A theoretical investigation of the lead-free double perovskites halides Rb2XCl6 (X = Se, Ti) for optoelectronic and thermoelectric applications

In this study, structural, electronic, optical, thermoelectric, and thermodynamics properties of vacancy-ordered double perovskites Rb2XCl6 (X = Se, Ti) were explored theoretically. The results revealed that Rb2SeCl6 and Rb2TiCl6 are indirect band gap (E-g) semiconductors with E-g values of 2.95 eV, and 2.84 eV respectively. The calculated properties (phonons, elastic constant, Poisson's ratio, and Pugh's ratio) revealed that both materials are dynamically and chemically stable and can exhibit brittle (Rb2SeCl6) and ductile (Rb2TiCl6) nature. From the analysis of optical parameters, it was noticed that the refractive index of the materials has a value of 1.5-2.0 where light absorption was found from the visible to the ultraviolet region. The thermoelectric properties determined by using the BoltzTrap code demonstrated that at room temperature, the Figure of merit (ZT) was found to be 0.74 and 0.76 for Rb2SeCl6 and Rb2TiCl6, respectively. Despite a moderate value of ZT in such materials, further studies might explore effective methods for tuning the electronic band gap and improving the thermoelectric response of the material for practical energy production applications.