Showing papers on "Perovskite (structure) published in 1991"

Earth's Core-Mantle Boundary: Results of Experiments at High Pressures and Temperatures

Abstract: Laboratory experiments document that liquid iron reacts chemically with silicates at high pressures (above 2.4 x 10 to the 10th Pa) and temperatures. In particular, (Mg,Fe)SiO3 perovskite, the most abundant mineral of earth's lower mantle, is expected to react with liquid iron to produce metallic alloys (FeO and FeSi) and nonmetallic silicates (SiO2 stishovite and MgSiO3 perovskite) at the pressures of the core-mantle boundary, 14 x 10 to the 10th Pa. The experimental observations, in conjunction with seismological data, suggest that the lowermost 200 to 300 km of earth's mantle, the D-double-prime layer, may be an extremely heterogeneous region as a result of chemical reactions between the silicate mantle and the liquid iron alloy of earth's core. The combined thermal-chemical-electrical boundary layer resulting from such reactions offers a plausible explanation for the complex behavior of seismic waves near the core-mantle boundary and could influence earth's magnetic field observed at the surface.

Effect of pressure, temperature, and composition on lattice parameters and density of (Fe,Mg)SiO3-perovskites to 30 GPa

Abstract: High-pressure, high-temperature properties of MgSiO3, (Fe01Mg09)SiO3, and (Fe02Mg08)SiO3 perovskites have been investigated using a newly developed X ray diffraction technique involving monochromatic synchrotron radiation The first direct measurements of unit cell distortions and equation-of-state parameters of the orthorhombic perovskite as functions of composition and simultaneous high pressure and high temperature were obtained The experiments were conducted under hydrostatic pressure up to 30 GPa, into the stability field of the perovskite The results demonstrate that the perovskite is elastically anisotropic, with the lattice parameter b being 25% less compressible than a and c Under increasing pressures the orthorhombic perovskite is distorted further away from the ideal cubic structure in agreement with theoretical predictions The 298-K isothermal equations of state of the three perovskites are indistinguishable within the uncertainty limits of the experiment The zero-pressure bulk modulus KT0 = 261 (±4) GPa with its pressure derivative KT0′ = 4 is close to that determined in previous static high pressure measurements The thermal expansion obtained from the high P - T experiments are consistent with previous measurements carried out at zero pressure but shows a strong volume dependence The temperature derivative of the isothermal bulk modulus at constant pressure (∂KT/∂T)p is −63(±05)×10−2 GPa/K Analyses of the high-temperature data give a value for the Anderson-Gruneisen parameter δT of 65–75, which is significantly higher than that used in recent lower mantle models

Microstructure of Solution‐Processed Lead Zirconate Titanate (PZT) Thin Films

Abstract: Lead zirconate titanate (PZT) thin films with a composition near the morphotropic phase boundary (Zr/Ti = 53/47) were fabricated by spin deposition of an alkoxide-derived solution and annealed at 650°C for 30 min. A complex microstructure is observed in which micrometer-scale rosettes of the desired perovskite phase are surrounded by nanocrystalline (10 to 15 nm) grains of pyrochlore structure. Transmission electron microscopy (TEM) demonstrates that the perovskite rosettes—features of approximately circular cross section which grow rapidly within the confined conditions of the thin film—are single crystals despite being highly porous. Pockets of lead-deficient pyrochlore extend throughout the thickness of the film. The only effects of Nb (2%) doping on the microstructure are to increase the fraction of the perovskite phase and the perovskite grain size. Despite the highly irregular shape of the perovskite particles and the presence of some pyrochlore, reasonable ferroelectric properties are measured (spontaneous polarization Ps∼ 0.2 C/m2).

Preparation and ferroelectric properties of PbZr0.53Ti0.47O3 thin films by spin coating and metalorganic decomposition

Abstract: Films with a composition PbZr0.53Ti0.47O3 were prepared on different platinum bottom electrodes using a spin‐coating process followed by metalorganic decomposition. The film morphology and structure were characterized by scanning electron microscopy and x‐ray diffraction analysis. The morphology was strongly influenced by the heat cycle used to form the PbZr0.53Ti0.47O3 layer. Fast heating and high‐fire temperatures produced smooth films, while slow heating and low‐fire temperatures gave films having a rosette perovskite phase and an inter‐rosette second phase. The films were characterized electrically by measuring hysteresis loops and capacitance and conductance versus bias voltage and by pulse‐switching measurements. The dependence of the switched and nonswitched polarization on the number of switching cycles (i.e., the fatigue behavior) is found to be much better for fast‐heated than for slowly heated films. Switching lifetimes exceeding 1011 cycles were measured. The type of platinum bottom electrode used was found to have a large influence on the ferroelectric properties of the lead zirconate titanate films.

Unquenchable high-pressure perovskite polymorphs of MnSnO3 and FeTiO3

Abstract: New high-pressure orthorhombic (GdFeO3-type) perovskite polymorphs of MnSnO3 and FeTiO3 have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO3 and FeTiO3 at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO3, with a volume change of -1.5%, and at 16 GPa in FeTiO3, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO3 perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) A, c=7.690 (8) A and V= 221.99 (15) A3. Volume compression data were collected between 7 and 20 GPa. The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO3 perovskite at 18.0 (5) GPa, cell parameters are a=5.022 (6) A, b=5.169 (5) A, c=7.239 (9) A and V= 187.94 (36) A3. Based on published data on the quench phases, the FeTiO3 perovskite breaks down to a rocksalt + baddelyite mixture of “FeO” and TiO2 at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides.

A new approach to laser heating in high pressure mineral physics

Abstract: Single crystals of olivine, forsterite, and enstatite were heated to over 2,000 K between 100 and 250 kbar with a 120 W CO{sub 2}-laser in a diamond anvil cell using a hydrostatic, inert gas pressure medium The conversion to some of their high pressure polymorphs, {beta}-phase, {gamma}-phase, perovskite, majorite, ilmenite, and clinoenstatite yielded single crystals large enough to obtain high quality Raman spectra, thus providing a quick, economic, and precise method to measure phase diagrams without pressure quenching The phase boundaries between olivine or forsterite and {beta}-phase and between {gamma}-phase and perovskite were accurately bracketed Preliminary results on the latter transition are within the P-T uncertainties of previous measurements using multi-anvil devices, constraining the temperature at 670 km depth to 1,900 {plus minus} 100 K, which is about 300 K higher than using their best estimate of the phase boundary The unquenchable phase of MgSiO{sub 3}-clinopyroxene was synthesized at {approximately}170 kbar; Raman spectra show that this phase is of C2/c symmetry Its stability field is between 50 and {approximately} 165 kbar at ambient temperature MgSiO{sub 3} perovskite was synthesized above 230 kbar and {approximately}2,000 K Raman spectra were measured from 75 to 470 kbar Above 370 kbar, two modes disappearmore » and two others change slope with pressure, suggesting a change in symmetry from Pbnm at room temperature« less

Thin-Layer Dielectrics in the Pb[(Mg1/3Nb2/3)1-xTix]O3 System

Abstract: Thin layers of Pb((Mg{sub 1/3}Nb{sub 2/3}){sub 1{minus}x})O{sub 3} (PMNT) were prepared by spin casting alkoxide-based solutions on platinized Si. The effects of additives, heat treatment, and composition (x = 0 to 0.9) on perovskite phase development, ceramic microstructure, and dielectric properties are reported. Depending upon the processing conditions, ceramic thin layers could be formed in a nonferroelectric pyrochlore phase (A{sub 2}B{sub 2}O{sub 6}) or in a ferroelastic perovskite phase (ABO{sub 3}). The dimensions of the pyrochlore and perovskite units cells were related and increased with Mg and Nb contents. To minimize pyrochlore formation, the most effective processing method involved rapid heat treatment between successive solution depositions. Phase development and microstructure were also affected by solution additives. Additions of benzoic acid were found to affect the structure in solution and the later organic pyrolysis behavior from thin layers. In this paper the effect of composition on the dielectric and ferroelectric properties is reported.

Simulation of the pre-melting behaviour of MgSiO3 perovskite at high pressures and temperatures

Abstract: MAGNESIUM-rich silicate perovskite is thought to be the dominant mineral phase in the Earth's lower mantle. The behaviour of MgSiO3 perovskite at high temperatures and pressures is therefore important for a wide range of geophysical problems, including the chemical and thermal evolution of the Earth, mantle convection, the thermal gradient in the mantle and the secular variations of the Earth's magnetic field. Experimental investigations at lower-mantle conditions are, however, difficult. We have performed computer simulations of MgSiO3 perovskite under typical lower mantle pressures and temperatures using the constant-temperature and constant-pressure molecular dynamics (MD) method. At pressures above 10 GPa, our simulations suggest that orthorhombic MgSiO3 perovskite undergoes a temperature-induced phase transformation to a cubic (or pseudo-cubic) phase before melting, and that the cubic phase is a solid electrolyte. The MD method tends to overestimate the temperature of melting and related phenomena, but should provide a reliable qualitative description of the ionic-conductivity behaviour. Quantitative determination of the structure and ionic conductivity of MgSiO3 perovskite at lower-mantle conditions must, however, await improvements in both experimental and simulation techniques.

Layered perovskite compounds Srn+1VnO3n+1 (n=1, 2, 3, and

Abstract: New layered perovskite compounds, Srn+1VnO3n+1 (n=1, 2 and 3) and SrVO3(n=∞ ), were synthesized and the physical properties of the compounds with n=1, 2 and ∞ were investigated Sr2VO4 was semiconductive and antiferromagnetic, but a weak ferromagnetic contribution to the susceptibility was observed below 45 K However, Sr3V2O7, exhibited a metallic electrical conduction and a Pauli-paramagnetism SrLaVO4 was also successfully synthesized and was found to be semiconductive The crystal structures and electrical and magnetic properties of Srn+1VnO3n+1 (n=1, 2, 3 and ∞ ) and SrLaVO4 were compared

Thermal diffusivity of MgSiO3 perovskite

Abstract: Thermal diffusivity of MgSiO3 perovskite has been measured in the temperature range of 160 K to 340 K using a sample synthesized in a uniaxial split-sphere high-pressure apparatus. At 300 K the thermal diffusivity of perovskite is 1.72 × 10−6 m2s−1, from which the thermal conductivity is evaluated to be 5.1 Wm−1K−1. Our model calculation shows that lattice thermal conductivity of the perovskite increases by a factor of 4 with depth throughout the lower mantle and reaches to 12 Wm−1K−1 in the vicinity of the mantle-core boundary. The D″ layer might not be a thermal boundary layer insulating the high core-temperature, if this layer mainly consists of the perovskite.

Phase Transition and Thermal Expansion of MgSiO3 Perovskite

Abstract: Results from in situ x-ray diffraction experiments with a DIA-type cubic anvil apparatus (SAM 85) reveal that MgSiO3 perovskite transforms from the orthorhombic Pbnm symmetry to another perovskite-type structure above 600 kelvin (K) at pressures of 7.3 gigapascals; the apparent volume increase across the transition is 0.7%. Unit-cell volume increased linearly with temperature, both below (1.44 x 10-5 K–1) and above (1.55 x 10–5 K–1) the transition. These results indicate that the physical properties measured on the Pbnm phase should be used with great caution because they may not be applicable to the earth9s lower mantle. A density analysis based on the new data yields an iron content of 10.4 weight percent for a pyrolite composition under conditions corresponding to the lower mantle. All current equation-of-state data are compatible with constant chemical composition in the upper and lower mantle; thus, these data imply that a chemically layered mantle is unnecessary, and whole-mantle convection is possible.

Thermal expansion of SrZrO3 and BaZrO3 perovskites

Abstract: 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.

Crystal Chemistry of Six-Coordinated Silicon: a Key to Understanding the Earth's Deep Interior*

Abstract: A survey of high-pressure silicates reveals 12 distinct high-density structural topologies with octahedral Si. Seven of these structure types - stishovite, perovskite, ilmenite, hollandite, calcium ferrite, pyrochlore and K2NiF4 type - contain only six-coordinated silicon. Other high-pressure silicates, including those with the garnet, pyroxene, wadeite, anhydrous phase B and phase B structures, contain both tetrahedral and octahedral Si. Five systematic trends among these dozen structures suggest the existence of other, as yet unobserved, possible mantle Si phases. The criteria are: (1) structures like rutile, hollandite and calcium ferrite formed from edge-sharing chains of silicon octahedra; (2) germanates synthesized at room pressure with octahedral Ge; (3) isomorphs of roompressure oxides with 3+ or 4+ transition-metal cations; (4) high-pressure magnesium silicates related to room-pressure aluminates by the substitution 2AI Mg+ Si; and (5) the homologous structures in system Mg-Si-O-H that includes phase B and anhydrous phase B. Each of these criteria can be used to predict other potential octahedral Si phases. Of special interest are predicted structure types that fulfill more than one criterion: diaspore-type (MgSi)O2(OH)2, aerugite-type Mgl0Si3Oi6 , sphene-type CaSi2Os, benitoite-type BaSi409, gibbsite-type MgSi(OH)6 and pseudobrookite-type Fe2SiOs.

The antiferroelectric crystal structure of the highly ordered complex perovskite pb(yb1/2nb1/2)o3

Abstract: The crystal structure of antiferroelectric Pb(Yb1/2Nb1/2)O3 has been characterized. Both X-ray diffraction and transmission electron diffraction show two sets of superlattice reflections originating respectively from the B-site atom ordering and the antiparallel lead-atom displacements which suffer within the orthorhombic ao-bo plane along the ao direction. The room-temperature crystal symmetry is considered to be orthorhombic with space group Pbnm (D2h16) and lattice parameters ao=5.918 AA, bo=23.453 AA and co=8.221 AA. High-resolution electron microscopy confirms the diffraction results. A structural phase transition from the lower-symmetry phase to the cubic paraelectric phase occurs at about 302 degrees C on heating.

Phase diagram of LaTiO x : from 2D layered ferroelectric insulator to 3D weak ferromagnetic semiconductor

Abstract: LaTiO x compounds are structurally related to perovskites and there are two known phases. The first,x=3.50, is a 2D layered-type ferroelectric. The second,x=3.00, is a weak ferromagnet with a 3D orthorhombic distorted perovskite structure. 20 samples with varying oxygen stoichiometry between these end members were prepared by floating zone melting, and then characterized by means of X-ray powder diffraction, electron microscopy, thermogravimetric analysis, resistivity and magnetic measurement. A phase diagram is established which displays the following physical and structural properties. A structural phase boundary atx=3.20 separates a new series of 2D layered structures from the 3D orthorhombic one. The former series represents the first conducting titanium oxides with a 2D layered structure to be reported. Atx=3.10 a phase boundary exists between a metallic and a weak ferromagnetic state where the magnetic transition temperatureT c can be sensitively tuned by the oxygen stoichiometryx. Samples withT c between 100 K and 130 K exhibit a metal-semiconductor transition whereas samples with higherT c , up to 149 K, are semiconductors between room temperature and 4.2 K.

Evolution of the Distortion of Perovskites Under Pressure: An EXAFS Study of BaZrO3, SrZrO3 and CaGeO3

Abstract: We have investigated the evolution of the distortion of several oxide perovskites with increasing pressure, using EXAFS in the diamond anvil cell. Cubic perovskite BaZrO3 remains cubic up to 52 GPa. Orthorhombic perovskite CaGeO3 becomes less distorted as pressure increases, becomes tetragonal at about 12 GPa and evolves toward cubic structure, still not obtained at 23 GPa. The distortion of orthorhombic perovskite SrZrO3 first increases with pressure up to 8 GPa, then decreases until the perovskite becomes cubic at 25 GPa. The results are interpreted in terms of a systematics, relating the distortion to the ratio f of the volumes of the AO12 dodecahedron and the BO6 octahedron, and to the compressibilities of the polyhedra. For cubic perovskites, f=5, which may correspond to a situation where the compressibilities of octahedra and dodecahedra are equal.

Residual Stress in Pzt Thin Films and its Effect on Ferroelectric Properties

Abstract: The residual stress in solution derived Pb(Zr.53Ti.47)O3, PZT 53:47, films was determined by measuring the bending of the substrate due to the stress. The substrate consisted of an oxidized (100) silicon wafer with 300 nm coating of platinum. In all cases the stress was tensile. Films fired at a temperature in the range where pyrochlore formation occurs (500° to 575°C) had the highest residual stresses, 200 to 350 MPa, whereas those fired at higher temperatures, 600° to 650°C, where the perovskite phase forms had stresses of 100 to 200 MPa. Stress measurements made during film firing indicate that the pyrochlore containing films had higher residual stress because their coefficient of thermal expansion was much larger than that of predominantly perovskite films. The effect of the amount of stress on ferroelectric properties was studied by making measurements on a film with and without the application of an external stress. The external stress was applied by bending a circular section of the substrate, which effectively lowered the amount of tensile stress in the film by ~30%. Decreasing the stress in this manner was found to increase the remanent polarization by ~11% and the dielectric constant by ~2%.

Hydroxy acid-aided synthesis of perovskite-type oxides of cobalt and manganese

Abstract: The hydroxy acid-aided process using citric or malic acid produced a perovskite phase at relatively low calcination temperatures. The adjustment of the starting solution to an appropriate pH value in the malic acid-aided process resulted in reducing the temperature for the formation of single-phase perovskite-type oxides and in producing large surface-area oxides.