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Defect structure examination of Sn-doped indium oxide (ITO)

Tin doped indium oxide (ITO) is one of the most commonly used transparent conducting oxides (TCOs). It is used in various commercial applications: transparent electrodes in solar collectors and solar cells, transparent heating elements for aircraft windows, for antistatic coating on instrument windows and in production of flat-panel displays. However, the structure of ITO is not completely understood yet, nor the correlation between the structure and its properties. In_2O_3 crystallizes in Ia-3 space group. It is isostructural with mineral bixbyite. The structure possesses two octahedral cation sites, denoted as B and D sites. Structural changes in indium oxide lattice due to doping with tin were studied by X-ray powder diffraction. The local environment of tin in ITO was investigated by ^(119)Sn Mössbauer spectroscopy. Nanocrystalline ITO samples containing 0-14 at% Sn were prepared by a sol-gel technique from InCl_3 and SnCl_4, followed by thermal treatment at 300 oC for 2 h. Diffraction patterns confirmed the In_2O_3-type structure. Diffraction lines were broadened, the line broadening increased with tin content. Unit-cell parameter was determined by UNITCELL program and refined by WPPF program. Unit-cell parameter increased with tin doping level up to 8 at% Sn and decreased above. Such behavior of the unit-cell parameter indicated that tin substitution for indium on sites B and D is non-uniform and is dependent on tin content. ^(119)Sn Mössbauer spectra showed that tin is present only in Sn^(4+) state in the samples. The best fits of ^(119)Sn Mössbauer spectra were obtained by assuming the presence of two doublets, corresponding to Sn^(4+) on B and D sites of the original In_2O_3 structure. The pronounced difference was observed in the relative area of the two doublets. The ratio of relative area of component D over relative area of component B decreased with the increase in tin content. For the sample containing 8 at% Sn this ratio was equal to unity. It was supposed that non-uniform tin occupation of sites B and D influenced crystallite sizes and lattice strain for ITO, so the analysis of diffraction line broadening was undertaken. Size-strain line-broadening analysis showed that crystallite sizes decreased and strain increased with increasing tin content.

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

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