Iodine Anions beyond −1: Formation of Li_<i>n</i>I (<i>n</i> = 2–5) and Its Interaction with Quasiatoms

Novel phases of Li_<i>n</i>I (<i>n</i> = 2, 3, 4, 5) compounds are predicted to form under high pressure using first-principles density functional theory and an unbiased crystal structure search algorithm. All of the phases identified are thermodynamically stable with respect to decomposition into elemental Li and the binary LiI at a relatively low pressure (≈20 GPa). Increasing the pressure to 100 GPa yields the formation of a high pressure electride where electrons occupy interstitial quasiatom (ISQ) orbitals. Under these extreme pressures, the calculated charge on iodine suggests the oxidation state goes beyond the conventional and expected −1 charge for the halogens. This strange oxidative behavior stems from an electron transfer going from the ISQ to I^– and Li⁺ ions as high pressure collapses the void space. The resulting interplay between chemical bonding and the quantum chemical nature of enclosed interstitial space allows this first report of a halogen anion beyond a −1 oxidation state

Ground-State Surface of All-Inorganic Halide Perovskites

All-inorganic halide perovskites CsPbX3 (X = Cl, Br, and I) are attracting intensive attention for their outstanding optoelectronic properties and good stability. Surface energy plays a vital role in determining surface-related properties and phenomena, such as surface stability, equilibrium crystal shape, and the nucleation and growth of materials. There is a lack of thorough understanding of the surface energies and surface stability of CsPbX3. Here, we systematically explore these properties of CsPbX3 using first-principles calculations. We first deal with the convergence issue about the surface energy of the X-terminated (110)c surface of the cubic phase. By avoiding artificial octahedral tilts, we obtain convergent surface energy for the X-terminated (110)c surface from a view of pseudo-cubic perovskites. We then create stability phase diagrams and identify the ground state of CsPbX3 surfaces. The effects of octahedral tilts on the surface energies and the stability are evaluated by making a comparison of the surface energies between cubic and orthorhombic phases. Notably, we obtain the absolute surface energies of halide perovskites, which are difficult to be accessed from experiments. Our results can be a basis for further understanding and exploring the properties of passivated surfaces by ligands in all-inorganic halide perovskites