The cooperative spin transition in [FexZn1-x(ptz)(6)](BF4)(2) .2. Structural properties and calculation of the elastic interaction

The cooperativity of the thermal spin transition in the Fe(II) spincrossover compound [Fe(ptz)(6)]BF4)(2) (ptz = 1-propyltetrazole) and in isomorphous mixed crystals with the isostructural zinc complex is investigated. From powder X-ray measurements the lattice deformation (tensor epsilon) accompanying the spin transition is determined. For diluted mixed crystals with x < 0.44 the lattice deformation is directly related to the spin transition, whereas in concentrated mixed crystals with, x > 0.44 a first order crystallographic phase transition (R3i --> P1i) is observed on cooling, which is triggered by the spin transition and can be suppressed by cooling rapidly. The thermal spin transition is measured with UV/VIS optical absorption spectroscopy on mixed single crystals in the R3i structure. From this metal dilution experiment an interaction constant of 169 cm(-1) for the cooperativity of the spin transition is determined. Furtheron, this interaction constant is calculated on the grounds of elasticity theory: The lattice deformation due to the spin transition is traced back to anisotropic elastic point defects, which directly interact with each other via their stress fields and indirectly via the surface of the elastic crystal by an image pressure. The elastic properties of the crystalline matrix are taken in the isotropic approximation. They are derived from the complete sets of anisotropic elastic constants of the pure iron and zinc compounds, which have been measured previously by Brillouin spectroscopy. The contribution of elastic energy calculated this way is similar or equal to 80% of the experimental value of the interaction constant, i.e. the cooperativity in crystalline spincrossover compounds is quantitatively of elastic nature.