A model of spin crossover in crystals containing pentanuclear clusters [CoIII(CN)6]2[FeII(tmphen)2]3 with a trigonal bipyramidal structure as structural units is presented. The model takes into consideration the spinorbital coupling operating within the cubic ( 4 2 ) 2 2 5T t e term of the hs-FeII ions and ( 5 ) 2 2 2T t term of the ls-FeIII ions. The cooperative nature of the phenomenon is attributed to longrange electron-deformational interaction via the field of accoustic phonons and short-range interaction via the optic phonon field. The interplay between short- and long-range interactions is shown to determine the gradual type of the spin-crossover transition in the titled compound. A new method of estimation of the parameter of electrondeformational interaction in spin-crossover compounds is suggested. The method takes into account the elongation of the Fe-N bonds when passing fron the ls 1A1 ( 6 2 t ) state to the hs 5T2 ( 4 2 2t e ) state of the FeII ions. The developed model explains the static magnetic susceptibility and temperature dependence of the Mössbauer spectra (Fig.1) of the FeII 3CoIII 2 compound. Quite good agreement with the experimental data is observed.
figure
Fig.1. Mössbauer spectra for FeII 3CoIII 2 complex calculated at T=5 and 300K with the set of the best fit parameters (thick solid lines).
Financial support of the STCU (project N5062) is highly appreciated. B.T. and K.D. gratefully acknowledge financial support from the BSF (grant 2006498). B.T. thanks ISF (grant 168/09).