Infrared and permittivity studies on alkali perchlorates

The infrared spectroscopic studies and the dielectric constant measurements during the orthorhombic to cubic transformation of NaClO₄, KClO₄, RbClO₄, and CsClO₄ are reported here to investigate the possibility or not of rotation of ClO₄⁻ in the cubic form. The multiplicity of ν₃, the triply degenerate stretching frequency, change at T_t as it should go from a triplet to a singlet when the crystal structure changes from Cmcm to . The symmetric stretching frequency of ClO₄⁻ is active in Cmcm but symmetry forbidden in . At the respective transition temperature there is a sharp increase in the dielectric constants of these perchlorates proving the transition to be of order-disorder type with the possibility of rotation of ClO₄⁻ at least in NaClO₄ and KClO₄, but involves the discontinuity of properties characteristic of a first-order transition