Microstructural Insights into the Transformation of Cubic, Low-Temperature, Disordered Cu2ZnSnS4 into the Tetragonal Form

Multinary earth-abundant chalcogenides, like kesterite, Cu2ZnSnS4 (CZTS), have attracted attention in sustainable energy applications like photovoltaics and thermoelectrics. High-energy ball milling provides a facile way for the synthesis of pure cubic CZTS. This sulfide crystallizes in a sphalerite-type structure with complete occupational disorder in the cationic substructure and a considerable amount of stacking faults. Heating of the material leads to the slow and irreversible transition into disordered, tetragonal kesterite, which is associated with a significant decrease in thermoelectric properties. Hence, a deep understanding of the phase transition process and its kinetics is a prerequisite for further crystal engineering. In situ X-ray powder diffraction and Raman spectroscopy supported by density functional theory calculations and ab initio molecular dynamics simulations (AIMD) were employed to gain microstructural insights into the phase transition process. Heating leads to a growth of the crystalline domains, which is associated with a reduction of strain. The domain growth reduces the stabilization of the metastable cubic phase by nanostructuring. This eventually leads to the segregation of tin cations, which corresponds to the beginning of the transition into the tetragonal phase. AIMD simulations indicate that the presence of faulting planes promotes the tin diffusion. As stacking faults appear to be energetically less favorable in the tetragonal disordered form, the stacking fault disorder is reduced upon heating, leading to an additional strain reduction.