We report the discovery of Zn 0.456 In 1.084 Ge 0.460 O 3 , a material closely related to bixbyite. In contrast, however, the oxygen atoms in this new phase occupy 4 Wyckoff positions, which result in 4 four-coordinate, 24 sixcoordinate (2 different Wyckoff positions), and 4 eight-coordinate sites as compared to the 32 six-coordinate (also 2 different Wyckoff positions) sites of bixbyite. This highly ordered material is related to fluorite, Ag 6 GeSO 8 , and γ-UO 3 and is n-type with a bulk carrier concentration of 4.772 × 10 14 cm −3 . The reduced form displays an average room temperature conductivity of 99(11) S· cm −1 and an average optical band gap of 2.88(1) eV. These properties are comparable to those of In 2 O 3 , which is the host material for the current leading transparent conducting oxides. The structure of Zn 0.456 In 1.084 Ge 0.460 O 3 is solved from a combined refinement of synchrotron X-ray powder diffraction and time-of-flight neutron powder diffraction and confirmed with electron diffraction. The solution is a new, layered, tetragonal structure in the I4 1 /amd space group with a = 7.033986(19) Å and c = 19.74961(8) Å. The complex cationic topological network adopted by Zn 0.456 In 1.084 Ge 0.460 O 3 offers the potential for future studies to further understand carrier generation in ∼3 eV oxide semiconductors.

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Abstract:The bixbyite structure of In 2 O 3 has two nonequivalent, 6-coordinate cation sites and, when Sn is doped into In 2 O 3 , the Sn prefers the "b-site" and produces a highly conductive material. When divalent/tetravalent cation pairs are cosubstituted into In 2 O 3 , however, the conductivity increases to a lesser extent and the site occupancy is less understood. We examine the site occupancy in the Mg x In 2−2x Sn x O 3 and Zn x In 2−2x Sn x O 3 systems with high resolution X-ray and neutron diffraction and density functional theory computations, respectively. In these sample cases and those that are previously reported in the M x In 2−2x Sn x O 3 (M = Cu, Ni, or Zn) systems, the solubility limit is greater than 25%, ensuring that the b-site cannot be the exclusively preferred site as it is in Sn:In 2 O 3 . Prior to this saturation point, we report that the M 2+ cation always has at least a partial occupancy on the d-site and the Sn 4+ cation has at least a partial occupancy on the b-site. The energies of formation for these configurations are highly favored, and prefer that the divalent and tetravalent substitutes are adjacent in the crystal lattice, which suggests short range ordering. Diffuse reflectance and 4-point probe measurements of Mg x In 2−x Sn x O 3 demonstrate that it can maintain an optical band gap >2.8 eV while surpassing 1000 S/cm in conductivity. Understanding how multiple constituents occupy the two nonequivalent cation sites can provide information on how to optimize cosubstituted systems to increase Sn solubility while maintaining its dopant nature, achieving maximum conductivity.