On Phase Transition of NH₄H₂PO₄-Type Crystals by Cluster Variation Method

Abstract

The cluster variation method (CVM) is applied to the Ishibashi model for ammonium dihydrogen phosphate (NH₄H₂PO₄) of a typical hydrogen bonded anti-ferroelectric crystal. The staggered and the uniform susceptibility without hysteresis are calculated at equilibrium. On the other hand, by making use of the natural iteration method (NIM) for the CVM, hysteresis phenomena of uniform susceptibility versus temperature observed in experiments are well explained on the basis of local minimum in Landau type variational free energy. The polarization P curves against the uniform field are also calculated.