STRUCTURAL AND MAGNETIC PHASE-TRANSITIONS IN RBVBR3

Structural and magnetic phase transitions in RbVBr3 have been investigated by neutron scattering. A structural phase transition occurs at T-t = 89.6 K. The unit cell below T-t is described by enlarging that of CsNiCl3 to root 3a, root 3n, c. The intensity of the nuclear Bragg peak for Q = (1/3, 1/3, 2) varies continuously at T-t, implying that the transition is of the second kind or very close to it. RbVBr3 undergoes two magnetic phase transitions at T-N1 = 28.1 K and T-N2 = 21.0 K due to the inequality of the interchain exchange couplings which arises from the structural phase transition. The critical exponent of the sublattice magnetizations for T-N1 is found to be beta(1) = 0.32 +/- 0.01 which is a little smaller than the exponent of the three dimensional XY universality. The intensity of the magnetic (1, 1, 1) reflection which is very weak in the intermediate phase grows steeply below T-N2. This indicates that the partially disordered collinear structure is acquired in the intermediate phase.