The kesterite–stannite structural transition as a way to avoid Cu/Zn disorder in kesterites: the exemplary case of the Cu2(Zn,Mn)SnSe4

The solid solution series between Cu₂ZnSnSe₄, crystallizing in the kesterite type structure, and Cu₂MnSnSe₄, adopting the stannite type structure, i. e. Cu₂(Zn_1-xMn_x)SnSe₄, was studied by a combination of neutron and X-ray powder diffraction. Powder samples with with 0 £ x £ 1 were synthesized by solid state reaction of the pure elements and qualified by wavelength-dispersive X-ray spectroscopy to contain a homogeneous, off-stoichiometric quaternary phase each. The lattice parameter and cation site occupancy factors were determined by simultaneously Rietveld analysis of neutron and X-ray powder diffraction data. The refined site occupancy factors were used to determine the average neutron scattering length of the cation sites in the crystal structure of the Cu₂(Zn_1-xMn_x)SnSe₄ mixed crystals, from which a cation distribution model was derived. For the end members Cu₂ZnSnSe₄ the disordered kesterite structure and for Cu₂MnSnSe₄ the stannite structure were approved. The cross-over from the kesterite to stannite type structure by Zn²⁺ « Mn²⁺ substitution in the Cu₂Zn_1-xMn_xSnSe₄ solid solution can be seen as a cation re-distribution process among the positions (0, 0, 0), (0, ½, ¼) and (0, ¼, ¾), including Cu⁺, Zn²⁺ and Mn²⁺. The Sn⁴⁺ cation does not take part in this process and remains on the 2b site. Moreover, the cross-over is also visible in the ratio of the lattice parameter c/(2a), showing a characteristic dependence on chemical composition. The order parameter Q, the quantitative measure of Cu/B^II disorder (B^II=Zn and Mn), shows a distinct dependency on the Mn/(Mn+Zn) ratio. In Zn-rich Cu₂(Zn_1-xMn_x)SnSe₄ mixed crystals the order parameter shows Q~0.7 and drops to Q~0 (complete Cu/B^II disorder) in the compositional region 0.3³x³0.7. In Mn-rich Cu₂(Zn_1-xMn_x)SnSe₄ mixed crystals, adopting the stannite type structure, the order parameter reaches almost Q~1 (order). Thus it can be concluded, that only Mn-rich Cu₂(Zn_1-xMn_x)SnSe₄ mixed crystals do not show Cu/B^II disorder. A similar trend of the dependency on chemical composition for both Cu/B^II-disorder and band gap energy E_g in Cu₂(Zn_1-xMn_x)SnSe₄ mixed crystals was observed.