Ferroelectric phase transition in (CH₃NH₃)₅Bi₂Cl₁₁ and (CH₃NH₃)₅Bi₂Br₁₁ crystals

Abstract

Temperature dependence of the linear birefringence Δn and excess entropy ΔS in a phase transitions regions are presented for the (CH₃NH₃)₅Bi₂Cl₁₁ and (CH₃NH₃)₅Bi₂Br₁₁ ferroelectric crystals. We have confirmed that there is a linear relation between Δn and ΔS in the temperature range from 285 to 306 K and from 295 to 306 K for (CH₃NH₃)₅Bi₂Cl₁₁ and (CH₃NH₃)₅Bi₂Br₁₁ crystals, respectively. Our results imply that relation between the linear birefringence and excess entropy can be described by Landau theory.

Keywords:

• Interfering polarizing method
• MAPCB
• MAPBB
• Phase transitions