Indirect-to-direct band gap transition induced by d−d coupling between cations in rare-earth chalcogenide perovskites

Chalcogenide perovskite materials have been shown to exhibit excellent properties for optoelectronics and photovoltaics. The research, however, has been focused on the II-IV-${\mathrm{S}}_3$ series of compounds. Here, by theoretical calculation, we predict that in the III-III-${\mathrm{S}}_3$ perovskites, there could exist a transition between the indirect and direct band gaps induced by the coupling strength of the $d$ orbitals between the A-site and B-site cations. We validate this prediction by synthesizing ${\mathrm{LaScS}}_3$ through solid state reaction from three elemental materials. Micro-Raman analysis combined with Raman tensor calculations are used to identify the perovskite phase of micrometer-size grains, from which photoluminescence can be observed. The emitted light peaks at about 519 nm (or 2.39 eV), which corresponds to the largest band gap among the sulfide perovskites. The discovery of light-emitting ${\mathrm{LaScS}}_3$ enriches the family of chalcogenide perovskites for optoelectronic applications.