Experimental and theoretical investigations on the composition-dependent structural phase transition in Cu₂Cd_xZn_1−xSnS₄

Samples of the Cu₂Cd_xZn_1−xSnS₄ solid solution series were synthesized by a mechanochemical process, exhibiting high crystallinity due to an annealing step under flowing H₂S gas. The composition-dependent structural transition between the kesterite- and stannite-type phases was determined for Cd content close to x_Cd ≈ 0.4 by means of x-ray diffraction and Raman spectroscopic probes, in excellent agreement with earlier investigations. Our DFT calculations predicted a critical Cd concentration value of x_Cd = 0.5 as the 'border' between the stannite- and kesterite-type structure in the Cu₂Cd_xZn_1−xSnS₄ solid solution series. The somewhat higher calculated Cd content value compared to the experimental case can be accounted by partial Cu/Zn disorder present in the synthesized samples. The measured optical band gaps E_g of the Cu₂Cd_xZn_1−xSnS₄ solid solution series decrease by ∼0.3 eV upon immediate introduction of Cd into the lattice, with E_g being almost constant with varying Cd concentration. All of our observations are interpreted within the framework of earlier reports on composition-dependent kesterite-to-stannite transition, where local structural variations due to the Zn²⁺ substitution by the larger Cd²⁺ cations appear to dictate the transition process. (paper)