Effect of Fe(II) spin crossover on charge distribution in and lattice properties of thiospinels

The spinels AgFe0.5Sn1.5S4 and CuFe0.5Sn1.5S4 belong to the rare examples of purely inorganic compounds in which Fe(II) exhibits a thermally induced transition from a low spin 3d(t2g)6(eg)0 to a high spin 3d(t2g)4(eg)2 electronic ground state. The extremely rare situation of having 119Sn as a second Mössbauer isotope in the lattice besides 57Fe is used to obtain deeper insight in the consequences the spin transition has on the lattice properties. To this end, 119Sn Mössbauer spectra were recorded between 5 and 500 K. The temperature dependence of the 119Sn hyperfine parameters is analysed. The data are compared to those obtained for the spinels AgMn0.5Sn1.5S4 and CuMn0.5Sn1.5S4 which are characterised by a temperature-independent high spin ground state. The results are discussed in terms of Fe–Sn charge transfer, local distortions on the tin site and changes of the vibrational lattice properties induced by the Fe spin transition.

graphical abstractUpper panel: temperature dependence of χmT of AgFe0.5Sn1.5S4, reflecting the progressive transition of iron from the low spin to the high spin state. Lower panel: 119Sn isomer shift. Below 400 K, i.e. in the temperature domain of the iron spin crossover, the isomer shift shows a much stronger variation with temperature than expected from the second order Doppler shift (dashed line).Figure optionsDownload as PowerPoint slideHighlights
► Effect of Fe spin crossover on the tin atoms studied by 119Sn Mössbauer spectroscopy.
► The 119Sn isomer shift shows an unusual dependence on temperature.
► Explanation: a charge transfer between Fe and Sn induced by the spin crossover.
► Fe spin state also influences the spatial charge distribution around tin.