Experimental characterization methods and Rietveld refinement parameters; TEM analysis; computational structural (RDFs), electronic (densities of states and unfolded band structures) and optical (hybrid DFT absorption spectra) analysis; XRD patterns after exposure to various environments; calculated chemical stability regions (chemical potentials); computational analysis of Cs 2 Ti 1-x Sn x Cl 6 vacancy-ordered perovskites.