Thermodynamic Properties of Alkaline Earth Titanates, Zirconates, and Hafnates

We shall review the problems arising during the critical analysis (aimed at selecting recommended values) of the thermodynamic constants of the alkaline-earth titanates, zirconates, and hafnates – a very important class of ferroelectric perovskites, widely used in radio electronics. Recommended values of the following standard thermodynamic quantities are given, with a statement of the uncertainty limits: the specific heat C_p,298, the enthalpy change H₂₉₈–H₀, the entropy S₂₉₈, the heat of formation ΔH_f,298, the free energy of formation ΔG_f,298 and the temperature and heat of phase transformation. The effect of impurities on the thermal constants of the phase transformations is discussed. The relationship between the thermodynamic characteristics and the crystal structure of perovskites is considered, with special reference to the effect of orthorhombic distortion of the ideal lattice on the entropy of the perovskite. In addition to thermodynamic methods we shall consider also other physical methods used to identify phase transformation temperatures, Curie points, and the temperature of the transition from the ferroelectric to the paraelectric state. Physical methods are especially important when the energy changes associated with the phase transformations are very small and cannot be detected by measurements of specific heat. 289 references.