Improved Photoactivity of Pyroxene Silicates by Cation Substitutions

We investigated the possibility of band structure engineering of pyroxene silicates with chemical formula A⁺¹ B⁺³ Si₂ O₆ by proper cation substitution. Typically, band gaps of naturally formed pyroxene silicates such as NaAlSi₂ O₆ are quite high (≈5 eV). Therefore, it is important to find a way to reduce band gaps for these materials below 3 eV to make them usable for optoelectronic applications operating at visible light range of the spectrum. Using first-principles calculations, we found that appropriate substitutions of both A⁺ and B³⁺ cations can reduce the band gaps of these materials to as low as 1.31 eV. We also discuss how the band gap in this class of materials is affected by cation radii, electronegativity of constituent elements, spin-orbit coupling, and structural modifications. In particular, the replacement of Al³⁺ in NaAlSi₂ O₆ by another trivalent cation Tl³⁺ results in the largest band-gap reduction and emergence of intermediate bands. We also found that all considered materials are still thermodynamically stable. This work provides a design approach for new environmentally benign and abundant materials for use in photovoltaics and optoelectronic devices.