Theory of Elastic and Electric Dipolar Orderings in NaCN and KCN

Abstract

The theory of the successive phase transitions in NaCN and KCN is presented. The elastic and electric dipole moments of a CN⁻ ion are treated in a unified way using the symmetry adapted orientational variables with E_g, T_2g and T_1u symmetries. Taking into account of the electronic polarizability of CN⁻ and K⁺ ions, the ground state of the electric dipolar arrangement is examined. It is shown that the electronic polarizability of a K⁺ ion α_K plays an essential role and that the experimentally observed antiferroelectric arrangement has the lowest energy when α_K>3.5 ų. The successive phase transition and the temperature dependence of the order parameters are examined.