The crystal structure of kesterite type compounds: A neutron and X-ray diffraction study

The atomic structure of the potential photovoltaic materials Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) is discussed on the basis of a structural analysis of neutron powder diffraction data refined by the Rietveld analysis. Both compounds were found to crystallize in the kesterite type structure, but with a disorder within the Cu-Zn layers at z=1/4 and 3/4. The latter causes Cu_Zn and Zn_Cu anti-site defects, whose concentration depends on the sample growth conditions.The temperature dependent structural phase transition in CZTS was studied by in-situ high temperature diffraction experiments using high energy synchrotron X-rays. The transition from the tetragonal kesterite to the cubic sphalerite type structure is discussed by means of the interplay of increasing and decreasing cation-anion-cation bond angles in the coordination tetrahedra, reflected by the shift of the anion atomic coordinates to more ideal (x and y) or non-ideal (z) values. This striking behavior may be connected with frustrations that occur due to the triangular geometry of the coordination tetrahedra.