Evolution of cation ordering and crystal defects controlled by Zn substitutions in Cu2SnS3 ceramics

Abstract

The microstructures of a series of Cu2ZnxSn1-xS3 (x = 0, 0.05, 0.10, 0.15,0.20) ceramic samples are investigated by a combination of selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field imaging (HAADF) and X-ray energy dispersive spectroscopy (EDS) techniques. The pure Cu2SnS3 sample takes the monoclinic structure with the ordering of eight 3Cu-Sn and four 2Cu-2Sn clusters, which obey the octet rule. With the increase of Zn substitution, unique mosaic-type nanostructures comprising well-defined cation-disordered domains coherently bonded to a surrounding network phase with semi-ordered cations are formed in the matrix grains. The atomic structures of the semi-ordered phases are revealed as CuInS2-like phase (Zn < 5 atom%), Cu6ZnSn3S10 (Cu2SnS3: ZnS = 3:1) and Cu4ZnSn2S7 (Cu2SnS3: ZnS = 2:1), respectively. These ordered structures derive from the zinc blende structure (201) superlattice of -(Cu-S)2(Zn-S)(Sn-S)- in the kesterite Cu2ZnSnS4 (Cu2SnS3:ZnS = 1:1). Meanwhile, point defects, dislocations, stacking faults, and finally Cu2-xS nanoprecipitates are formed sequentially to compromise the excessive Cu ions when the Zn contents increase from 5 atom% to 20 atom%. Understanding of the concurrence and evolution of the cation ordering and crystal defects are important to tailor their microstructures and physical properties in the Cu-Zn-Sn-S quaternary system.