A Theory of Successive Phase Transitions in K₂Ba(NO₂)₄ —Dipolar Frustration—

Abstract

A simple theory has been developed to explain various interesting aspects of successive phase transitions in K₂Ba(NO₂)₄. K₂Ba(NO₂)₄ crystal undergoes two phase transitions successively associated with orientational ordering of NO₂ ionic groups. The first transition is characterized by the transition from disordered phase to a POD (partially ordered) phase, while the second transition is characterized by two-dimensional ordering in one of the sublattices. It has been shown that the successive transition scheme in K₂Ba(NO₂)₄ can be satisfactorily explained within the framework of non-collinear Ising spin system with interactions up to third nearest neighbours. The most interesting features of the transitions; partial ordering at the first transition and low dimensionality of the second transition are both understood as due to ‘dipolar frustration’ occuring in an Ising system arranged on a Kagome lattice.