Ferroic phase transitions in β-LiNH4SO4

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in beta-LiNH4SO4 and its deuterated analogue. beta-LiNH4SO4 undergoes succesive phase transitions: a paraelectric-ferroelectric phase transition at T, congruent to462 K, a ferroelectric-ferroelastic phase transition at T-2 congruent to 283 K and a transition from one ferroelastic phase to the other at T-3 congruent to 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 T-1, is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.