The Behaviour of (NH₄)₂Cd₂(SO₄)₃ under Pressure

Abstract

Abstract The effect of hydrostatic pressure up to 1400 bar on (NH 4 ) 2 Cd 2 (SO 4 ) 3 from 300 to 20 °K was studied. For pressures up to 824 bar the shift of the ferroelectric phase transition temperature, T tr , was found ∂ T tr /∂ p = + 3.35 × 10 −3 deg/bar. The permittivity, ϵ, above T tr fulfilled ∂ϵ/∂ p < 0. These experimental results cannot be explained by the electrostrictive interaction only. This can be done, however, using the thermodynamic potential taking into account that the phase transition is caused by the instability in non‐homogeneous lattice vibration modes. For pressures higher than 824 bar two phase transitions occurred. It was found that ∂ T tr /∂ p = −0.62 × 10 −3 deg/bar and for the other transition ∂ T tr /∂ p = + 10.3 × 10 −3 deg/bar. At a pressure of 824 bar a triple point exists.