High pressure effect on the crystal and magnetic structure of Pr0.1Sr0.9MnO3 manganite

The crystal and magnetic structure of Pr_0.1Sr_0.9MnO₃ manganite has been studied by the neutron diffraction at high pressures up to 5 GPa in the temperature range 10?C295 K. At normal pressure and decreasing temperature the appearance of the C-type (T _N = 220 K) and G-type (T _N = 180 K) antiferromagnetic states occurs, which is accompanied by a structural phase transition from the cubic structure (Pm $ \bar 3 $ m space group) to the tetragonal structure (I4/mcm space group). It is shown that the temperature of the transition to the C-type antiferromagnetic phase increases with pressure with the pressure coefficient dT _N/dP = 4.0(5) K/GPa and the temperature of the transition to the G-type antiferromagnetic phase weakly depends on pressure.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Structural Phase Transitions and the Equation of State in SnSe at High Pressures up to 2 Mbar

The crystal structure of tin selenide SnSe has been studied under quasihydrostatic compression at pressures up to 205 GPa created in diamond anvil cells at room temperature. Two structural phase transitions have been detected at and P ≈ 2.5 32 GPa. The former phase transition is continuous from the GeS-type structure (space group Pbnm) to the TlI-type structure (space group Cmcm). The phase transition to the CsCl-type cubic structure (space group Pm\(\bar 3\)m) occurs at 32 GPa and is accompanied by a stepwise decrease in the volume of the unit cell by 7%. The pressure dependence of the specific volume of the unit cell at room temperature has been constructed up to 205 GPa.     

Crystal structure phase separation in anion-deficient La0.70Sr0.30MnO3 − δ manganite system

The crystal structure of anion-deficient La_0.70Sr_0.30MnO_{3 ? δ} manganite powders (δ = 0, 0.10, 0.15, 0.20) has been investigated at room temperature by the neutron diffraction method using a high-resolution Fourier diffractometer. The structure data have been refined by the Rietveld method. The crystal structures of the stoichiometric La_0.70Sr_0.30MnO₃ and anion-deficient La_0.70Sr_0.30MnO_2.90 manganites are satisfactorily described by the rhombohedral space group R $\bar 3$ c. A small amount of the non-perovskite MnO phase (space group Fm $\bar 3$ m) has been found for the anion-deficient La_0.70Sr_0.30MnO_2.90 sample. It has been found that the anion-deficient La_0.70Sr_0.30MnO_2.85 sample consists of two perovskite phases described by space groups R $\bar 3$ c and I4/mcm and the MnO phase (space group Fm $\bar 3$ m). The crystal structure of the anion-deficient La_0.70Sr_0.30MnO_2.80 sample is described by one perovskite phase with space group I4/mcm. The volume fraction of the MnO phase is ?1% for all anion-deficient samples. Oxygen vacancies in the anion-deficient La_0.70Sr_0.30MnO_{3 ? δ} manganite system stimulate the structure phase separation at 293 K.     

Structures and phase transitions in some Pb2B′B″O6 complex perovskite-type compounds

In order to study the influence of substitutions and of the ionic radii of B’ ions on structure and phase transitions, we present some physical measurements and data on Pb₂B′B″O₆ (B”=W, Nb, Mo) complex perovskite compounds. By Mn and Fe substitution for Mg in Pb₂MgWO₆ compound a change in lattice parameters and shift of antiferroelectric transition temperature is observed. It appears that in all three series of compounds a linear dependence between ferroelectric or antiferroelectric transition temperature and B’ ionic radii is observed.     

Magnetic ordering in Fe<Subscript>1.087</Subscript>Te under pressure

The crystal and magnetic structures of Fe_1.087Te have been studied by neutron powder diffraction in the temperature range from 1.7 to 80 K at pressures of  ≈0.4 and ≈1.2 GPa. No symmetry change of the tetragonal paramagnetic ambient pressure phase (space group P4/nmm) was observed for temperatures above 60 K and pressures up to  ≈1.2 GPa. A novel pressure-induced phase of Fe_1.087Te having orthorhombic symmetry (space group Pmmn) and incommensurate antiferromagneticbicollinear order was observed in the temperature range from 50 to 60 K at  ≈1.2 GPa. The known monoclinic ambient pressure phase of Fe_1.087Te (space group P2 ₁/n) with commensurate antiferromagnetic order was found to be stable up to at least  ≈1.2 GPa at low temperature.     

Concentration-dependent structural transition in the La0.70Sr0.30MnO3−δ system

The crystal structure of anion-deficient La_0.70Sr_0.30MnO_3?δ manganites (δ = 0, 0.10, 0.15, and 0.20) has been studied by powder neutron diffraction at room temperature. The crystal structures of the stoichiometric La_0.70Sr_0.30MnO₃ and anion-deficient La_0.70Sr_0.30MnO_2.90 samples are satisfactorily described by the rhombohedral R $\bar 3$ c space group. The anion-deficient La_0.70Sr_0.30MnO_2.85 sample separates into two perovskite phases with the R $\bar 3$ c and I4/mcm space groups. The crystal structure of La_0.70Sr_0.30MnO_2.80 cannot be described by a single perovskite phase in the I4/mcm space group. It is found that the clustering of oxygen vacancies is the cause of structural phase separation at 293 K in anion-deficient La_0.70Sr_0.30MnO_3?δ manganites.     

A novel Sr_{3}Pb_{6}Ce_{2}Ti_{12}O_{36} ferroelectric thin film grown by pulsed laser ablation

Complex perovskite oxide ferroelectric thin films are of great technological interest because of their high dielectric constant and large tunability. In this paper, we report the structural and electrical properties of Sr \(_{3}\) Pb \(_{6}\) Ce \(_{2}\) Ti \(_{12}\) O \(_{36}\) (SPCTO) thin films grown by pulsed laser deposition. The role of oxygen pressure and substrate temperature on the microstructure, dielectric properties and leakage current mechanism of SPCTO thin films was investigated. Strong oxygen partial pressure dependence on the microcrystalline properties and leakage current conduction mechanism was observed. Both Raman spectra and C-V characteristics show a ferroelectric phase rather than paraelectric phase for the deposited thin films. Investigations on the leakage current showed that SPCTO thin films deposited at different oxygen pressure have different dominant conduction mechanism at various electric fields. The low field conduction mechanism is governed by Ohmic and space charge limited conduction mechanisms, whereas at high fields, the conduction process is dominated by Schottky emission mechanism. The dielectric constant as well as the tunability is found to increase with increase in the crystallite size.     

Structural changes in chlorpropamide at high pressure

The crystalline structure of a molecular crystal of chlorpropamide C₁₀H₁₃ClN₂O₃S is studied by X-ray diffraction at high pressures of up to 4.2 GPa at room temperature. Under normal conditions the structure of chlorpropamide has orthorhombic symmetry with the space group P2₁2₁2₁. At high pressures P > 1.2 GPa, a polymorphic phase transition into the monoclinic phase with the space group P2₁ is observed. The baric dependences of the lattice parameters and unit-cell volumes are obtained for both phases of chlorpropamide.     

Pyroelectric and electromechanical properties of the antiferroelectric liquid crystal MHPOBC

We study pyroelectric and electromechanical effects in the prototype antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl-phenyl)4′-octylbiphenil-4-carboxylate (MHPOBC). The linear electromechanical effect in the freely suspended liquid crystal films of MHPOBC has been detected in the broad temperature range inclusive of the antiferroelectric SmC _A ^* as well as paraelectric SmA. The anomalous behavior of the hysteresis loop of SmC _β ^* in the (pyroelectric coefficient, dc bias electric voltage) coordinates has been found.     

Structure of the intermediate high-pressure phases of ternary lead tellurides

In chambers with diamond anvils, the structure of high-pressure phases of ternary lead tellurides Pb_1?x Sn_xTe (x = 0.29) and Pb_1?x Mn_xTe (x = 0.05) and nonstoichiometric crystals Pb_0.55Te_0.45, Pb_0.45Te_0.55 is analyzed by the synchrotron radiation diffraction method at pressures of P up to 14 GPa. The orthorhombic structure of the intermediate high-pressure phase (space group Pnma) is determined for all the samples above 6 GPa. Models of the phase transition in PbTe from the initial rock salt structure to the orthorhombic phase, which constitutes a distorted variant of NaCl, as well as the properties of this phase, are discussed.     

The influence of deuteration on the phase transitions in (NH4)3 Me 3+F6 cryolites (Me 3+=Sc and Ga)

The heat capacity of partially deuterated crystals with a cryolite structure, namely, (NH₄)₃ScF₆ and (NH₄)₃GaF₆, is measured in the temperature range from 80 to 370 K. The p-T phase diagrams of these compounds are investigated at pressures up to p=0.6 GPa. It is revealed that the deuteration does not affect the sequences of phase transitions observed in the proton-containing ammonium cryolites studied earlier. The isotope effect most clearly manifests itself in significant changes in the thermodynamic parameters of the $I12/m1 - P\bar 1$ low-temperature transformation in scandium cryolite.     

High-pressure X-ray diffraction study of 1T-TaS2

We present a room temperature high-pressure X-ray diffraction study of the layered compound 1T-TaS₂ up to 20 GPa. This material is known to exhibit a variety of structural phase transitions that are ascribed to the stabilization of charge density wave states. It has been recently shown that at pressures larger than 3 GPa and up to 25 GPa, 1T-TaS₂ becomes superconductor below about 5 K. It was suggested that this superconductivity coexists with different CDW states, an hypothesis that can be tested by X-ray diffraction. Our first results at room temperature show that at around 1.9 GPa, the nearly-commensurate (NCCDW) phase transforms into a phase similar to the high temperature incommensurate phase (ICCDW). Above 9 GPa, we show the existence of another IC phase, still discernable up to 20 GPa despite the pressure-induced crystal damage above 13 GPa. These results are consistent with resistivity measurements, but call for a complete exploration of the P–T phase diagram of 1T-TaS₂.     

Polar smectic subphases: Phase diagrams, structures and X-ray scattering

We analyze a discrete phenomenological model accounting for phase transitions and structures of polar Smectic-C* liquid-crystalline phases. The model predicts a sequence of phases observed in experiment: antiferroelectric SmC _A ^* –ferrielectric SmC _FI1 ^* –antiferroelectric SmC _FI2 ^* (three-and four-layer periodic, respectively)–incommensurate SmC _α ^* –SmA. We find that, in the three-layer SmC _FI1 ^* structure, both the phase and the module of the order parameter (tilt angle) differ in smectic layers. This modulation of the tilt angle (and therefore of the layer spacing d) must lead to X-ray diffraction at the wave vectors Q _s =2πs/d(s=n±1/3) even for the nonresonant scattering.     

Mechanism of formation of cellular dislocation structures during propagation of intense shock waves in crystals

The mechanism of formation of a cellular dislocation structure in face-centered cubic (fcc) metal crystals subjected to shock compression at strain rates \(\dot \varepsilon \) > 10⁶ s^?1 has been considered theoretically within the dislocation kinetic approach based on the kinetic equation for the dislocation density (dislocation constitutive equation). A dislocation structure of the cellular type is formed in the case of a two-wave structure of the compression wave behind its shock front (elastic precursor). It has been found that, at pressures σ > 10 GPa, the dislocation cell size Λ _c depends on the pressure σ and the density ρ _G of geometrically necessary dislocations generated at the shock front according to the relationship Λ _c ～ ρ _G ^?n ～ σ^?m , where n = 1/4–1/2, m = 3/4–3/2, and m = 1, for different pressures and orientations of the crystal. It has been shown that, in copper and nickel crystals with the shock loading axis oriented along the [001] direction, the cellular structure is not formed after reaching the critical pressures σ _c equal to 31 and 45 GPa, respectively.     

Temperature dependence of Mn2+ EPR in Sn2P2S6 near the phase transition

The X-band EPR spectrum of Mn²⁺ in Sn₂P₂S₆ was studied in the temperature rangeT=223–363 K. At room temperature the spin-Hamiltonian constants areg=2.00±0.01,B ₂ ⁰ =(163±3)·10^?4 cm^?1,B ₂ ² =(159±3)·10^?4 cm^?1,A=?(75±1)·10^?4 cm^?1. The effect of the invariance in temperature of the resonance magnetic fields in the narrow temperature rangeT=337–340 K and the model of the paramagnetic centre are discussed. According to EPR data a phase transition occurs atT=337 K. This transition from the paraelectric phase to the ferroelectric one is accompanied by a dramatic change in value of the spin-Hamiltonian constantB ₂ ⁰ .     

Equation of state and structural phase transition in FeBO3 at high pressure

The evolution of X-ray diffraction patterns in FeBO₃ under high pressures up to 63 GPa has been investigated at room temperature in a diamond anvil cell. A structural phase transition at a pressure of 53±2 GPa was found for the first time. The transition is of the first-order type with a hysteresisless drop of the reduced unit cell volume of about 8.6%. Apparently, the transition is isostructural. At pressures below the transition, the equation of state for FeBO₃ was fitted. In the third-order approximation of the Birch-Murnagan equation of state, the bulk modulus K and its first pressure derivative K′ were found to be 255±25 GPa and 5.0±1.2, respectively.     

Equation of state and high-pressure irreversible amorphization in Y3Fe5O12

The change of crystal structure in yttrium iron garnet Y₃Fe₅O₁₂ was studied at room temperature at high pressures up to ∼55 GPa by the x-ray diffraction technique in diamond anvil cells. At a pressure of about ∼50 GPa, a drastic change in the x-ray diffraction pattern was observed indicating the transition into an amorphouslike state. When the pressure was increased, the bulk modulus of YIG was found to be 193 ± 4 GPa. It was also found that the amorphous state was retained after decompression down to ambient pressure. From the shape of x-ray patterns in the “amorphous” phase, it was concluded that the local atomic structure consists of iron-oxygen FeO₆ octahedral complexes with disordered orientations of local axis and of randomly arranged others ion fragments with the overall Y₃Fe₅O₁₂ composition. For the amorphous phase, it was evaluated that the bulk modulus of FeO₆ octahedral complexes is about 260 GPa. The text was submitted by the authors in English.     

Influence of Pr dopant on the dielectric properties and Curie temperatures of Ba1−3x Pr2x Ti0.95Sn0.05O3 (0.01≤x≤0.05) ceramics

Three perovskite-structured ceramic phases Ba_1?3x Pr_2x Ti_0.95Sn_0.05O₃ (BPTS-x) with x=0.01, 0.03, and 0.05 have been synthesized by solid-state reaction method. Obtained BPTS-x ceramics were investigated by X-ray diffraction and dielectric properties measurements. Analysis of all the ceramic samples using X-ray diffraction method at room temperature and its Rietveld refinement inferred rhombohedric structure with the space group $R\bar{3}c$ . The temperature variation of real permittivity gives evidence of the ferroelectric phase transition and of the relaxation behavior. With increasing Pr concentration, the degree of diffuse phase transition was enhanced, and a linear reduction in the transition temperature was produced. The conductivity spectra have been investigated by the Jonscher universal power law (σ(ω)=σ _dc+Aω ^s ) and attributed to hopping conduction mechanism.