Dilatometric Investigations of Overcritical Behaviour in [N(C$\text{}_{2}$H$\text{}_{5}$)$\text{}_{4}$]$\text{}_{2}$CuCl$\text{}_{4}$ Crystals

Abstract

The results of experimental investigation of thermal expansion of [N(C$\text{}_{2}$H$\text{}_{5}$)$\text{}_{4}$]$\text{}_{2}$CuCl$\text{}_{4}$ crystals in a wide temperature range are presented. The anomaly of thermal deformation typical of first order phase transition at 258 K and additional continuous anomaly at 195 K were observed. It was shown that low temperature anomalies of physical properties of [N(C$\text{}_{2}$H$\text{}_{5}$)$\text{}_{4}$]$\text{}_{2}$CuCl$\text{}_{4}$ crystals can be described as overcritical trace of isomorphous phase transitions. Results of numerical analysis of phenomenological model of improper ferroelastic exhibiting isomorphous phase transitions are presented and compared with experimentally determined temperature dependencies of spontaneous deformation of [N(C$\text{}_{2}$H$\text{}_{5}$)$\text{}_{4}$]$\text{}_{2}$Cu Cl$\text{}_{4}$ crystals