Structural phase transition and octahedral tilting in the calcium titanate perovskite CaTiO3

Crystal structure of the calcium titanate perovskite CaTiO₃ was refined by the Rietveld analysis of neutron diffraction data collected over the temperature range of 296 to 1720 K. The orthorhombic Pbnm-tetragonal I4/mcm phase transformation reversibly occurred at 1512 ± 13 K and the tetragonal I4/mcm-cubic Pm3?m transition was reversibly observed at 1635 ± 2 K. The structural phase transitions are characterized by the tilt of the TiO₆ octahedron. The tilt systems of the Pbnm, I4/mcm, and Pm3?m CaTiO₃ are ?_x^??_y^??_z⁺ (?_x^? = ?_y^?), ?_x⁰?_y⁰?_z^?, and ?_x⁰?_y⁰?_z⁰, respectively. Here the ?_x^?, ?_z⁺, and ?_x⁰ stand for the out-of-phase tilt angle of TiO₆ octahedron along x axis, the in-phase octahedral tilt angle along z axis, and no octahedral tilting along x axis, respectively. All the tilt angles, ?_x^?, ?_z⁺, and ?_z^? decreased with increasing temperature where the critical exponents were about 0.25. The ?_x^? and ?_y^? of the orthorhombic structure decreased discontinuously to ?_z^? of the tetragonal structure or 0° through the Pbnm-I4/mcm transition. The tilt angle, the unit-cell parameters and unit-cell volume discontinuously changed at the Pbnm-I4/mcm transition temperature, indicating the first-order nature. The increase of the cell volume in the Pbnm-I4/mcm transition was 0.088 vol.%. In contrast, the unit-cell volume continuously increased and the tilt angle ?_z^? continuously decreased with increasing temperature and became 0° at the I4/mcm-Pm3?m transition temperature, indicating a continuous nature of the transformation. The Pbnm-I4/mcm and I4/mcm-Pm3?m phase transitions are induced by the tilting of TiO₆ octahedra, indicating the displacive nature of the transitions. The larger thermal motion of O2 atom in I4/mcm CaTiO₃ indicates the larger positional disorder of O2 atom, which is consistent with the tilt system ?_x⁰?_y⁰?_z^?.     

Elasticity of BaFCl single crystal under hydrostatic pressure

The sound velocities for longitudinal and transverse waves have been measured in single crystalline BaFCl at room temperature using ultrasonic pulse echo and Brillouin scattering techniques. The complete set of elastic constants is deduced and lead to the bulk moduli values of BaFCl at ambiant conditions (, , ) which are compared with those obtained by a shell model. Moreover, using the ultrasonic technique under pressure, the pressure derivatives of the second order elastic constants at 298 K have been determined up to 0.3 GPa. All moduli increase linearly with pressure in this pressure range, allowing to determine directly and separately the first derivative of the bulk modulus B'₀ = 5.8. These data are used to calculate a Murnaghan equation of state. A detailed comparison is given between our results with those recently obtained by X-ray diffraction on powder or calculated using the local density approximation method. Finally, the anisotropy of BaFCl under pressure is discussed. Received: 19 March 1998 / Revised: 15 May 1998 / Accepted: 19 May 1998     

High-pressure effects on the crystal and magnetic structure of managanites

Nuetron diffraction studies of high-pressure effects on the crystal and magnetic structure of A_1-x A _x ^′ MnO₃ manganites (A = Pr, La; A′ = Sr, Ca, Na) are reviewed. High pressure leads to various changes in the magnetic structure of manganites: the appearance of a new A-type antiferromagnetic (AFM) state in compounds with the initial ferromagnetic (FM) or pseudo-CE type AFM state, such as La_{1 ? x} Ca _x MnO₃ (x = 0.25, 0.33), Pr_{1 ? x} Sr _x MnO₃ (x = 0.48), Pr_0.7Ca_0.3Mn_{1 ? y} Fe _y O₃ (y = 0, 0.1), Pr_{1 ? x} Na _x MnO₃ (x = 0.2, 0.25); and the appearance of a new C-type AFM state in the Pr_0.44Sr_0.56MnO₃ compound with the initial A-type AFM state. The observed changes in the magnetic structure and the behavior of the transition temperature to the FM state under high pressure are discussed in the framework of the current theoretical concepts.     

High-pressure crystal structure studies of Fe,Ru and Os

In order to reveal structural trends with increasing pressure in d transition metals, we performed full potential linear muffin-tin orbital calculations for Fe, Ru, and Os in the hexagonal close packed structure. The calculations cover a wide volume range and demonstrate that all these hexagonal close-packed metals have non-ideal c/a at low pressures which, however, increases with pressure and asymptotically approaches the ideal value at very high compressions. These results are in accordance with most recent experiment for Ru and Os. The experimental data for iron is not conclusive, but it is believed that the c/a ratio decreases weakly with increasing pressure at moderate compression. Since, the experimental and calculated equations of state for iron are in increasingly good agreement with increasing pressure, it is possible that either the negative c/a trend is valid only for a restricted pressure range, or related to the experimental difficulties (e.g. non-hydrostaticity).     

High-pressure crystal structure of thorium disulfide and diselenide and uranium disulfide

Abstract

The crystal structure of ThS₂, ThSe₂ and US₂ has been investigated for pressure up to 60GPa using x-ray powder diffraction. The bulk moduli are 175(10), 155(10) and 155(20) GPa, respectively. A pressure-induced phase transformation occurs at about 40 GPa for ThS₂, 30 GPa for ThSe₂ and 15 GPa for US₂. The results for ThSe₂ indicate that its high-pressure phase has a monoclinic structure. The same structure is compatible with the observed high-pressure spectra of ThS₂ and US₂. However, the crystal system assignment is less certain for these compounds.     

A martensitic phase transition in nanocrystalline 3Y-TZP powders under hydrostatic pressure conditions

The aim of this study was to produce yttria-stabilized zirconia nanopowders from zirconium oxychloride and zirconium oxynitrate salts using a co-precipitation technique, and to investigate the influence of hydrostatic pressure on the phase transition in these powders. It is shown that synthesis conditions and calcination temperature have a strong influence on the nanopowder's agglomeration, as well as on the stability of the tetragonal phase to phase transition under pressure conditions. Doped zirconia nanopowders synthesized from oxynitrate salts are more agglomerated and more stable than the oxychloride-based powders. Increasing the role of interfacial energy in agglomerated nanopowders leads to an increase in the stability of the tetragonal phase in doped zirconia nanopowders systems obtained at low and high calcination temperatures. Formation of separated nanoparticles at middle calcination temperature leads to a decrease in the stability of the tetragonal phase to phase transition under hydrostatic pressure conditions.     

Fine crystal structure of electrolytic iron deposits obtained under various electrolysis conditions

An x-ray study has been made of the fine crystal structure of iron deposits obtained at various current densities and various electrolysis temperatures. The microdistortion and grain size were calculated by Fourier analysis of the line profile and the line width. Electrolytic iron deposits have considerable microdistortion, more extensive, the higher the current density and the lower the electrolyte temperature. The linear dimensions of the grains decrease under these conditions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 2, pp. 40–44, February, 1969.     

High-pressure effects on the crystal and magnetic structure of the Nd0.78Ba0.22CoO3 cobaltite

The crystal and magnetic structure of the Nd_0.78Ba_0.22CoO₃ cobaltite is studied by neutron diffraction at high pressures up to 4.2 GPa in the temperature range 10–300 K. The pressure dependences of structural parameters are obtained. Ferromagnetic ordering of the Co sublattice is observed at normal pressure below T _C ～ 140 K, and ferrimagnetic ordering of the Co and Nd sublattices with an antiparallel direction of magnetic moments appears at T _F ～ 40 K. The magnetic moment of Co and the temperature T _C change slightly under pressure, which points to the stability of the initial intermediate-spin (S = 1) state of Co³⁺ ions. This behavior differs considerably from the characteristic behavior of cobaltites that are close in chemical composition and structure and exhibit ferromagnetic ordering of only the Co sublattice. In these cobaltites, the magnetic moment of Co is substantially suppressed and T _C decreases under pressure, which is related to the change in the state of Co³⁺ ions from the intermediate spin state to the nonmagnetic low-spin state (S = 0). The interplay between the appearance of the magnetic interaction of the R-Co sublattices and the stability of the spin state of Co³⁺ ions in the Nd_0.78Ba_0.22CoO₃ cobaltite is discussed.     

Doping effect on crystal structure and conduction property of fast oxide ion conductor LaGaO3-based perovskite

In order to discuss oxide ion conduction mechanism for LaGaO₃-based perovskite compounds, doping effects were investigated using two kinds of solid solutions whose oxygen vacancy concentrations are the same: one is La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 with A-site and B-site substitutions and the other is LaGa_0.8Mg_0.2O_2.9 with only B-site substitution. Conductivity measurements showed that La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 had a circumstance whereby oxide ion could more easily diffuse in the perovskite structure than in LaGa_0.8Mg_0.2O_2.9. Structural analyses using neutron diffraction found out the following three differences: the first finding was that the saddle point formed by two A-site cations and one B-site cation in La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 was larger than that in LaGa_0.8Mg_0.2O_2.9 due to larger displacements of A-site and B-site cations; the second was that the doubly doping with Sr and Mg was more effective for reduction of GaO₆ octahedral tilt angles than the doping with Mg; the last was that La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 had larger oxygen displacement than LaGa_0.8Mg_0.2O_2.9. It was considered that these structurally related parameters dominated the high oxide ion conduction in LaGaO₃-based perovskite compounds.     

First principles study of electronic structure and optical properties of CaTiO3

The electronic-energy band structure, site and angular momentum decomposed density of states (DOS) and charge-density contours of perovskite CaTiO ₃ are calculated by the first principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows an indirect (R-Γ) band gap of 1.5 eV. The total DOS as well as the partial density of states (PDOS) are compared with the experimental photoemission spectra. The calculated DOS are in reasonable agreement with the experimental energy spectra and the features in the spectra are interpreted by a comparison of the spectra with the PDOS. The origin of the various experimentally observed bands have been explained. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ca and TiO ₃ is mainly ionic and that the TiO ₃ entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of CaTiO ₃ . The real and imaginary parts of the dielectric function and hence the optical constants such as refractive index and extinction coefficient are calculated. The calculated spectra are compared with the experimental results for CaTiO ₃ and are found to be in good agreement with the experimental results. The effective number of electrons per unit cell participating in the interband transitions are calculated. The role of band structure calculation as regards the optical properties of CaTiO ₃ is discussed. Received 1 February 2000 and Received in final form 21 July 2000     

High-pressure synthesis of BaVO3: A new cubic perovskite

A new cubic perovskite BaVO₃ was synthesized by high-pressure synthesis at 15 GPa, and 1350 °C. Contrary to our expectations that lattice expansion by Ba substitution for Sr would lead to non-centrosymmetric tetragonal distortion, BaVO₃ preserved its cubic crystal structure with a=3.94288(3) Å at room temperature and had Fermi-liquid characteristics as SrVO₃ down to the lowest temperature.     

High-pressure structure and elastic properties of tantalum single crystal:First principles investigation

Since knowledge of the structure and elastic properties of Ta at high pressures is critical for addressing the recent controversies regarding the high-pressure stable phase and elastic properties, we perform a systematical study on the highpressure structure and elastic properties of the cubic Ta by using the first-principles method. Results show that the initial body-centered cubic phase of Ta remains stable even up to 500 GPa and the high-pressure elastic properties are excellently consistent with the available experimental results. Besides, the high-pressure sound velocities of the single- and polycrystals Ta are also calculated based on the elastic constants, and the predications exhibit good agreement with the existing experimental data.     

Laser-induced helical structure in the isotropic phase of nematic liquid crystal

We report on the first observation of laser-induced helical structure in the isotropic phase of a nematic liquid crystal. The helical structure was induced by two nearly counter-propagating waves circularly polarized in the same sense. Bragg reflection in the optical tunneling region was studied. We made an extensive investigation on the analogy between Bragg reflection from a cholesteric liquid crystal slab and four-wave mixing. It provides a deeper insight into the physics of the optical properties of cholesteric liquid crystal.     

Magnetic structure of LaCrO3 perovskite under high pressure from in situ neutron diffraction

The temperature-pressure phase diagram for both the crystal and magnetic structures of LaCrO(3) perovskite has been mapped out by in situ neutron-diffraction experiments under pressure. The system offers the opportunity to study the evolution of magnetic order, spin direction, and magnetic moment on crossing the orthorhombic-rhombohedral phase boundary. Moreover, a microscopic model of the superexchange interaction has been developed on the basis of the crystal structure obtained in this work to account for the behavior of T(N) under high pressure.     

Tuning of crystal phase structure in hydrated WO3 nanoparticles under wet chemical conditions and studies on their photochromic properties

Hydrated tungsten oxide nanoparticles have been synthesized using a simple wet chemical method while varying the concentration of HCl. XRD studies show that the variation in HCl concentration from 1 M to 6 M in the reaction results into gradual change in crystal structure of hydrated WO₃ from hexagonal (WO₃·0.33H₂O) to pure orthorhombic (WO₃·H₂O), through a series of samples with mixed phase of the two indifferent ratios. The similar variations in the degree of hydration and phase variations have also been observed from Raman, FTIR and TGA studies. The average crystallite size of the hydrated WO₃ particles was estimated to be ～26 nm from XRD line broadening and AFM studies showed the formation of spherical shaped particles for all the samples. The photochromic studies were carried out on the composite films of these materials in the polymeric matrix of polyvinyl alcohol (PVA) while exposing to UV light. The composite films show interesting variations in the photochromic behavior depending on the crystal structure of hydrated WO₃ filler. The photochromic behavior has been explained on the basis of EPR spectra of hydrated WO₃.     

The Born-model and the effects of hydrostatic and non-hydrostatic stresses on rubidium halide structural phase transitions

An analytic Born-model, with the same set of repulsive parameters for both phases in each salt, has been used to calculate the properties of the NaCl-CaCl structural phase transformation in three rubidium halides. The treatment required a careful evaluation of the three repulsive parameters by comparison with equilibrium conditions in both phases and measured bulk moduli, and involved a self consistent analysis which takes into account the experimental uncertainties in reported values of CsCl-phase lattice parameters. Calculated values for the equilibrium transition pressure, lattice parameters and lattice energies are in satisfactory agreement with reported experimental results. The model has also been used to calculate the lattice energy continuously from the NaCl to the CsCl phases, as a function of both hydrostatic and non-hydrostatic stresses. These calculations give a semiquantitative estimate of an energy barrier between the two stable structures, which is consistent with reported measurements of elastic constant and hysteresis effects near the transition pressure. The calculated effects of a uniaxial stress are found to be as much as three times larger than those of a hydrostatic stress, and the effects of the uniaxial stress on the barrier height are found to be approximately the same as the effects on the equilibrium energy differences. Measurements of the effect of this uniaxial stress on the forward transition pressure of RbI were carried out and the measured variations were found to be in excellent agreement with the calculated change in equilibrium transition pressure—as expected from the energy barrier calculations.     

静水压对立方钙钛矿CsSnX3(X=I, Br, Cl)光电性质的第一性原理探究

全无机无铅卤化物钙钛矿已经成为重要的新一代太阳能电池材料.采用密度泛函理论的第一性原理研究了不同静水压下CsSnX₃(X=I, Br, Cl)材料的晶体结构,电子结构和光学性能,并分析了其内在联系.结果表明施加静水压可使材料中Sn-X键长减小,使原子之间的耦合增强,带隙值减小,且随着卤族元素半径的增大,压力效应越明显;随着压力的增加,材料的吸收系数和复折射率增大,吸收光谱出现红移现象,在可见光区和近红外光区吸收增强.相比CsSnBr₃和CsSnCl₃,CsSnI₃在可见光区吸收最佳且受压力作用影响最小,更适用于钙钛矿太阳能电池材料.     

Surface structure of magnetite (111) under hydrated conditions by crystal truncation rod diffraction

X-ray crystal truncation rod (CTR) diffraction under hydrated conditions at circum-neutral pH was used to determine the surface structure of Fe₃O₄(111) following a wet chemical mechanical polishing (CMP) preparation method. The best-fit model to the CTR data shows the presence of two oxygen terminated domains that are chemically inequivalent and symmetrically distinct in the surface contribution ratio of 75% oxygen octahedral-iron (OOI) termination (^aO_2.61–^aO_1.00–^oh1Fe_2.55–^bO_1.00–^bO_3.00–^td1Fe_1.00–^oh2Fe_1.00–^td2Fe_1.00–R) to 25% oxygen mixed-iron (OMI) termination (^bO_1.00–^bO_3.00–^td1Fe₀–^oh2Fe_1.00–^td2Fe_1.00–^aO_3.00–^aO_1.00–^oh1Fe_3.00–R). An adsorbed water layer could not be constrained in the best-fit model. However, bond-valence analyses suggest that both of the surfaces are hydro-oxo terminated. Furthermore, the topmost iron layers of both domains are inferred to be occupied with the redox active Fe²⁺ and Fe³⁺ cations indicating that these irons are the principle irons involved in controlling the surface reactivity of magnetite in industrial and environmentally relevant conditions.     

High-pressure crystal chemistry of scheelite-type tungstates and molybdates

Unit-cell parameters and crystal structures of CaWO₄(scheelite) and CaMoO₄ (powellite) have been determined at several pressures to 5 8 GPa, and unit-cell parameters of PbMoO₄ (wulfemte), PbWO₄ (stolzite) and CdMoO₄ have been measured at pressures to 6 0 GPa All five tetragonal scheelite-type compounds compress anisotropically, with the (itc) axis 1 2 to 1 9 times more compressible than a. In both CaWO₄ and CaMoO₄ the cation tetrahedra (with W⁶⁺ or Mo⁶⁺) behave as rigid structural elements with no observed cation-oxygen compression (W-O and Mo-O bond compression < 0001 GPa^?1) Compression of the eight-coordinated calcium polyhedron, on the other hand, is comparable to bulk compression of the compounds (Ca-O bond compression = 0005 ± 0 001 GPa^?1) Amsotropies in the pressure response of the calcium polyhedron, which is more compressible parallel to c than perpendicular to c, result in the amsotropic unit-cell compression Bulk moduli of the five compounds (with K' assumed to be 4) are CaWO₄ (68 ± 9 GPa), CaMoO₄ (81 5 ± 0 7 GPa), PbWO₄ (64 ± 2 GPa), PbMoO₄ (64 ± 2 GPa), and CdMoO₄ (104 ± 2 GPa) No reversible transitions to the monoclinic (fergusomte) distortion of scheelite were observed in these compounds Pressure-volume data for PbWO₄, however, display strong positive curvature (K'_calc) = 23 ± 2) up to about 5 GPa, at which pressure crystals appear to undergo a first-order phase transition The relatively large curvature may be a premonitory effect pnor to a reconstructive transition Structural changes in these compounds with increasing pressure are qualitatively similar to changes that result from isobanc cooling or substitution of a smaller cation in the eight-coordinated site.