Ferroic phase transitions in β-LiNH₄SO₄

Abstract

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in β-LiNH₄SO₄ and its deuterated analogue. β-LiNH₄SO₄ undergoes succesive phase transitions: a paraelectric - ferroelectric phase transition at T₁ ≃ 462 K, a ferroelectric - ferroelastic phase transition at T₂ ≃ 283 K and a transition from one ferroelastic phase to the other at T₃ ≃ 28 K. Attention is focused on the influence of the order of phase transitions on the pattern of ferroelectric and ferroelastic domain structure, and also on the role played by the dynamics of molecular groups in the mechanism of transitions. The pre-transition effect connected with the ferroelectric-paraelectric transition: heterophase, capable of accounting for anomalies in different physical properties present 1-3 K below T₁ is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.

Key Words:

• Keywords
• Phase transitions
• Ferroelectricity
• Ferroelasticity
• Domain structure
• Pre-transition effect
• Hetero-phase
• β-LiNH₄SO₄
• α-LiNH₄SO₄