Exploring optical isotropy in halide double perovskites X2MgCl4 (X = Na, K, Rb, and Cs) with anisotropic structures

The family of halide double perovskites X2MgCl4, where X represents alkali metals like Na, K, Rb, and Cs, reveals a fascinating class of materials with unique structural and optical properties. Featuring a layered structure that enhances stability, X2MgCl4 is investigated using first-principles methods to uncover its structural, electronic, and optical characteristics. The electronic band structure analysis shows that X2MgCl4 has significant band gaps in the range of 6.37-6.46 eV, with Cs2MgCl4 having the largest band gap of 6.46 eV, influenced by the electronegativity of the alkali metal cations. Notably, substituting different alkali metals alters the electronic properties, which can be utilized in designing materials with tailored functionalities. Charge density analysis confirms the ionic nature of X2MgCl4 compounds. Specifically, our calculations indicate that Cs2MgCl4 possesses an optimal band gap (E-VC < E-g) for effective radiative recombination, benefiting from considerable core-valence excitations. Moreover, despite their inherent anisotropic nature, these materials display varying degrees of optical isotropy and have the ability to be produced as large single crystals, enhancing their suitability for diverse optoelectronic applications and fast scintillation. Overall, our findings suggest that X2MgCl4 holds promise as a high-performance candidate for scintillating applications, paving the way for advancements in radiation detection technologies.