Structure and phase relations in the 2(CuInS2)-Cu2ZnSnS4 solid solution system

The intermixing of roquesite (CuInS₂) and kesterite (Cu₂ZnSnS₄), i. e. Cu(In_x(ZnSn)_1−xS₂ was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ × ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 800 °C and cooled with a 10 K/h rate after the final annealing. The structural parameters of CuIn_x(ZnSn)_1−xS₂ were determined by simultaneous Rietveld refinement of neutron and X-ray diffraction data. The microstructure and chemical composition of the samples were investigated by electron microprobe analysis. A broad miscibility gap exists in the region 0.4 ≤ × < 0.8 indicated by the coexistence of two phases, an In-rich (x ~ 0.77) and a Zn-Sn-rich (x ~ 0.33) phase. Cu(In_x(ZnSn)_1−xS₂ mixed crystals with 0 ≤ x < 0.4 crystallize in the kesterite type structure, and with 0.8 ≤ × ≤ 1.0 in the chalcopyrite type structure. In the latter In, Zn and Sn are disordered on the In site. In the mixed crystals the lattice constant a and c show a linear dependence on chemical composition, whereas the tetragonal deformation Δ = 1−c/2a varies nonlinearly. Moreover in the mixed crystal with x ~ 0.15 the tetragonal deformation is equal zero and thus its structure is characterized by a pseudo-cubic unit cell.