sc2MgAl, 31.404 GPa for Ti2MgAl and − 11.759 GPa for Y2MgAl which displays that Sc2MgAl and Y2MgAl are brittle and Ti2MgAl is ductile. The phonon dispersion curves are calculated along the lines of high symmetry within the first Brillouin region. Phonon frequencies are completely positive in the full Brillouin region, which proves the dynamic stability of the L21 phases of these alloys. Several thermodynamic properties such as Debye entropy, temperature and vibrational free energy are also computed and analysed. Debye entropies of alloys follow Ti2MgAl > Y2MgAl > Sc2MgAl relationship.Graphical Abstract
The electronic and thermodynamic variation of the δ-Ni3Nb phase with pressure in the range of 0–1200 GPa under hydrostatic pressure was investigated by DFT. By comparing the nucleation phase γ with the transition phase γ", it is found that the magnitude of the stability of the three phases remains the same under 0–1200 GPa, and the difference between the stability of δ and γ" phases is small and both are larger than that of γ. This indicates that the δ phase has the precipitation law of γ→δ and γ″→δ under 0–1200 GPa under hydrostatic pressure. The calculated data show that the results obtained by using the GGA-PW91 generalized function have the smallest error with the experimental value, and the properties of energy, cell volume, density of states, differential charge, elastic constant and phonon of γ, γ" and δ phases are calculated after the convergence of each parameter, and the δ phase is a strongly anisotropic material and the most dense vibration at 4.8 and 6.6 THz frequencies and the heat capacity δ>γ ">γ relationship.
New methods and materials for storing hydrogen efficiently and safely have been studied intensively recently since hydrogen is being a great energy carrier for future green energy usage. One of these material groups is lightweight perovskite hydrides. The current study deals with LiCaH3 and NaCaH3 perovskite hydrides with regards to structural, elastic, electronic, mechanical stability, and hydrogen storage properties. All computations have been done using first principles calculations. Both perovskite hydrides are found to be mechanically stable and ductile in nature. Electronic band structures of hydrides depict indirect band gaps for both perovskite hydrides, indicating that they are non-metallic. LiCaH3 and NaCaH3 have band gaps as 2.37 eV and 2.23 eV, respectively. The gravimetric hydrogen density of LiCaH3 is found to be 5.7 wt% and 4.38 wt% for NaCaH3which are reasonable in line with the goal that is set by US Energy Department which is 4.5 wt%.
The elastic and thermodynamic properties of NdGaO3 and NdInO3 with the space group Pm3¯m (no. 221) are studied for the first-time using density functional theory. The computed equilibrium lattice constants at stable phase are found to be 3.89 and 4.14 Å which are in a good agreement with literature. Elastic constants are computed to evaluate mechanical behaviours of materials. By applying Born stability criteria for elastic constants, it is found that both materials are mechanically stable. Moreover, bulk and shear modulus, Poisson’ ratio, Cauchy pressures, and Young modulus are examined. Ductile and brittle behaviour analysis indicates that both materials are ductile in nature. The thermodynamic properties such as thermal expansion coefficient, Grüneisen parameter, bulk modulus, specific heat capacities, and entropy are computed at a temperature range of 0–1000 K. The calculated elastic and thermodynamic properties can serve as a reference for future investigations as some of these physical properties can be difficult to determine experimentally.
This study has investigated ab initio pseudopotential calculations on the structural, electronic, elastic, vibrational and thermodynamic properties of the full-Heusler X2ScGa (X = Ir and Rh) alloys. The calculations have taken place under consideration of the generalized gradient approximation (GGA) of the density functional theory (DFT) with using the plane-wave ab initio pseudopotential method. According to the calculations, the major contribution to electronic states at the Fermi energy has been achieved by d orbitals, revealing a more active role for transition metals Ir (Rh) and Sc atoms. The reckonings point out that the Ir2ScGa and Rh2ScGa have metallic behavior at the equilibrium lattice constant with the density of states (DOS) at the Fermi level (N (EF)) of 1.412 states/eV and 1.821 states/eV, respectively. The results of the elastic constants showed that these compounds met the criteria for Born mechanical stability. It was also observed that they have a ductile structure and exhibit anisotropic behavior according to Pugh criteria. Besides, the full phonon spectra and their projected partial density of states of the alloys have been analyzed with the first-principle linear-response approach of the density-functional perturbation theory. All the alloys behaved dynamically stable in the L21 phase. Furthermore, internal free energy, entropy, specific heat capacity at constant volume and vibrational free energy changes of Ir2ScGa and Rh2ScGa alloys were analyzed and discussed between the temperature range of 0–800 K using the quasi harmonic approximation. According to the results, these alloys are potential candidate for industrial use.
This study adopts density functional theory to predict and thoroughly investigate new types of perovskite compounds for solid state storage of hydrogen. CaTiH 3 and MgTiH 3 perovskite hydrides are chosen and investigated using density functional theory in terms of ground state properties, electronic, mechanical, and thermodynamic properties for solid state storage of hydrogen. Stability of compounds are verified by calculating formation energies. Several crucial parameters; elastic constants, bulk, Young, Shear modulus, and Cauchy pressures are computed and analysed in great detail. Mechanical stability evaluation indicated that both compounds are mechanically stable whereas MgTiH 3 is ductile whilst CaTiH 3 is a brittle material. In addition, mechanical anisotropy is analysed using 2D surfaces. Both compounds showed anisotropic behaviour in all directions except for linear compressibility. Electronic band structures and their corresponding density of states of compounds are obtained. The results indicate that both compounds have metallic nature. From the results presented here, it can be predicted that MgTiH 3 is a better material for hydrogen storage with a gravimetric density of ∼4.01 wt %.
The results obtained from ab initio calculations on ZnM 2 O 4 (M = Co, Rh and Ir) compounds have been reported. The elastic constants, Bulk, Shear and Young modulus, and Poisson's ratios of the compounds are presented. In addition, full phonon dispersion curves and projected density of states of the compounds have been computed using the direct method. The lattice parameters (a) and internal parameters (u) are found to be in a good agreement with experimental results. According to both the B/G values and the Poisson's ratio, these compounds have covalent bondings. The analysis of the band structure of these compounds have indicated indirect band gaps of 1.25 eV for ZnCo 2 O 4 and 1.14 eV for ZnRh 2 O 4 and 0.86 eV for ZnIr 2 O 4 . The full phonon spectra of these compounds show that they are dynamically stable in the cubic spinel structure.
Ab initio calculations of structural, electronic, elastic, and phonon properties of TiRu3 and TiOs3 compounds have been studied using the density functional theory (DFT) within the generalized gradient approximation (GGA). The basic structural properties such as lattice constants, bulk modulus and pressure derivative of bulk modulus of these compounds were studied and compared with the previous theoretical data. Electronic band structures and partial densities of states for TiRu3 and TiOs3 compounds were computed and analyzed. The electronic band calculations showed that the TiRu3 and TiOs3 compounds have metallic nature. Phonon spectra, their total and projected densities of states for these compounds were computed by using a linear-response method in the framework of the density functional perturbation theory. The specific heat capacities at a constant volume CV and Debye temperature of TiCr3 and TiOs3 compounds were also calculated and discussed.
To study the structural, electronic and thermodynamic behavior of CeMgTl, full-potential linear augmented plane wave plus local orbital (FP-LAPW þ lo) method has been used. The lattice parameters (a 0 , c 0), bulk modulus (B 0) and its first order pressure derivative (B 0 0) have been calculated for CeMgTl. Band structure and density of states histograms depicts that "5d" orbital electrons of Tl have dominant character in the electronic contribution to CeMgTl. Impact of the temperature and pressure on unit cell volume, bulk modulus, Debye temperature, Grüneisen parameter, specific heat and thermal expansion coefficient (a) have been studied in wide temperature range (0e300 K) and pressure range (0e15 GPa).
Four series of ternary compounds RPdSb (R=Y,Ho,Er) , RPdBi (R=Nd,Y,Dy,Ho,Er) , RPd2Sb (R=Y,Gd-Er) , and RPd2Bi (R=Y,Dy-Er) were studied by means of magnetization, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermoelectric power, and Hall effect measurements, performed in the temperature range 1.5-300K and in magnetic fields up to 12T . All these ternaries, except for diamagnetic Y-based phases, exhibit localized magnetism of R3+ ions, and a few of them order antiferromagnetically at low temperatures (TN=2-14K) . The equiatomic compounds show half-metallic conductivity due to the formation of narrow gaps in their electronic band structures near the Fermi energy. Their Seebeck coefficient at room temperature is exceptionally high (S up to 200muV/K ), being promising for thermoelectric applications. In contrast, all the 1:2:1 phases are semimetals and their thermoelectric power is much lower (maximum S of 10-25muV/K ). The Hall effect in the compounds studied corroborates complex character of their electronic structure with multiple electron and hole bands with different temperature and magnetic field variations of carrier concentrations and their mobilities.
Heusler alloys ErPd2Sb, ErPd2Bi, ErPdSb and ErPdBi were studied by means of X-ray diffraction, magnetic susceptibility, electrical resistivity, thermoelectric power and Hall effect measurements. All these compounds are paramagnetic down to 1.7K due to localized magnetism of Er3+ ions. Their electrical behavior is governed by the formation of narrow gaps in the electronic band structures near EF. Both electrons and holes contribute to the conductivity, yet the dominant role is played by p-type carriers. The Hall response is strongly dependent on temperature and applied magnetic field, thus reflecting complex character of the underlying electronic structures. The Seebeck coefficient in ErPdSb is of the order of 150μV/K at room temperature making this material promising candidate for thermoelectric applications. Common to all four compounds studied is a rapid drop in the resistivity below 6–8K that seems manifest a phase transition of magnetic or superconducting origin, despite the lack of any corresponding anomalies in the low-temperature magnetic susceptibility and specific heat.
A theoretical investigation has been made on the phonon spectrum and heat capacity of polymorphs of carbon and boron nitride with special interests on the variation of Debye temperature and stiffness with temperature. A part of optical phonon branches of graphite exhibits higher frequencies than those of diamond. As a consequence, graphite shows smaller heat capacity and higher Debye temperature than diamond above a crossover temperature of 1000 K. This supports experimental reports of heat capacity although available experimental data are widely scattered. The higher Debye stiffness of graphite at above 1000 K is not contradictory to the fact that conventional stiffness of diamond is much larger than that of graphite, since the Debye stiffness is determined by both acoustic and optical phonons, whereas only acoustic phonons contribute to the conventional stiffness. The same trend was found between hexagonal and cubic boron nitrides with a crossover temperature of 600 K.
Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T ≈ 650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations. © 2000 American Institute of Physics.
A new compound UPd2Sb was prepared and studied by means of X-ray diffraction, magnetization, electrical resistivity, magnetoresistivity, thermoelectric power and specific heat measurements. The phase crystallizes with a cubic structure of the MnCu2Al-type (s.g. ). It orders antiferromagnetically at TN=55 K and exhibits a modified Curie–Weiss behaviour with reduced effective magnetic moment at higher temperatures. The electrical resistivity behaves in a manner characteristic of systems with strong electronic correlations, showing Kondo effect in the paramagnetic region and Kondo-like response to the applied magnetic field. The Seebeck coefficient exhibits a behaviour expected for scattering of conduction electrons on a narrow quasiparticle band near the Fermi energy. The low-temperature electronic specific heat in UPd2Sb is moderately enhanced being about 81 mJ/mol K2.
Martensitic and magnetic transformations of the Heusler Ni <sub>50</sub> Mn <sub>50-y</sub> X <sub>y</sub> ( X = In , Sn and Sb ) alloys were investigated by differential scanning calorimetry measurement and the vibrating sample magnetometry technique. In all these alloy systems, the austenite phase with the ferromagnetic state was transformed into the martensite phase, which means that these Heusler alloys have potential as Ga -free ferromagnetic shape memory alloys (FSMAs). Furthermore, multiple martensitic transformations, such as two- or three-step martensitic transformations, occur in all these alloy systems. It was confirmed by transmission electron microscopy observation that the crystal structure of the martensite phase is an orthorhombic four-layered structure which has not been reported in other FSMAs. Therefore, the present Ga -free FSMAs have the great possibility of the appearance of a large magnetic-field-induced strain.
Some general aspects of electronic structure of the XTZ semi-Heusler systems are discussed using the Korringa–Kohn–Rostoker computations within the LDA framework. An energy gap appears for a total number of 18 valence electrons, which corresponds to semiconductors. This gap is still maintained in the density of states in other XTZ compounds with different electron concentration (EC). This peculiar electronic structure gives rise to a variety of different physical properties, starting from EC=16 (FeTiSn) and EC=17 (FeTiSb, CoTiSn) (paramagnetic or ferromagnetic) metals, through semiconductors (NiTiSn, NiZrSn, NiYSb, FeVSb, CoTiSb, etc.) and EC=19 (CoVSb, CoNbSb, NiTiSb) metallic phases. The reason for the instability of the semi-Heusler phase of NiVSb (EC=20) is discussed using the total energy KKR analysis. The characteristic gap in the density of states is also maintained for the minority spin projection in the case of the half-metallic ferromagnetism in CoMnSb (EC=21) and NiMnSb (EC=22). Some of the semiconducting compounds (NiTiSn, FeVSb, NiYSb) seem to possess promising thermoelectrical properties.
This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.
This paper gives a short overview of the calculation of thermal properties of
materials from first principles, using the Quasi-Harmonic Approximation (QHA).
We first introduce some of the thermal properties of interest and describe how
they can be calculated in the framework of the QHA; then we briefly recall
Density-Functional Perturbation Theory as a tool for calculating phonons from
first principles, and present some codes that implement it; finally we review
recent applications of first-principle QHA.
The photoresponsivity spectra of double-barrier resonant tunneling diodes have been measured in a wide range of light wavelength as well as applied voltage. The complex behavior of measured spectra is analyzed taking into account different channels for electron injection into the quantum well (QW). It has been shown that the photoresponse in the infrared wavelength range could arise not only from the electrons excited inside the QW, but also from the two-dimensional electrons confined on the quantum level in the spacer accumulation layer. (C) 2000 American Institute of Physics. [S0003-6951(00)03127-2].
In this study, we perform first–principle calculations based on density functional theory (DFT) to obtain the ground state structural, elastic and dielectric properties of various ABO3 type ceramics and their {AxA′(1-x)}BO3 and A{BxB′(1-x)}O3 alloys. To represent alloy perovskites, we employ supercells with species A, A’ = Ba, Sr, Pb; B, B’ = Ti, Zr. The effects of composition and atomic configuration/order on lattice structure, thermodynamics, elastic constants and dielectric properties are evaluated. In calculations, we have used linear response and homogeneous field methods and we have also provided an assessment of the performance of these approaches in the determination of aforementioned properties.
We have computed dielectric and piezoelectric properties for the cubic form of alloy perovskites. Even though cubic form of alloy perovskites does not have any piezoelectric properties, owing to crystallographic site occupied by different type of atoms, the inversion symmetry breaks down and the structures develop a small tetragonality, in turn a small polarization and non-zero but quite small piezoelectric coefficients emerge as expected. For instance the observed maximum piezoelectric constant for
is . The magnitudes are smaller than the feasible ranges for actual application needs, but they may increase substantially upon phase to lower symmetry tetragonal forms transformation.
The structural, electronic, anisotropic elastic, and lattice dynamical properties of the M2AB (M = Ti, Zr, Hf;
A = Al, Ga, In) compounds belong to the family of MAX phases have been investigated by accomplishing
the first principles density functional theory (DFT) calculations with utilizing the generalizedegradient
approximation (GGA). Structural parameters, formation enthalpies, and X-ray diffraction patterns have
been calculated for all compounds. Electronic band structure and corresponding density of states (DOS)
have been obtained. Having negative formation enthalpy showed that, all compounds could be experimentally
synthesized. Also, among the nine different M2AB compounds, the most stable one has been
found as Hf2InB with respect to the formation enthalpies and band filling theory calculations. Moreover,
the elastic constants have been predicted using the stress-finite strain technique. The numerical estimations
of the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, Pugh's modulus, hardness,
thermal conductivities, and anisotropy factors have been studied. All compounds are found to have
low thermal conductivity and all compounds (except Zr involved ones) are hard materials and mechanically
stable. Furthermore, the phonon dispersion curves as well as corresponding phonon PDOS
have been plotted
The pressure dependent behaviour of the structural, electronic, mechanical, vibrational, and thermodynamic properties of Pd2TiX (X=Ga, In) Heusler alloys was investigated by ab initio calculations. The lattice constant, the bulk modulus and its first pressure derivative, the electronic band structure and the density of states (DOS), mechanical properties such as elastic constants, anisotropy factor, Young’s modulus, etc., the phonon dispersion curves and phonon DOS, entropy, heat capacity, and free energy were obtained under pressure. It was determined that the calculated lattice parameters are in good agreement with the literature, the elastic constants obey the stability criterion, and the phonon dispersion curves have no negative frequency which shows that the compounds are stable. The band structures at 0, 50, and 70 GPa showed valence instability at the L point which explains the superconductivity in Pd2TiX (X=Ga, In).
Starting from a 3×3 spectral problem, a Darboux transformation (DT) method for coupled Schrödinger (CNLS) equation is constructed, which is more complex than 2×2 spectral problems. A scheme of soliton solutions of an integrable CNLS system is realised by using DT. Then, we obtain the breather solutions for the integrable CNLS system. The method is also appropriate for more non-linear soliton equations in physics and mathematics.
First principles density functional theory (DFT) calculations have been used to investigate the structural, anisotropic elastic and electronic properties of ruthenium doped tungsten-diboride ternary compounds (W1-xRuxB2) for an increasing molar fraction of Ru atom from 0.1 to 0.9 by 0.1. Among the nine different compositions, W0.3Ru0.7B2 has been found as the most stable one due to the formation energy and band filling theory calculations. Moreover, the band structures and partial density of states (PDOS) have been computed for each x composition. After obtaining the elastic constants for all x compositions, the secondary results such as Bulk modulus, Young’s modulus, Poisson’s ratio, Shear modulus, and Vickers Hardness of polycrystalline aggregates have been derived and the relevant mechanical properties have been discussed. In addition, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young’s and Shear moduli.
The implications of the mathematical format of the embedded-atom method of computer modeling of metals have been studied with use of a simple nearest-neighbor analytic model for the fcc lattice. The physical inputs into the model are the atomic volume, the cohesive energy, the bulk modulus, the average shear modulus, the vacancy-formation energy, and the slope at the nearest-neighbor distance of the spherically averaged free-atom electron density calculated with Hartree-Fock theory. The model employs an exponential repulsion between nearest-neighboring atoms, an exponentially decreasing function for the free-atom electron density, and a universal equation relating the crystal energy and the lattice constant. The anisotropy ratio of the cubic shear moduli is constrained to be 2 with this model. The dependence of the energies for unrelaxed configurations for vacancy formation, divacancy binding, and low-index plane surfaces on the model parameters has been analyzed. The average shear modulus plays a dominant role in determining these energies relative to the bulk modulus or the cohesive energy because the slope of the embedding function at the equilibrium electron density is linear in the average shear modulus. Embedding functions are not uniquely determined in specific models, and it is shown that the embedding functions used in several models are essentially equivalent.
Phase equilibria in Al-Ti-Zr ternary system at 1273 K were experimentally investigated through alloy sampling combined with electron probe micro-analyzer (EPMA) and X-ray diffraction (XRD). No ternary compound was detected. Experimental results show that there is a continuous solid solution beta(Ti, Zr) which dissolves up to 25.5 at.% Al, and Ti can substitute Zr in most Al-Zr binary intermediate phases to a certain degree while the maximum solubility of Zr in alpha(2)(Ti3Al) and gamma(TiAl) reaches up to 25 at.% and 9 at.%, respectively. The isothermal section consists of 16 single-phase regions, 26 two-phase regions and 14 three-phase regions.
First principles calculations using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method in the framework of density functional theory (DFT) were performed to study the electronic structures and magnetic properties of new full-Heusler compounds Ti2VZ (Z=Al, Ga, and In). Electronic structure calculations showed that Ti2VZ (Z=Al, Ga, and In) compounds in AlCu2Mn-type are conventional ferrimagnets. The Ti2VAl, Ti2VGa, and Ti2VIn compounds in the CuHg2Ti-type structure have half-metallic characteristics with a respective majority band gap of 0.52, 0.51, and 0.59 eV at the equilibrium lattice parameter. The origin of half-metallicity in these compounds was also discussed. The total magnetic moments of Ti2VZ (Z=Al, Ga, and In) compounds in the CuHg2Ti-type structures were 2 μB per formula unit which were in agreement with Slater–Pauling rule (Mtot=18−Ztot). The Ti2VAl, Ti2VGa, and Ti2VIn compounds in the CuHg2Ti-type structure respectively showed half-metallic characteristics at lattice constants ranges of 6.12–7.17 Å, 5.99–7.12 Å, and 6.31–7.06 Å, indicating the lattice distortion did not affect the half-metallic properties of these compounds which makes them interesting materials in the spintronics field.
The full-potential linearized augmented plane wave method with the generalized gradient approximation for the exchange and correlation potential (LAPW-GGA) is used to understand the electronic and elastic properties of the first thorium-containing nitride perovskite TaThN3. Total and partial density of states, charge distributions as well as the elastic constants, bulk modulus, compressibility, shear modulus, Young modulus and Poisson ratio are obtained for the first time and analyzed in comparison with cubic ThN. The chemical bonding in TaThN3 is a combination of ionic Th–N and of mixed covalent–ionic Ta–N bonds. The cubic TaThN3 is semiconducting with the direct gap at about 0.65 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Full-Heusler compound Ti2NiAl with Hg2CuTi-type structure is demonstrated to be a new half-meltal ferromagnet by first-principles calculations. The compound has a complete (100%) spin polarization around the Fermi level in the total density of state. The band structure calculations show that the majority spin is strongly metallic, while the minority spin shows an insulating behavior. The compound has a total magnetic moment of −3.0μB per formula on first-principles calculations which complies well with the Slater–Pauling (SP) rule. Though having different atomic surroundings, the profiles of atom-projected density of states of Ti(A) and Ti(B) are similar. The half-metallic character is retained when the lattice constant ranging from −12.8% to +4.9%.
Mineralogists and geophysicists need to understand and predict the properties of solids and liquids at normal and especially at high pressures and temperatures. For example, they need to know the equilibrium structure, equation of state, phase transitions, and vibrational properties of solids, and the interatomic or intermolecular interaction needed for a molecular dynamics study of liquids (Stixrude et al. 1994; Soederlind and Ross 2000; Karki et al. 2001; Alfè et al. 2002; Steinle-Neumann et al. 2004; Sha and Cohen 2006; Carrier et al. 2007). This information, in sufficient detail, is not always available from experiment. Increasingly, it comes from the simple first-principles Kohn-Sham density functional theory (Kohn and Sham 1965; Parr and Yang 1989; Dreizler and Gross 1990; Perdew and Kurth 2003; Perdew et al. 2009a). Often the ground-state version of this theory suffices, since the electrons can stay close to their ground state even when the nuclear motion is thermally excited; there is however also a temperaturedependent version of die theory (Mermin 1965).
This paper deals with the ground state of an interacting electron gas in an external potential v(r). It is proved that there exists a universal functional of the density, Fn(r), independent of v(r), such that the expression Ev(r)n(r)dr+Fn(r) has as its minimum value the correct ground-state energy associated with v(r). The functional Fn(r) is then discussed for two situations: (1) n(r)=n0+n(r), n/n01, and (2) n(r)= (r/r0) with arbitrary and r0. In both cases F can be expressed entirely in terms of the correlation energy and linear and higher order electronic polarizabilities of a uniform electron gas. This approach also sheds some light on generalized Thomas-Fermi methods and their limitations. Some new extensions of these methods are presented.
The ground state of the Heusler heavy fermion compound CeInCu2 has been investigated by 63Cu nuclear magnetic resonance. Temperature dependences of the linewidth of spectrum and spin-lattice relaxation time, T1, reveal two antifferomagnetic-like transitions at TN1 = 1.6 and TN2 = 1.1 K.
CeInCu2 is a heavy fermion compound with specific heat coefficient gammaMax = 1.4J/mol.K2, but the ground state is antiferromagnetic. There exists a disorder between the Ce atoms and the In atoms in the Heusler structure. Order parameter is 0.8.
High resolution specific heat measurements have been performed on a single crystal NdCu6 in applied magnetic fields up to 5 T. A magnetic field versus temperature phase diagram is presented.
Relations between the elastic and plastic properties of pure polycrystalline metals are discussed and a systematic relation between shear modulus, Burgers vector and plastic shear strength of metals possessing the same lattice structure is proposed. In addition reasons are given for believing that in a limited temperature range malleability is related to Poisson's ratio.
A measurement of crystal parameters around TN for Cr2O3 by neutron diffraction was performed. The dependence on crystal parameters of the magnetic interactions in Cr2O3 discussed in the literature was recalculated using a modified Oguchi theory and the known experimental data of the change of TN with pressure. The controversy in sign in these experiments is discussed. The change across TN of the largest exchange parameter Δ|J1| and the double sum Δ|Y| of the remaining parameters were estimated as well as their change under pressure.
Almost 50 ternary transition-metal aluminides have been reported in the ordered BiF3 structure, representing roughly half of the observed ordered ternaries. To investigate the possible occurrence of other aluminides in this structure, the heats of formation of 38 ordered M2NAl ternaries, most not reported to occur, have been calculated. While all but one is stable relative to the elemental metals, the test for ternary stability requires comparison of the ternary heat with competing two and three phase mixtures of binary phases. Of the BiF3 ternaries hitherto unreported, nine are estimated to occur while eighteen are found to be unstable. The pattern of occurrence obtained from experiments and calculations taken together suggests that on the order of another 20 compounds beyond those predicted here could exist. Thus, this important ordered phase is more pervasive than previously suggested by experiment.
The continuum theory of elasticity has been used for more than a century and has applications in many fields of science and engineering. It is very robust, well understood and mathematically elegant. In the isotropic case elastic properties are easily represented, but for non-isotropic materials, even in the simple cubic symmetry, it can be difficult to visualise how properties such as Young's modulus or Poisson's ratio vary with stress/strain orientation. The ElAM (Elastic Anisotropy Measures) code carries out the required tensorial operations (inversion, rotation, diagonalisation) and creates 3D models of an elastic property's anisotropy. It can also produce 2D cuts in any given plane, compute averages following diverse schemes and query a database of elastic constants to support meta-analyses. distributed program, including test data, etc.: 43 848 No. of bytes in distributed program, including test data, etc.: 2 498 882 Distribution format: tar.gz Programming language: Fortran90 Computer: Any Operating system: Linux, Windows (XP, Vista) RAM: Depends chiefly on the size of the arrays representing elastic properties in 3D Classification: 7.7 Nature of problem: Representation of elastic moduli and ratios, and of wave velocities, in 3D; automatic discovery of unusual elastic properties. Solution method: Stiffness matrix (6 × 6) inversion and conversion to compliance tensor (3 × 3 × 3 × 3), tensor rotation, dynamic matrix diagonalisation, simple optimisation, postscript and VRML output preparation. Running time: Dependent on angular accuracy and size of elastic constant database (from a few seconds to a few hours). The tests provided take from a few seconds for test0 to approximately 1 hour for test4.
A continuous-wave laser action at 929 nm from a Nd3+:LiNbO 3 nonlinear laser crystal co-doped with ZnO was reported. The parameters of relevance in the quasi-three-level laser performances were investigated. It was shown that the emission cross section at laser wavelength was found to be similar to that of the Nd:YAG laser line at 946 nm. Using the QPM self-frequency doubling of the fundamental laser line, it was observed that the Nd3+ doped LINbO3:ZnO emerged as a promising laser around 930 and 465 nm.
It is shown that the alloys Co2Ni1−xGa1+x, x = 0.06, 0.09, 0.12, 0.15, are ferromagnetic shape memory alloys. In the as-solidified state their martensite start temperatures vary in the range 20 °C<T<60 °C as the concentration parameter x decreases. The high and low temperature phases are body centered cubic and orthorhombic and/or monoclinic, respectively, The transformation hysteresis, i.e. the difference between the martensite and austenite start temperatures, equals approximately 30 degrees. The saturation magnetization of the alloys resembles that of nickel while their coercive force is of the order of 100mT.
From a theory of Hohenberg and Kohn, approximation methods for treating an inhomogeneous system of interacting electrons are developed. These methods are exact for systems of slowly varying or high density. For the ground state, they lead to self-consistent equations analogous to the Hartree and Hartree-Fock equations, respectively. In these equations the exchange and correlation portions of the chemical potential of a uniform electron gas appear as additional effective potentials. (The exchange portion of our effective potential differs from that due to Slater by a factor of 23.) Electronic systems at finite temperatures and in magnetic fields are also treated by similar methods. An appendix deals with a further correction for systems with short-wavelength density oscillations.
The band structure of Mn-based Heusler alloys of the C1b crystal structure (MgAgAs type) has been calculated with the augmented-spherical-wave method. Some of these magnetic compounds show unusual electronic properties. The majority-spin electrons are metallic, whereas the minority-spin electrons are semiconducting.
We present a fully [ital ab] [ital initio] calculation of the thermodynamic properties for silicon within the quasiharmonic approximation, making use of volume-dependent phonon frequencies obtained from pseudopotential local-density calculations. The temperature dependence of the thermal-expansion coefficient, specific heat (at constant volume), and other related quantities are studied. We confirm that the thermal-expansion coefficient behaves differently in three temperature regions: positive for temperature below 15 K, negative between 15 and 125 K, and positive again above 125 K. This finding agrees with experiment. The abnormal (negative) thermal-expansion coefficient at low temperatures is explained through a detailed study of mode Grueneisen parameters. Both specific-heat and thermal-expansion-coefficient values calculated are in excellent agreement with experiment up to a few hundred kelvin.
We present a sampling method for Brillouin-zone integration in metals which converges exponentially with the number of sampling points, without the loss of precision of normal broadening techniques. The scheme is based on smooth approximants to the delta and step functions which are constructed to give the exact result when integrating polynomials of a prescribed degree. In applications to the simple-cubic tight-binding band as well as to band structures of simple and transition metals, we demonstrate significant improvement over existing methods. The method promises general applicability in the fields of total-energy calculations and many-body physics.
We present a new scheme to study the linear response of crystals which combines the advantages of the dielectric-matrix and supercell (``direct'') approaches yet avoids many of their drawbacks. The numerical complexity of the algorithm is of the same order as that of a self-consistent calculation for the unperturbed system. The method is not restricted to local perturbations as the dielectric-matrix one nor to short wavelengths as the direct one. As an application, we calculate the long-wavelength optical phonons in Si and GaAs, both transverse and longitudinal, using norm-conserving pseudopotentials, and without any use of supercells.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
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