Study of Order Parameters Coupling in Uniaxial Ferroelectrics on the Basis of Pyroelectric and Dilatometric Measurements

Abstract

A method is devised of extracting the explicit form of the coupling between the primary and secondary order parameters with the use of experimental data and respecting classical and non-classical values of the effective critical exponents. The corresponding equation of state stems from the Ising model on a compressible lattice treated within the mean field approximation supplemented with terms ascertaining scaling invariance in the vicinity of the critical point. The theory is exemplified by the molecular ferroelectric crystals (CH3NH3)5Bi2Cl11 (MAPCB) and (CH3NH3)5Bi2Br11 (MAPBB).