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 a t T1 = 311 K and a first-order one - at T2 = 39 K (DELTAT2 = 11 K). Be sides 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 additi onal one was found at T2 = 287.0 K, both on the cooling and heating r uns. On the basis of the data about the peculiarities of dielectric an d optical properties in the vicinity of T2 one can made the conclusion about the ferroelectric character of the phase, situated between T2 and T2cool.