Searching for Photoactive Polymorphs of CsNbQ₃ (Q = O, S, Se, Te) with Enhanced Optical Properties and Intrinsic Thermodynamic Stabilities

Nowadays, materials design efforts to obtain nontoxic and cost-effective photoactive semiconductors with a given chemical composition face the challenge of the coexistence of more than one configuration or crystal structure, so-called polymorphism. Polymorphs for multicomponent materials might exhibit various crystal structures by unique connectivity modes, hence creating polyhedral networks extended across the three-dimensional space or restricted along specific directions. A key component in photoactive materials design consists in the assessment of the thermodynamic stability of the various polymorphs along with their targeted properties, such as the optical band gap and the photon absorption efficiency. In this work, we conduct density functional theory calculations on cesium-niobate and cesium-niobium-chalcogenide CsNbO_3-xQ_x (Q = S, Se, Te, and x = 0, 1, 2, 3) compounds aiming at identifying intrinsically stable polymorphs with a high ability to absorb visible light. The connectivity between niobium-cation-centered polyhedra in the different polymorphs favors low dimensionality due to the large radius of the Cs cation. We identify unreported compounds, CsNbS₃ and CsNbSe₃, in the orthorhombic phase, where the polyhedra compose networks of low-dimensional connectivity as thermodynamically stable and strong visible-light absorbers.