First-principles investigation of triaxial strain effects on the physical properties of Rb2TiBr6 double perovskite for photocatalytic and thermoelectric applications

Abstract

In this study, we conduct a comprehensive computational investigation of the compound Rb₂TiBr₆, focusing on the influence of triaxial strain on its mechanical, electronic, optical, photocatalytic and thermoelectric properties. All calculations were performed within the framework of density functional theory (DFT) using the Wien2k computational package. Structural optimization was carried out with the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA), while the modified Becke–Johnson (mBJ) potential was employed to evaluate the electronic and optical properties. In addition, the thermoelectric performance was assessed by combining DFT results with semi-classical Boltzmann transport theory (SBT). The optimized structure of Rb₂TiBr₆ adopts a cubic phase with a lattice parameter of 10.594 Å. The calculated elastic constants confirm its mechanical stability under different strain conditions. Further analysis indicates that the compound exhibits ductile and anisotropic characteristics, with bonding predominantly of ionic nature. The electronic band structure and density of states (DOS) confirm that Rb₂TiBr₆ is a semiconductor with an indirect band gap (Γ–X), which increases from 1.952 eV to 2.115 eV as the applied strain varies from 0 % to 6 %. Optical analysis reveals strong absorption above 10⁴ cm⁻¹ and low reflectivity in the visible region, highlighting its potential for photovoltaic applications. Furthermore, the material exhibits excellent photocatalytic activity, underscoring its suitability for water-splitting processes. Thermoelectric analysis reveals that the application of strain enhances the figure of merit (ZT), reaching ∼3.5 at low temperature, which underscores the strong potential of Rb₂TiBr₆ for efficient thermoelectric energy conversion.