Phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>NbOF<Subscript>5</Subscript> oxyfluoride

The thermal and dielectric properties of the (NH₄)₂NbOF₅ oxyfluoride have been investigated. It has been established that the structural phase transitions Cmc2₁ → C2 → Ia observed at the temperatures T ₁ = 258.0 K and T ₂ = 218.9 K exhibit a nonferroelectric nature. The hydrostatic pressure, which stabilizes the initial phase and destabilizes the low-temperature phase, hardly affects the temperature range of stability of the intermediate phase. The model of sequential ordering of the structural elements due to phase transitions has been analyzed using experimental data on the entropies of the phase transitions.     

Ferroelastic phase transition in elpasolite Tl2KInF6

A new elpasolite Tl₂KInF₆ has been synthesized. This compound undergoes at 228 K a ferroelastic phase transition from a room-temperature Fm3m variety to a monoclinic variety. X-ray, optical, calorimetric and DTA studies under hydrostatic pressure have been performed and compared with other elpasolite-type fluorides.     

Heat capacity of PMN near an electric-field-induced phase transition

The heat capacity of a single crystal of Pb(Mg_1/3Nb_2/3)O₃ (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 ΔS = 0.028R in the field-induced phase transition suggests an insignificant change in the volume fraction of existing polar nanoregions.     

Low-temperature specific heat of the Rb<Subscript>2</Subscript>KScF<Subscript>6</Subscript> elpasolite

The specific heat of single-crystal Rb₂KScF₆ is measured using ac calorimetry in the range 4–280 K. The results are discussed in the context of a group-theoretical analysis of possible distortions of the elpasolite structure and of experimental data obtained earlier with an adiabatic calorimeter in a narrower temperature region.     

Studies of the thermodynamic properties of the ordered perovskites Pb2CdWO6 and Pb2YbTaO6 within a broad temperature range

This paper reports on measurements of the specific heat of Pb₂CdWO₆ made at temperatures ranging from 80 to 750 K and of Pb₂YbTaO₆ within the 350-to 700-K temperature range. First-order phase transitions from the cubic phase at 677.3 and 581 K, respectively, were observed, and their thermodynamic characteristics were determined. The entropy change on the phase transitions is close to Rln4 for both compounds. The results obtained are discussed in terms of the model of position disordering of the lead ions. It was established that below 350 K, Pb₂CdWO₆ can exist in two states, stable and metastable, depending on the sample thermal prehistory.     

Electrocaloric effect in triglycine sulfate under equilibrium and nonequilibrium thermodynamic conditions

The direct and indirect measurements of intensive electrocaloric effect in a triglycine sulfate ferroelectric crystal are performed under equilibrium and nonequilibrium thermodynamic conditions implemented in the adiabatic calorimeter. The effect of the electric field parameters (frequency, profile, and strength) on the value of the effect and degree of its reversibility are studied. The difference between the temperature variation values in a switched-on and switched-off dc field under quasi-isothermal conditions is established. The low-frequency periodic electric field induces the temperature gradient along the electrocaloric element and heat flux from its free end to the thermostated base. A significant excess of the field switching-off rate over the switching-on rate leads to a noticeable intensification of the cooling effect.     

Thermophysical study of structural phase transitions in Na<Subscript>0.95</Subscript>Li<Subscript>0.05</Subscript>NbO<Subscript>3</Subscript> solid solution

Temperature dependences of specific heat C_p(T) and coefficient of thermal expansion ;(T) for Na_0.95Li_0.05NbO₃ sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The C_p(T) and α(T) anomalies at T₃ = 310 ± 3 K, T₂ = 630 ± 8 K, and T₁ = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the C_p(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.     

Thermal expansion of (Ba1 ? xLax)Ti1 ? x/4O3 solid solutions

Deformation and the thermal expansion coefficient of ceramic samples of (Ba_{1 − x} La _x )Ti_{1 − x/4}O₃ solid solutions (x = 0, 0.026, 0.036, 0.054) were studied in the temperature range 120–700 K. Based on an analysis of the data obtained, the temperature-composition phase diagram is refined, and the temperature dependence of the polarization is calculated. The results are discussed in combination with the dielectric measurement data. Original Russian Text ? M.V. Gorev, I.N. Flerov, Ph. Sciau, S. Guillemet-Fritsch, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 4, pp. 746–752.