[en] 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#Minus Sign#y/2}O₁₆ (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.

[en] Highlights: • Cesium incorporation increased hollandite's radiation and leaching stability. • Critical dose for amorphization doubled after full cesium substitution. • Critical amorphization temperature for hollandite was between 200 and 300 °C. • Irradiation doubled fractional cesium released during leaching. - Abstract: The radiation damage tolerance of nuclear waste forms is dependent on the material's resistance to defect formation and its ability to accommodate structural distortions that arise from defect creation. This study illustrates how the radiation tolerance of hollandite can be improved thorough compositional control of cesium stoichiometry. A hollandite series with the general form Ba_xCs_yZn_x+y/2Ti_8-x-y/2O₁₆ (0 2+) irradiation at 27 °C, 100 °C, 200 °C and 300 °C followed by characterization with grazing incidence X-ray diffraction, transmission electron microscopy, and aqueous leaching tests. After exposure to 400 keV or 1 MeV Kr²⁺ irradiation, hollandite exhibited an onset of amorphization near 0.14 dpa and full amorphization ranging from 0.21 to 0.54 dpa depending on the cesium content. The radiation tolerance increased at elevated temperatures with a critical amorphization temperature between 200 °C and 300 °C. Elemental leaching decreased with increasing cesium content. Irradiated samples exhibited twice the fraction of cesium release compared to pristine samples. Experimental results also showed that cesium release from irradiated samples was at a minimum for the Ba_0.33Cs_1.00Zn_0.83Ti_7.17O₁₆ sample.