GROWTH AND PHYSICAL PROPERTIES OF POTASSIUM ZINC CHLORIDE MODULATED­ FERROELECTRIC CRYSTALS

Abstract

In recent years the incommensurate (IC) materials have attracted much •attention, because they constitute an intermediate class between crystalline .and non-crystalline systems in that they exhibit perfect long-range order without translational periodicity at least in one direction. This intermediate class gives rise to many new phenomena that were not observed in crystals. One example is the appearance of phase solitons in the IC phase. It is generally believed that an IC phase consists of regularly placed discommensurations (DC's) or solitons, and the distance between these solitons increases continuously and eventually and transition to a commensurate (C) phase occurs when temperature is lowered.

A number of compounds of the family A2BJ4; where A and B are cations and X is an anion, have been subjected to great interest because of variety of physical properties found in their sequence of transitions including normal, incommensurate, commensurate, superionic, ferroelectric and ferroelastic phases.

K2ZnCl4 hereafter (KZC) crystal is a member of the A2B]4 family showing a sequence of structural phase transitions. Above 553 K the crystal is in a normal paraelectric phase (N) with orthorhombic symmetry and space group (S.G.) Pmcn having 4 molecules per unit cell (Z=4). On cooling it turns to incommensurate phase (IC) at 553 K extending allover a temperature range of about 150 K, that is longer than other members of this family.