Properties of a new ferroelectric crystal family: MCd(NO2)3 [M=NH4, K,Rb, Cs,Tl]

Single crystals of MCd(NO₂)₃, with M=NH₄, K, Rb, Cs, and Tl, respectively, were grown and investigated by means of optical and dielectric methods. The crystals show rhombohedral symmetry (possible space group R 3) and multi-domain patterns at ambient temperature. The compounds exhibit phase transitions above room-temperature of first (M=K) and second order (M=Rb, Cs, Tl), respectively, into cubic high-temperature phases. Observed dielectric anomalies and hysteresis loops point to ferroelectric properties and suggest a relaxational order-disorder mechanism of the nitrite ions to be responsible for the transitions. The MCd-(NO₂)₃-compounds are seen to represent a new family of ferroelectrics with a cubic prototypic phase closely related to the structural type of the perovskites. The transions are classified by symmetry considerations, and a possible structure of the cubic high-temperature phase with space group Pm3 is proposed.