On Phase Diagrams of Mixed Crystals of the KH 2 PO 4 -Family

Phase diagrams of mixed crystals between a ferroelectric and an antiferroelectric crystals, both belonging to the KH 2 PO 4 -family, were discussed on the basis of the Slater model. The free energy function for the antiferroelectric transition was analyzed in some detail. The role of the intersublattice interaction, which is substantial for giving rise to the antiferroelectric transition, was reconsidered and it is pointed out that a ferroelectric phase with the polarization along the [110] direction may be stabilized when the interaction changes its sign. Also the role of the intrasublattice interaction is mentioned. The results are discussed in the relationship with the RbH 2 PO 4 –NH 4 H 2 PO 4 mixed crystal system.