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.