Crystal Chemistry and Leaching Behavior of Sn-Based Hollandite Waste Forms for Cs Immobilization

Sn-containing hollandite has been highlighted by recent machine learning-guided design as a promising host for radioactive cesium (Cs), yet waste form validation remains limited. Here, a Cs-bearing Sn-based hollandite series, (Ba,Cs) 1.66 (Fe,Sn) 8 O 16 , was synthesized in air by solid-state reaction, with hollandite formation at 1125°C followed by pressureless sintering at 1200°C. x-Ray diffraction confirms hollandite as the major phase across the series, with minor Ba-bearing secondary phases only at low Cs contents. Rietveld refinement reveals anisotropic lattice expansion and a tetragonal-to-monoclinic symmetry change at the Cs-rich end member, accompanied by progressive tightening of tunnel descriptors, indicating a Sn-specific Cs accommodation response distinct from many Ti-based hollandites. Chemical durability was evaluated by monolithic leaching in deionized water at 90°C. At Cs loadings up to ∼17 wt%, 7-day Cs release is comparable to representative Ti-based hollandites tested under similar conditions. Sn exhibits ultralow normalized release, while Cs and Ba show higher release without detectable degradation of the hollandite matrix. Extended 28-day interval leaching on Ba 0.8 Cs 0.86 Fe 2.46 Sn 5.54 O 16 (HS2) and Ba 0.4 Cs 1.26 Fe 2.06 Sn 5.94 O 16 (HS3) separates rapid early wash-off from slower, diffusion-controlled release. By 14–28 days, Cs and Ba leaching rates converge to ∼2–4 × 10 −2 g·m −2 ·day −1 , with effective diffusion coefficients of ∼1–2 × 10 −11 cm 2 /s and leaching indices of ∼11. Benchmarking against Ti-based hollandites places the Sn-based series within a competitive loading–durability window under a comparatively mild processing route, supporting Sn-based hollandite as a readily fabricated candidate for high-loading Cs immobilization.