S. V. Mel’nikova

Ferroelectric phase transition in Cs3Bi2I9

Single crystals of Cs3Bi2I9 have been grown. Polarized-optical, dielectric, and calorimetric studies have been performed, along with measurements of birefringence, and the coefficients of elasticity c44, and linear expansion a over a wide temperature range. It is found that an intrinsic (pseudo-intrinsic) second-order phase transition to the ferroelastic phase takes place at T0=220.0±0.6 K accompanied by a change of the crystal symmetry from 6/mmc to 2/m. The transition parameter depends linearly on the shear strain x5 and varies with temperature according to a power law with exponent β=0.5.

Optical and x-ray diffraction studies of the symmetry of distorted phases of the (NH4)3ZrF7 crystal

Abstract(NH4)3ZrF7 single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T1↑ = 280 K, T2↑ = 279.6 K, T3↑ = 260–265 K, and T4↑ = 238 K on heating and at T1↓ = 280 K, T2↓ = 269–270 K, T3↓ = 246 K, and T4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: Oh5(Z = 4) ↔ D2h25(Z = 2) ↔ C2h3(Z = 2) ↔ Ci1(Z = 108) ↔ monoclinic2(Z = 216).

Search for and study of phase transitions in some representatives of the APb2 X 5 family

Single crystals of KPb2Cl5, RbPb2Cl5, and RbPb2Br5 are grown and studied using optical polarization methods. The heat capacity of the crystals is investigated by differential scanning microcalorimetry, and the birefringence and the angle of rotation of the optical indicatrix are measured. The measurements are performed in the temperature range 270–640 K. It is found that KPb2Cl5 undergoes a first-order ferroelastic phase transition at T0↑ = 530 K, T0↓ = 528 K, and ΔH = 1000 ± 200 J/mol. The transition is accompanied by twinning and a change in symmetry mmm ⇄ P21/c. The RbPb2Cl5 crystal remains monoclinic up to the melting temperature. The RbPb2Br5 compound belongs to the I4/mcm tetragonal modification and does not undergo structural transformations.

Phase transitions in oxyfluoride (NH4)2WO2F4

Abstract(NH4)2WO2F4 single crystals are grown, and their polarization-optical, calorimetric, and birefringence properties are studied in the temperature range 90–350 K. A first-order structural phase transition is found to occur at T01↑ = 202 K with thermal hysteresis of ΔT01 ≈ 6–12 K. The phase transition is accompanied by twinning and modification of the symmetry

Investigation into phase diagrams of the fluorine-oxygen system: Ferroelastic-antiferroelectric (NH4)2WO2F4-(NH4)2MoO2F4

Cmcm \leftrightarrow \bar 1

Optical studies of phase transitions in oxyfluoride (NH4)2NbOF5

. An additional weak anomaly in the differential scanning calorimeter signal is found at T02 ≈ 170 K. The total thermal effect of both anomalies is ∑ΔHi = 3200 ± 400 J/mol and ∑ΔSi = 16.5 ± 2.0 J/mol K. The phase transition at T01 is of the order-disorder type.

Investigation of the influence of gradual substitution K ↔ Rb on the structure and phase transition in KxRb1 − xPb2Br5 solid solutions

Crystals of ammonium lithium sulfate NH4LiSO4 in α and β modifications are studied, and conditions of their nucleation and growth are determined. The α modification of NH4LiSO4 and α → β phase transitions are investigated using polarized light microscopy, x-ray diffraction, and differential scanning calorimetry in the temperature range 80–530 K. It is found that, depending on the conditions of growth and storage, there exist two temperature ranges (Tα → β ≈ 340–350 and ≈440–450 K) in which the crystals can undergo an α → β reconstructive phase transition. The enthalpy of this transformation depends on the symmetry of the final phase. In the former case (340–350 K), the reconstructive phase transition leads to rapid destruction of the sample. In the latter case (440–450 K), the crystal structure undergoes a slow transformation (recrystallization) without noticeable distortions. The results obtained indicate that no structural phase transition occurs in the α modification of NH4LiSO4 at 250 K.

Phase transitions in orthorhombic oxyfluoride (NH4)2MoO2F4

Thermal, physical, structural, optical, and dielectric investigations have been performed for oxyfluoride solid solutions (NH4)2W1 — xMoxO2F4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1). The character of the influence of the chemical and hydrostatic pressures on the stability of the parent (space group Cmcm) and distorted ferroelastic and antiferroelectric phases has been determined by analyzing the temperature-pressure, unit cell volume-composition, and temperature-composition phase diagrams. The specific features of the nature and mechanism of the phase transitions have been discussed using the available data on the structural, entropy, and dielectric parameters.