Theoretical prediction of the structural, electronic and optical properties of vacancy-ordered double perovskites Tl2TiX6 (X = Cl, Br, I)

Abstract

Recently, more and more vacancy-ordered double perovskites have been proposed as functional materials in optoelectronic devices. In this paper, a simulation work on the structural, electronic and optical properties of vacancy-ordered double perovskites Tl2TiX6 (X ​= ​Cl, Br, I) has been carried out by using first-principles calculations to reveal their potential application in optoelectronic devices. The Goldschmidt's tolerance factors of these compounds are very close to the ideal value of 1 for cubic perovskites, which confirm their structural stability. Moreover, the formation energies of these compounds were also calculated to evaluate their thermodynamic stability. The theoretically predicted lattice constants of Tl2TiCl6, Tl2TiBr6 and Tl2TiI6 are 10.060 ​Å, 10.553 ​Å and 11.320 ​Å, respectively, which can provide references for future experimental research. The calculated electronic properties indicate that Tl2TiCl6, Tl2TiBr6 and Tl2TiI6 have direct bandgaps of 3.39 ​eV, 2.57 ​eV and 1.58 ​eV, respectively, showing ideal bandgap nature for optoelectronic applications. Compared with many vacancy-ordered double perovskites, the three Tl2TiX6 compounds all show the advantage of small effective mass of holes. In addition, the investigation of optical properties, such as reflectivity, absorption coefficient and energy loss function in the photon energy range of 0–25 ​eV further confirms the potential utilization of these Tl2TiX6 compounds in optoelectronic devices.