A2AgCrCl6 (A = Li, Na, K, Rb, Cs) Double Perovskites: A Transition Metal-based Semiconducting Material Series with Appreciable Optical Characteristics

We examined the geometry, electronic density of states and band structures of the A2AgCrCl6 (A = Cs, Rb, K, Na, Li) series using the recently proposed 'strongly constrained and appropriately normed' (SCAN) meta-GGA functional paired with the non-local correlation part from the rVV10 vdW density functional. The lattice density for the entire series was found varying between 2.68 and 4.08 gcm-3, and the electronic band gap (indirect) was lying between 1.81 and 1.84 eV. The VBM and CBM of A2AgCrCl6 were confirmed to be essentially of Cr(3d) characters, but contributions from the Ag(4d) and Cl(3p) orbital states to the VBM were also appreciable. The A2AgCrCl6 (A = Cs, Rb, K) systems were identified to possess genuine perovskite stoichiometry, evaluated using various geometry-based indices (viz. octahedral factor, tolerance factors, and global instability index). This was not so for A2AgCrCl6 (A = Na, Li), which was due to the small size of the Na and Li cations, causing the critical strain of the CrCl6 octahedra and significant decrease in the cell volumes. However, all five perovskites studied were shown to be characterized by nearly similar optical properties, including the nature of the oscillator peaks in the dielectric function, absorption coefficient, energy-loss function, photoconductivity, and reflectivity spectra. The refractive index of A2AgCrCl6 (A = Cs, Rb, K, Na, Li) was about 2.3, and the extinction coefficient was very small. The latter is expected for semiconductor materials. The indirect nature of the electronic band gap and optical transition peaks obtained from this study of Cs2AgCrCl6 agreed well with the experiments.