Calorimetric Study of (CH₃NH₃)₅Bi₂Br₁₁ Ferroelectric Crystals

Abstract

(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_v(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_v 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 ΔS connected with the low temperature anomaly reaches the magnitude of 2Rln2.

Keywords:

• Ferroelectrics
• phase transition
• specific heat