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).

Nonclassical Critical Behaviour and Low Temperature Anomalies in Ferroelectric DMACA as Described by Many-Sublattice Model

“…(5c) The values of the coecients in Eqs. (5a), (5b) and (5c) are given in Table. ll of the values of the coecients in the cases 16 and presented in Table are chosen to be close to the real ones [14,16,29,39]. One sees that for the case of compressible model the saturation of the order parameter is much faster.…”

“…To obtain a saturation of the spontaneous polarisations in temperatures similar as in the real materials one has to replace the constant coecients j, k s and k r with functions depending on the secondary order parameter, as in the case of uniaxial ferroelectric materials MAPCB and MAPBB [14,16,29,39]. Then one deals with polarisation-strain coupling.…”