Thermal expansion, phase diagrams and barocaloric effects in (NH(4))(2)NbOF(5).

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.

Thermal expansion, polarization and phase diagrams of Ba(1-y)Bi(2y/3)Ti(1-x)Zr(x)O(3) and Ba(1-y)La(y)Ti(1-y/4)O(3) compounds.

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.

Heat capacity study of relaxors BaTi(0.65)Zr(0.35)O(3) and BaTi(0.60)Zr(0.40)O(3).

The heat capacity of two relaxors BaTi(1-x)Zr(x)O(3) (x = 0.35,0.40) was measured using adiabatic calorimetry in the temperature range 100-360 K. The C(p)(T) dependence of both compositions is characterized by the presence of two diffuse anomalies near the Burns temperature T(d) and the temperature of the maximum in permittivity T(m) in the temperature ranges 250-350 K and 120-200 K. The anomalous heat capacity near T(d) was analysed taking into account the distribution of Zr concentration in nanoregions leading to the distribution of their transition temperatures into the polar phase. Excess heat capacity near T(m) was discussed in the framework of the spherical random bond-random field model. The results are compared with the data obtained by the same procedures for PbMg(1/3)Nb(2/3)O(3) studied experimentally earlier.