Effects of composition and frozen-in conditions on bulk and grain boundary conductivities of Yb2Ti2O7-based materials

The present work provides a comprehensive interpretation of factors which affect cation and/or anion disordering in Yb2Ti2O7-based materials, and their effects on transport properties. It also provides guidelines for further improvements of transport properties. Powder processing and thermal history may play major roles, especially when anti-site cation exchange is likely to occur at typical sintering temperatures, as confirmed by structural refinement. One found that the cooling rate may also determine changes in frozen-in conditions. Excessive sintering temperatures affect both the bulk and grain boundary properties of samples with composition Yb2Ti2O7; this was ascribed to the formation of a Ti-rich amorphous phase, and corresponding excess of Yb in the bulk phase. Anti-site occupancy of B-site positions was found to affect ionic conductivity, as demonstrated for conditions that cause cation heterogeneities in Yb2Ti2O7, and also by analysing conductivity data for compositions with excess of Yb. The conductivity data obtained for Yb2 + xTi2 − xO7 − d become less dependent on sample preparation, possibly because deliberate composition changes play a prevailing role, compared to intrinsic cationic and anionic disorders. Samples with slight excess of Ti retain prevailing ionic conductivity, which differs significantly from the effects of a typical donor additive on ionic and electronic conductivities.