Compositional control of tunnel features in hollandite-based ceramics: structure and stability of (Ba,Cs)_1.33(Zn,Ti)₈O₁₆

The impact of composition on the tunnel features of hollandite materials for the purpose of radioactive cesium (Cs) immobilization was evaluated. The barium (Ba) to cesium (Cs) ratio was varied in the tunnel sites referred to as the A-site of the hollandite structure. Zinc (Zn) was substituted for titanium (Ti) on the B-site to achieve the targeted stoichiometry with a general formula of Ba_xCs_yZn_x+y/2Ti_8−x−y/2O₁₆ (0 < x < 1.33; 0 < y <1.33). The tunnel cross-section depended on the average A-site cation radius, while the tunnel length depended on the average B-site cation radius. Substitution of Cs resulted in a phase transition from a monoclinic to a tetragonal structure and an increase in unit cell volume of 1.8% across the compositional range. Cs loss due to thermal evaporation was found to decrease in compositions with higher Cs content. The enthalpies of formation from binary oxides of Zn-doped hollandite measured using high-temperature oxide melt solution calorimetry were strongly negative, indicating thermodynamic stability with respect to their parent oxides. The formation enthalpies became more negative, indicating hollandite formation is more energetically favorable, when Cs was substituted for Ba across the range of Zn-doped compositions investigated in this study. Compositions with high Cs content exhibited lower melting points of approximately 80 °C. In addition, high Cs content materials exhibited a significant reduction in Cs release from the solid to liquid phase by leaching or aqueous corrosion as compared to low Cs content materials. These property changes would be beneficial for applications in radioactive cesium immobilization in a multi-phase ceramic by allowing for decreased processing temperatures and higher cesium weight loadings. More broadly, these results establish the link between composition, structural symmetry, and thermodynamic stability for tunnel structured ceramics with implications in the design of new energy conversion and storage materials.