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Structure and microstructure of Sn-doped indium oxide (ITO)

Tin doped indium oxide (ITO) is a transparent conductive oxide (TCO) with excellent electrical and optical properties. Both indium oxide (In2O3) and ITO crystallize in a cubic bixbyite-type structure, space group Ia. The unit cell contains 80 atoms, where 32 sites are occupied by cations in two non-equivalent six-fold coordinated sites (B and D sites, respectively). Structural changes in indium oxide lattice due to tin doping were studied by XRD. The local environment of Sn inside a host lattice was investigated by 119Sn Mössbauer spectroscopy. Microstructure was studied by means of TEM and XRD line broadening analysis. Nanocrystalline ITO samples containing 0-14 at% Sn were prepared by a sol-gel technique. Unit-cell parameter a of samples was determined by UNITCELL program and refined by WPPF program. The unit-cell parameter increased with tin doping level up to 7.8 at% Sn and decreased at higher levels. This suggested that Sn substitution for In on B and D sites of the original In2O3 structure is non-uniform and dependant on Sn content. Rietveld refinement showed the presence of interstitial oxygen in the ITO samples. The position of interstitial oxygen indicated on D site preference for Sn at low doping level and subsequent increase in B site occupancy with the increase in Sn content, which agreed well with the 119Sn Mössbauer spectroscopy results. Simultaneously with the Rietveld structure refinement, the analysis of diffraction line broadening was carried out using Si powder as a standard for instrumental line broadening. The crystallite size decreased and lattice strain increased with the increase in Sn doping level. The particle sizes as determined by TEM followed the behavior of crystallite sizes obtained by XRD line broadening analysis.

tin-doped indium oxide; X-ray powder diffraction; rietveld method

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