The high-pressure phase of the ScF3 crystal has been studied using synchrotron radiation diffraction and Raman scattering. This phase existing in the pressure range 0.6–3.2 GPa is optically anisotropic: its structure is described by space group R \(\bar 3\) c, Z = 2, and the transition is associated with the rotation of ScF6 octahedra around the threefold axis. The pressure dependences of the lattice parameters and the rotation angle have been determined. The number of lines in the Raman spectrum corresponds to the expected number for this structure; the recovery of soft modes has been observed above the phase transition.

The structures of three phases of the K3WO3F3 crystal have been determined from X-ray diffraction data obtained for a powder sample. The profile and structural parameters have been refined according to the technique implemented in the DDM program. The results obtained have been discussed using the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic displacements and allows the interpretation of the experimental data. The sequence of structural transformations is found to be as follows: \(Fm\bar 3m\xrightarrow[{(\eta _1 ,0,0)}]{{11 - 10(\Gamma _4^ - )}}I4mm\xrightarrow[{(\eta _1 ,\eta _2 ,0)}]{{11 - 10(\Gamma _4^ - )}}Cm\).

The low-temperature ferroelectric G2 polymorph of K3WO3F3 oxyfluoride is formed by chemical synthesis. The electronic parameters of G2-K3WO3F3 have been measured by X-ray photoelectron spectroscopy under excitation with Al Kα radiation (1486.6 eV). Detailed spectra have been recorded for all element core levels and Auger lines. The chemical bonding effects in the WO3F3 and WO6 octahedrons are considered by using the binding energy difference ΔBE(O–W)=BE(O1s)−BE(W4f7/2).

The influence of a homogeneous electrical field E on the characteristics and propagation conditions of the Lamb wave in a piezoelectric crystalline plate is considered on the basis of the theory of acoustic wave propagation in piezocrystals under the effect of an external electrical field.

Paper is presented the results of computer simulation. Effect of the dc electric field influence on the propagation of Lamb and SH waves and its temperature coefficients of delay in piezoelectric langasite crystal plate for a lot of cuts and directions have been calculated. There were found the cuts possessing the thermostability and sufficient electromechanical coupling.

Polarization-optical studies and measurements of the birefringence Δ n and the angle of rotation of the optical indicatrix for the (NH 4 ) 2 NbOF 5 crystal have been carried out in the temperature range 100–350 K. Two anomalies of the birefringence have been revealed at the temperatures T 01 = 258 K and T 02 ≈ 219 K. According to the twinning pattern, the crystal undergoes successive changes in symmetry: orthorhombic ↔ monoclinic 1 ↔ monoclinic 2. The twofold axis of the monoclinic phases (or the normal to the plane) is directed along [001] or . The effect of the uniaxial compression along [011] or and the electric field E ≈ 25 kV/cm along [100] or on the twin structure has been studied. The ferroelastic phase transition at T 01 is due to the appearance of the shear deformation x 4 ( T ) and is accompanied by significant anomalies of the birefringence. Strong pretransition phenomena mask the jumps in the birefringence Δ n ( T ) and in the angle of rotation of the indicatrix φ( T ) at T 01 .

Low-temperature ferroelectric G2 polymorph of K3WO3F3 has been prepared by chemical synthesis. Structural and chemical properties of the final product have been evaluated with X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Structure parameters of G2–K3WO3F3 are refined by the Rietveld method from XRD data measured at room temperature (space group Cm, Z=2, a=8.7350(3)Å, b=8.6808(5)Å, c=6.1581(3)Å, β=135.124(3)Å, V=329.46(3)Å3; RB=2.47%). Partial ordering of oxygen and fluorine atoms has been found over anion positions. Mechanism of ferroelectric phase transition in A2BMO3F3 oxyfluorides is discussed.

This paper reports the results of our crystal-chemical analysis of structures from the delafossite family A+B3+X2, where A and B are cations, and is oxygen. The family is represented by structures of two types, CuFeO2 and α-NaFeO2 (R \(\bar 3\) m, Z = 1). Predictions of new compounds are presented. About a hundred new crystals can be synthesized.

Pressure-induced phase transitions in the ScF 3 crystal were studied using synchrotron radiation diffraction, polarization microscopy, and Raman spectroscopy. The phase existing in the range 0.6–3.0 GPa is optically anisotropic; its structure is described by space group R 3 c ( Z = 2), and the transition is due to rotation of ScF 6 octahedra around a threefold axis. The pressure dependence of the structural parameters and angle of rotation are determined. The number of Raman spectral lines corresponds to that expected for this structure; above the phase transition point, a recovery of soft modes takes place. At a pressure of 3.0 GPa, a transition occurs to a new phase, which remains metastable as the pressure decreases. The results are interpreted using an ab initio method based on the Gordon-Kim approach.

The structures of all three phases of the Rb2KInF6 crystal have been determined from the experimental X-ray diffraction data for the powder sample. The refinement of the profile and structural parameters has been carried out by the technique implemented in the DDM program, which minimizes the differences between the derivatives of the calculated and measured X-ray intensities over the entire profile of the X-ray diffraction pattern. The results obtained have been discussed using the group-theoretical analysis of the complete order-parameter condensate, which takes into account the critical and noncritical atomic displacements and permits the interpretation of the experimental data obtained previously. It has been reliably established that the sequence of changes in the symmetry during phase transitions in Rb2KInF6 can be represented as \(Fm\bar 3m\xrightarrow[{0,0,\phi }]{{11 - 9\left( {\Gamma _4^ + } \right)}}{{I114} \mathord{\left/ {\vphantom {{I114} {m\xrightarrow[{\left( {\psi ,\phi ,\phi } \right)}]{{11 - 9\left( {\Gamma _4^ + } \right) \oplus 10 - 3\left( {X_3^ + } \right)}}{{P12_1 } \mathord{\left/ {\vphantom {{P12_1 } {n1}}} \right. \kern-\nulldelimiterspace} {n1}}}}} \right. \kern-\nulldelimiterspace} {m\xrightarrow[{\left( {\psi ,\phi ,\phi } \right)}]{{11 - 9\left( {\Gamma _4^ + } \right) \oplus 10 - 3\left( {X_3^ + } \right)}}{{P12_1 } \mathord{\left/ {\vphantom {{P12_1 } {n1}}} \right. \kern-\nulldelimiterspace} {n1}}}}\).

Basic equations that describe the conditions of reflection and refraction of bulk acoustic waves at the boundary between acentric crystalline media under uniaxial stress are presented. The effect of uniaxial stress on the anisotropy of reflection and refraction of bulk acoustic waves is numerically analyzed for boundaries of the crystal-vacuum, piezoelectric crystal-piezoelectric crystal, and piezoelectric crystal-elastic isotropic medium types.

The results of crystal-chemical analysis of layered hexagonal structures related to InFeZn2O5 and having space group P6(3)/mmc are reported. It was found that more than 250 new compounds could be synthesized. Multilayered compounds of other compositions are possible.

Main theory results concerned with bulk elastic wave reflection/refraction on the boundary between two piezoelectric crystals subjected to the action of bias electric field or mechanical stress have presented. Some calculations for LiNbO 3 and Bi 12 GeO 20 crystals have made.

Formulas describing the influence of non-homogeneous mechanical pressure on propagation of bulk acoustic waves in crystals have derived. Phase velocity dependence for small amplitude waves in Bi12SiO20 crystal subjected the action of non-homogeneous pressure has been investigated. Under those conditions the behavior of the wave front has researched. Numerical calculation of phase velocity of waves and their directions propagation changing have obtained.

Raman spectra of perovskite-like (NH4)(3)WO3F3 (assigned below as A3), (NH4)(2)KWO3F3 (A2K), Cs-2(NH4)WO3F3 (C2A) oxyfluorides are obtained in a wide temperature range including transition points for A3 and A2 K crystals. Transition anomalies are found and analyzed. Transformation in A3 crystal is shown to be bound with octahedron groups ordering and H-bond formation, that is accompanied by cell volume multiplication, hardening and formation of complex discrete low frequency lattice spectrum. Same cell doubling was found in A2 K crystal, but without lattice ordering. C2A lattice stays disordered down to 15 K.

Using the group-theoretical methods, the complete set of critical and noncritical order parameters arising at phase transitions in crystals with the O-h(5)- Fm3m space group has been obtained and analyzed. The mechanical and permutation representations have been constructed, and their composition for all the right systems of points of the O-h(5) h space group has been determined. Experimental data on structural phase transitions in ammonium cryolites and elpasolites have been considered.

The heat capacity of a single crystal of Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) in an electric filed with E = 3 kV/cm applied along the [111] direction has been measured using adiabatic calorimetry over the temperature range 170–250 K. Anomalies in C p have been found, which correspond to a field-induced phase transition from a relaxor to a ferroelectric state at 225 K under field cooling conditions or at 235–240 K on the subsequent field heating. The field-induced ferroelectric phase persists in a metastable state at low temperatures and is destroyed on zero-field heating at 210 K. The small entropy change Δ S = 0.028R in the field-induced phase transition suggests an insignificant change in the volume fraction of existing polar nanoregions.

Crystallochemical analysis of skutterudite-type structures in the BX 3 , AB 4 X 12 , and AB ′ 3 B 4 O 12 compositions ( A and B are metals; X = P, As, Sb) has been performed. Probable regions of structure formation are determined, thereby indicating that more than 270 new compounds of the AB 4 X 12 composition can be synthesized.

The pulse - phase method for examination of ultrasonic waves propagation in materials, including mediums with high attenuation of a sound was designed. By automation methods of measurings the opportunity of determination the elastic waves velocities with an frequency range 100 kHz - 30 MHz with the resolution on frequency of 10-6 Hz is implemented. The phase method and a method of the frequency strobing for recording impulses are realized. These methods allow to define with precision not worse than 10-9 second the time of ultrasonic waves propagation. Requirements to the minimum linear dimensions of samples are restricted by the method sensitivity only. Velocities of elastic waves and their signal attenuation in ceramic and composites are measured.