Site identity and importance in cosubstituted bixbyite In 2 O 3

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. © 2017 by the authors; licensee MDPI, Basel, Switzerland.U.S. Department of Energy National Science Foundation: DGE-1324585 CA060553 Basic Energy Sciences Office of Science: DMR-1121262, DE-AC02-06CH11357 Higher Education Research Council Northwestern University Basic Energy Sciences: DE-FG02-08ER46536 Argonne National LaboratoryKarl Rickert recognizes that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1324585. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. N.S. gratefully acknowledge that this study was partially supported by the Council of Higher Education (CoHE) of Turkey. Karl Rickert, Nazmi Sedefoglu, Jeremy Harris, and Kenneth R. Poeppelmeier gratefully acknowledge additional support from the Department of Energy Basic Energy Sciences Award No. DE-FG02-08ER46536. A portion of this research was performed at Oak Ridge National Laboratory?s Spallation Neutron Source at POWGEN, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, and the U.S. Department of Energy. Use of 11BM at the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work made use of the J. B. Cohen X-ray Diffraction Facility which is supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University. A portion of this work was supported by the NU Keck Biophysics Facility and a Cancer Center Support Grant (NCI CA060553)