Abstract
The bixbyite structure of In2O3 has two nonequivalent, 6-coordinate cation sites and, when Sn is doped into In2O3, the Sn prefers the “b-site” and produces a highly conductive material. When divalent/tetravalent cation pairs are cosubstituted into In2O3, however, the conductivity increases to a lesser extent and the site occupancy is less understood. We examine the site occupancy in the MgxIn2−2xSnxO3 and ZnxIn2−2xSnxO3 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 MxIn2−2xSnxO3 (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:In2O3. Prior to this saturation point, we report that the M2+ cation always has at least a partial occupancy on the d-site and the Sn4+ 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 MgxIn2−xSnxO3 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.
Highlights
Bixbyite indium oxide is the basis of industrially important n-type transparent conducting oxides, which are in turn fundamental components of flat panel displays, touch screens, and solar cells [1,2,3]
As higher conductivities are being demanded of transparent conductors, increasing the solubility of tin in the bixbyite structure is of interest
Sn4+ into In2 O3, where M = Mg, Ca, Ni, Cu, Zn, or Cd, to form Mx In2−2x Snx O3 which maintains the bixbyite structure of In2 O3
Summary
Bixbyite indium oxide is the basis of industrially important n-type transparent conducting oxides, which are in turn fundamental components of flat panel displays, touch screens, and solar cells [1,2,3]. Tin-doped indium oxide (ITO) is a highly effective transparent conducting oxide that derives its desirable conductivity from the formation of Frank-Köstlin clusters [4]. The concentration of these clusters are directly influenced by the solubility of tin in the bixbyite lattice. As higher conductivities are being demanded of transparent conductors, increasing the solubility of tin in the bixbyite structure is of interest.
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