Unraveling Cs substitution‐induced evolution in Fe‐hollandite: Linking crystal chemistry to leaching resistance

Abstract This study systematically explored the crystal chemistry of Fe‐substituted hollandite (Fe‐hollandite) solid solutions (Ba x Cs y )(Fe 2 x + y Ti 8−2 x − y )O 16 ( x + y = 1.33) yielding reliable results through meticulous Rietveld analysis of XRD data. Continuous Cs substitution triggered a monoclinic‐tetragonal (M‐T) phase transition, enhancing structural stability, as evidenced by increased tolerance factor ( t H ). The transition boundary, controlled by t H = .985, determines the symmetry of the hollandite phase. Unit cell parameters and the local bonding environment were refined, indicating enlarged tunnel sizes/cavities with higher Cs content, attributed to a larger ionic radius (Cs + vs. Ba 2+ ). Comparison between Cs/Ba‐O bond distances and tunnel cross‐section sizes confirmed effective A‐site cation immobilization, hindered by oxygen bottlenecks. Additionally, a refined model was proposed and validated to predict lattice constants of Fe‐hollandite precisely. Beyond crystal chemistry evolution with Cs substitution, leaching resistance in the same sample suites was assessed. As Cs content increased, normalized elemental release of A‐site cations initially decreased until reaching a minimum at intermediate Cs substitution, then rose again. Ba 0.33 Cs 1.0 Fe 1.66 Ti 6.34 O 16 (i.e., HF5) displayed optimal leaching resistance. Variations in leaching resistance might be influenced by the distortion in [BO 6 ] octahedra and the bonding environment of tunnel cations. This comprehensive analysis highlights intricate correlations between crystal chemistry and leaching resistance, offering insights to guide the design of advanced crystalline nuclear waste forms with enhanced corrosion resistance.