Potts model and phase transition in lead phosphate Pb₃(PO₄)₂

Abstract

An attempt is made to explain the first-order phase transition (180.4 degrees C) and the presence of an intermediate phase (180.4 degrees C-300 degrees C) between the para- and ferro-phases in lead phosphate, Pb3(PO4)2, by invoking the basic ideas of the three-dimensional renormalised Potts model. This involves the introduction of three order parameters with two different critical temperatures. A relaxational type flip mode is proposed as the driving soft mode. The abnormal behaviour of the spontaneous strain between 160 degrees C and 180 degrees C, as well as the anomalous behaviour of the elastic stiffness constants C11 and C22 during the phase transition, are explained.