RESUMEN
Thermal and dielectric studies of (ND4)2MoO2F4 crystals undergoing successive phase transitions at T1 = 272 K and T2 = 181 K showed that deuteration is accompanied by an increase in the chemical pressure in the crystal lattice (Δp ≈ 0.02 GPa), which shifts the Cmcm â Pnma transformation for the first order to the tricritical point. The direct participation of ammonium groups in the mechanism of structural distortions is demonstrated by a decrease in the entropy of the high-temperature phase transition (ΔS1 = R ln 6.0). An external hydrostatic pressure leads to an expansion of the temperature interval of the intermediate antiferroelectric Pnma phase. The triple point on the T-p phase diagram, where the Cmcm, Pnma, amd Pnma* phases coexist, can be realized at a negative pressure of ptrp ≈ -0.8 GPa.
RESUMEN
The thermal expansion along the a, b and c axes of a (NH(4))(2)NbOF(5) crystal was measured from 120 to 300 K. Anomalies of α(T) associated with previously reported phase transitions Cmc2(1) --> C2 --> 1a were observed along all directions at T(1) = 259.3 K and T(2) = 220.5 K. The analysis of thermal expansion and heat capacity in the frame of the Pippard relations has permitted us to determine the uniaxial pressure derivatives of the transition temperatures dT/dσ(i). The T-σ(i) phase diagrams have shown a tendency for the intermediate phase to disappear at high stress along the b and c axes. Intensive and extensive barocaloric effects near the structural phase transitions were found to be comparable with the caloric parameters of some ferroelectrics, ferromagnets and antiferromagnets.
RESUMEN
The thermal expansion properties of the ceramic compositions Ba(1-y)La(y)Ti(1-y/4)O(3) (y = 0.0, 0.026, 0.036, 0.054) and Ba(1-y)Bi(2y/3)Ti(1-x)Zr(x)O(3) (y = 0.10; x = 0.0, 0.04, 0.05, 0.10, 0.15) were determined in the temperature range 120-700 K. We report the temperature-dependent measurements of the strain, thermal expansion coefficient and the magnitude of root mean square polarization. The results obtained are discussed together with the data on the structure and dielectric properties.