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Titanate‐based hollandite ceramics are promising nuclear waste forms for Cs immobilization. In this work, a series of Al‐substituted hollandite (Ba,Cs)1.33(Al,Ti)8O16 was investigated across a broad compositional range with varying Cs content. Powder X‐ray diffraction showed that all samples exhibited a tetragonal hollandite phase. Enthalpies of formation determined by high‐temperature melt solution calorimetry indicated enhanced thermodynamic stability with increased Cs content, which generally agreed with sublattice‐based thermodynamic calculations. Moreover, enthalpies of formation of the samples were primarily affected by three factors: (a) relative sizes of cations on the A‐sites and B‐sites, (b) tolerance factor, and (c) optical basicity. Fractional element release revealed that Cs retention was significantly improved for the high Cs‐containing hollandite compositions, which were supported by the evolution of microstructure of the pre and postleach particles. Elution studies of Al‐substituted hollandite spiked with radioactive 137Cs indicated that transmutation of Cs to Ba in the hollandite was accompanied by an increase in the retention of the Cs decay product, suggesting long‐term stability of Al‐substituted hollandite phase. [ABSTRACT FROM AUTHOR]

A series of single-phase Fe-substituted hollandite (Ba,Cs)1.33(Fe,Ti)8O16 compositions with the chemical formula Ba1.33−xCsxFe2.66−xTi5.34+xO16 (x = 0, 0.1, 0.2, 0.667, and 1.33) were systematically investigated using both experimental and computational methods to establish possible links between crystal chemistry, Cs retention, thermochemistry and chemical durability. A phase transition from monoclinic to tetragonal was observed as a function of both Cs content and temperature. Elemental analysis revealed that Cs retention was significantly improved for the hollandite with higher Cs content. High-temperature melt solution calorimetry and sublattice-based thermodynamic simulations confirmed a high degree of thermodynamic stability in the Fe-substituted compounds which was enhanced in compositions with higher Cs content. This trend can be primarily attributed to two factors: (1) a decreasing ratio of the average ionic radii of B-site cations to that of A-site cations and (2) an increasing tolerance factor. Based on a reoptimized sublattice model, a pseudo-ternary phase diagram was generated to predict the optimal composition possessing the highest Cs content and A-site occupancy, which had not been experimentally explored. Leaching tests further verified that high Cs-containing compositions have significant chemical durability. [ABSTRACT FROM AUTHOR]

Hollandite materials, as a class of crystalline nuclear waste forms, are promising candidates for the immobilization of radioactive elements, such as Cs, Ba, as well as a variety of lanthanide and transition‐metal fission products. In this study, three Ga‐doped titanate hollandite‐type phases, Ba1.33Ga2.67Ti5.33O16, Ba0.667Cs0.667Ga2Ti6O16, and Cs1.33Ga1.33Ti6.67O16, were synthesized using a solid‐state reaction route. All synthesized phases adopted a single phase tetragonal structure, as determined by powder X‐ray diffraction (XRD), and elemental analysis confirmed the measured stoichiometries were close to targeted compositions. Extended X‐ray absorption fine structure spectroscopy (EXAFS) was used to determine the local structural features for the framework of octahedrally coordinated cations. EXAFS data indicated that Cs1.33Ga1.33Ti6.67O16 possessed the most disordered local structure centered around the Ga dopant. The enthalpies of formation of all three hollandite phases measured using high‐temperature oxide melt solution calorimetry were found to be negative, indicating enthalpies of formation of these hollandites from oxides are thermodynamically stable with respect to their constituent oxides. Furthermore, the formation enthalpies were more negative and hence more favorable with increased Cs content. Finally, aqueous leaching tests revealed that high Cs content hollandite phases exhibited greater Cs retention as compared to low Cs content hollandite. While preliminary in nature, this work draws attention to the links between the capacity for radionuclide retention, atomistic level structural features and bulk thermodynamic properties of materials. [ABSTRACT FROM AUTHOR]