Multivalency in Zr₆Nb₂O₁₇

We use an aliovalent cation co-substitution approach to study defect formation tendencies and stability limits in Zr₆Nb₂O₁₇. We choose In³⁺, Sc³⁺, Y³⁺, and Ga³⁺ to probe a spectrum of ionic size mismatches relative to Zr⁴⁺ and Nb⁵⁺. Solubility trends track well with expectations based on radii matching. We stabilize In³⁺ and Sc³⁺ in the A₆B₂O₁₇ phase when co-substituting with Nb⁵⁺ at several mol%; these trivalent cations are not dissolved when directly substituted without the charge-compensating pentavalent species. Our results suggest that charge compensation by holes, cation interstitials, or anion vacancies in response to aliovalent cation doping is energetically unfavorable and limits solubility, thus requiring extrinsic charge-compensation through co-substitution to accommodate the trivalent species at several mol% concentrations.