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

• Two triple points were found on the T–p phase diagram of (NH4)3TiF7.
• A cubic parent phase G0  Pm−3m is observed under pressure.
• (NH4)3TiF7 undergoes successive nonferroelectric transitions G3 → G2 → G1 → G0.
• Structural distortions are accompanied by a huge total entropy change ΔS > Rln48.
• Pressure increase leads to a direct phase transition between the cubic phases G3 → G0.

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 G0  Pm−3m cubic phase. A direct transformation between high (G0) and low (G3) temperature cubic phases takes place at p > 0.41 GPa with a baric coefficient dTG0 → G3/dp = −40 K/GPa.

Two triple points found on the T–p phase diagram of (NH4)3TiF7 show the existence of a parent cubic Pm−3m phase above the decomposition temperature. In the succession of nonferroelectric phase transitions G3-cubic → G2-P4/mnc → G1-4/m → G0-Pm−3m, the transformation G1 → G0 is characterized by a huge baric coefficient dTG0 → G1/dp = −600 K/GPa what leads to the highest instability of G1 phase in relation to pressure compared to other phases. A direct transformation between two cubic phases G0 → G3 exists at pressures above 0.41 GPa.Figure optionsDownload as PowerPoint slide