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) spincro ssover compound [Fe(ptz)(6)]BF4)(2) (ptz = 1-propyltetrazole) and in i somorphous mixed crystals with the isostructural zinc complex is inves tigated. From powder X-ray measurements the lattice deformation (tenso r epsilon) accompanying the spin transition is determined. For diluted mixed crystals with x < 0.44 the lattice deformation is directly rela ted to the spin transition, whereas in concentrated mixed crystals wit h, x > 0.44 a first order crystallographic phase transition (R3i --> P 1i) 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 singl e crystals in the R3i structure. From this metal dilution experiment a n interaction constant of 169 cm(-1) for the cooperativity of the spin transition is determined. Furtheron, this interaction constant is cal culated on the grounds of elasticity theory: The lattice deformation d ue to the spin transition is traced back to anisotropic elastic point defects, which directly interact with each other via their stress fiel ds and indirectly via the surface of the elastic crystal by an image p ressure. 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 con tribution of elastic energy calculated this way is similar or equal to 80% of the experimental value of the interaction constant, i.e. the c ooperativity in crystalline spincrossover compounds is quantitatively of elastic nature.