A-site occupancy effects on structure, ionic conductivity, and thermodynamic stability of K_xMg_x/2Ti_8−x/2O₁₆

K_xMg_x/2Ti_8−x/2O₁₆ (1.54 ≤ x ≤ 2) hollandite samples synthesized using a conventional solid-state route were used to investigate the effect of structure, ionic conductivity, and thermodynamic stability as a function of A-site (K) occupancy. The ionic conductivity, as measured using AC impedance spectroscopy, decreased as the A-site occupancy (and K content) increased. Titanate hollandite, of the form K_xMg_x/2Ti_8−x/2O₁₆ , was measured using high-temperature oxide melt solution calorimetry in sodium molybdate (3Na₂O \(\bullet\) $\bullet$ 4MoO₃) solvent at 804 \(^\circ{\rm C}\) ${}^{\circ }\mathrm{C}$ . The enthalpies of formation are strongly exothermic indicating that they are thermodynamically stable relative to their constituent oxides. Previous studies have demonstrated increased stability with an increase in Cs content at fixed A-site occupancy. This work demonstrated an increase in the stability for the K-bearing hollandite samples with an increase in K content and tunnel occupancy. The measured enthalpies of formation (∆H_f,ox) were in good agreement with density functional theory (DFT) predictions.