Quantitative Determination of Charge Transport and Interstitial Alkali-Ion Migration in Alkali-Metal-Substituted Scheelite-Type CaWO4

Alkali-metal ion substitution has emerged as a promising strategy for engineering advanced solid-state ionic conductors. In this work, a series of A-site-substituted CaWO4 ceramics (A = Na, K, Cs; 5% nominal substitution) was synthesized via solid-state reaction. A systematic investigation of phase formation, structural stability, and electrical properties reveals that partial substitution of Ca2+ by alkali ions enhances the total conductivity by one to 2 orders of magnitude. Quantitative transference-number analysis demonstrates that charge transport at 800 °C is dominated by alkali-cation migration in the Na- and K-substituted samples, with estimated cationic contributions of approximately 87% and 83%, respectively, whereas the Cs-substituted sample exhibits comparable cationic and electronic contribution. Comparative defect-energy calculations indicate that Na-related interstitial configurations are energetically more accessible than the corresponding K- and Cs-related configurations among the examined models. AIMD simulations further visualize a continuous Na+ hopping pathway through the available interstitial space of the scheelite framework. These results suggest that alkali-cation migration, rather than oxygen-vacancy migration, is primarily responsible for the enhanced conductivity in Na- and K-substituted CaWO4. These results indicate the size-dependent regulation of charge carriers in alkali-metal-substituted CaWO4 and provide mechanistic insights for the rational design of tungstate-based mixed ion–electron conductors.