Ferridistortive Phase-Transition in [n(ch3)4]2cobr4 Crystals

The orthorhombic-monoclinic phase transition in [N(CH3)4]2CoBr4 crystals has been studied by measuring static dielectric constants along the a, b and c axes in the wide range of temperature from 370 K down to 10 K including the transition temperature T(c) = 287.0 K. Anomalous parts of dielectric constants due to the phase transition are found to show a broad maximum around 70 K for b plate samples, while a shallow minimum around 120 K for c plate samples. The phase transition in [N(CH3)4]2CoBr4 is regarded as the ferridistortive phase transition in which the spontaneous shear strain consists of two kinds of sublattice strains with opposite signs and different magnitudes.