Unified description of nonclassical uniaxial ferroelectric materials in order–disorder phase transitions

Abstract

An equation of state accounting for coupling of the primary order parameter (here the polarization) with a secondary order parameter (here a strain ϵ) is used to describe the temperature dependences of the spontaneous polarization and of the electric susceptibility on external biasing field. A new scaling invariant and its explicit expression are given. The theory is exemplified by the effective critical behavior of the molecular ferroelectric crystals (CH₃NH₃)₅Bi₂Cl₁₁ (MAPCB) and (CH₃NH₃)₅Bi₂Br₁₁ (MAPBB).

Keywords:

• compressible model of phase transition
• strain–polarization coupling
• scaling invariants
• critical exponents
• order–disorder phase transition

Acknowledgement

This work was supported by the Polish State Committee for Scientific Research (project register number N N202 1050 33).