Magnetoelectric and multiferroic properties of ternary copper chalcogenides Cu2MIIMIVS4

We investigate theoretically the ternary copper chalcogenides with the general formula (Cu2MMS4)-M-II-S-IV. This family of compounds can crystallize in two different non-centrosymmetric structures, I (4) over bar 2m or Pnm2(1). We show that all the reported members of (Cu2MMS4)-M-II-S-IV having the Pnm2(1) symmetry exhibit a large spontaneous polarization. This result suggests that several of these materials are likely to be multiferroics since they order magnetically at low temperature. We discuss in detail in the framework of Landau theory the members Cu2MnSnS4 and Cu2MnGeS4 which should present both a linear magnetoelectric effect and multiferroic behavior.