The unusual magnetic properties of kuramite-stannite pseudobinary series: a SQUID and EPR survey

The magnetic properties of the synthetic Cu3SnS4 (kuramite)-Cu2FeSnS4 (stannite) pseudobinary series were investigated by means of electron paramagnetic resonance (EPR) spectroscopy, at room temperature, and by magnetometry, in the range 2-300 K. The system is particularly complex, from both chemical and crystal chemical points of view, in particular with respect to the metal valence states and the local ordering in the different terms of the series. Nevertheless, recent successes in synthesising nanostructured kuramite foster the interest to ascertain the bulk magnetic properties of these important semiconducting phases. The obtained results allowed to ascertain that a variable lack of local ordering in the Cu3-x Fe (x) SnS4 (x < 0.85) samples induce the raise of strong metal-sulphur-metal superexchange interactions, that result in the appearance of marked deviations from the single-ion behaviour, typical for pure stannite. Ferro- and antiferromagnetic interactions are in fact observed at relatively high temperatures (similar to 150 K). A possible role played by Cu(I)-Fe(III) was revealed by the EPR measurements. The Cu-rich terms of the series (x < 0.1) are characterised by dynamic resonant disorder (i.e. time-evolving delocalisation of the formally divalent valence state for Cu among the nearest neighbouring Cu-sites), in addition to the Cu-Fe-Sn static disorder verified along the whole series. Both factors concurring to a non-periodic arrangement of paramagnetic ions in the lattice have the main effect to drastically broaden the EPR lines.