Monte Carlo study of a compressible pseudospin model for (CH3NH3)(5)Bi2Cl11 - art. no. 224110

A two-sublattice peudospin model with distance-dependent interaction parame ters is constructed for the ferroelectric phase transition at 307 K and for the isomorphous anomaly at 180 K in pentakis (methylammonium) undecachloro dibismuthate (III) (CH3NH3)(5)Bi2Cl11. A series of Monte Carlo simulations involving continuous degrees of freedom of deformation is performed for the model. The critical temperatures turn out about 23% lower than those resul ting from the mean-field treatment for the three-dimensional fee lattice in the absence of coupling with strain. An upward shift in the critical tempe rature then is found due to the coupling with the strain. The multiple hist ogram method is used to obtain the theoretical predictions for the temperat ure dependence of the spontaneous polarization, dielectric susceptibility, and specific heat. The existence of a narrow peak in the specific heat and the ratio of the Curie constants close to 4 are confirmed. Evidence is give n of the usefulness of the Monte Carlo method with fairly small simulation boxes for the studies of isomorphous anomalies.