Correlations of the distribution of spin states in spin crossover compounds

Short range correlations of the distribution of high spin (HS) and low spin (LS) states show up in thermal spin transition curves, decay curves of the light induced metastable HS state (LIESST state), and in structural featur es during the spin transitions. Correlations are due to short range interac tions between the spin crossover molecules. Short range interactions may co mpete with omnipresent long range interactions and give rise to interesting spin transition phenomena. In this paper, the effect of correlations on th e thermal spin transition in the mixed crystal system [FexZn1-x(pic)(3)]Cl- 2. EtOH (pic = picolylamine) is discussed. In particular the step in the th ermal transition curve is a direct consequence of such correlations. In add ition, the decay of the metastable HS state of the pure iron compound at ca . 20 K can be significantly changed by preparing metastable HS states with a random distribution over the lattice sites. Both experiments could be wel l reproduced by Monte Carlo simulations. Tn the orthorhombic modification o f the compound Fe[5NO(2)-sal-N(1,4,7,10)]([2,2'-(2,5,8,11-tetraazadodeca-1, 11-diene-1,12-diyl)4-nitrophenolato] (2-)-N2, N2',N2",N2"',O1,O1']Fe(II)) a commensurable superstructure has been found. This compound represents the first example of a stable infinite range correlation of the spin states ove r the lattice sites. (C) 1999 Elsevier Science: S.A. All rights reserved.