NEUTRON-SCATTERING STUDY OF STRUCTURAL AND MAGNETIC PHASE-TRANSITIONS IN RBVBR3

RbVBr3 transforms from the CsNiCl3 structure to a new hexagonal structure at T-t = 89.6 K. The unit cell is enlarged to root 3a, root 3a,c below T-t. The structural phase transition is found to be second order 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. The spin ordering in the intermediate phase is found to be a partially disordered collinear structure. The order parameter exponent beta(1) = 0.32 is obtained for T-N1. The intrachain exchange constant is estimated as J(0)/K-B = -130 K.