Lattice relaxation and order in the low-spin to high-spin transitions in molecular crystals

Abstract

A series of the d6 iron(II) complexes with bulky organic ligands (like [Fe(bipy)2(NCS)2]) can exist in two spin forms: in the low-spin (S = 0) form at low temperature and in the high-spin (S = 2) form at high temperature. In the crystal phase, the transition between these two forms may be either smooth or abrupt. Recently, the abrupt spin transitions were identified with the first-order transitions between different ordered phases occurring in the binary mixtures of the two spin forms of the complex. Here, we apply the method widely used in the field of binary metal alloys to the analysis of the spin transitions. The molecules undergoing the spin transition are modeled by octahedra of variable size which interact when they are immediate neighbors in the crystal lattice. We show that some simple assumptions concerning the intermolecular interaction and crystal geometry relaxation allows one to get the desired first-order phase transitions together with a satisfactory description for the crystal compressibility as a function of temperature. © 1996 John Wiley & Sons, Inc.