No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Electronic, magnetic and transport properties of anti-ferromagnetic semiconductor BaGd2X4(X S, Se)
... Furthermore, the studied structure exhibits dynamical stability, as evidenced by the absence of imaginary frequencies. For an in-depth discussion on dynamical stability [53], refer to the supplementary materials provided. The mechanical properties of the materials, focusing on aspects such as modulus, are illustrated in Fig. 3. Analysis of the bulk modulus data indicates a pronounced resistance to compression, suggesting that substantial pressure is needed to compress these materials significantly. ...
The tunable optical characteristics and superior thermal stability of Indium and Thallium-based quaternary chalcogenides are significant. We studied the intricate relationship between the optoelectronic, and thermoelectric features of notable BaXCu3Se4 (X = In, Tl) quaternary chalcogenides. Both the maximum of the valence band and conduction band coincide at the Γ-point, confirming these materials as direct band gap materials. By substituting Indium for thallium, the calculated band gap decreases from 0.71 eV to 0.53 eV. These anions have a considerable impact and contribute to a decrease in the energy gap via valence electrons. Partially filled d orbitals of copper play an important role in electronic states at the Fermi level. The components of the complex dielectric function, as well as other important optical parameters, are examined and analyzed for the potential usage in optoelectronic devices. The ε 1(ω) becomes negative at 6.32 eV, suggesting that the medium is reflecting all of the incident light. Thallium affects the absorption spectrum because it changes the density of states and electronic transitions. The absorption spectra indicated that the material absorbs in the visible and near-ultraviolet parts of the spectrum, which is fascinating and might have applications in optoelectronics. The investigated materials are appropriate to be used for thermoelectric devices confirmed by their significant and notable thermoelectric properties. Because the Seebeck coefficient is negative, most charge carriers, typically electrons, flow from the higher temperature area to the lower temperature region. At both low and high temperatures, thallium is accountable for BaTlCu3Se4’s higher thermal conductivity than BaInCu3Se4 material.
In this study, the computational functionals GGA and mBj of density functional theory (DFT) were employed to explore the electronic structure of tantalum based double perovskite oxide Ba2CoTaO6. Computed tolerance (t) and octahedral (µ) factors deduce the cubic stability of the perovskite crystal. Negative formation energy indicates that the system is thermodynamically stable. The elastic constants satisfy the conditions for cubic stability and the Pagh’s index (B/G) evinces the brittle nature. The band structure and total density of states reveal the half-metallic nature with a 4.7 eV insulator indirect energy band gap at the Fermi level in the spin up direction. An integer-valued (4µB) magnetic moment is observed to favor the half-metallic ferromagnetism in the double perovskite oxide Ba2CoTaO6. Optical properties play a significant role in the design and functionality of optical devices and systems. In this aspect, the two parts of the dielectric function (real and imaginary) and the related parameters have been explored. The observed properties provide insights into possible applications of this system in spintronics and optoelectronics.
The structural, electronic, and magnetic properties of novel half-Heusler alloys ScXGe (X = Mn, Fe) are investigated using the first principle full potential linearized augmented plane wave approach based on density functional theory (DFT). To attain the desired outcomes, we employed the exchange–correlation frameworks, specifically the local density approximation in combination with Perdew, Burke, and Ernzerhof's generalized gradient approximation plus the Hubbard U parameter method (GGA + U) to highlight the strong exchange–correlation interaction in these alloys. The structural parameter optimizations, whether ferromagnetic (FM) or nonmagnetic (NM), reveal that all ScXGe (where X = Mn, Fe) Heusler alloys attain their lowest ground state energy during FM optimization. The examination of the electronic properties of these alloys reveals their metallic character in both the spin-up and spin-down channels. The projected densities of states indicate that bonding is achieved through the hybridization of p–d and d–d states in all of the compounds. The investigation of the magnetic properties in ScXGe (where X = Mn, Fe) compounds indicates pronounced stability in their ferromagnetic state. Notably, the Curie temperatures for ScXGe (X = Mn, Fe) are determined to be 2177.02 K and 1656.09 K, respectively. The observation of metallic behavior and the strong ferromagnetic characteristics in ScXGe (X = Mn, Fe) half-Heusler alloys underscores their potential significance in the realm of spintronic devices. Consequently, our study serves as a robust foundation for subsequent experimental validation.
Lead-free inorganic Ge-based perovskites GaGeX3 (X = Cl, Br, and I) are promising candidates for solar cell applications due to their structural, mechanical, electrical, and optical properties. In this work, we performed density functional theory (DFT) calculations using the CASTEP module to investigate these properties in detail. We found that the lattice parameters and cell volumes increase with the size of the halogen atoms, and that all the compounds are stable and ductile. GaGeBr3 has the highest ductility, machinability, and lowest hardness, while GaGeCl3 has the highest anisotropy. The band gap values, calculated using the GGA-PBE and HSE06 functionals, show a direct band gap at the R–R point, ranging from 0.779 eV and 1.632 eV for GaGeCl3 to 0.330 eV and 1.140 eV for GaGeI3. The optical properties, such as absorption coefficient, conductivity, reflectivity, refractive index, extinction coefficient, and dielectric function, are also computed and discussed. We observed that the optical properties improve with the redshift of the band gap as Cl is replaced by Br and I. GaGeI3 has the highest dielectric constant, indicating the lowest recombination rate of electron–hole pairs. Our results suggest that GaGeX3 (X = Cl, Br, and I) can be used as effective and non-toxic materials for multijunction solar cells and other semiconductor devices.
In our previous works [J. Phys. Chem. Solids 163 (2022) 110600], the CrLaCoAl alloy had been predicted to be half-metallic ferrimagnet with high Curie temperature, and it met the dynamic, mechanical and thermal stabilities. Furthermore, a smaller convex hull indicated that it was likely to be prepared in experiment. However, the half-metallicity of CrLaCoAl alloy is possible to be destroyed by defect. Therefore, it is necessary to study the stability, electronic and magnetic properties of defective CrLaCoAl. Results show that the formation energies of the Cr–La, Co–Al swap and Al(Cr), Co(La) antisite are negative, indicating that they are likely to be formed in the process of crystal growth. Among them, the formation energy of the Al(Cr) antisite is the lowest in defects, and the spin polarization is as high as 93.72%. In addition, the spin polarization of all antisites is over 60% except for the Cr(Al), La(Cr) and La(Al) antisite. High spin polarization in defect is favorable in spintronic application.
High-throughput computational screening of materials with targeted thermal conductivity (κ) plays an important role in promoting the advancement of material design and enormous applications. The Slack model has been widely applied for the fast evaluation of κ with minimal time and resources, showing the potential capability of high-throughput screening of κ. However, after examining the Slack model on a large set of 353 materials, a huge discrepancy is found between the predicted κ and the correspondingly measured κ in experiments for some materials in addition to the generally overestimated κ by the Slack model. Thus, it is necessary to optimize the Slack model for efficiently and accurately evaluating κ. In this study, based on the high-throughput comparison of the κ predicted by the Slack model using elastic properties and those measured in experiments, an optimized Slack model is proposed. As a result, the κ predicted by the optimized Slack model agrees reasonably with the κ measured in experiments, which is much better than the previous prediction. The optimized Slack model proposed in this study can be used for further high-throughput computational evaluation of κ, which would be helpful for finding materials of ultrahigh or ultralow κ with broad applications.
Thermoelectrics, which can generate electricity from a temperature difference, or vice versa, is a key technology for solid‐state cooling and energy harvesting; however, its applications are constrained owing to low efficiency. Since the conversion efficiency of thermoelectric devices is directly obtained via a figure of merit of materials, zT, which is related to the electronic and thermal transport characteristics, the aim here is to elucidate physical parameters that should be considered to understand transport phenomena in semiconducting materials. It is found that the weighted mobility ratio of the majority and minority carrier bands is an important parameter that determines zT. For nanograined Bi–Sb–Te alloy, the unremarked role of this parameter on temperature‐dependent electronic transport properties is demonstrated. This analysis shows that the control of the weighted mobility ratio is a promising way to enhance zT of narrow bandgap thermoelectric materials. Suppression of the bipolar conduction in narrow bandgap thermoelectric materials is crucial for improving their device efficiency. This work correlates the weighted mobility ratio in thermoelectric figure of merit and bipolar conduction in bismuth‐telluride‐based alloys. The results suggest that increasing weighted mobility ratio suppresses the bipolar conduction most effectively among other parameters like carrier concentration and bandgap.
The electronic and magnetic properties of strontium hexa-ferrite (SrFe12O19) are studied in pure state (SrFe12O19) and with dopant in the positions 2 and 3 of Fe atoms (SrGdFe11O19-I and SrGdFe11O19-II, respectively) by utilizing a variety of the density functional theory (DFT) approaches including the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) and GGA plus Hubbard U parameter (GGA+U). The pure SrFe12O19 is a hard magnetic half-metal with an integer magnetic moment of 64.00μ B, while using the GGA+U functional, the magnetic intensity increases, resulting in a magnetic semiconductor with a high integer magnetic moment of 120μ B. By doping the Gd atom in the two different positions of Fe, the magnetic moment is increased to 71.68μ B and 68.00μ B, respectively. The magnetic moment increases and remains an integer; hence, SrGdFe11O19-II can be very useful for application in magnetic memories. Moreover, applying the Hubbard parameter turns SrGdFe11O19-I and SrGdFe11O19-II to magnetic semiconductors with a magnetic moment of 124μ B, and the energy gap of both doped structures at spin down is found to be less than the pure case. By studying the electronic density diagram of the atoms of the crystal, it is found that the major effect to create magnetization in the pure case is due to the Fe atom. However, in the doped case, the elements Gd and Fe have the highest moment in the crystal respectively.
By using WIEN2k code, we investigated the mechanical and thermoelectric properties of magnesium based MgX2O4 (X = Rh and Bi) spinels. To compute the mechanical behavior of MgX2O4 (X = Rh and Bi), the Perdew-Bruke-Ernzerhof (PBEsol) flavor of generalized gradient approximation is used. From structural optimization, ground state lattice constant (a0) show a comparable with the previously evaluated theoretical and experimental values. The Born stability criterion represents that the investigated spinels are stable in the cubic phase and their ductile behaviors are observed by calculating Pugh’s ratio as well as Poisson ratio. Besides, thermodynamic behavior is concluded in terms of the Debye temperature. To investigate the electronic and thermoelectric behavior, the modified Becke and Johnson (mBJ) potential is employed. Finally, we investigated the thermoelectric behavior to represent the importance of studied spinels in thermoelectric appliances by calculating the figure of merit (ZT). High values of the See-beck coefficient and ZT at room temperature explores the potential of the studied spinels in thermoelectric devices.
Based on high-throughput density functional theory calculations, we performed a systematic screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys XX′YZ (X, X′, and Y are transition metal elements except Tc, and Z is one of B, Al, Ga, In, Si, Ge, Sn, Pb, P, As, Sb, and Bi). Following an empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12 000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we have identified 70 so far unreported SGSs, confirmed by explicit electronic structure calculations with proper magnetic ground states, of which 17 candidates have a distance to the convex hull smaller than 0.10 eV/atom. It is demonstrated that all four types of SGSs can be realized, defined based on the spin characters of the bands around the Fermi energy. Type-II SGSs show promising transport properties for spintronic applications. The effect of spin-orbit coupling is investigated, resulting in large anisotropic magnetoresistance and anomalous Nernst effects.
We investigated the thermoelectric transport mechanism of ZrNiSn and ZrNiPb Half-Heuslers within the Boltzmann transport theory, under constant relaxation time approximation. The prediction of band structure behavior along the high symmetry Brillouin zone and density of states are used to establish relations among various transport coefficients like Seebeck coefficient, electrical conductivity, thermal conductivity and power factor which have been found to be in good agreement with the experimental results. The Seebeck coefficient variation show the n-type behavior of heat carriers. The electrical and thermal conductivity show a semiconducting nature of bands along the Fermi level. The lattice part of thermal conductivity show an exponential decreasing trend along the high temperatures. Power factor variation and figure of merit show the heavier element doping of ZrNiSn results in descent increase in efficiency triggering its applicability. The overall measurements show that semi classical Boltzmann transport theory has well behaved potential in predicting the transport properties of the compounds.
We report on the implementation of a tool for the analysis of second-order elastic stiffness tensors, provided with both an open-source Python module and a standalone online application providing visualization tools of anisotropic mechanical properties. After describing the software features, how we compute the conventional elastic constants and how we represent them graphically, we explain our technical choices for the implementation. In particular, we focus on why a Python module is used to generate the HTML web page with embedded Javascript for dynamical plots.
The Open Quantum Materials Database (OQMD) is a high-throughput database currently consisting of nearly 300,000 density functional theory (DFT) total energy calculations of compounds from the Inorganic Crystal Structure Database (ICSD) and decorations of commonly occurring crystal structures. To maximise the impact of these data, the entire database is being made available, without restrictions, at www.oqmd.org/download. In this paper, we outline the structure and contents of the database, and then use it to evaluate the accuracy of the calculations therein by comparing DFT predictions with experimental measurements for the stability of all elemental ground-state structures and 1,670 experimental formation energies of compounds. This represents the largest comparison between DFT and experimental formation energies to date. The apparent mean absolute error between experimental measurements and our calculations is 0.096 eV/atom. In order to estimate how much error to attribute to the DFT calculations, we also examine deviation between different experimental measurements themselves where multiple sources are available, and find a surprisingly large mean absolute error of 0.082 eV/atom. Hence, we suggest that a significant fraction of the error between DFT and experimental formation energies may be attributed to experimental uncertainties. Finally, we evaluate the stability of compounds in the OQMD (including compounds obtained from the ICSD as well as hypothetical structures), which allows us to predict the existence of ~3,200 new compounds that have not been experimentally characterised and uncover trends in material discovery, based on historical data available within the ICSD.
Phonon plays essential roles in dynamical behaviors and thermal properties,
which are central topics in fundamental issues of materials science. The
importance of first principles phonon calculations cannot be overly emphasized.
Phonopy is an open source code for such calculations launched by the present
authors, which has been world-widely used. Here we demonstrate phonon
properties with fundamental equations and show examples how the phonon
calculations are applied in materials science.
Here we propose a new concept in which incorporated magnetic ions create spin entropy, a narrowed band-gap and stronger anharmonic phonon coupling to obtain a larger Seebeck coefficient, higher electrical conductivity and lower thermal conductivity for significant improvement of the ZT value. This idea is experimentally achieved both with magnetic ion doping and full substitution of quaternary chalcogenide nanocrystals: the ZT value of Ni-doped Cu2ZnSnS4 is extraordinarily enhanced by 7.4 times compared to that of pure Cu2ZnSnS4, while Cu2CoSnS4 shows a 9.2 times improvement.
Batteries that shuttle multi-valent ions such as Mg2+ and Ca2+ ions are promising candidates for achieving higher energy density than available with current Li-ion technology. Finding electrode materials that reversibly store and release these multi-valent cations is considered a major challenge for enabling such multi-valent battery technology. In this paper, we use recent advances in high-throughput first-principles calculations to systematically evaluate the performance of compounds with the spinel structure as multivalent intercalation cathode materials, spanning a matrix of five different intercalating ions and seven transition metal redox active cation. We estimate the insertion voltage, capacity, thermodynamic stability of charged and discharged states, as well as the intercalating ion mobility and use these properties to evaluate promising directions. Our calculations indicate that the Mn2O4 spinel phase based on Mg and Ca are feasible cathode materials. In general, we find that multivalent cathodes exhibit lower voltages compared to Li cathodes; the voltages of Ca spinels are ~ 0.2V higher than those of Mg compounds (versus their corresponding metals), and the voltages of Mg compounds are ~1.4 V higher than Zn compounds; consequently, Ca and Mg spinels exhibit the highest energy densities amongst all the multivalent cation species. The activation barrier for the Al3+ ion migration in the Mn2O4 spinel is very high (~1400 meV for Al3+ in the dilute limit); thus, the use of an Al based Mn spinel intercalation cathode is unlikely. Amongst the choice of transition metals, Mn-based spinel structures rank highest when balancing all the considered properties.
Motivated by the recent findings of potential topological insulators and good high-temperature thermoelectric materials in the half-Heusler compounds based on heavy metals, we combine first-principles and Boltzmann transport theory to systematically study the electronic structure and thermoelectric performance of half-Heusler compounds MYSb (M = Ni, Pd, Pt). We find that among these three compounds PtYSb is a direct band-gap semiconductor, while the remaining two show indirect band-gaps. Chemical potential dependence of Seebeck coefficients and power factors (PFs) are discussed in terms of electronic band structure and density of states near the Fermi level. Calculated Seebeck coefficients of these compounds are found to be about 245 µV K−1 at room temperature, and n-type doping would present a better PF. NiYSb has a high PF of 9.5 × 1011 W K−2 m−1 s−1 at 900 K and thus exhibits good high-temperature thermoelectricity. The temperature dependence of Seebeck coefficients under different carrier concentrations are also presented, which are in good agreement with available theories and experiments.
A theoretical formalism to calculate the single crystal elastic constants for orthorhombic crystals from first principle calculations is described. This is applied for TiSi2 and we calculate the elastic constants using a full potential linear muffin-tin orbital method using the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. From this the calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young's modulus, and Poisson's ratio for ideal polycrystalline TiSi2 are also calculated and compared with corresponding experimental values. The directional bulk modulus and the Young's modulus for single crystal TiSi2 are estimated from the elastic constants obtained from LDA as well as GGA calculations and are compared with the experimental results. The shear anisotropic factors and anisotropy in the linear bulk modulus are obtained from the single crystal elastic constants. From the site and angular momentum decomposed density of states combined with a charge density analysis and the elastic anisotropies, the chemical bonding nature between the constituents in TiSi2 is analyzed. The Debye temperature is calculated from the average elastic wave velocity obtained from shear and bulk modulus as well as the integration of elastic wave velocities in different directions of the single crystal. The calculated elastic properties are found to be in good agreement with experimental values when the generalized gradient approximation is used for the exchange and correlation potential.
We demonstrate how by taking better account of electron correlations in the 3d shell of metal ions in nickel oxide it is possible to improve the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin density functional theory.
We report on the effects of partial substitution of nickel by palladium on the thermoelectric properties of ZrNiSn-based half-Heusler compounds. It is shown that the substitution of palladium for nickel results in a significant, beneficial reduction of the thermal conductivity. The Seebeck coefficient also decreases, but only by a small amount. In the Hf0.5Zr0.5Ni0.8Pd0.2Sn0.99Sb0.01Hf0.5Zr0.5Ni0.8Pd0.2Sn0.99Sb0.01 compound, a power factor of 22.1 μW K−2 cm−1 and a thermal conductivity as low as 4.5 W/m K are measured at room temperature. The dimensionless figure of merit ZT increases with increasing temperature and reaches a maximum value of 0.7 at about 800 K. © 2001 American Institute of Physics. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/70206/2/APPLAB-79-25-4165-1.pdf
Quaternary Heusler alloys with high spin polarization and high Curie temperature, and lower Gilbert damping parameters have broad application prospects in spintronics field, which motivates us to design the alloys with above-mentioned properties. In the current manuscript, we systematically studied the electronic structure, magnetic properties and Gilbert damping parameters of quaternary Heusler alloys CrYCoX (X= Si, Ge, Sn, Pb). Electronic calculations show that the CrYCoX alloys are half-metallic ferrimagnets, and their integer total magnetic moments obey the Slater–Pauling rule: Mt = Zt-18. Origin of half-metallic gap is discussed by hybridization, occupation and distribution of electronic states. According to the Heisenberg model, we calculated the exchange coupling parameters of CrYCoX alloys, and find that the competition and cooperation between d−d exchange, super-exchange and RKKY exchange lead to the emergence of ferrimagnetic phase. Curie temperatures of CrYCoX alloys calculated by mean-field approximation and Monte Carlo simulation are significantly higher than room temperature. Finally, Gilbert damping parameters of CrYCoX alloys calculated by via linear response theory are localized in 0.0051∼0.0136 at 300 K, indicating they have lower energy loss and higher response speed as microwave and spintronic materials. Our results show that the CrYCoX alloys are promising candidates in room microwave and spintronic devices.
Using the first-principles calculations, we study the stability, electronic and magnetic properties of monolayer, bilayer and trilayer Cr2NO2 and Cr2CO2. Stability is estimated from the thermal, mechanical and dynamical perspectives. Calculations show that the monolayer, bilayer and trilayer Cr2NO2 and Cr2CO2 are ferromagnetic ground states, and Cr2CO2 are nearly half-metals, and Cr2NO2 are half-metals with an integer total magnetic moment. For the monolayer, bilayer and trilayer Cr2NO2 and Cr2CO2, the Z-axis direction is their easy magnetization axis, and their magnetic anisotropy energies are obviously increased with the increase of layered number, the improvement of magnetic anisotropy energy origins from the enhancement of spin–orbit coupling strength. In addition, we also discuss the origin of gaps for monolayer, bilayer and trilayer half-metallic Cr2NO2.
The spin-dependent magnetic, thermoelectric, and electronic characteristics of Mg-based spinels MgGd2X4 (X = S, Se) are investigated using density functional theory (DFT) calculations. The modified Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol-GGA) is utilized to compute the structural and elastic characteristics. In addition, the magnetic, thermoelectric, and electronic characteristics are explored employing the modified-Becke and Johnson (mBJ) potential. After the energy difference calculations among the ferromagnetic (FM) and nonmagnetic (NM) phases, the stability of the structures is confirmed in the FM phase. Further, thermodynamic stability is determined by calculating formation energies and phonon dispersion spectra. The Charpin method is used to analyze elastic characteristics in detail. The total magnetic moments are produced by 2p-states of chalcogenides and f-states of Gd atoms, which result in substantial hybridization at the Fermi level. Finally, electronic-thermal coefficients, such as the power factor, Seebeck coefficient, and thermal and electrical conductivity, are investigated, concluding that investigated spinels can be used in thermoelectric devices.
We systematically study the stability, Gilbert damping parameters, electronic structure, exchange interaction and Curie temperature of CrYCoZ (Z = Al, Ga, In, Tl) alloys. Stability is estimated from the perspective of thermodynamics, dynamics and mechanics. Gilbert damping parameters are calculated by using the linear response formalism, and the values 0.6∼4.1× 10⁻³ are observed at room temperature. Electronic calculations show that the CrYCoZ alloys are half-metallic ferrimagnets, and their integer magnetic moments follow the Mt=Zt−18. Origin of gap is discussed by the hybridization, distribution and occupation of electronic states as well as the charge transfer. In addition, the effect of GGA+U on electronic structure is also discussed. Based on the Heisenberg model, we calculate the exchange interactions between atomic components, and analyze the competition and collaboration between d–d exchange, super-exchange and RKKY exchange, finally offer the dominant mechanism of appearance of ferrimagnetic order phase. Furthermore, we calculate the Curie temperatures of CrYCoZ alloys, which are much higher than room temperature. Overall, the CrYCoZ alloys can be considered as a prospective candidates in spintronics application.
Using first-principles calculations, we predict the CrLaCoZ (Z = Al, Ga, In, Ge, Sn, Pb) Heusler alloys to be half-metals, and their total magnetic moments follow the Slater-Pauling rule Mt = Zt − 18. Structural stability is estimated in terms of thermodynamics, dynamic, mechanical and thermal. A comparison in total energy shows that the CrLaCoZ alloys have ferrimagnetic ground states, and the competition between d-d exchange, super-exchange and RKKY exchange interactions leads to the appearance of magnetic order phase. Phase separation shows that these alloys may be synthesized in a non-equilibrium process. The formation of half-metallic gap mainly ascribes to the d-d and p-d orbital hybridizations. Furthermore, we find that the value of gap is decided by the strength of exchange splitting between eg and t2g states according the distribution and occupation of electronic states. Presence of X and DO3 disorders destroys their half-metallicity. From the calculated exchange interactions, it is found that the Cr(A)-Cr(A), Cr(A)-La(B) and Cr(A)-Co(C) exchanges play a leading role in all interactions, and finally determine the Curie temperatures. Apart from the CrLaCoSn alloy, other alloys have evidently higher Curie temperatures than room temperature, which is favorable as an electrode materials in magnetic tunnel junction.
Thermoelectrics is attractive as a green and sustainable way for harnessing waste heat and cooling applications. Designing high performance thermoelectrics involves navigating the complex interplay between electronic and heat transports. This fundamentally involves understanding the scattering physics of both electrons and phonons, as well as maximizing symmetry-breaking in entropy and electronic transports. In the last two decades, thermoelectrics have progressed in leaps and bounds thanks to parallel advancements in scientific technologies and physical understandings. Figure of merit zT of 2 and above have been consistently reported in various materials, especially Chalcogenides. In this review, we provide a broad picture of physically driven optimization strategies for thermoelectric materials, with emphasis on electronic transport aspect of inorganic materials. We also discuss and analyzes various newly coined metrics such as quality factors, electronics quality factor, electronic fitness function, weighted mobility, and Fermi surface complexity factor. More importantly, we look at the non-trivial interdependencies between various physical parameters even at a very fundamental level. Moving forward, we discuss the outlook for the potential of 3D printing and device oriented research in thermoelectrics. The intuition derived from this review will be useful not only to guide materials selection, but also research directions in the coming years.
In the present study, NaXH3 (X = Mn, Fe, Co) perovskite type hydrides have been investigatednby performing first-principles calculation. The results of the structural optimizations show that all these compounds have negative formation energy implying the thermodynamic stability and synthesisability. The mechanical stability of these compounds has been studied with the elastic constants. Moreover, the polycrystalline properties like bulk
modulus, Poisson's ratio, etc. have been obtained using calculated elastic constants of
interest compounds. The electronic properties have been studied and band structures have
been drawn with the corresponding partial density of states. These plots indicated that
NaXH3 hydrides show metallic characteristics. The charge transfer characteristics in these
compounds have been studied with the Bader partial charge analysis. The phonon
dispersion curves and corresponding density of states indicated that NaXH3 compounds
are dynamically stable compounds. The investigation on hydrogen storage characteristics
of NaXH3 compounds resulted in hydrogen storage capacities of 3.74, 3.70 and 3.57 wt% for
X = Mn, Fe and Co, respectively. The present study is the first investigation of NaXH3
perovskite type hydrides as known up to date and may provide remarkable contribution to
the future researches in hydrogen storage applications.
The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for spin-gapless semiconductors (SGSs). However, to date, there have been no experimental attempts at fabricating a junction device. This paper reports a fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction measurements show that the (001) CoFeCrAl electrode films with atomically flat surfaces have a B2-ordered phase. The saturation magnetization is 380 emu/cm3, almost the same as the value given by the Slater–Pauling–like rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87% and 165%, respectively. Cross-sectional electron diffraction analysis shows that the MTJs have MgO interfaces with fewer dislocations. The temperature- and bias-voltage dependence of the transport measurements indicates magnon-induced inelastic electron tunneling overlapping with the coherent electron tunneling. X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic arrangement of the Co and Fe magnetic moments of B2-ordered CoFeCrAl, in contrast to the ferrimagnetic arrangement predicted for the Y -ordered state possessing SGS characteristics. Ab-initio calculations taking account of the Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect of the Cr-Fe swap disorder on the ability for electronic states to allow coherent electron tunneling is discussed.
Magnesium-ion batteries are a promising energy storage technology because of their higher theoretical energy density and lower cost of raw materials. Among the major challenges has been the identification of cathode materials that demonstrate capacities and voltages similar to lithium-ion systems. Thiospinels represent an attractive choice for new Mg-ion cathode materials owing to their interconnected diffusion pathways and demonstrated high cation mobility in numerous systems. Reported magnesium thiospinels, however, contain redox inactive metals such as scandium or indium, or have low voltages, such as MgTi2S4. This article describes the direct synthesis and structural and electrochemical characterization of MgCr2S4, a new thiospinel containing the redox active metal chromium and discusses its physical properties and potential as a magnesium battery cathode. However, as chromium(III) is quite stable against oxidation in sulfides, removing magnesium from the material remains a significant challenge. Early attempts at both chemical and electrochemical demagnesiation are discussed.
Aurophilicity, known as aurophilic interaction, is a strong attractive van der Waals interaction between cationic gold(I) centers, whose strength is comparable to the hydrogen bond. Here, we show that aurophilicity can serve as an engineering approach to expand structural dimensionality for nanomaterials design. Specifically, based on a global-structure search method and density functional theory calculations, we predict a series of stable two-dimensional (2D) AuMX2 (M=Al, Ga, In; X=S, Se) structures featuring intracrystalline aurophilic interactions. All AuMX2 monolayers designed are semiconductors with moderate bandgaps, excellent carrier mobilities, and good optical properties. The intriguing chemistry of aurophilicity coupled with novel electronic properties render AuMX2 monolayers a potentially new series of 2D materials that are of fundamental importance in gold chemistry and technology importance for nanoelectronics.
BoltzTraP2 is a software package for calculating a smoothed Fourier expression of periodic functions and the Onsager transport coefficients for extended systems using the linearized Boltzmann transport equation. It uses only the band and $k$-dependent quasi-particle energies, as well as the intra-band optical matrix elements and scattering rates, as input. The code can be used via a command-line interface and/or as a Python module. It is tested and illustrated on a simple parabolic band example as well as silicon. The positive Seebeck coefficient of lithium is reproduced in an example of going beyond the constant relaxation time approximation.
We determine the elastic properties of the layered thermoelectrics BiOCuSe and LaOCuSe using first-principles density functional theory calculations. To predict their stability, we calculate six distinct elastic constants, where all of them are positive, and suggest mechanically stable tetragonal crystals. As elastic properties relate to the nature and the strength of the chemical bond, the latter is analyzed by means of real-space descriptors, such as the electron localization function (ELF) and Bader charge. From elastic constants, a set of related properties, namely, bulk modulus, shear modulus, Young's modulus, sound velocity, Debye temperature, Grüneisen parameter, and thermal conductivity, are evaluated. Both materials are found to be ductile in nature and not brittle. We find BiOCuSe to have a smaller sound velocity and, hence, within the accuracy of the used Slack's model, a smaller thermal conductivity than LaOCuSe. Our calculations also reveal that the elastic properties and the related lattice thermal transport of both materials exhibit a much larger anisotropy than their electronic band properties that are known to be moderately anisotropic because of a moderate effective-electron-mass anisotropy. Finally, we determine the lattice dynamical properties, such as phonon dispersion, atomic displacement, and mode Grüneisen parameters, in order to correlate the elastic response, chemical bonding, and lattice dynamics.
The diffusion of ions in solid materials plays an important role in many aspects of materials science such as the geological evolution of minerals, materials synthesis, and in device performance across several technologies. For example, the realization of multivalent (MV) batteries, which offer a realistic route to superseding the electrochemical performance of Li-ion batteries, hinges on the discovery of host materials that possess adequate mobility of the MV intercalant to support reasonable charge and discharge times. This has proven especially challenging, motivating the current investigation of ion mobility (Li+, Mg2+, Zn2+, Ca2+, and Al3+) in spinel Mn2O4, olivine FePO4, layered NiO2, and orthorhombic δ-V2O5. In this study, we not only quantitatively assess these structures as candidate cathode materials, but also isolate the chemical and structural descriptors that govern MV diffusion. Our finding that matching the intercalant site preference to the diffusion path topology of the host structure controls mobility more than any other factor leads to practical and implementable guidelines to find fast-diffusing MV ion conductors.
Semiconducting large bandgap oxides are considered as interesting candidates for high-temperature thermoelectric power generation (700–1,200 °C) due to their stability, lack of toxicity and low cost, but so far they have not reached sufficient performance for extended application. In this review, we summarize recent progress on thermoelectric oxides, analyze concepts for tuning semiconductor thermoelectric properties with view of their applicability to oxides and determine key drivers and limitations for electrical and thermal transport properties in oxides based on our own experimental work and literature results. For our experimental assessment, we have selected representative multicomponent oxides that range from materials with highly symmetric crystal structure (SrTiO3 perovskite) over oxides with large densities of planar crystallographic defects (Tin
O2n−1 Magnéli phases with a single type of shear plane, NbOx
block structures with intersecting shear planes and WO3−x
with more defective block and channel structures) to layered superstructures (Ca3Co4O9 and double perovskites) and also include a wide range of their composites with a variety of second phases. Crystallographic or microstructural features of these oxides are in 0.3–2 nm size range, so that oxide phonons can efficiently interact with them. We explore in our experiments the effects of doping, grain size, crystallographic defects, superstructures, second phases, texturing and (to a limited extend) processing on electric conductivity, Seebeck coefficient, thermal conductivity and figure of merit. Jonker and lattice-versus-electrical conductivity plots are used to compare specific materials and material families and extract levers for future improvement of oxide thermoelectrics. We show in our work that oxygen vacancy doping (reduction) is a more powerful driver for improving the power factor for SrTiO3, TiO2 and NbOx
than heterovalent doping. Based on our Seebeck-conductivity plots, we derived a set of highest achievable power factors. We met these best values in our own experiments for our titanium oxide- and niobium oxide-based materials. For strontium titanate-based materials, the estimated highest power factor was not reached; further material improvement is possible and can be reached for materials with higher carrier densities. Our results show that periodic crystallographic defects and superstructures are most efficient in reducing the lattice thermal conductivity in oxides, followed by hetero- and homovalent doping. Due to the small phonon mean free path in oxides, grain boundary scattering in nanoceramics or materials with nanodispersions is much less efficient. We investigated the impact of texturing in Ca3Co4O9 ceramics on thermoelectric performance; we did not find any improvement in the overall in-plane performance of a textured ceramic compared to the corresponding random ceramic.
Here we explore the applicability of the two current model in understanding
the transport behavior of Fe 2 CoSi compound by using the first principles
calculations in combination with the Boltzmann transport theory. The
spin-unpolarized calculation shows large density of states (DOS) at Fermi level
(E F) and is unable to provide the correct temperature dependence of transport
coefficients. The spin-polarised calculation shows reduced DOS at the E F in
the spin-up channel, whereas spin-dn channel have almost zero DOS at the E F .
The absolute value of Seebeck coefficient in the spin-up channel shows linear
increment with the temperature and in the spin-dn channel it varies
non-linearly. The electrical conductivity also shows non-linear temperature
dependence in both the spin channels whereas, the electronic thermal
conductivity shows linear temperature dependence. The values of transport
coefficients and their temperature dependence obtained by using the two current
model are found to be in fairly good agreement with the experimental data.
Present work clearly suggests the importance of two current model in
understanding the transport properties of the compound.
Computer Simulation Software and 173 page Book.
Good thermoelectric materials possess low thermal conductivity while maximizing electric carrier transport. This article looks at various classes of materials to understand their behavior and determine methods to modify or “tune” them to optimize their thermoelectric properties. Whether it is the use of “rattlers” in cage structures such as skutterudites, or mixed-lattice atoms such as the complex half-Heusler alloys, the ability to manipulate the thermal conductivity of a material is essential in optimizing its properties for thermoelectric applications.
The n-type 0.1 wt% SbIâ-doped biâTe{sub 2.85}Se{sub 0.15} compounds were fabricated by hot extrusion in the temperature range 300--510 C under an extrusion ratio of 20:1. The extruded compounds were highly dense. The grains were small, equiaxed (â¼ 1.0 μm), and contained many dislocations due to the dynamic recrystallization during the extrusion. The grains were also preferentially oriented through the extrusion. The bending strength and the figure of merit of the compounds, hot-extruded at 440 C, were 97 MPa and 2.62 à 10â»Â³/K, respectively.
A sol-gel method has been applied to synthesize Ca3Co4O9 powders by using calcium and cobalt nitrates as raw materials and citric acid as complexing agent. The Pyrolytic decomposition mechanism of the dried gel and formation process of Ca3Co4O9 were investigated by TG-DTA, XRD, FT-IR, SEM and TEM. The results show pure Ca3Co4O9 powders can be prepared after the dried gel was calcined at 750–900 °C for 2 h. 2% of polyethylene glycol (PEG) 400 as a dispersant was added to the nitrate solution during the formation of sol precursor. Ca3Co4O9 nanoparticles with a size of 30–50 nm can be obtained by the dried gel synthesized in the presence of PEG 400 calcined at 800 °C for 2 h.
Chalcopyritelike quaternary chalcogenides, Cu2ZnSnQ4 (Q=S,Se), were investigated as an alternative class of wide-band-gap p-type thermoelectric materials. Their distorted diamondlike structure and quaternary compositions are beneficial to lowering lattice thermal conductivities. Meanwhile, partial substitution of Cu for Zn creates more charge carriers and conducting pathways via the CuQ4 network, enhancing electrical conductivity. The power factor and the figure of merit (ZT) increase with the temperature, making these materials suitable for high temperature applications. For Cu2.1Zn0.9SnQ4, ZT reaches about 0.4 at 700 K, rising to 0.9 at 860 K.
A linear correlation between the Grüneisen parameter and ratio of the velocities of longitudinal (ν
l)and transverse (ν
t) acoustic waves in crystals is found. It is assumed that velocities ν
l and ν
t are severally harmonic parameters, while their ratio ν
l/ν
t is an anharmonic quantity and depends on the ratio between the shear and flexural rigidities of interatomic bonds.
It is shown that the low-temperature elastic constants of vitreous silica can account for only a small fraction of the low-temperature heat capacity. On the other hand, for quartz the elastic constants account for all the low-temperature heat capacity. The data for cristobalite are not complete, but there is evidence to suggest that the anomaly also exists in this phase of silica. It is shown that the long-wave vibrational properties of vitreous silica, optical and acoustical, are consistent with the analogous properties of ionic solids.
Density functional calculations in the generalized gradient
approximation are used to study the transport properties of the
clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30. The
band structures of these clathrates indicate that they are all
semiconductors. Seebeck coefficients, conductivities and Hall
coefficients are calculated, to assess the effects of carrier
concentration on the quantity S2σ/τ (where S is the
Seebeck coefficient, σ is the conductivity, and τ the electron
relaxation time) which is proportional to the thermoelectric power
factor. In each compound we find that both p- and n-doping will
significantly enhance the thermoelectric capabilities of these
compounds. For p-doping, the power factors of all four clathrates are of
comparable magnitude and have similar temperature dependence, while for
n-doping we see significant variations from compound to compound. We
estimate that room-temperature ZT values of 0.5 may be possible for
optimally n-doped Sr8Ga16Ge30 or Ba8In16Sn30; at 800 K ZT
values as large as 1.7 may be possible. For single crystals of high
quality, with substantially increased scattering times, the power factor
of these materials will be significantly higher. Recent experiments are
reviewed in the light of these calculations.
To determine solid-state structures, the electron localization function (ELF) must be interpreted somewhat differently than for molecules. The example of the diamond structures of C (right, top), Si, Ge, and α- and β-Sn (right, bottom) show clearly that ELF depicts the electronic changes in regional space as the covalent bond gives way to metallic bonding.
The most recent data on the thermoelectric properties of cooling materials suitable for use near room temperature have been reviewed. The materials discussed include Bi2Te3 and its pseudo-binary Bi2Te3-Sb2Te3 and Bi2Te3-Bi2Se3 alloys, with the major emphasis on the pseudo-ternary alloys in the system Bi2Te3-Sb2Te3-Sb2Te3. The data presented include (1) the Seebeck coefficient, thermal conductivity, and figure of merit as a function of electrical resistivity; (2) the variations in the lattice thermal conductivity with alloying; (3) the temperature dependence of thermoelectric properties of the pseudo-ternary alloys. Presented also are the results of a recent study of growth variables on the thermoelectric properties of these alloys.The pseudo-ternary Bi2Te3-Sb3-Sb2Te2-Sb2Se3 system provided the best n- and p-type materials. These materials, which gave an average figure of merit of 3·3 × 10−3 deg−1 at room temperature, achieved a maximum Peltier cooling from room temperature of 78°K in a single-stage refrigerating couple. Furthermore, a temperature as low as 159°K was attained continuously from 300°K by a six-stage Peltier refrigerator constructed from these ternary alloys. The superior thermoelectric properties of these ternary alloys were interpreted on the basis of a large reduction and a small temperature dependence of the lattice thermal conductivity, and an increase in the energy band gap of the alloys with additions of Sb2Se3.
Nonmetallic crystals transport heat primarily by phonons at room temperature and below. There are only a few nonmetallic crystals which can be classed as high thermal conductivity solids, in the sense of having a thermal conductivity of > 1 W/cmK at 300K. Thermal conductivity measurements on natural and synthetic diamond, cubic BN, BP and AIN confirm that all of them are high thermal conductivity solids. Studies have been made of the effect on the thermal conductivity of nitrogen impurities in diamond, and oxygen impurities in AIN. The nitrogen impurities scatter phonons mostly from the strain field, the oxygen impurities scatter phonons mostly from the mass defects caused by aluminum vacancies. Pure A1N as well as pure SiC, BeO, BP and BeS conduct heat almost as well as does copper at room temperature, while pure natural and synthetic diamonds conduct heat five times better than copper.All of the nonmetallic solids that are known to possess high thermal conductivity have either the diamond-like, boron carbide, or graphite crystal structure. There are twelve different diamond-like crystals, a few boron carbide-type crystals, and two graphite structure crystals that have high thermal conductivity. Analyses of the rock-salt, fluorite, quartz, corundum and other structures show no candidates for this class. The four rules for finding crystals with high thermal conductivity are that the crystal should have (1) low atomic mass, (2) strong bonding, (3) simple crystal structure, and (4) low anharmonicity. The prime example of such a solid is diamond, which has the highest known thermal conductivity at 300K.
Polycrystalline samples of (Na1−yMy)xCo2O4 (M=K, Sr, Y, Nd, Sm and Yb; y=0.01∼0.35) were prepared by a solid state reaction method. In this study, in order to improve the thermoelectric properties of NaxCo2O4, the effects of partial substitution of other metals for Na on the thermoelectric properties of NaxCo2O4 from room temperature to 1073 K were investigated. For M=Sr, the thermoelectric power and the electrical resistivity increased, and the electronic and lattice contribution to the thermal conductivity decreased compared to the non-substituted sample. These effects suggest that the carrier density was reduced by the substitution of Sr for Na. As a result, the figure of merit of the sample for M=Sr was improved. On the other hand, for other samples in spite of the increase in the electrical resistivity, the thermoelectric power decreased. These results are anomalous effects, which cannot be described merely by a change of the carrier density. For all samples, except for M=Y, the lattice contribution to the thermal conductivity decreased and for all samples, except for M=K, the electronic contribution slightly decreased.
The effect of spark plasma sintering (SPS) temperature on thermoelectric properties of (Ti,Zr,Hf)NiSn half-Heusler compounds were studied. Four kinds of samples were prepared: an arc-melted sample, a sample sintered at 973 K by SPS (SPS-973), a sample sintered at 1173 K by SPS (SPS-1173), and a sample sintered at 1373 K by SPS (SPS-1373). The power factor of SPS-1173 is several % lower than that of the arc-melted sample, but the thermal conductivity is reduced to 70% of that of the arc-melted sample. As a result, the dimensionless figure of merit ZT of SPS-1173 is improved and it reaches 0.43 at 760 K.
We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N-atoms(2) scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special 'preconditioning' optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package), The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
Thermoelectric materials are solid-state energy converters whose combination of thermal, electrical, and semiconducting properties
allows them to be used to convert waste heat into electricity or electrical power directly into cooling and heating. These
materials can be competitive with fluid-based systems, such as two-phase air-conditioning compressors or heat pumps, or used
in smaller-scale applications such as in automobile seats, night-vision systems, and electrical-enclosure cooling. More widespread
use of thermoelectrics requires not only improving the intrinsic energy-conversion efficiency of the materials but also implementing
recent advancements in system architecture. These principles are illustrated with several proven and potential applications
of thermoelectrics.
A program to compute many functions dependent on the electron density rho(r) from the results of ab initio molecular calculations is presented. The program allows the generation of different one-, two-, and three-dimensional grids for further graphical representation or numerical analysis. Other options like extracting separate atom contributions to the function computed or locating maximum and minimum values are also implemented. A number of illustrative applications regarding different rho(r)-dependent functions are presented and the performance and portability of the program is discussed.
An introduction to the mechanical properties of ceramics
- Green