A model for the mechanism of incorporation of Cu, Fe and Zn in the stannite -: Kesterite series, Cu2FeSnS4-Cu2ZnSnS4

In order to clarify the symmetry problem along the stannite - kesterite join [Cu2FeSnS4 - Cu2ZnSnS4], a structural study of synthetic Cu2Fe1-xZnxSnS4 single crystals was performed (x = 0, 0.2, 0.5, 0.7, 0.8 and 1, respectively). The metal distribution among the tetrahedral cavities was determined by refining different models in both the I (4) over bar and I (4) over bar 2m space groups. The best agreement was obtained in I (4) over bar 2m, even for the Zn-rich members of the series. However, two different mechanisms of incorporation take place along the stannite-kesterite join. For pure stannite and zincian stannite (x = 0, 0.2, 0.5), the 2a position (0,0,0) is mainly occupied by (Fe,Zn), whereas Cu is the dominant species at 4d (0,1/2,1/4). For ferroan kesterite and pure kesterite (x = 0.7, 0.8, 1), the 2a position is fully occupied by Cu, whereas (Zn,Fe) and the remaining Cu are disordered at 4d. On the basis of the structural results, pure Me-S bond-distances are proposed for Fe, Cu, Zn in both 2a and 4d sites, and the metal distribution among the tetrahedral sites is obtained accordingly. For x greater than or equal to 0.7, the Me-S distance found for the atom located at 2a closely approaches that found for the atom located at 4d, thus producing a more regular framework. Accordingly, distortion parameters lambda and sigma(2) of the S(Me3Sn) tetrahedron decrease with increasing Zn. This feature, in turn, is the reason for the pseudocubic symmetry of the lattice observed in the Zn-rich region (2a close to the c parameter). The unit-cell volume linearly increases with increasing Zn, thus confirming the mainly covalent character of the bonds in these compounds. The previously noted inversion of slope in the unit-cell parameters at x = 0.7 corresponds to the point of the series wherein Cu becomes predominant at the 2a site. The proposed model accounts for the structural and geometrical variations observed along the stannite-kesterite series, even if no change of space group is assumed.