1. (a) Atomic structure and (b) band structure and projected density of states (pDOS) of Cs₂SnI₆Colorful figures are available on website

2. Seven supercell models of Cs₂SnI₆ surfaces (a) For (001) surface: CsI₂-terminated and SnI₄-terminated slabs; (b) For (011) surface: I₄-terminated and Cs₂SnI₂-terminated slabs; (c) For (111) surface: non-stoichiometric Sn-terminated, CsI₃-terminated and stoichiometric CsI₃-terminated slabs

3. Calculated total cleavage, relaxation and surface energies of two complementary non-stoichiometric terminations in (001), (011) and (111) orientations, which are compared with the cleavage, relaxation and surface energies of the stoichiometric CsI₃-terminated (111) surface

4. Illustration of the accessible chemical potential region for Cs₂SnI₆Constraints imposed by the formation of competing secondary phases resulting in the allowed region shaded in green

5. Stability of low-index surfaces of Cs₂SnI₆ as a function of chemical potentials (a) Analysis of stability of the two terminations of Cs₂SnI₆ (001) surface with respect to the allowed region for maintaining equilibrium with the primary phase Cs₂SnI₆. The orange and blue regions indicate the stable region for CsI₂- and SnI₄-terminations, respectively; (b) Similar to (a) for the Cs₂SnI₆ (011) surface. The orange and blue regions are for the I₄- and Cs₂SnI₂-terminations, respectively; (c) Similar to (a) for the Cs₂SnI₆ (111) surface. The orange and blue regions are for the Sn- and stoichiometric CsI₃-terminations, respectively; (d) Surface energies of the seven surface models of Cs₂SnI₆ as a function of the chemical potentials colorful figures are available on website