A series of spinel compounds with composition CuFe0.5(Sn(1−x)Tix)1.5S4 (0≤x≤1) is analysed by X-ray diffraction, measurements of magnetic susceptibilities and 57Fe Mössbauer spectroscopy. All samples show a temperature-dependent equilibrium between an electronic low spin 3d(t2g)6(eg)0 and a high spin 3d(t2g)4(eg)2 state of the Fe(II) ions. The spin crossover is of the continuous type and extends over several hundred degrees in all samples. The Sn/Ti ratio influences the thermal equilibrium between the two spin states. Substitution of Sn(IV) by the smaller Ti(IV) ions leads to a more compact crystal lattice, which, in contrast to many metal–organic Fe(II) complexes, does not stabilise the low spin state, but increases the residual high spin fraction for T→0 K. The role played by antiferromagnetic spin coupling in the stabilisation of the high spin state is discussed. The results are compared with model calculations treating the effect of magnetic interactions on spin state equilibria.
Comparison of fractions of high spin Fe(II) from Mössbauer spectra (circles) with plots of χmT (dots) versus T. Discrepancies between both methods indicate anti-ferromagnetic spin coupling.
► Many Fe(II) complexes show thermally induced high spin–low spin crossover. ► Spin crossover in spinel compounds is extremely scarce. ► Usually, lattice contraction favours the low spin state in Fe(II) complexes. ► In these spinels, lattice contraction favours the high spin state. ► The stabilisation of the high spin state is explained by spin–spin interactions.