Doping Mechanisms of Sn in In₂O₃ Powder Studied Using ¹¹⁹Sn Mössbauer Spectroscopy and X-Ray Diffraction

The doping mechanism of Sn in In₂O₃ (ITO) powder was investigated using ¹¹⁹Sn transmission Mössbauer spectroscopy (TMS) and X-ray diffraction (XRD) in view of the chemical state of Sn. Deconvolution analyses of TMS spectra revealed that there was substitutional Sn⁴⁺ coordinated not only by 6 oxygen atoms [Sn_In^·] but also by 7 or 8 oxygen atoms, for the samples with doping concentrations higher than 5 at.%. The amount of such electrically deactivated Sn⁴⁺ (coordination number of 7 or 8) increased with increasing doping concentration, which was quantitatively consistent with the decrease in doping efficiency. Precise XRD analyses indicated a systematic increase in the lattice constant with increasing doping concentration from 0.5 to 7 at.%. The increase in lattice constant was explained in terms of a repulsive force among tetravalent [Sn_In^·] (coordination number of 6) with higher effective charge than In³⁺, which was also consistent with the results on the coordination of Sn⁴⁺ obtained through TMS analyses.