Observation of ferroelectricity in paramagnetic copper octacyanomolybdate.

We report the observation of ferroelectricity in a copper octacyanomolybdate-based paramagnet, Cu(2)[Mo(CN)(8)].8H(2)O (Cu(II), S = (1)/(2); Mo(IV), S = 0). This compound has a freezing point for the fixation of hydrogen bonding at 150 K. Around this temperature, an enhancement in the ferroelectricity and an increase in the dielectric constant are observed. The ferroelectricity of this system is classified into amorphous ferroelectrics; i.e., the electric poling effect induces an electric polarization. The electric polarization is maintained by the structural local disorder of hydrogen bonding and the three-dimensional CN network. In this ferroelectricity, the crystal structure is a polar group of C(infinityv) after application of an electric field.