Lattice mediated interactions and ferroelectric anomalies in the crystal (CH3NH3)(5)Bi2Cl11 (PMACB)

A two-sublattice compressible pseudospin model has been constructed to desc ribe the behaviour of the spontaneous polarisation of PMACB in the temperat ure region comprising the ferroelectric phase transition (T=307K) and the i somorphous anomaly (T= 180 K). A strong coupling of pseudospins with a "pan cake" strain -(2 epsilon(33) - epsilon(11) - epsilon(22))/3 resulting from the comparison of the model with the experimental spontaneous polarisation agrees with the existing data on the dielectric susceptibility, and on the specific heat. The ferroelectric phase transition then turns out to be clos e to a tricritical point. The numerical values of the parameters of the mod el show how the ferroelastic phase transition (T=260 K) can occur in this c rystal without any discernible effect on the specific heat and on the diele ctric susceptibility.