PHASE-TRANSITION OF (NH4)2SO4-K2SO4 MIXED-CRYSTAL

The peculiar temperature dependence of a remanent polarization in (NH4)2SO4K2SO4 mixed crystal which changes the sign at low temperature has been studied by extending the unified model in the ferroelectric crystals to the ferrielectric crystals consisting of two sublattices. The phase transition in this crystal system has been analyzed on the basis of an idea that two kinds of NH4+ ions, NH4+(I) and NH4+(II), with different dipole moments form two sublattices. In the mixed crystal, NH4+(II) with larger dipole moment than NH4+(I) is replaced preferentially by K+ ion with no dipole moment. We consider that the replacement of ions produces modifications in the two kinds of potential in each sublattice, two kinds of ferroelectric intrasublattice interactions and weak antiferroelectric intersublattice interaction, leading to the peculiar thermal property of the ferrielectric phase.