A2AgCrCl6 (A = Li, Na, K, Rb, Cs) halide double perovskites: a transition metal-based semiconducting material series with appreciable optical characteristics†
We have theoretically examined the geometries, electronic density of states and band structures of cubic and hexagonal A2AgCrCl6 (A = Cs, Rb, K, Na, Li) using meta-GGA SCAN-rVV10. The optimized lattice density was found to vary between 2.68 and 4.08 g cm−3 for cubic-A2AgCrCl6, with the fundamental electronic bandgap (direct) in the range of 0.66–0.69 eV. The cell density of hexagonal A2AgCrCl6 was between 2.97 and 3.93 g cm−3, but with an indirect bandgap of 0.93–1.02 eV. The valence band maximum and the conduction band minimum of A2AgCrCl6 were confirmed to be essentially of Cr(3d) character, but the contributions from the orbital states of Cl(3p) to the VBM were also appreciable. Cubic A2AgCrCl6 (A = Cs, Rb, K) was identified to possess genuine perovskite stoichiometry, evaluated using various geometry-based indices (viz. octahedral factor, tolerance factor, and global instability index). This was not so for A2AgCrCl6 (A = Na, Li), and was due to the small size of Na and Li cations that caused the critical strain of CrCl6 octahedra and a significant decrease in the cell volume. However, all the five A2AgCrCl6 displayed nearly similar optical properties, including the nature of the oscillator peaks in the dielectric function, absorption coefficient, photoconductivity, reflectivity, and Tauc spectra. The zero-limit of the refractive index was calculated around 2.25 and 2.00 for cubic and hexagonal A2AgCrCl6, respectively, and the extinction coefficient was very small for all cases. The nature of the optical bandgap and transition peaks discussed in this study of cubic and hexagonal Cs2AgCrCl6 agreed well with the experiment. The examination of phonon band dispersion led to the conclusion that cubic-A2AgCrCl6 (A = Cs, Rb) are the only halide double perovskites of the entire series that are dynamically stable.