The Impact of Local Coordination Environments on Transparency and Conductivity in Select Fluorite-Related Systems

The 6-coordinated cation site is the fundamental building block of the most effective transparent conducting oxides, but higher coordination environments have been largely ignored as a result of these successful materials. The Fluorite aristotype, albeit anion-deficient, is maintained by Sn:In 2O3, one of the most successful transparent conducting oxides, and offers the greatest potential for discovering new transparent conducting oxides with diverse coordination environments. Herein, the tetragonal, fluorite- and scheelite-related structure of Ga2In6Sn2O 16 and the bixbyite structure of In2O3 are used as starting points for investigating the impact of higher coordination environments on optical and electrical properties in quaternary and quinary metal oxide systems within the oxygen-to-cation ratio range of MO1.5≤x<2.0 . The substitution of Ge into these materials also leads to a novel anion-deficient fluorite structure and a delafossite-like family.;The coordination environments of Ga2In6Sn 2O16 are investigated with the replacement of up to 35% of the Sn content with Ti, Zr, or Hf, and up to 28% (100% for Sc) of the In content with Sc, Cr, Fe, or Lu. The tetravalent substitutions illustrate the dual role of Sn as dopant and structural element and that the majority of the conductivity originates from the 7-coordinate sites. The empty valence d orbitals of the substitutes promote spd covalency with surrounding oxygen atoms, decreasing conductivity and revealing that the superior doping ability of Sn lies in its filled, core-like d orbitals. The replacement of In and optical studies of the isostructural Mn3Ta2O8 suggest that the 6-coordinate site has a major impact on the band structure, which can affect both transparency and conductivity. Surpassing the solubility limit of Ti or introducing Ge departs from the fluorite aristotype and produces orthorhombic and monoclinic delafossite-like phases. The delafossite-like Ga0.4In1.1Ge0.3Sn0.2O 3.25 has a conductivity (582 S·cm-1) and transparency (2.86 eV) that are comparable to those of Ga2In6Sn 2O16. Cosubstitution of Zn and Ge into In2O 3, however, produces a new fluorite-related structure with four distinct coordination environments. The conductivity (99 S·cm-1) and transparency (2.88 eV) are comparable to In2O3. This is atypical behavior, as the cosubstitution of In2O3 commonly produces isostructural solid solutions, such as MgxIn 2-2xSnxO3, but rarely new structures.