Calorimetric Study of (CH3NH3)5Bi2Br11 Ferroelectric Crystals.

(CH₃NH₃)₅Bi₂Br₁₁ (MAPBB) crystals exhibits ferroelectrics phase transitions at 311 K and additional anomaly of physical properties at 77 K. The results of temperature study of the specific heat for the (CH₃NH₃)₅Bi₂Br₁₁ crystals are presented at temperature range of 6-277 K. The characteristic anomaly of temperature dependence of specific heat was observed at 72.4 K. It was established, that description of the experimental C_υ(T) dependence at temperatures below anomaly can be achieved with two Debye functions (Θ_D(1) = 47 K and Θ_D(2) = 215 K) and one low-frequency Einstein-type term (Θ_E = 73 K). In order to fit C_υ in the temperature range above the anomaly another Einstein-like term with Θ_E = 233 K must be included with double number of modes. The entropy AS connected with the low temperature anomaly reaches the magnitude of 2Rln2. [ABSTRACT FROM AUTHOR]