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The heat capacity and the effect of hydrostatic pressure on structural phase transitions in ammonium hexafluoroaluminate, (NH4)3AlF6, have been studied. Two heat capacity anomalies have been found with maxima at T 1 = 218.5 ± 0.5 K and T 2 = 179 ± 2 K. Respective entropy changes of phase transitions are ΔS1 = 15.3 ± 0.5 J mol−1 K−1 and ΔS2 = 2.5 ± 0.5 J mol−1 K−1. It is shown that the p–T phase diagram of this compound is rather complex and contains a triple point (ptp = 0.12 GPa, Ttp = 221 K) and high-pressure phase. The mechanism of transformations and generalized p–T phase diagram for compounds of the (NH4)3MIIIF6 family are discussed within the framework of the rotational order–disorder model.

The heat capacities of Pb2 MgWO6 and Pb2 CoWO6 and the effect of hydrostatic pressure on the phase transitions have been studied on powdered samples by means of an adiabatic calorimeter and DTA performed under pressure, respectively. Thermodynamic parameters, such as change of total excess entropy, latent heat and hysteresis of the phase transition temperature were determined. Entropies and p -T phase diagrams are discussed in connection with structural data.

The effect of hydrostatic pressure on structural phase transitions in some fluorotitanates and fluorosilicates of the ABF6.6H2O family is studied. It is shown that it is possible to divide the crystals of this family into two groups differing from each other in their values of entropy change and shifts of the phase transition temperature T0 under pressure. In some compounds the high-pressure phases and triple points were established. The phase diagram for FeSiF6.6H2O can be considered as a unified diagram describing the possible phase transitions in the crystals studied. The effect of deuteration is investigated in different phases.

The pressure dependences of the heat capacity and T0 of the bromo-elpasolites Cs2NaYBr6 and Cs2NaTmBr6 were investigated. It is shown that it is possible to describe the experimental results on the grounds of thermodynamic theory.

The structural, magnetic, and thermophysical properties of cation-substituted sulfides Gd(X)Mn(1-X)S (0.04 ≤ X ≤ 0.25) with the NaCl-type face-centered cubic lattice have been investigated. The range of existence of long-range antiferromagnetic order has been established. The anomalies observed in the temperature dependence of the specific heat correspond to the temperatures of the magnetic phase transition. The anomaly in the specific heat caused by electron transitions between the 4f levels and d band states has been observed. It has been found that the coefficient of thermal expansion decreases with increasing concentration of substituents in the magnetically ordered region and remains nearly invariable in the paramagnetic phase.