Ground and excited state properties of I–V dipoles and alkaline earth oxides at the (0 0 1) surfaces of alkali chlorides: DFT calculations

Ground and excited state properties of I–V dipoles (Be2+V−, Mg2+V− and Ca2+V−) and alkaline earth oxides (BeO, MgO and CaO) at the (0 0 1) surfaces of the alkali chlorides (LiCl, NaCl and KCl) were examined using an embedded cluster model and density functional theory DFT calculations with effective core potentials. A finite lattice in which the Coulomb potential in the central region closely fits the Madelung potential in the unit cell of the host crystal is constructed and followed by generating the crystal surface. The 〈1 1 0〉 orientations of the I–V dipoles were more stable than the 〈1 0 0〉 orientations by ca. 0.157–0.310 a.u. in the singlet ground state configurations and by ca. 0.002–0.104 a.u. in the triplet excited state configurations. The optimal relaxation mode of the nearest neighbor ions to the I–V dipoles lowers the total electronic energies of the ground state configurations by ca. 0.89–2.99 eV and the excited state configurations by ca. 1.01–3.17 eV. The Glasner–Tompkins relation is generalized to include the positive ion species in both the ground and excited state configurations. The defect-free surfaces can be made semiconducting by low-lying triplet excitations, and the defect-containing surfaces by doping Ca2+ in the excited state configurations. The unstable O2− species binds very strongly at the alkaline earth metal sites of the I–V dipoles to form stable alkaline earth oxides. The most preferred substrates for interaction with O2− were Be2+-doped LiCl in the excited state and Be-doped KCl in the ground state. The strength of the alkaline earth oxide bond is directly proportional to the amount of charge transferred from O2− to the surface. The results of the adsorbate–substrate interactions are explainable in terms of electrostatic potential curves, Coulombic attractions and spin pairing.