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Interstitial oxygen in nanocrystalline Sn-doped indium oxide

Tin doped indium oxide (ITO) exhibits high electrical conductivity, high transparency in visible range and high infrared reflectivity for wavelengths higher than 1 micro m. ITO has extensive commercial applications, including transparent heating elements of aircraft and car windows, antistatic coatings on electronic instrument display panels, transparent electrodes in solar collectors and solar cells [1]. Both In_2O_3 and ITO crystallize in cubic bixbyte structure, Ia-3 space group [2]. The In_2O_3 lattice is similar to that of fluorite, but only three-fourths of the anions found in fluorite are present. The unit cell contains 80 atoms. The 32 cations occupy the 8b and 24d positions, referred to as B and D positions. The 48 anions occupy the general positions 48e. Each cation resides at the center of a distorted cube, with six corners occupied by oxygen anions. The remaining two corners are empty and they are the possible locations for oxygen interstitial anions. In_2O_3 samples doped with 0-14 at% Sn were prepared by a sol-gel technique from InCl_3 and SnCl_4 reagent grade chemicals, followed by thermal treatment at 300 ^oC for 2 h. The samples were examined by X-ray powder diffraction and 119Sn Mössbauer spectroscopy. Diffraction patterns confirmed the bixbyte-type structure for all samples. Diffraction lines were broadened, the line broadening increased with tin content. Unit-cell parameters increased with tin doping level up to 8 at% Sn and decreased above indicating the non-uniform tin substitution on the B and D sites. The 119Sn Mössbauer spectra showed that tin is present only in Sn^4+ state. The best fits of 119Sn Mössbauer spectra were obtained by assuming the presence of two doublets, corresponding to B and D positions in cation sublattice of the In_2O_3 structure. The ratio of relative area of component corresponding to D site over relative area of component corresponding to B site decreased with increase in tin content. For all samples the Rietveld structure refinement was performed. The difference between the indium and tin scattering factors is too low to allow discriminating indium from tin sites. Nevertheless, the overall occupancy of the interstitial oxygen site was determined. It increased with tin doping level. The oxygen interstitial anions compensate the charge of ionized tin donors [3]. On the other hand, they cause a distorsion of the In_2O_3 structure [4]. Size-strain line-broadening analysis showed that crystallite sizes decreased and strain increased with increase of tin content. [1] Chopra, K.L., Major, S., Pandya, D. K., Thin Solid Films. 102 (1983) 1. [2] Marezio, M., Acta Cryst. 20 (1966) 723. [3] De Witt, J. H. W., J, Solid State Chem. 20 (1977) 143. [4] Quaas, M., Eggs, C., Wulff, H., Thin Solid Films. 332 (1998) 277.

tin doped indium oxide (ITO); X-ray powder diffraction; Rietveld method

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