Phase transitions and dielectric properties of hexagonal KNiCl3-type antiferromagnets

It is found that KNiCl3, RbFeBr3, RbMnBr3, RbCoBr3 and TlFeCl3 belonging to a KNiCl3-type antiferromagnet undergo novel class of structural phase tran sitions which comprise magnetic phase transitions accompanied by spin-frust ration in the c-plane and ferroelectric one. They have a prototype crystal structure of space group symmetry P6(3) /mmc and change this symmetry to he xagonal P6(3)cm or trigonal P (3) over bar c1. Simultaneously the configura tion of the three kinds of the linear chains of face-sharing NiCl6-octahedr a (h-h-h-packing) is displaced along the c-axis at the structural phase tra nsitions. These compounds exhibit characteristic variations of the dielectr ic constant epsilon(c), lattice constant c and spontaneous polarization P-s in the structural and magnetic transitions. The compounds mentioned above and RbVBr3 are classified into three groups with respect to their dielectri c and magnetic properties associated with the transitions. It is in particu lar interesting that RbCoBr3 develops simultaneously the ferroelectric and antiferromagnetic phase transitions at 37.0K.