Vibronic Model for Cooperative Spin-Crossover in Pentanuclear {[MIII(CN)6]2[M′ II(tmphen)2]3} (M/M′ = Co/Fe, Fe/Fe) Compounds

In this paper we present a microscopic model for the cooperative spin-crossover (SCO) phenomenon in crystals containing cyano-bridged pentanuclear clusters {[MIII(CN)6]2[M′ II(tmphen)2]3} (M/M′ = Co/Fe, 1; Fe/Fe, 2) with a trigonal bipyramidal (TBP) structure. The low-spin to high-spin (ls–hs) transition is considered as a cooperative phenomenon that is driven by the interaction of electronic shells of FeII ions with totally symmetric deformation of the local environment that is extended over the crystal lattice via the acoustic phonon field. Due to proximity of FeII ions in the cluster, the short-range intracluster interaction between these ions via the optical phonon field is included as well. The model takes into consideration also the spin–orbit coupling operating within the cubic 5T2(t24e2) term of the hs-FeII ions. For cluster 2 the Heisenberg-type exchange interaction between the hs-FeII and ls-FeIII ions is taken into account. The competition between short- and long-range interactions is shown to ...