Strain–polarisation coupling and elastic constant determined from polarisation and strain curves in order–disorder uniaxial ferroelectrics

Abstract

AbstractA practical method is developed to determine the strain dependence of the pseudospin–pseudospin interaction parameters in discrete models of the continuous order–disorder improper ferroelastic phase transitions from experimental data on the primary order parameter (polarisation). The temperature dependence of the mechanical strain can be then deduced from the spontaneous polarisation data only. It is also shown that the dependence of the elastic constant on the temperature can be obtained from the polarisation and the strain data. The temperature dependences of the coupling constant and the elastic constant between the molecular dipoles of the methylammonium ions in the uniaxial ferroelectrics (CH3NH3)5Bi2Cl11 (MAPCB) and (CH3NH3)5Bi2Br11 (MAPBB) are determined and shown to be at the origin of the particular shape of the polarisation and the strain curves.