PHASE-TRANSITIONS OF KNICL3-TYPE TRIANGULAR LATTICE ANTIFERROMAGNETS

Structural and magnetic phase transitions of triangular lattice antife rromagnets of xy-like RbFeBr3, RbVBr3 and Ising-like RbCoBr3 have been studied by measuring thermal expansions and dielectric constants. RbF eBr3 shows a very small cusp-like anomaly of the thermal expansion alo ng the c-axis in the ferroelectric phase transition (39.5 K). It is su ggested that a lattice distortion of RbVBr3 along the c-axis is an ord er parameter of the structural phase transition, which can correspond to displacements of the two linear chains, which are columns of VBr6-o ctahedra. New structural and magnetic phase transitions of RbCoBr3 wer e found at 90 and 11 K respectively. Strong dielectric anomalies attri buted to the onset of the magnetic ordering process were measured for RbVBr3 and RbCoBr3 but not for RbFeBr3.