Exploring the links between crystal chemistry, cesium retention, thermochemistry and chemical durability in single-phase (Ba,Cs)1.33(Fe,Ti)8O16 hollandite

A series of single-phase Fe-substituted hollandite (Ba,Cs)(1.33)(Fe,Ti)(8)O-16 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.