Emerging piezochromism in transparent lead free perovskite Rb3X2I9 (X=Sb, Bi) under compression: A comparative theoretical insight

Hydrostatic pressure has proven to be an alternative tool to chemical pressure to tune the crystal structure and physical and chemical properties, paving the way for a novel functional materials design. Halide perovskites consist of a large family of perovskites, with tremendous potential for energy applications. Their soft lattices and distinct behavior under pressure make them necessary to be studied under compressed conditions. Ternary antimony halides are one such class of perovskites that act as an alternative to lead (Pb) halide perovskites due to the lesser toxicity of Sb/Bi in comparison with Pb. In this letter, we have theoretically envisaged the effects of compression via hydrostatic pressure on the structural, electronic, and optical properties of Rb3X2I9, where X=Sb and Bi-. The bandgaps not only decrease but are also direct in the desired pressure range. The calculated effective masses decrease with pressure, thereby indicating higher mobility of the charge carriers in the compressed material. The presence of piezochromism has also been confirmed from the variation of optical properties under hydrostatic compression. Our investigations shed light on the pressure dependent sensitive tuning of electronic and optical properties of Rb3Bi2I9 and Rb3Sb2I9, rendering these materials to have significant photovoltaic applications.