Experimental and theoretical investigations on the composition-dependent structural phase transition in Cu2CdxZn1−xSnS4
Heppke, Eva M. and Küllmey, Tim and Efthimiopoulos, Ilias and Avci, Fatma D. and Appelt, Oona and Paulus, Beate and Lerch, Martin – 2019
Samples of the Cu2Cd x Zn1−x SnS4 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 x Zn1−x SnS4 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 x Zn1−x SnS4 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.