A neutron diffraction study of the stannite-kesterite solid solution series

The intermixing of stannite and kesterite, i.e. Cu₂Fe_1−xZn_xSnS₄, was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ x ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 750°C and quenched after the final annealing. The lattice parameter, cation site occupancies and the isotropic temperature factors were determined by simultaneously Rietveld analysis of X-ray and neutron powder diffraction data. The refined lattice constants are in agreement with the literature. The refined site occupancy factors were used to determine the average neutron scattering length of the cation sites in stannite, kesterite and Cu₂Fe_1−xZn_xSnS₄ solid solutions, giving new insight into the cation distribution. For Cu₂FeSnS₄ the stannite type cation ordering was approved, whereas a new ordering was obtained in Cu₂ZnSnS₄. In the latter Cu occupies the 2a site, Zn and the remaining Cu are disordered on 2c (0, ½, ¼) and 2d (0, ¼, ¾). The disorder may be due to sample synthesis, i.e. to the quenching of the samples. The cross-over from stannite to kesterite by Fe²⁺ ↔ Zn²⁺ substitution in the Cu₂Fe_1−xZn_xSnS₄ solid solution can be foreseen as a 3-stage process of cation exchange among the positions at (0, 0, 0), (0, ½, ¼) and (0, ¼, ¾), including Cu⁺, Zn²⁺ and Fe²⁺. The Sn⁴⁺ cation does not take part in this process and remains on the 2b site. Moreover the crossover is also visible in the ratio of the lattice parameter c/(2a), showing a characteristic dependence on chemical composition.

The refined isotropic temperature factors were also taken into account. They give a consistent picture and to a degree support the developed cation distribution.