Heat capacity, entropy, dielectric properties and T–p phase diagram of (NH4)3TiF7

Abstract

Successive phase transitions G1→G2→G3 in the double salt (NH4)3TiF7 have been studied by detailed calorimetric, DTA under hydrostatic pressure and dielectric measurements. Rather large entropy jumps at phase transition points were found to be followed by large additional contributions associated with the temperature dependence of the excess heat capacity. The permittivity behaviour and the tangent of the dielectric losses proved the nonferroelectric nature of both transformations. Two triple-points were found on the temperature-pressure phase diagram, suggesting the existence of a hypothetical parent G0Pm−3m cubic phase. A direct transformation between high (G0) and low (G3) temperature cubic phases takes place at p>0.41GPa with a baric coefficient dTG0→G3/dp=−40K/GPa.