Dielectric behaviour and phase transitions in perchlorates of K, Rb and Cs

The alkali metal perchlorates show structural phase transitions and the observed changes in their physical properties have been ascribed to a disordering of the structure and onset of free rotation of the ClO4 groups at the transition temperature Tsub(c). Since the earlier study on the dielectric properties was with polycrystalline samples, a detailed study on single crystals was taken up. The dielectric constants of single crystals of KClO4, RbClO4 and CsClO4 were measured along their longest axis of the stable room temperature phase (Esub(b)) at different frequencies (1 to 30 KHz) and also the loss. It is found that peak values of Esub(b) near Tsub(c) are approximately 800, 100 and 53 in K, Rb and Cs perchlorates while the volume expansion is roughly 6.27, 5.21 and 5.8 x 10-4/deg C. The dielectric loss along b axis also shows an increase near Tsub(c), which is more pronounced at lower frequencies. The dielectric anomaly at Tsub(c) and the large value of E in the cubic phase is discussed in terms of the disorder of the ClO4- groups in cubic phase and the possible contribution from Frenkel defects. (author)