The deformation tensor ∊ of the spin transition in the mixed crystal [Fe0.46Zn0.54(ptz)6](BF4)2

Abstract

The conversion of the spin state of complexes exhibiting thermal spin crossover from the1A1low-spin (LS) state to the5T2high-spin (HS) state is accompanied by a deformation of the lattice due to the larger bond lengths in the HS state as compared with the LS state. In a previous work [Kuszet al.(2000).J. Appl. Cryst.33, 201–205], it has been shown that the deformation of the lattice, corrected for its temperature dependence, can be described by an almost temperature-independent tensor ∊ multiplied by the fraction of molecules in the HS state, γHS. Here the dependence of ∊ in a mixed single crystal of [Fe0.46Zn0.54(ptz)6](BF4)2(ptz = propyltetrazole) with a transition temperature near 110 K is reported. In order to study the dependence of the metal–ligand bond length on the concentration of Zn, five structures were determined at 10 K: the neat iron and the mixed-crystal compound in both the excited HS state (LIESST) and the LS state, and the neat Zn compound. The comparison of the structures shows that neither the bond lengths nor their differences in the HS and LS states depend linearly on the Zn concentration. These deviations can be considered as consequences of elastic interactions directly observed by X-ray diffraction analysis. The lattice constantsaandcalso do not depend linearly on the Zn concentration, contrary to the unit-cell volume and the differences ofaandcbetween the HS and LS states, a fact which is in agreement with the observed linear dependence of these parameters on the temperature-dependent HS fraction.