Stability and Optoelectronic Properties of Alkali–Magnesium Trihalide Perovskites FrMgX 3

ABSTRACT This study unveils various physical properties of non‐toxic, halide‐based, and novel perovskite FrMgX 3 (X = Cl, Br, and I) without external temperature and pressure using density functional theory (DFT) calculations. The negative formation enthalpy, along with suitable octahedral and tolerance factors, confirms structural stability, with a lattice parameter of 5.17, 5.48, and 5.94 Å for FrMgCl 3 , FrMgBr 3 , and FrMgI 3 , respectively. Finally, the positive phonon frequencies of FrMgCl 3 and FrMgBr 3 confirm the possibility of experimental synthesis of these compounds, whereas FrMgI 3 shows negative phonon spectra. The anisotropic materials FrMgCl 3 and FrMgBr 3 exhibit a brittle nature, whereas FrMgI 3 shows ductility. The band structure, obtained with generalized gradient approximation‐Perdew–Burke–Ernzerhof (GGA‐PBE) and hybrid HSE06 functional, indicates an indirect band gap and a semiconductor‐to‐nearly‐insulator range of band gap. The ionic and covalent bond structures are revealed through the analysis of CDD, population analysis, and bond‐length data. Notably, the highest absorption peak lies in the ultraviolet (UV) region, with values ranging between 12 and 18 eV. Moreover, the excellent conductivity and reflectivity of the compounds make them superior in deep‐UV optoelectronics, photodetection, and transparent‐conductive applications. In terms of thermal properties, FrMgCl 3 exhibits a higher melting point and Debye temperature, whereas FrMgI 3 demonstrates lower minimum thermal conductivity.