부산대학교 도서관

A two-sublattice peudospin model with distance-dependent interaction parameters is constructed for the ferroelectric phase transition at 307 K and for the isomorphous anomaly at 180 K in pentakis (methylammonium) undecachlorodibismuthate (III) (CH{sub 3}NH{sub 3}){sub 5}Bi{sub 2}Cl{sub 11}. 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 resulting from the mean-field treatment for the three-dimensional fcc lattice in the absence of coupling with strain. An upward shift in the critical temperature then is found due to the coupling with the strain. The multiple histogram method is used to obtain the theoretical predictions for the temperature 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 given of the usefulness of the Monte Carlo method with fairly small simulation boxes for the studies of isomorphous anomalies.