Dielectric and Birefringent Properties of [(CH3)2NH2]5Cd2CuCl11 Solid Solutions in the Region of Their Phase Transitions

On the basis of the optical and dielectric investigations of [(CH3)2NH2]5Cd2CuCl11 solid solutions the existence of phase transitions at T1 = 175 K and T2 = 117.5 K was confirmed. Both phase transitions were found to be shifted toward lower temperatures with respect to the corresponding transitions in the “host” [(CH3)2NH2]5Cd3Cl11 crystals. It was found that the proton conductivity in [(CH3)2NH2]5Cd2CuCl11 crystals is realized through the Grotthus mechanism. The investigations of the birefringent properties confirmed existence of the structural changes at T0 = 320 K connected with the complex co-operative effect involving weakening of the hydrogen bonds and modification of the Jahn–Teller distortion with temperature.