The kesterite-stannite structural transition as a way to avoid Cu/Zn disorder in kesterites: the exemplary case of the Cu-2(Zn,Mn)SnSe4

The solid solution series between Cu2ZnSnSe4, crystallizing in the kesterite type structure, and Cu2MnSnSe4, adopting the stannite type structure, i.e. Cu-2(Zn1-xMnx)SnSe4, was studied by a combination of neutron and X-ray powder diffraction. Powder samples with 0 <= x <= 1 were synthesized by the solid state reaction of the pure elements and it was confirmed by wavelength-dispersive X-ray spectroscopy that each contained a homogeneous, off-stoichiometric quaternary phase. The lattice parameters and cation site occupancy factors were determined simultaneously by the Rietveld analysis of the 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 Cu-2(Zn1-xMnx)SnSe4 mixed crystals, from which a cation distribution model was derived. For the end member Cu2ZnSnSe4, the disordered kesterite structure was confirmed and for Cu2MnSnSe4, the stannite structure was confirmed. The cross-over from the kesterite to stannite type structure by Zn2+ <-> Mn2+ substitution in the Cu2Zn1-xMnxSnSe4 solid solution can be seen as a cation re-distribution process among the positions (0, 0, 0), (0, 1/2, 1/4) and (0, 1/4, 3/4), 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 parameters c/(2a), showing a characteristic dependence on the chemical composition. The order parameter Q, the quantitative measure of Cu/B-II disorder (B-II = Zn and Mn), shows a distinct dependence on the Mn/(Mn + Zn) ratio. In Zn-rich Cu-2(Zn1-xMnx)SnSe4 mixed crystals, the order parameter Q similar to 0.7 and drops to Q similar to 0 (complete Cu/B-II disorder) in the compositional region 0.3 >= x >= 0.7. In Mn-rich Cu-2(Zn1-xMnx)SnSe4 mixed crystals, adopting the stannite type structure, the order parameter reaches almost Q similar to 1 (order). Thus, it can be concluded that only Mn-rich Cu-2(Zn1-xMnx)SnSe4 mixed crystals do not show Cu/B-II disorder. A similar trend of the dependence on the chemical composition of both Cu/B-II-disorder and the band gap energy E-g in Cu-2(Zn1-xMnx)SnSe4 mixed crystals was observed.