crystals Article Site Identity and Importance in Cosubstituted Bixbyite In2O3 Karl Rickert 1, Jeremy Harris 1, Nazmi Sedefoglu 2, Hamide Kavak 3, Donald E. Ellis 4 and Kenneth R. Poeppelmeier 1,* 1 Department of Chemistry, Northwestern University, Evanston, IL 60208, USA; [email protected] (K.R.); [email protected] (J.H.) 2 Department of Physics, Osmaniye Korkut Ata University, Osmaniye 80000, Turkey; [email protected] 3 Physics Department, Cukurova University, Adana 01330, Turkey; [email protected] 4 Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-847-491-3505 Academic Editor: Stevin Snellius Pramana Received: 14 December 2016; Accepted: 6 February 2017; Published: 9 February 2017 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. Keywords: bixbyite; indium oxide; transparent conducting oxide 1. Introduction 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 [1–3]. Specifically, 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. 2+ One proven method to increase the solubility of tin in In2O3 is to perform a cosubstitution of M 4+ and Sn into In2O3, where M = Mg, Ca, Ni, Cu, Zn, or Cd, to form MxIn2−2xSnxO3 which maintains the bixbyite structure of In2O3. In so doing, solubility limits as high as x = 0.5 can be achieved, which Crystals 2017, 7, 47; doi:10.3390/cryst7020047 www.mdpi.com/journal/crystals Crystals 2017, 7, 47 2 of 10 is approximately double the amount of tin in ITO [5,6]. This does not double the conductivity of ITO, however,Crystals as 2017 the, 7, cosubstitution47 is not true doping because the tetravalent tin is counterbalanced2 of 10 by a divalentapproximately cation when double replacing the amount two of trivalenttin in ITO cations.[5,6]. This A does complete not double counterbalancing the conductivity effect of ITO, would substantiallyhowever, decreaseas the cosubstitution the conductivity is not true when doping compared because the to ITO,tetravalent but in tin some is counterbalanced cases an inherent by a off stoichiometrydivalent cation in the when divalent replacing and tetravalent two trivalent cation cations. is observed, A complete which counterbalancing results in a dopanteffect would effect and high conductivitysubstantially decrease [7]. the conductivity when compared to ITO, but in some cases an inherent off Instoichiometry ITO, the dopant in the favorsdivalent one and of tetravalent the two nonequivalent cation is observed, cation which sites results present in in a thedopant bixbyite effectstructure. and Both siteshigh conductivity are 6-coordinate [7]. and can be described as a cube with two anion positions vacant along either a body diagonalIn ITO, the (b-site) dopant or afavors face diagonalone of the ( dtwo-site). none Computationalquivalent cation and sites experimental present in the studies bixbyite agree structure. Both sites are 6-coordinate and can be described as a cube with two anion positions vacant that the tin in ITO favors the b-site [8–10]. The b-site comprises 25% of the cation sites in the bixbyite along either a body diagonal (b-site) or a face diagonal (d-site). Computational and experimental structure, shown in Figure1, which would inherently limit the solubility of tin, but the solubility limit studies agree that the tin in ITO favors the b-site [8–10]. The b-site comprises 25% of the cation sites is actuallyin the lower,bixbyite resulting structure, in shown mixed in indiumFigure 1, and which tin would occupancy inherently of the limitb-site. the solubility The solubility of tin, limitsbut of cosubstitutedthe solubility bixbyite, limit is however,actually lower, can occupyresulting up in tomixed 50% indium of the and cation tin occupancy sites and thereforeof the b-site. must The alter the d-sitesolubility either limits exclusively of cosubstituted or in combination bixbyite, however, with thecan boccupy-site. The up to impact 50% of of the multiple cation sites constituents and (i.e., onlytherefore Sn vs.must M/Sn alter pairs)the d-site on theeither electronic exclusively properties or in combination has already with drawn the b-site. investigative The impact interest, of particularlymultiple for constituents M = Zn, but (i.e., the only importance Sn vs. M/Sn of site pair occupancys) on the electronic has not yet properties been tied has to already either the drawn multiple constituentsinvestigative or the interest, electronic particularly properties for M [ 11= Zn,,12 ].but The the constituent importance of identities site occupancy and their has not site yet preferences been havetied been to linkedeither the to multiple crystal structure,constituents however, or the elec astronic exhibited properties by the[11,12]. cosubstitution The constituent of identities Zn/Ge pairs. and their site preferences have been linked to crystal structure, however, as exhibited by the In this instance, the system forms a single defined phase instead of a solid solution and departs from cosubstitution of Zn/Ge pairs. In this instance, the system forms a single defined phase instead of a the bixbyite structure in order to provide a 4-coordinate site for Ge [13]. solid solution and departs from the bixbyite structure in order to provide a 4-coordinate site for Ge [13]. FigureFigure 1. The 1. The unit unit cell cell (black (black lines) lines) of of bixbyite bixbyite InIn22O3 asas viewed viewed along along the the b axis,b axis, showing showing the locations the locations of theofb -sitesthe b-sites (opaque, (opaque, orange, orange, 25% 25% of of cation cation sites) sites) andand the dd-sites-sites (transparent, (transparent, blue, blue, 75%). 75%). A thorough understanding of how multiple constituents occupy the two nonequivalent cation A thorough understanding of how multiple constituents occupy the two nonequivalent cation sites in bixbyite can provide information on how to optimize the cosubstituted systems to increase sites in bixbyite can provide information on how to optimize the cosubstituted systems to increase tin solubility while maintaining its dopant nature. Herein, the cosubstituted bixbyite system tin solubility while maintaining its dopant nature. Herein, the cosubstituted bixbyite system MxIn2−2xSnxO3, with M = Mg or Zn, is evaluated and the site occupancies and electronic properties are MxIn2compared−2xSnxO 3to, witheach Mother = Mg and or reported Zn, is evaluated cosubstituted and systems. the site occupanciesThe ZnxIn2−2xSn andxO electronic3 system (commonly properties are comparedreferred to eachto as either other ZITO and reported or IZTO) cosubstitutedis widely studied systems. as a transparent The ZnxIn conductor2−2xSnxO and3 system the electronic (commonly referredproperties to as either have been ZITO previously or IZTO) reported, is widely but studied here it is as investigated a transparent with conductora computational and theapproach electronic propertiesthat examines have been local previously site structure reported, [6,11,14–19]. but here MgxIn it2 is−2xSn investigatedxO3, in contrast, with has a computational been the subject approach of a single bulk study and a thin film study and here is the subject of a more thorough structural and that examines local site structure [6,11,14–19]. MgxIn2−2xSnxO3, in contrast, has been the subject of a singleproperty bulk studystudy and[5,20]. a thinLocal film coor studydination and environments here is the subject and site of aoccupancies