MODELING OF 2-STEP HIGH-SPIN REVERSIBLE-ARROW LOW-SPIN TRANSITIONS USING THE CLUSTER VARIATION METHOD

A thermodynamic description of high spin (HS) reversible arrow low spi n (LS) transition curves beyond the Bragg-Williams approximation is gi ven using the Kikuchi cluster variation method (CVM). Transition curve s of unusual behaviour(i.e. two step transition) are reproduced by sho rt range interaction energies which are present in addition to long ra nge elastic interaction between the spin changing molecules. The corre lations in the distribution of the spin changing centers can be expres sed analytically. They give rise to a reduced mixing entropy which was found experimentally in compounds with two step transitions. (C) 1998 Elsevier Science Ltd. All rights reserved.