Phase Transitions In Perovskites Research Articles

Correlation analysis between subduction in the last 180 Myr and lateral seismic structure of the lower mantle: geodynamical implications

We have calculated the correlation between four recent tomographic models and reconstructions of past subduction sites to estimate the present positions of the lithospher subducted during the Cenozoic and the Mesozoic. A significant correlation is found at a depth of about 1000 km for subduction younger than 120 Myr, while remnants of subduction older than 120 Myr are detected close to the core‐mantle boundary (CMB). Somewhere in the depth range of 1300–2600 km a gap exists where no significant correlation is found in any time. The results suggest that the slabs do not pass through the lower mantle continuously. It is possible that both regimes, the catastrophic and the steady‐state, may be important for different time periods. Fast seismic anomalies close to the CMB may be associated with a past catastrophic event, while the good correlation in the upper part of the lower mantle may be explained by a steady‐state model. Other explanations for the very good correlation found at a depth of about 1000 km are either a predicted perovskite phase transition or a chemical boundary suggested by seismic array analysis.

Calculations of pressure-induced phase transitions in mantle minerals

The crystal structures and energies of SiO2 stishovite, MgO periclase, Mg2SiO4 spinel, and MgSiO3 perovskite were calculated as a function of pressure with the polarization-included electron gas (PEG) model. The calculated pressures of the spinel to perovskite phase transitions in the Mg2SiO4 and MgSiO3 systems are 26.0 GPa and 27.0 GPa, respectively, compared to the experimental zero temperature extrapolations of 27.4 GPa and 27.7 GPa. The two oxide phases are found to be the most stable form in the pressure range 24.5 GPa to 31.5 GPa, compared to the experimental zero temperature extrapolation of 26.7 GPa to 28.0 GPa. The volume changes associated with the phase transitions are in good agreement with experiment. The transition pressures calculated with the PEG model, which allows the ions to distort from spherical symmetry, are in much better agreement with experiment than those calculated with the modified electron gas (MEG) model, which constrains the ions to be spherical.

Femtosecond time-resolved spectroscopy of soft modes in structural phase transitions of perovskites.

Femtosecond time-domain observations of soft-mode dynamics in crystals near structural phase transitions have been conducted. Impulsive stimulated Raman scattering (ISRS) experiments are reported for both the orthorhombic phase of ${\mathrm{KNbO}}_{3}$ and the tetragonal phase of ${\mathrm{BaTiO}}_{3}$. The data from potassium niobate clearly show a heavily damped soft mode of ${\mathit{B}}_{2}$ symmetry and relaxational modes of ${\mathit{A}}_{1}$ symmetry but not ${\mathit{B}}_{2}$ symmetry. Similarly, the data from barium titanate clearly show a heavily damped E-symmetry soft mode and no relaxational modes of this symmetry. The absence of relaxational modes of the same symmetries as the soft modes is consistent with an eight-site order-disorder model of the phase transitions in this crystal class. The present results demonstrate significant advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

Nature of phase transitions in lead-containing perovskites with regard to specific applications

Abstract The physical background of field-induced first-order ferroelectric-paraelectric, antiferroelectric-ferroelectric phase transitions and diffused phase transition in modified lead-containing perovskite ceramic compositions is discussed. Examination of radiation effects has proven to be useful in finding ways to control the properties of perovskites. Some specific applications of the materials are considered.

Perovskite at high P‐T conditions: An in situ synchrotron X ray diffraction study of NaMgF3 perovskite

The lattice distortion and structural phase transition of NaMgFa perovskite (Neighborite) have been studied using synchrotron X ray powder diffraction at high pressure and temperature. Changes in the unit cell dimensions of the perovskite are determined by conventional peak indexing and least squares routines. The stress field within the high‐pressure cell assembly is analyzed, and the yield strength of the NaMgF3 perovskite is determined at high P and T. The pressure‐ and temperature‐induced dimensional changes of the NaMgF3 perovskite structure are expressed empirically as a combination of compression/expansion of the [Mg‐F] bond length and tilting of the MgF6 octahedral framework. The linear thermal expansions of the NaMgF3 perovskite observed at different pressures show significant anisotropy with αa > αc > αb which reflects the decrease of structural distortion and the development of a phase transition in the perovskite with increasing temperature. The tilting angle of the MgF6 octahedral framework is observed to decrease rapidly toward zero, in a manner expected for a ferroelastic phase transition, as the temperature approaches the transition point Tc. The apparent [Mg‐F] bond lengths of the MgF6 octahedra experience drastic shrinkage with increasing temperature just prior to the transition. Despite a 12% change in volume due to compression, the experimental results on NaMgF3 perovskite show that the thermal expansivity is independent of pressure, i.e., dα/dP ≈ 0, and, compatibly, that the compressibility is independent of temperature, i.e., dβ/dT 0. However, the dominant compression mechanism is the compression of the octahedral bond length, whereas the dominant mechanism for thermal expansion is the diminishing of octahedral tilting. The Earth's mantle may be isochemical if the thermal expansion of MgSiO3 perovskite at high pressure behaves like NaMgF3, of which the Anderson‐Grüneisen parameter is near zero, i.e., δs ≈ 0. It is observed that crystal structure of the NaMgF3 perovskite transforms directly from the orthorhombic Pbnm phase to the cubic phase at all pressures. The transition temperature is observed to increase with increasing pressure with a positive slope of 45 K/GPa.

Pressure-Induced Migration of Ferroelectric Interfaces

AbstractThe study of the migration of the first-order paraelectric-ferroelectric interfaces in BaTiO3 under the hydrostatic pressure is provided in the framework of the time-dependent Ginzburg-Landau theory. The analytical solution describing the interphase boundary is applied for the calculations of its width and velocity at different pressures. The calculations are based on the experimental data for BaTiO3. The parameters of the power law of the temperature and pressure dependences of the interface velocity under the temperature and pressure were obtained. Using the BaTiO3 example we illustrate the ability of the suggested approach in the description of the kinetics of the first-order ferroelectric phase transition in perovskites.

Thermo- and magnetic-field-induced dynamics of ferroelectric interphase boundaries

Thermo- and magnetic-field-induced dynamics of first-order ferroelectric and antiferroelectric phase transitions are considered, taking into account the inertia effect. The width and velocity of interphase boundaries are calculated as functions of temperature and magnetic field strength. The results obtained here are essentially different from those of the Ginzburg-Landau theory and of the case of the small kinetic energy term, and they may also be used for the description of the kinetics of ferroelectric phase transitions in high-temperature superconductive perovskites.

Electron microscopy study of domain structure due to phase transitions in natural perovskite

Transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90° rotation twins about the [001] axis (referred to as [001]90° twin). Single crystals that were heattreated and quenched from above 1585 K exhibit a dramatic change in domain structure compared with the starting material and specimens quenched from T < 1470 K. Mutually perpendicular {110} and [001]90° twins are observed throughout the crystal, forming a cross-hatched domain texture. 1/2[001] antiphase domains, which are very rarely observed in the starting material, also become dominant in the crystal. This change in domain structure is interpreted as due to a structural phase transition in perovskite at a temperature below 1585 K. From the point symmetry elements that describe the twin laws and the translational elements that relate the antiphase domains, the most likely phase near 1585 K is tetragonal with space group P4/mbm. These results are consistent with high-temperature powder X-ray diffraction study. On the other hand, density of the {112} twins is increased significantly in the crystal quenched from 1673 K. Twin domains are either bound by mutually perpendicular {110} and (001) walls, or by {112} walls with {110} twin domains within the polygonal {112} domains. Both twin density variation and domain morphology suggest that the crystal may be cubic at this temperature. Microstructure of a single crystal deformed at 1273 K and 3.5 GPa (within the orthorhombic stability field) is morphologically quite distinct from that of the heat-treated specimens. Dislocations dominate the microstructure and often interact with twin domain boundaries.

Stability of orthorhombic MgSiO3 perovskite in the Earth's lower mantle

MAGNESIUM-rich silicate perovskite is thought to be the primary constituent of the Earth's lower mantle: experiments have shown1 MgSiO3 perovskite to be stable at lower-mantle pressures, and the elastic properties of perovskite-dominated assemblages agree well with seismological observations2–4. It has also been suggested5–8 that the observed orthorhombic structure will undergo displacive phase transitions to higher-symmetry structures at lower-mantle conditions. The presence of such transitions would have important consequences for mantle convection9, and could provide an explanation for some of the weak seismic discontinuities observed10–12 in the lower mantle. However, the determination of the phase behaviour of MgSiO3 perovskite at lower-mantle conditions has so far eluded both experimental and theoretical efforts. Here we report the results of electronic-structure calculations of the energetics of displacive phase transitions in MgSiO3 perovskite, and demonstrate that the lower-symmetry orthorhombic phase should be highly favoured throughout the lower mantle. Our results are consistent with recent experiments13on MgSiO3 perovskite encompassing the temperatures and pressures of the uppermost regions of the lower mantle.

Simulation Study of Off-Center Shifts in Perovskite Systems

An orientational order-disorder phase transition in perovskites has been studied by simulation using the FEFF-5 ab initio program. Off-center shifts of the central atoms toward the <100> directions in perovskite systems have been modeled for three different phases: (i) totally disordered, (ii) partially ordered, and (iii) totally ordered structures. XAFS spectra have been simulated as a function of temperature from these modeled structures. The relative Debye-Waller factors extracted from the simulated XAFS spectra have been quantitatively analyzed using the Einstein model and a single-shell approximation.

Multiple phase transitions and the style of mantle convection

Multiple phase transitions, based on experimental and theoretical results, have been incorporated into an extended Boussinesq model of mantle convection. A formulation is used which facilitates the numerical computations of convection with multiple phase transitions and a triple point near the 670 km discontinuity. The propensity for layered convection is enhanced by including the additional phase transitions. The location of the triple point of the spinel to perovskite phase transitions plays an important role in controlling the fate of the ascending hot plumes. A plume with a core temperature above this triple point temperature T c tends to pass easily through the phase boundary at 670 km. Plumes with a core temperature lower than T c are blocked by the multiple phase transitions. Both depth-dependent thermal expansivity and internal heating increase the tendency of the convective system to layering. The dynamical time-scales in the transition zone are much shorter than those of the top and bottom boundary layers. Simulations with increasing Rayleigh number (Ra) from 10 6 to 5 × 10 7 show unambiguously the increased tendency toward layering with larger Ra. In the early Earth, when Ra was higher and internal heating was several times stronger, layered convection might have been the preferred mode of mantle convection. With time, as both Ra and radioactive heating decreased, the style of mantle convection would have become less layered.

Thermal expansion of SrZrO3 and BaZrO3 perovskites

High-temperature X-ray diffraction studies of SrZrO3 and BaZrO3 perovskites have been carried out to 1200° C. The diffraction patterns are analyzed with Rietveld method so as to refine the unit cell dimensions. The volumetric thermal expansion coefficient are observed to be 2.98*10-5K-1 for orthorhombic Pbnm phase, 3.24*10-5K-1 for orthorhombic Cmcm phase, 3.75*10-5K-1 for tetragonal I4/mcm phase of SrZrO3 perovskite, and 2.06*10-5K-1 for cubic Pm3m phase of BaZrO3 perovskite, respectively. The linear thermal expansion coefficients of SrZrO3 perovskite show considerable anisotropy of α a >α c >α b for orthorhombic Pbnm phase, which reflect the decrease of distortion of the perovskite. It is demonstrated that thermal expansion of the centrosymmetrically distorted ABX3 perovskite can be empirically expressed as a combination of the changes of [B-X] bond length and tilting angle of BX6 octahedral framework. The octahedral tilting is considered to be the primary order parameter for the ferroelastic type of structural phase transitions in perovskite. Thermodynamically, the tilting induced volume change denotes the “excess volume” and the corresponding thermal expansion represents the “excess thermal expansion” for the lower symmetry phase with respect to its prototype of the cubic perovskite.

Substrates for high - temperature superconductors

Potential applications as well as fundamental studies of high - temperature superconductors (HTSC) require improvements in epitaxial processes and development of novel substrates. The substrates used so far (SrTiO 3, MgO, Al 2O 3, ZrO 2:Y, LaGaO 3, Si) do not fulfil the requirements for optimized growth and HTSC properties. One of the substrate problems is structural phase transitions in perovskites leading to twinning and surface corrugations. A detwinning procedure for LaGaO 3 will be described. Furthermore, a possible route towards suitable substrates will be indicated.

Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction, thermal expansion and heat capacity

High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO3 (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc=520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10-5 (K-1) below Tc and 3.5x 10-5 (K-1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.

Mantle layering from ScS reverberations: 2. The transition zone

This is the second paper in a four‐part sequence that examines the nature of mantle layering using the ScSn phases and internal reflections observed within the reverberative interval of SH‐polarized seismograms. Mantle reflectivity profiles for 18 seismic corridors sampling portions of Indonesia, Australia, and the western and central Pacific are constructed from low‐frequency (25‐mHz) ScS reverberations; i.e, ScSn‐type phases reflected once from internal mantle discontinuities. Modeling the reflectivity profiles using synthetic seismograms yields precise, path‐specific estimates of the travel times to the 410‐km and 660‐km discontinuities and their reflection coefficients. The data are consistent with an olivine‐dominated model of the transition zone, marked by two major phase transitions having average apparent depths of 414±2 km and 660±2 km and impedance contrasts of 0.046±0.010 and 0.072±0.010, respectively. The interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km (peak to peak) and is negatively correlated, consistent with an endothermic transition at 660 km whose Clapeyron slope magnitude is comparable to that of the 410‐km (exothermic) transition. In the case of the 660‐km discontinuity, we have been able to supplement these long‐wavelength (1500–5000 km) observations with an estimate of intrapath topography, inferred from the frequency dependence of the 660‐km reflection coefficient, which gives peak‐to‐peak variations less than 40 km on scale lengths of 500–1500 km. For the 660‐km discontinuity, the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction. However, we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660‐km discontinuity which may involve small compositional variations. Plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition, loss of reflected energy due to local curvature (or roughness) of the reflector, and/or extreme topography occurring on a small compositional component to the discontinuity (≤40% of the total impedance contrast). By stripping the 410‐km and 660‐km peaks from the composite path reflectivity profile, we identify three minor reflectors at mean depths of 520, 710, and 900 km. The shallowest may mark the β‐phase → γ‐spinel phase transition. The 710‐km discontinuity can be attributed to the ilmenite (gamet) → perovskite phase transition, although changes in perovskite symmetry can potentially provide explanations for both the 710‐km and 900‐km features.

Off-center displacement of the Nb ions below and above the ferroelectric phase transition of KTa0.91Nb0.09O3.

We have measured the off-center displacements of the Nb ions in ${\mathrm{KTa}}_{0.91}$${\mathrm{Nb}}_{0.09}$${\mathrm{O}}_{3}$ as a function of temperature both below and above the ferroelectric transition temperature ${\mathit{T}}_{\mathit{c}}$=85.6 K. The purpose of these measurements was to elucidate the basic nature of the ferroelectric phase transitions in mixed perovskites. The off-center displacements were determined by x-ray-absorption fine-structure (XAFS) measurements. The data were analyzed by calculating the XAFS spectra theoretically as a function of structural and other parameters and by refining these parameters to obtain the best fit with the experimental results up to the third coordination shell. In these calculations we used curved photoelectron wave functions and took into account multiple scattering up to and including, the third-order collinear scattering. It is found that these scattering contributions are both necessary and sufficient in order to account for all the scattering contributions with a total scattering path length of 8 \AA{} or less. At 70 K the off-center displacement, relative to the oxygen octahedra, is found to be 0.145 \AA{} in the [111] direction. These displacements change by less than 20% from 70 K to room temperature, namely, from below to far above the phase transition. These results lead to the conclusion that the magnitude of the displacements hardly changes over a very wide temperature range. Thus, the ferroelectric transition is not displacive and the main change taking place at the phase transition is from a disordered to orientationally ordered arrangement of displacements.

Theory of tetragonal twin structures in ferroelectric perovskites with a first-order phase transition.

A three-dimensional Landau-Ginzburg model has been constructed to describe the tetragonal twin structures resulting from a first-order ${\mathit{O}}_{\mathit{h}\mathrm{\ensuremath{-}}}$${\mathit{C}}_{4\mathit{v}}$ proper ferroelectric phase transition in perovskites. The model takes into account the nonlinear and nonlocal characteristics of the polarization (order parameter) as well as the electromechanical coupling. Quasi-one-dimensional (Q1D) analytic solutions for the space profiles of the order parameter are obtained for a 180\ifmmode^\circ\else\textdegree\fi{} twin and for a charge-neutral 90\ifmmode^\circ\else\textdegree\fi{} twin with a special choice of parameters. Without the presence of interfacial defects, such as dislocations, the Q1D solutions require the support of inhomogeneous mechanical constraints. Elastic deformation and dimensional changes associated with the twin structures, and their implications on the piezoelectric effect in ferroelectric ceramics, are also addressed.

On the Landau theory of structural phase transitions in layered perovskites (CH3NH3)2MCl4(M = Mn, Cd, Fe) : comparison with experiments

On the Landau theory of structural phase transitions in layered perovskites (CH₃NH₃)₂MCl₄(M = Mn, Cd, Fe) : comparison with experiments

Surface phonons near structural phase transitions of fluoridic perovskites

Surface phonons near structural phase transitions of fluoridic perovskites

Surface phonons near structural phase transitions of fluoridic perovskites

The fluoridic perovskite KMnF 3 exhibits an antiferrodistortive phase transition which goes along with a soft mode at the R-point of the Brillouin zone. We investigate in this paper the surface phonons of the KF(001) surface at temperatures near this phase transition. The calculations are carried out for relaxed and reconstructed (001) slabs.