Epitaxial strain induced ferroelectricity in rocksalt binary compound: Hybrid functional Ab initio calculation and soft mode group theory analysis

We have studied the detailed mechanism of epitaxial strain induced ferroelectricity in rocksalt binary compound by ab initio calculation and soft mode group theory analysis. By applying compressive strain, cubic binary rocksalt (F_m3m) transforms into tetragonal (I_4/mmm) structure. With increasing compressive strain, tetragonal structure becomes unstable against spontaneous transformation to lower symmetry tetragonal structure (I_4/mm), evident both from ab initio calculation and from soft mode group theory analysis. For the tensile strain, phase transition sequence can be cubic binary rocksalt to tetragonal (I_4/mmm) and to orthorhombic structure (I_m2m). From ab initio calculation and space group analysis, we propose that the epitaxial strain induced ferroelectricity of rocksalt binary compound is the generic property.     

Electronic structure and phase stability of ZrO2

The electronic structures of both cubic and tetragonal ZeO₂ were calculated by the discrete-variational (DV)-Xα cluster method. The results show that ZrO₂ is fairly ionic, and its ionicity is different for the cubic and the tetragonal phases. The bond order and the magnitude of the energy gap between the highest occupied O-2p level and the lowest unoccupied Zr-4d level also depend on the crystal structure. These changes in the electronic states seem to be closely related to the phase stability of ZrO₂.     

Defect cascades and point defects in low-temperature neutron irradiated α-tin monitored by Mössbauer spectroscopy

Neutron irradiation of cubic α-tin at 4.6 K leads to a new component in the Mössbauer spectra similar to tetragonal β-tin, disappearing again during annealing between 90 and 160 K. From the striking difference between the fractions of the new component in absorber spectra and in source spectra we conclude that it is due (a) to defect cascades from fast neutron collisions and (b) to point defects from the (n, γ) recoil.     

Molecular dynamics simulations of nanoindentation of monocrystalline germanium

Three-dimensional molecular dynamics simulations using the Tersoff potential are conducted to investigate the nanoindentation process of monocrystalline germanium (Ge). It is found that a phase transformation from fourfold-coordinated diamond cubic phase (Ge-I) to sixfold-coordinated β-tin phase (Ge-II) occurs during the nanoindentation process. The simulation results suggest that a pressure-induced phase transformation instead of dislocation-assisted plasticity is the dominant deformation mechanism of monocrystalline Ge thin films during the nanoindentation process.     

Raman and X‐ray investigations of the incorporation of Ca2+ and Cd2+ in the ZrO2 structure

The formation of solid solution and ZrO₂ phase stabilization were investigated by Raman spectroscopy and X‐ray diffraction (XRD) in calcium‐containing and cadmium‐containing zirconium oxide samples heated at 1073 K in air. The adopted preparation procedure led to the incorporation of calcium and cadmium in solid solution into the zirconia structure. The solid solution favored the tetragonal and cubic zirconia phases at the expense of the thermodynamically stable monoclinic modification. Combined macro‐ and micro‐Raman spectroscopy disclosed that instead of forming a homogeneous phase t″, intermediate between the tetragonal t′ and the cubic phase, the tetragonal and cubic phases coexisted in the range 9.49–13.89 mol% for Ca and 11.88–17.23 mol% for Cd. At higher dopant contents the cubic form stabilized. The impurity content necessary to stabilize the high‐symmetry phases was defined. Copyright © 2007 John Wiley & Sons, Ltd.     

Ferromagnetism in the high-pressure phases of (GaSb)1−x Mnx

The electrophysical and magnetic properties of recently discovered high-pressure phases in the GaSb-Mn system with simple cubic and tetragonal structures have been examined. It has been shown that samples with the primitive cubic structure for low temperatures are in the ferromagnetic state and the Curic temperature depends on the initial manganese content and reaches T _c = 280 K for x = 0.6. It has been shown that these samples for a manganese content x ≤ 0.5 are in the semiconducting state with large impurity conduction and pass to the metallic state as x increases. The GaSbMn phase with the tetragonal structure has ferromagnetic properties up to temperatures of T ～450 K (at which the phase begins to decay) and exhibits metallic properties. The magnetization at T = 77.3 K is equal to M = 0.58 μ_B and 0.28 μ_B per manganese ion for the simple cubic and tetragonal phases, respectively.     

First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O3

Electronic and optical properties of Sr(Ti,Zr)O₃ crystals in the cubic (Pm-3m) and tetragonal (I4/mcm) phase were calculated by the first-principles calculations using the density functional theory and the local density approximation. The band structure of cubic and tetragonal phases show an indirect band gap at (R-Γ) point and at (M-Γ) point in the Brillouin zone, respectively. The linear photon-energy dependent dielectric functions and some optical properties such as the absorption coefficient, energy-loss function and reflectivity are calculated for both phases. The optical properties of tetragonal phase of Sr(Ti,Zr)O₃ were investigated by theoretical methods for the first time. We have also made some comparisons with the available related experimental and theoretical data.     

First principles study on the structural and optical properties of the high-pressure ZnO phases

A new high-pressure tetragonal phase (B10) of ZnO is investigated with an ab initio calculation based on density functional theory and is compared with the cubic B1 (rocksalt structure) and B2 (CsCl structure) phases at high pressure. It is found that the B10 phase has a more covalent nature than the B2 phase. The B1, B2, and B10 phases are semiconductors and their band gap energies are determined to be 3.73, 3.15, and 3.27 eV, respectively. The B10 phase has a similar optical response to the B2 phase, but not the B1 phase. The similarity of dielectric function between B10 and B2 phases are the result of the similar profiles of electronic density of state.     

First-principles study of martensitic phase transformation of TiRh alloy

Geometric structures and atomic positions were studied with plane wave pseudo-potential method based on density functional theory for cubic, tetragonal, and monoclinic phases of TiRh alloy. Their phonon dispersion curves were obtained with frozen phonon method at harmonic approximation using density-functional perturbation theory. Our calculations revealed that both B2 and L1₀ phases are thermodynamically unstable. Jahn-Teller effect triggers the occurrence of Bain transformation from cubic to tetragonal phase, and then soft-mode phonon further leads to the transition from tetragonal to monoclinic phase on cooling. The monoclinic phase was predicted to be P2/m space group through atomic vibrational movement along [001] direction of virtual frequency modes of L1₀ phase. The temperature from B2 to L1₀ and then to P2/m were predicted to be about T=1100.53 K and T=324.48 K through free energy calculations with the electronic plus vibrational energy of formation, respectively, which is in good agreement with experimental observations.     

Molecular dynamics simulation of zirconia melting

The melting point for the tetragonal and cubic phases of zirconia (ZrO₂) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.     

Enhancing the phase stability and ionic conductivity of scandia stabilized zirconia by rare earth co-doping

Effect of co-doping Yb, Gd and Ce in scandia stabilized zirconia (SSZ) on the phase stability, high temperature aging behavior and ionic conductivity was studied. Both binary (10 mol% SSZ) and the ternary (co-doped) compositions were found to be in single cubic phase in the as-processed condition. However, the binary composition exhibited the rhombohedral ‘β’ phase after sintering whereas the ternary compositions remained in the single cubic phase. The sintered pellets were aged at 900 °C for 500 h in air to study the phase stability at high temperature. Transmission electron microscopy revealed that the aged samples of Yb and Gd co-doped compositions contain small amount of the tetragonal phase which resulted in considerable degradation in conductivity (more than 20%). The Ce co-doped sample, on the other hand, was in single cubic phase after aging and this ensured that conductivity reduction was minimal in this composition. The co-doped samples however, showed higher conductivity before and after aging compared to the binary composition. The rhombohedral ‘β’ phase was absent in all the co-doped ternary compositions even after high temperature aging.     

Melting curve of silicon to 15 GPa determined by two-dimensional angle-dispersive diffraction using a Kawai-type apparatus with X-ray transparent sintered diamond anvils

The melting curve of silicon has been determined up to 15 GPa using a miniaturized Kawai-type apparatus with second-stage cubic anvils made of X-ray transparent sintered diamond. Our results are in good agreement with the melting curve determined by electrical resistivity measurements [V.V. Brazhkin, A.G. Lyapin, S.V. Popova, R.N. Voloshin, Nonequilibrium phase transitions and amorphization in Si, Si/GaAs, Ge, and Ge/GaSb at the decompression of high-pressure phases, Phys. Rev. B 51 (1995) 7549] up to the phase I (diamond structure)—phase II (β-tin structure)—liquid triple point. The triple point of phase XI (orthorhombic, Imma)—phase V (simple hexagonal)—liquid has been constrained to be at 14.4(4) GPa and 1010(5) K. These results demonstrate that the combination of X-ray transparent anvils and monochromatic diffraction with area detectors offers a reliable technique to detect melting at high pressures in the multianvil press.     

Modeling of Phase Transitions of Graphites to Diamond-Like Phases

The method of the density functional theory is used to study structural transformations between graphites and diamond-like phases. The calculations have been carried out in two approximations: a local density approximation and a generalized gradient approximation. It is found that the phase transitions of hexagonal graphene layers to a cubic diamond and diamond-like phases must occur at uniaxial compressions of ~57–71 GPa, whereas some diamond-like phases can be obtained from tetragonal graphene layers at significantly lower pressures of 32–52 GPa. The X-ray diffraction patterns have been calculated for the phase transition of graphite I4₁/amd to tetragonal LA10 phase that takes place at the minimum pressure that can be used for experimental identification of these compounds.     

High-pressure phases in the GaSb-Mn system

The effect of high pressure (6 GPa) on the formation of new phases in a polycrystalline mixture GaSb: Mn = 1: 1 upon heating was studied. Sphalerite-type solid solutions with a small amount of Mn form at temperatures below 520–600 K. At higher temperatures, new crystalline GaSbMn phases are synthesized: a phase with a simple cubic structure with a lattice parameter a = 2.946 ± 0.001 Å (at 620–670 K) and a phase with a tetragonal CuAl₂-type structure (space group I4/mcm) with lattice parameters a = 6.426 ± 0.004 Å and c = 5.349 ± 0.004 Å (at 690–870 K). These new phases are metastable under normal conditions and have magnetic properties. The structure, conductivity, and thermal stability of the synthesized phases are investigated, and the products of decomposition of these new phases upon annealing are analyzed.     

Structural phase transition of GeAs under pressure

Abstract

We have measured the effect of pressure on structural properties and the optical response of the layered compound GeAs by means of x-ray powder diffraction and optical reflectivity. The low-pressure semiconducting phase is found to be stable up to 13 GPa. At higher pressures the compound crystallizes in the rocksalt structure. Reflectivity measurements show that the structural change is associated with a semiconductor-to metal-transition. Returning to lower pressures from the region of the cubic phase results in the formation of a metastable tetragonal phase.     

Characterization of laser sealed coatings of yttria partially stabilized zirconia

The present paper reports an experimental investigation based on X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) analyses of phases formed after laser sealing of plasma sprayed coatings of 8.5 wt% yttria partially stabilized zirconia (YPSZ). X-ray diffraction and X-ray step-scanning analyses showed that the plasma sprayed and sealed coatings consisted mainly of t′ phase with a very small amount of monoclinic phase (m phase) in the plasma sprayed coatings. It was also found that the small amounts of m and cubic phases (c phase) present in the sealed coatings were dependent on laser processing specific energies (specific energy is equal to laser power/traverse speed x beam diameter). It was also found that rhombohedral (r) phase formed after laser sealing of coatings at higher specific energies. A direct relationship between c/a ratio of transformable tetragonal phase (t phase) produced by thermal treatment of sealed coatings and nontransformable tetragonal phase (t′ phase) and the amount of Yttria was obtained. A new equation was derived, which describes the relationship between Yttria concentration and the c/a ratio of tetragonal phases (t or t′).     

Third-order elastic constants and pressure derivatives of the second-order elastic constants of β-tin

The second- and third-order elastic constants and pressure derivatives of second-order elastic constants of tetragonal β-tin have been obtained using the deformation theory. The strain energy density derived using the deformation theory is compared with the strain dependent lattice energy obtained from the elastic continuum model approximation to get the expressions for the second- and third-order elastic constants. Higher order elastic constants are a measure of the anharmonicity of a crystal lattice. The 12 non-vanishing third-order elastic constants and the six pressure derivatives of the second-order elastic constants in tetragonal β-tin are obtained in the present work and are compared with the available experimental values. The second-order elastic constant C₃₃ obtained in the present study is in reasonable agreement with the experimental values. The third-order elastic constants are generally one order of magnitude greater than the second-order elastic constants as expected of a crystalline solid. The third-order elastic constant C₃₃₃ is higher in magnitude than all other values. This shows a greater anharmonicity of β-tin along the c-axis direction of the crystal.     

First-principle study of phase stability,electronic structure and thermodynamic properties of cadmium sulfide under high pressure

By employing first principle and a quasi-harmonic Debye model, we study the phase stability, phase transition, electronic structure and thermodynamic properties of cadmium sulfide (CdS). The results indicate that CdS is a typical ionic crystal and that the zinc-blende phase in CdS is thermodynamically unstable. Moreover, the heat capacity of the wurtzite and rocksalt phases of CdS decreases with pressure and increases with temperature, obeying the rule of the Debye T³ law at low temperature and the Dulong–Petit limit at high temperature.     

Pressure-induced phase transition in wurtzite ZnS: An ab initio constant pressure study

We study the pressure-induced phase transition of wurtzite ZnS using a constant pressure ab initio technique. A first-order phase transition into a rocksalt state at 30–35 GPa is observed in the constant pressure simulation. We also investigate the stability of wurtzite (WZ) and zinc-blende (ZB) phases from energy–volume calculations and Gibbs free energies at zero temperature and find that both structures show nearly similar equations of state and transform into a rocksalt structure around 14 GPa, in agreement with experiments. Additionally, we examine the influence of pressure on the electronic structure of the wurtzite and zinc-blende ZnS crystals and find that their band gap energies exhibit similar tendency and increase with increasing pressure. The calculated pressure coefficients and deformation potential are found to be comparable with experiments.     

A Raman and NMR study of KSbF6

The ¹⁹F spin-lattice relaxation results in the laboratory frame show that upon cooling from the tetragonal phase, KSbF₆(I), the transition to the cubic phase, KSbF₆(II), occurs over a wide temperature range (~38 K) in which the two phases co-exist. The Raman results using powdered samples agree with this observation but co-existence of phases has of course not been observed in single crystal Raman measurements. Upon rapid cooling of the powdered samples in the Raman studies the tetragonal phase could be super-cooled. Upon heating from the cubic phase, the transition was observed at 302 ± 2 K in all the measurements. The Raman spectra of KSbF₆(I) give no evidence of a non-centrosymmetric structure but it is shown that this is so because the Sb-atoms are only very slightly displaced from centro-symmetrical positions. ¹⁹F second moment results are in agreement with a model in which the SbF^?₆-octahedra are stationary below 180 K and reorient isotropically above 260 K. The importance of scalar spin-spin coupling between fluorine and antimony nuclei is reflected by the T_1p results in the vicinity of the T₁ minimum. The Raman spectra of the cubic phase at higher and lower temperatures are different and the polarized spectra of single crystals are used to assign the bands in terms of a C₃-site group symmetry for the SbF^?₆-groups and a T unit cell group symmetry.