Theory of the two-step spin conversion induced by the cooperative molecular distortions in spin-crossover compounds

Gütlich et al. [Chem. Phys. Lett. 91, 348 (1982)] observed an ``unusual'' two-step high-spin⇄low-spin transition in the spin crossover compound [Fe(2-pic)₃]Cl₂.C₂H₅OH. Recently, Petrouleas and Tuchagues [Chem. Phys. Lett. 137, 21 (1987)] have shown that the other compound Fe[5NO₂-sal-N(1,4,7,10)] exhibits a two-step spin conversion with a wider middle step. The mechanism of these unusual spin conversions are studied theoretically on the basis of the cooperative molecular distortion model [Kambara, J. Phys. Soc. Jpn. 49, 1806 (1980) and Sasaki and Kambara, J. Chem. Phys. 74, 3472 (1981)]. The couplings of the spin states of the Fe ion with a molecular distortion and a lattice strain are taken into account in the model. The spin conversion is induced by the intermolecular spin coupling mediated by the lattice vibration mode and the lattice strain. In the present model the two inequivalent sites occupied by equivalent iron complexes are assumed in a unit cell of the lattice. Molecular distortions of a pair of the two complexes occupying the two inequivalent sites couple with each other symmetrically and antisymmetrically. The origin of the two-step spin conversion (LL-->LH-->HH) comes mainly from the differences between the effects of the interpair interaction to the symmetric distortion and on the antisymmetric distortion. Various patterns of two-step spin conversions are obtained by changing the coupling strengths of the Fe ion with molecular distortion and with lattice strain and also the strength of interpair interaction. The essential features of the observed thermally induced two-step transitions and also the observed pressure effect to the transition may be reproduced by the present model.