The elastic interaction of high-spin and low-spin complex molecules in spin-crossover compounds

Several transition metal compounds show a transition from the low-spin (LS) to the high-spin (HS) electronic state with increasing temperature. The cooperative nature of the transition is usually parametrised by an interaction constant Gamma , the origin of which is still under discussion. In the frame of the lattice expansion mode, the interaction Gamma is attributed to the elastic interaction between the spin-changing ions as a result of the deformation of the crystal accompanying the transition. The authors calculate the complete elastic energy originating from the so-called image pressure in closed form by considering the crystal as an isotropic homogeneous elastic medium with the spin-changing ions as incompressible inclusions described by the full elastic dipole tensors P^HS and P^LS, respectively. The calculated values of Gamma based on X-ray data and reasonable estimates of the elastic constants of the compounds (Fe(2-pic)₃)Cl₂.Sol (2-pic=2-aminomethylpyridine, Sol=MeOH,EtOH) and (Fe(2-pic-ND₂)₃)Cl₂.EtOD are compared with the experimental values of Gamma .