BIREFRINGENT AND DIELECTRIC-PROPERTIES OF [N(C2H5)4]2ZNBR4 AND [N(C2H5)4]2CDBR4 CRYSTALS

Dielectric and birefringent properties of [N(C2H5)4]2ZnBr4 and [N(C2H5)4]2CdBr4 crystals were investigated. It has been ascertained that two phase transitions manifest themselves in the temperature dependencies epsilon(T) for [N(C2H5)4]2CdBr4 crystal - a second-order transition at T1 = 311 K and a first-order one - at T2 = 39 K (DELTAT2 = 11 K). Besides two earlier known phase transitions in [N(C2H5)4]2ZnBr4 at T1 = 350 K and T2cool = 282.8 K (DELTAT2 almost-equal-to 2.3 K), the additional one was found at T2* = 287.0 K, both on the cooling and heating runs. On the basis of the data about the peculiarities of dielectric and optical properties in the vicinity of T2 one can made the conclusion about the ferroelectric character of the phase, situated between T2* and T2cool.