Electronic Structure Studies of Main Group Oxides Possessing Edge-Sharing Octahedra:  Implications for the Design of Transparent Conducting Oxides

Experimental and computational studies were performed to investigate the electronic structure of main group oxides containing edge-sharing octahedra. Compounds belonging to four structure types were investigated: rutile (SnO2, PbO2), trirutile (MgSb2O6, ZnSb2O6, MgBi2O6, ZnBi2O6), PbSb2O6-type (CdSb2O6, CaSb2O6, SrSb2O6, BaSb2O6, SrBi2O6, BaBi2O6), and ilmenite (ZnSnO3, CdSnO3, CaSnO3, NaSbO3, KSbO3, NaBiO3, AgBiO3). For application as an n-type transparent conductor, the conduction band (CB) properties are of primary importance. In all compounds investigated here, the CB arises primarily from the antibonding M ns to O 2p interaction (M = Sn, Pb, Sb, Bi). To be an effective transparent conductor the lowest energy band in the CB should be wide so that carriers introduced upon doping will be highly mobile. For this to occur, the antibonding M ns to O 2p interactions must be offset by a bonding interaction at the CB minimum (normally at the Γ point). In ternary oxides, the bonding interaction involves overlap between the empty valence shell s orbital on the charge compensating A cation (A = Mg2+, Ca2+, Sr2+, Ba2+, Na+, K+, Ag+, Cd2+, Zn2+) with unfilled antibonding states originating on the edge-sharing octahedral framework. A disperse CB is obtained when the electronegativity of the A cation is such that it enables a covalent interaction with oxygen, and the oxygen coordination environment is symmetric. These conditions are met in ZnSb2O6 (trirutile), CdSb2O6 (PbSb2O6-type), CdSnO3 (ilmenite), ZnSnO3 (ilmenite), and AgBiO3 (ilmenite), as well as the binary oxides SnO2 and In2O3 (corundum polymorph). When M is Bi5+ or Pb4+, relativistic effects reduce the spatial overlap between M 6s orbital and the oxygen orbitals. This effect lowers the energy of the conduction band, thereby narrowing the band gap. Consequently, in these materials, it is generally not possible to simultaneously obtain a disperse CB and a wide band gap, limiting applications as transparent conductors.