Elastic interaction of high-spin and low-spin complex molecules in spin-crossover compounds. II

Abstract

For pt.I see ibid., vol.21, p.1423 (1988). Several transition-metal compounds show a transition from the low-spin to the high-spin electronic state with increasing temperature. The cooperative nature of the transition in the solid state is attributed to the elastic interaction between the spin-changing ions as a result of the deformation of the crystal accompanying the transition. In a previous work the long range part of the elastic interaction due to the image pressure on a spherical surface was treated. In this work the elastic energy originating from the direct elastic interaction between the high-spin and low-spin complex molecules randomly distributed over the lattice sites is computed. The crystal is considered as an isotropic homogeneous elastic medium with the spin-changing ions as point defects described by elastic dipole tensors P^HS and P^LS, respectively. The interaction depends on the components of the tensor difference P^HL=P^HS-P^LS which are unequivocally determined based on the X-ray data of the compounds (Fe(2-pic)₃) Cl₂.Sol (2-pic=2-aminomethylpyridine, Sol=MeOH, EtOH). Collecting both contributions (the image and the direct part) the value of the interaction constant Gamma can be explained in a consistent way for both compounds.