Tin-rich nano-fluorite evolution in bixbyite indium tin oxide ceramics

This study presents an unprecedented experimental observation of fluorite-structured nano-precipitates (NPs) within bulk ITO grains. These NPs, containing higher Sn concentrations than the ITO matrix, form coherent interfaces with the bixbyite (ITO matrix) structure, representing nanoscale compositional heterogeneities that have remained undetected. The NPs form through charge compensation, as local Sn enrichment drives oxygen interstitials to fill intrinsic oxygen vacancies of the bixbyite structure, and their features can be tuned via reduction treatment or heat treatment temperature. EXAFS analysis of samples with high NP density revealed elevated coordination numbers for Sn ions, supporting the presence of oxygen interstitials. Formation of NPs occurs during post-sintering cooling when Sn ions, initially dissolved at high temperatures, are expelled due to decreased solubility. While 2Sn_In^•O_i^″⁢ defect clusters in ITO have been previously discussed only through theoretical predictions, this study provides the experimental observation of such clusters in the form of NPs.