High-Pressure Behavior and Disorder for Ag₂ZnSnS₄ and Ag₂CdSnS₄

We carried out first-principles calculations to simulate Ag₂ZnSnS₄ and Ag₂CdSnS₄ and calculated enthalpies of different plausible structural models (kesterite-type, stannite-type, wurtzkesterite-type, wurtzstannite-type, and GeSb-type) to identify low- and high-pressure phases. For Ag₂ZnSnS₄, we predict the following transition: kesterite-type→[8.2GPa]→ GeSb-type. At the transition pressure, the electronic structure changes from semiconducting to metallic. For Ag₂CdSnS₄, we cannot decide which of the experimentally observed structures (kesterite-type or wurtzkesterite-type) is the ground-state structure because their energy difference is too small. At 4.7 GPa, however, we predict a transition to the GeSb-type structure with metallic character for both structures. Regarding the sensitivity of the material to disorder, a major drawback for solar cell applications, Ag₂CdSnS₄ behaves similar to Cu₂ZnSnS₄, both showing a high tendency to cationic disorder. In contrast, the disordered structures in Ag₂ZnSnS₄ are much higher in energy, and therefore, the material is less affected by disorder.