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.     

On the superconductivity in T1BiTe2

The temperature dependence of the magnetic susceptibility of T1BiTe₂ and T1Te, in the range 0.050 to 1.5K, has been investigated. The superconducting transition in the T1Te samples at T = 0.190K is discovered. In T1BiTe₂ superconductivity has not been seen down to 0.050 K. All this shows that the superconducting transition discovered earlier by Hein and Swiggard at 0.140 K in T1BiTe₂ is characteristic of the T1Te foreign phase.     

Thermal conductivity of the amorphous and nanocrystalline phases of the beech wood biocarbon nanocomposite

Natural composites (biocarbons) obtained by carbonization of beech wood at different carbonization temperatures T _carb in the range of 800–2400°C have been studied using X-ray diffraction. The composites consist of an amorphous matrix and nanocrystallites of graphite and graphene. The volume fractions of the amorphous and nanocrystalline phases as functions of T _carb have been determined. Temperature dependences of the phonon thermal conductivity κ(T) of the biocarbons with different temperatures T _carb (1000 and 2400°C) have been analyzed in the range of 5–300 K. It has been shown that the behavior of κ(T) of the biocarbon with T _carb = 1000°C is controlled by the amorphous phase in the range of 5–50 K and by the nanocrystalline phase in the range of 100–300 K. The character of κ(T) of the biocarbon with T _carb = 2400°C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.     

Phase transformations of Fe3N-Fe4N iron nitrides at pressures up to 30 GPa studied by in situ X-ray diffractometry

The parameters of the equation of state of the stable ?-Fe₃N _x (where x = 0.8) nitride in the Fe-N system have been determined at pressures up to 30 GPa and temperatures up to 1273 K. The parameters V ₀ = 81.48(2) Å^?3, K _T = 162(3) GPa, K _T = 4.0 = 1.66(2), γ₀ = 555 K, and q = 1 have been determined for ?-Fe₃N_0.8 by the approximation of the P-V-T data with the Vinet equation of state and the thermal parameters within the Mie-Grüneisen-Debye formalism. No anomalous change in the volume of the cell owing to possible magnetic transitions has been revealed. The instability of Fe₄N at high pressures and Fe₃N in the presence of a deficit of nitrogen in the system has been established. The stable phase in the temperature range of 300–673 K and the pressure range of 20–30 GPa is ?-Fe₃N rather than ?-Fe₃N_0.8.     

Structural property of CsCl-type sodium chloride under pressure

The structure and equation of state of CsCl-type sodium chloride have been determined using high-pressure powder X-ray diffraction from 32 to 134 GPa. The CsCl-type phase remains stable over this entire pressure range. Pressure-volume data can be fitted with a Vinet equation of state with K_30 GPa=135.1 GPa, K′_30 GPa=3.9, and V_30 GPa=27.70 Å³. The nearest-neighbour distance between sodium and chlorine atoms decreased as pressure increased. Significant discrepancies of nearest-neighbour distance between previous theoretical predictions and this study were observed at pressures higher than 70 GPa.     

Dielectric relaxation and magnetic characteristics of the Bi0.5La0.5MnO3 ceramics

The complex permittivity ?* of ceramics of bismuth-lanthanum manganite Bi_0.5La_0.5MnO₃ has been measured in ranges of temperatures T = 10–200 K and frequencies f = 10²–10⁶ Hz. Clearly pronounced regions of the non-Debye dielectric relaxation have been revealed at low temperatures (T < 90 K). To describe them, the possible mechanisms have been proposed and discussed. The temperature dependences of magnetization, the anomalous behavior of which can be associated with the phase transition from the paramagnetic phase into the ferromagnetic phase occurring at T ～ 40–80 K, have been measured in the temperature range T = 10–120 K.     

Infrared absorption investigation of the role of octahedral groups upon the phase transition in the Rb2KMoO3F3 crystal

The infrared absorption spectra of the oxyfluoride Rb₂KMoO₃F₃ have been measured in the frequency range corresponding to stretching vibrations of the Mo-O anion octahedron with the purpose of clarifying their role in the phase transition. A semi-empirical calculation of two possible configurations of quasioctahedral MoO₃F₃ groups has been performed. The results of the investigations have demonstrated that some octahedra in the crystal structure change the local symmetry from C _3v to C _2v due to the phase transition (T = 197 K).     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Damage of Dusty Optical Elements in the Field of Continuous-Wave Laser Radiation

The structure and properties of high-pressure phases of iron nitrides Fe₇N₃ in the pressure range of 50–150 GPa have been studied with ab initio calculations within the electron density functional theory. A new phase Amm2-Fe₇N₃, which is the most energetically favorable in the pressure range of 43–128 GPa, has been found using the USPEX (Universal Structure Predictor: Evolutionary Xtallography) algorithms. It has been thermodynamically shown that another high-pressure phase β-Fe₇N₃ is isostructural to a similar phase of iron carbide. The elastic properties have been calculated for all modifications ε-, β-, and Amm2-Fe₇N₃ stable at high pressures.     

Features of the Jahn-Teller transition in Ni1 − x Co x Cr2O4 solid solutions

The structure of Ni_{1 ? x} Co _x Cr₂O₄ solid solutions has been investigated by synchrotron X-ray powder diffraction in the temperature range of 90–350 K for compositions x = 0, 0.005, 0.01, 0.015, 0.02, 0.1, 0.5, and 0.8, and their structures have been refined using the Rietveld analysis. The T-x phase diagram for solid solutions in the studied range of temperatures and concentrations has been constructed based on the obtained data. The revealed structural phase transitions have been explained from the viewpoint of the crystal field theory and the Jahn-Teller cooperative effect. Distortions of polyhedra, which are the cause of the structural phase transition, have been evaluated. The applicability of the Landau phenomenological theory and the possibility of using the magnitude of spontaneous strains as the order parameter have been discussed.     

Studies of the heat capacity and thermal expansion of the Na0.95K0.05NbO3 solid solution

The heat capacity and thermal expansion of ceramic samples of the Na_0.95K_0.05NbO₃ solid solution have been investigated over a wide temperature range of 100–750 K. The observed anomalies in the heat capacity and thermal expansion at T ₄ = 297 K, T ₃ = 535 K, T ₂ = 665 K, and T ₁ ≈ 710 K correspond to the sequences of phase transitions N → Q → G → S → T1. It has been shown that, as a result of the phase transitions, the unit cell volume at T ₄ and T ₂ decreases, and at T ₃ and T ₁, increases with increasing temperature. The directions of the shift of the phase transition temperatures induced by hydrostatic pressure have been determined. It has been established that all structural transformations are accompanied by relatively small variations in the entropy. Different mechanisms of the structural distortions have been discussed.     

Simultaneous solution of Kompaneets equation and radiative transfer equation in the photon energy range 1-125 keV

Radiative transfer equation in plane parallel geometry and Kompaneets equation is solved simultaneously to obtain theoretical spectrum of 1-125 keV photon energy range. Diffuse radiation field are calculated using time-independent radiative transfer equation in plane parallel geometry, which is developed using discrete space theory (DST) of radiative transfer in a homogeneous medium for different optical depths. We assumed free-free emission and absorption and emission due to electron gas to be operating in the medium. The three terms n, n² and (∂n/∂x_k) where n is photon phase density and x_k=(hν/kT_e), in Kompaneets equation and those due to free-free emission are utilized to calculate the change in the photon phase density in a hot electron gas. Two types of incident radiation are considered: (1) isotropic radiation with the modified black body radiation I^MB[1] and (2) anisotropic radiation which is angle dependent. The emergent radiation at τ=0 and reflected radiation τ=τ_max are calculated by using the diffuse radiation from the medium. The emergent and reflected radiation contain the free-free emission and emission from the hot electron gas. Kompaneets equation gives the changes in photon phase densities in different types of media. Although the initial spectrum is angle dependent, the Kompaneets equation gives a spectrum which is angle independent after several Compton scattering times.     

Check of the T-invariance principle in 635 MeV pn-scattering

The “polarization-asymmetry” relation resulting from the T-invariance of strong interactions has been tested in a 635 MeV pn double-scattering experiment.No effects of T-invariance violation have been observed within the accuracy obtained of some percent in the angular range of (34–124)°. The T-odd scattering phase λ₁ has been obtained as λ₁=0.01 ± 0.02, which exceeds the accuracy of determining the λ₂ phase of pp-scattering at other energies.     

Effect of the rotational depolarization in flourescent measurements of the nematic order parameters

The effect of the rotational depolarization of the values of the order parameters <p₂ > … <p₄ > of nematic liquid crystals, determined by polarization flourescence measurements has been considered using the theory recently developed by Zannoni. For this purpose an approximate solution of the equation for the rotational diffusion in a nematic phase has been obtained in an analytical form. The order parameters of the nematic p-ethoxy-benzylidene-p-n-butylaniline have been determined experimentally and the neglect of the rotational molecular motion was found to lead to incorrect values of the order parameters.     

Pressure-induced transformations and optical properties of the two-dimensional tetragonal polymer of C60 at pressures up to 30 GPa

The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C₆₀ have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C₆₀ and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C₆₀ were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C₆₀ monomer. The deformation potential and the Grüneisen parameters of the 2D(T) polymeric phase of C₆₀ have been determined and compared with those of the pristine material.     

Analysis of thermoelastic behaviour of MgO at high temperatures and high pressures

The thermoelastic behaviour of MgO has been studied for the temperature range (300-3000 K) under different compressions down to V/V₀=0.3. It has been shown that a comprehensive study of the thermoelastic properties of MgO can be made with the help of the Anderson-Isaak equation for thermal expansivity and the Vinet equation of state taken together. We have estimated the values of thermal expansivity α, isothermal bulk modulus K_T, their variations with pressure and temperature, the Anderson-Gruneisen parameter and the change in entropy with compression for MgO along isotherms at different temperatures. The results have been discussed and compared with the corresponding values reported in the recent literature.     

Field dependent Hall coefficient of the Kondo-system (La, Ce)B6

The Hall coefficient R_H of dilute (La, Ce)B₆ single crystals has been measured in magnetic fields up to 7 T in the temperature range 10 mK- 4.2 K. The field dependence of R_H shows pronounced maxima, in accordance with theoretical predictions. From the size and position of the Hall maximum we obtain a consistent value for the s-wave phase shift δ = 15° of the Ce impurities in LaB₆.     

Investigation of the possibilities for increasing the thermoelectric figure of merit of nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions

The transport coefficients and thermoelectric figure of merit ZT for bulk nanostructured materials based on Bi₂Te₃-Sb₂Te₃ solid solutions have been investigated theoretically. Similar materials prepared by rapid quenching of the melt with the subsequent grinding and sintering contain amorphous and nanocrystalline regions with different sizes of particles. According to the performed estimations, the thermoelectric figure of merit of the amorphous phase can exceed the value of ZT for the initial solid solution by a factor of 2?C3 primarily due to the significant decrease in the thermal conductivity. The effective transport coefficients of the medium as a whole have also been investigated as a function of the parameters of each phase, and the concentration range of the amorphous phase, which corresponds to the effective values ZT > 1, has been determined.     

A raman study of hydrostatic pressure induced phase transitions in Rb2KInF6 crystals

The Raman spectra of the elpasolite (Rb₂KInF₆) crystal have been studied in the pressure range from 0 to 5.3 GPa at a temperature of 295 K. A phase transition at a pressure of approximately 0.9 GPa has been found. An analysis of the variations in the spectral parameters has led to the conclusion that the phase transition to a distorted phase is accompanied by the doubling of the volume of the primitive cell of the initial cubic phase. Numerical calculations of the lattice dynamics in the Rb₂KInF₆ crystal have been performed. The numerical simulation has established that the phase transition at a pressure of 0.9 GPa is associated with condensation of the F _lg mode. A probable high-pressure phase is the phase with space group C2/m.     

Thermal and physical properties of sodium niobate ceramics over a wide temperature range

The temperature dependences of the heat capacity C _p (T) and thermal expansion coefficient α(T) of NaNbO₃ ceramic samples have been investigated in the temperature range from 2 to 800 K. In addition to the anomalies associated with the known phase transitions at temperatures T ₆ ≈ 265 K, T ₅ ≈ 638 K, T ₄ ≈ 760 K, and T ₃ ≈ 793 K, anomalies in the behavior of C _p (T) and α(T) have been observed near T _5″ ≈ 500 K and T _5′ ≈ 600 K. It has been found that all the observed structural transformations, according to the values of the entropy change, are not related to the ordering of structural elements. It has been shown that, with an increase in the temperature, the unit cell volume during the phase transitions near 265, 515, 604, and 638 K decreases. The specific features of the transition to the phase R3c have been examined. Two possible scenarios of the sequence of phase transformations in the temperature range between T ₅ and T ₆ have been analyzed.