Specific heat overcritical behaviour in (CH₃NH₃)₅Bi₂Cl₁₁ (MAPCB) crystals

Department of Physics, Moscow State University, Moscow 119899, Russia
Institute of Physics, Technical University of Wroclaw, 50-370, Wroclaw, Poland

The results of the study of thermal properties of MAPCB crystals in the temperature interval 90-330 K are presented. Specific heat anomalies at the second order phase transition at 160 K and 307 K are revealed and discussed. Anomaly at 307 K is due to the second order ferroelectric phase transition of order-disorder type with the symmetry change Pcab - Pca2₁. The low-temperature anomaly in MAPCB crystals relates rather to continuous change of the crystal state without alteration of its symmetry which can be described by the phenomenological theory of isomorphic phase transition. The existence of different types of the crystal change without symmetry alteration (the first order phase transition, critical point, overcritical behaviour) was predicted and shown to relate closely to liquid-gas transformation behaviour. The phenomenological theory of isomorphic phase transitions is applied for the quantitative description of the second order phase transition at 307 K and overcritical behaviour in the region of 160 K in the framework of the single expression for the crystal free energy. Both anomalies can be explained within the modified theory of isomorphic phase transition in ferroelectrics when the term of the tenth degree in the free energy expansion is taken into account.