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Ab initio calculations of the wolframite MnWO4 under high pressure
Abstract
Ab initio calculations, based on the density functional theory, of the structural properties of MnWO4 wolframite compound are reported. We obtain the equilibrium volume from an equation of state with the anti-ferromagnetic (AF) and ferromagnetic configurations, AF being the lowest energy state, and the structural properties of the wolframite structure at zero pressure are obtained. We also study the wolframite structure up to a pressure of 31 GPa, and the pressure evolution of structural parameters is found to be in good agreement with the available experimental data.
... oil) [19] and at around 30 GPa on Raman spectroscopy experiments carried out using Ne as PTM [15]. Ab initio calculations on nonmagnetic wolframites ZnWO 4 and MgWO 4 [19,20] show that the CuWO 4 -type triclinic phase is energetically competitive with the low-pressure (LP) monoclinic wolframite phase from 1 atm to around 40 GPa. At this pressure, the monoclinic β-fergusonite phase is predicted to be more favorable. ...
... . This result is in agreement with the axial compressibilities of other wolframites [19,20] and agrees well with those reported by Macavei and Schulz [13] for MnWO 4 up to 9 GPa. It is also worth noting that as pressure increases the monoclinic structure distorts. ...
The pressure-induced phase transition of the multiferroic manganese tungstate MnWO4 is studied on single crystals using synchrotron x-ray diffraction and Raman spectroscopy. We observe the monoclinic P2/c to triclinic P1¯ phase transition at 20.1 GPa and get insight on the phase transition mechanism from the appearance of tilted triclinic domains. Selective Raman spectroscopy experiments with single crystals have shown that the onset of the phase transition occurs 5 GPa below the previously reported pressure obtained from experiments performed with powder samples.
The high-pressure behavior of FeVO4 was studied using X-ray diffraction and first-principles calculations. The occurrence of two first-order phase transitions was detected from experimental characterization in a range of 12.3 GPa: P1̅ (3 GPa) → α-MnMoO4-type (5 GPa) → wolframite, both being irreversible. Ab initio calculations supported the pressure evolution of experimental results. Theoretical results predict the following phase transition sequence at high pressures: wolframite (36 GPa) → raspite (66.5 GPa) → AgMnO4-type structure.
First-principles calculations have been carried 11 out to study the InVO 4 compound under pressure. In this 12 work, total energy calculations were performed in order to 13 analyze the structural behavior of the experimentally known 14 polymorphs of InVO 4 : α-MnMoO 4-type (I), CrVO 4-type (III), 15 and wolframite (V). In addition, in this paper we present our 16 results about the stability of this compound beyond the 17 pressures reached by experiments. We propose some new 18 high-pressure phases on the basis of the study of 13 possible 19 candidates. The quasi-harmonic approximation has been used 20 to calculate the sequence of phase transitions at 300 K: CrVO 4-type, III (the transition pressure is given in parentheses) → 21 wolframite, V (4.4 GPa) → raspite, VI (28.1 GPa) → AgMnO 4-type, VII (44 GPa). Equations of state and phonon frequencies as 22 a function of pressure have been calculated for the studied phases. In order to determine the stability of each phase, we also 23 report the phonon dispersion along the Brillouin zone and the phonon density of states for the most stable polymorphs. Finally, 24 the electronic band structure for the low-and high-pressure phases for the studied polymorphs is presented as well as the 25 pressure evolution of the band gap by using the HSE06 hybrid functional.
In situ Raman spectra of MnWO4 have been measured at high pressures up to 29.4 GPa and temperatures up to 1473 K. It is shown that a pressure-induced phase transition from the monoclinic to triclinic phase starts at about 17.7 GPa at room temperature, and the phase transition is confirmed to be reversible. A temperature induced melting from monoclinic to molten state starts at 1100 K and is completed at 1300 K. After cooling, molten MnWO4 recrystallizes into monoclinic MnWO4 crystals.
In this paper we report a theoretical study of the CaSeO 4 compound at ambient pressure and under pressure. Here we made a structural analysis of its three known polymorphsorthorhombic (Cmca), monoclinic monazite, and tetragonal scheelitewhere direct comparison with experimental measurements is done. Besides, the electronic and vibrational structures are reported for the first time for those structures. In addition, the behavior of CaSeO 4 as a function of pressure is studied, where phase transitions are investigated by considering a quasiharmonic approximation at 300 K. After a total energy study of 14 possible high-pressure phases of CaSeO 4 , the following sequence of pressure-driven structural transitions has been found: orthorhombic (Cmca) → tetragonal scheelite → monoclinic AgMnO 4 -type structure. It was observed that monazite is less stable as temperature increases, while the opposite occurs for the AgMnO 4 -type structure, this being a novel polymorph. This high-pressure structure is a distortion of the monazite structure and resembles the distorted barite-type structure (P2 1 /n) of CaSO 4 . The equation of state and the pressure evolution of the structural, electronic, and vibrational properties are also reported.
The community of material scientists is strongly committed to the research area of multiferroic materials, both for the understanding of the complex mechanisms supporting the multiferroism and for the fabrication of new compounds, potentially suitable for technological applications. The use of high pressure is a powerful tool in synthesizing new multiferroic, in particular magneto-electric phases, where the pressure stabilization of otherwise unstable perovskite-based structural distortions may lead to promising novel metastable compounds. The in situ investigation of the high-pressure behavior of multiferroic materials has provided insight into the complex interplay between magnetic and electronic properties and the coupling to structural instabilities.
In this work, we report on the microwave-assisted hydrothermal synthesis of Sn2+-doped ZnWO4 nanocrystals with controlled particle sizes and lattice structures for tunable optical and photocatalytic properties. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, UV–vis diffuse reflectance spectroscopy, and Barrett–Emmett–Teller technique. The effects of Sn2+ doping in ZnWO4 lattice on the crystal structure, electronic structure, and photodegradation of methylene orange dye solution were investigated both experimentally and theoretically. It is found that part of the Sn2+ ions were homogeneously incorporated in the ZnWO4 host lattice, leading to a monotonous lattice expansion, and part of Sn2+ ions were expelled at surface sites for decreased crystallinity and particle size reduction. By Sn2+ doping, ZnWO4 nanocrystals showed a significant XPS binding energy shift of Zn 2p, W 4f, and O 1s, which is attributed to the combination of electronegativity between Sn2+ and Zn2+, lattice variation, and particle size reduction. Meanwhile, the BET surface areas were also greatly enlarged from 40.1 to 110 m2·g–1. Contrary to the theoretical predictions of the quantum size effect, Sn2+-doped ZnWO4 nanocrystals showed an abnormal band gap narrowing, which can be well-defined as a consequence of bulk and surface doping effects as well as lattice variations. With well-controlled particle size, crystallinity, and electronic structure via Sn2+ doping, the photocatalytic performance of Sn2+-doped ZnWO4 nanocrystals was optimized at Sn2+ doping level of 0.451.
In this work we present a theoretical study of structural stability of CrVO4-type orthophosphates InPO4 and TiPO4 at ambient and high pressures. Total energy calculations and lattice dynamics were used to obtain structural and vibrational properties of these compounds. Also, we have studied at ambient pressure the orthophosphates TlPO4 and VPO4 in order to compare their structural and vibrational properties with InPO4 and TiPO4. Here we analyze the variation of the Raman and IR frequencies as functions of the reduced mass. Using the phonon dispersion relations we have calculated the Gibbs free energy and evaluated the phase transitions at 300 K, at which most experimental measurements are performed. By taking into consideration the Bastide's diagram and previous theoretical and experimental studies in APO4 compounds, we have considered 12 candidate structures for the high-pressure regime. We found the following sequence for pressure-driven structural transition: CrVO4 type → zircon → scheelite → wolframite, for InPO4 and TiPO4. The equations of state, phonon frequencies, and their behavior with pressure of the most stable polymorphs are also reported. We also included the phonon spectrum and the projected phonon density of states of each phase for both compounds. Finally, calculation of the evolution of magnetic moment is reported for TiPO4.
This work reports on the kinetic control of MnWO4 nanoparticles with an aim to achieve the tailored structural properties. Using citric acid as the capping agent, MnWO4 nanocrystals were prepared to show particle sizes ranging from 8 to 29 nm under hydrothermal conditions. The grain-growth kinetics for MnWO4 nanoparticles was determined to quantitatively follow the equation, D5 = (4.59 × 1020)te−17.0/T, where D is the particle size at given reaction time, t, and reaction temperature, T. Systematic sample characterization using combined techniques of X-ray diffraction, transmission electron microscope, selected area electron diffraction, Barret−Emmett−Teller technique, Fourier transformed infrared spectra, UV−visible diffuse reflectance spectra, and Raman spectra indicates that particle size reduction led to an apparent lattice expansion, which is followed by lowering of lattice symmetry and band gap broadening. Different from the bulk counterparts, new vibration modes were observed in both Infrared and Raman spectra at about 913 and 930 cm−1, respectively, which intensified monotonously with particle size reduction, leading to a picture that MnWO4 nanoparticles were terminated by surface disordered layers. All these size-dependent physical properties were closely related to the surface disorder and the relevant absorbates.
In this paper we report a density functional study of the structural, electronic, and vibrational properties of the EuWO4 compound in the scheelite structure. We use Raman spectroscopy to complement the study for this phase at ambient pressure. The first part of the paper is devoted to analyzing the results obtained with the Perdew-Burke-Ernzerhof for solids exchange-correlation functional within the GGA+U approximation and compare those with our experimental results and reported available data. We also present the evolution of these properties, for the same crystal structure, up to 8 GPa. The second part of the paper is devoted to discussing our study on the high-pressure phase transitions of EuWO4, for which we follow the evolution of the structural properties as a function of pressure. The results show that the high-pressure behavior of EuWO4 is very similar to that of CaWO4, SrWO4, BaWO4, and PbWO4, for which the scheelite structure undergoes a phase transition to the fergusonite structure, an observation also supported by our experimental data. Additionally, we can also conclude that at higher pressures, EuWO4 evolves to the BaWO4-II and Cmca crystal phases.
Raman scattering measurements and lattice-dynamics calculations have been performed on magnesium tungstate under high pressure up to 41 GPa. Experiments have been carried out under a selection of different pressure-media. The influence of non-hydrostaticity on the structural properties of MgWO 4 and isomorphic compounds is examined. Under quasi-hydrostatic conditions a phase transition has been found at 26 GPa in MgWO 4 . The high-pressure phase has been tentatively assigned to a triclinic structure similar to that of CuWO 4 . We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low-and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO 4 , MnWO 4 , FeWO 4 , CoWO 4 , NiWO 4 , ZnWO 4 , and CdWO 4 .
Angle-dispersive x-ray-diffraction and x-ray-absorption near-edge structure measurements have been performed on CaWO4 and SrWO4 up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of CaWO4, under hydrostatic conditions SrWO4 undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total-energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the AO8 polyhedra of ABO4 scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation 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 semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
Room-temperature Raman scattering was measured in ZnWO4 up to 45 GPa. We report the pressure dependence of all the Raman-active phonons of the low-pressure wolframite phase. As pressure increases new Raman peaks appear at 30.6 GPa due to the onset of a reversible structural phase transition to a distorted monoclinic b-fergusonite-type phase. The low- and high-pressure phases coexist from 30.6 GPa to 36.5 GPa. In addition to the Raman measurements we also report ab initio total-energy and lattice-dynamics calculations for the two phases. These calculations helped us to determine the crystalline structure of the high-pressure phase and to assign the observed Raman modes in both the wolframite and b-fergusonite phases. Based upon the ab initio calculations we propose the occurrence of a second phase transition at 57.6 GPa from the b-fergusonite phase to an orthorhombic Cmca phase. The pressure evolution of the lattice parameters and the atomic positions of wolframite ZnWO4 are also theoretically calculated and an equation of state reported.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
Angle-dispersive x-ray-diffraction and x-ray-absorption near-edge structure measurements have been performed on CaWO 4 and SrWO 4 up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of CaWO 4 , under hydrostatic conditions SrWO 4 undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total-energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the AO 8 polyhedra of ABO 4 scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.
Single-crystal structure determinations of wolframite-type structures were performed using a diamond anvil cell with beryllium gaskets. Unit cell parameters of MgWO
All investigated compounds compress anisotropically with the
In all compounds, the WO
With increasing pressure the oxygen atomic positions remain constant within one standard deviation, while the heavy atoms positions show a different pressure dependence: a shift of
With increasing pressure the A and W cations approach each other, indicating a tendency of a transition from the wolframite structure to the scheelite structure, where the A and W cations lie in the same plane.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
A laser diode transversely pumped compact, passively Q-switched, self-stimulating Nd3+:PbWO4/Cr4+:YAG Raman laser is proposed. With 4.5 mJ diode pumping, fundamental and Raman output energies of 2 and 2.5 μJ have been obtained simultaneously at 1.058 and 1.17 μm, respectively. The frequency conversion efficiency is estimated to be 56%. The Raman pulse width of 8 ns is 40% of the fundamental laser pulse width of 20 ns.
Studies at high pressures and temperatures are helpful for understanding the physical properties of the solid state, including such classes of materials as, metals, semiconductors, superconductors, or minerals. In particular, the phase behaviour of ABX4 scintillating materials is a challenging problem with many implications for other fields including technological applications and Earth and planetary sciences. A great progress has been done in the last years in the study of the pressure-effects on the structural and electronic properties of these compounds. In particular, the high-pressure structural sequence followed by these compounds seems now to be better understood thanks to recent experimental and theoretical studies. Here, we will review studies on the phase behaviour of different ABX4 scintillating materials. In particular, we will focus on discussing the results obtained by different groups for the scheelite-structured orthotungstates, which have been extensively studied up to 50 GPa. We will also describe different experimental techniques for obtaining reliable data at simultaneously high pressure and high temperature. Drawbacks and advantages of the different techniques are discussed along with recent developments involving synchrotron X-ray diffraction, Raman scattering, and ab initio calculations. Differences and similarities of the phase behaviour of these materials will be discussed, on the light of Fukunaga and Yamaoka’s and Bastide’s diagrams, aiming to improve the actual understanding of their high-pressure behaviour. Possible technological and geophysical implications of the reviewed results will be also commented.
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.}