Microscopic Origin of the Ferroelastic Phase Transition in LiNH₄C₄H₄O₆·H₂O

Abstract

The microscopic theory proposed by Kurihara et al. on the elastic anomalies of K₂Pt(CN)₄Br_0.3·3H₂O (KCP) was applied to the ferroelastic phase transition of LiNH₄C₄H₄O₆·H₂O (LAT). Replacing the role of the water molecule in KCP by that of the ammonium ion, the anomalous temperature dependence of the elastic constant c₅₅ in LAT above T_c was well explained by the theory. The anomalies of the ultrasonic absorption α₅₅ and the specific heat near T_c were also examined on the basis of the theory. It was found that these experimental results could be explained satisfactorily by choosing three parameters in the theory as V\simeq50 K, λ\simeq1.9 and c₅₅⁰\simeq10.6×10¹⁰ dyn/cm².