Structural Evolution in Hollandite Solid Solutions Across the A‐Site Compositional Range from Ba_1.33Ga_2.66Ti_5.34O₁₆ to Cs_1.33Ga_1.33Ti_6.67O₁₆

Abstract

Hollandite solid solutions along the A‐site compositional range from the pure barium end‐member Ba1.33Ga2.66Ti5.34O16 to the pure cesium end‐member Cs1.33Ga1.33Ti6.67O16 have been synthesized using a solid‐state reaction technique. The crystal structure of the hollandite across the entire compositional range remained in the I4/m space group. Structural evolution was resolved by neutron diffraction, total scattering data, and density functional theory calculations. A trend of decreasing thermodynamic stability with smaller tunnel cations was attributed to increased structural distortion observed in the system. In addition, the tunnel cations' local environment was studied in the eightfold coordinated oxygen cavities. Local binding features of the tunnel cations reveals that the hollandite structure can strongly stabilize tunnel cations, even at elevated temperatures up to 500 K.

Thermodynamic assessment of the hollandite high‐level radioactive waste form

“…Again, the reported 1473 K temperature at which the cooling rate of melt processed samples started slowing was adopted as the equilibrium temperature. Equilibrium calculations using assessed thermochemical models and values predict the hollandite phase is stable for the experimental compositions of Amoroso et al, Xu et al, Aubin‐Chevaldonnet et al, and Costa et al (Table ).…”

“…Ti‐substituted hollandite, one of the SYNROC phases, is an alternative candidate for Cs immobilization. In these waste forms, 137 Cs (and other constituent radionuclides, ie 137 Ba, 87 Rb) is incorporated into the crystalline structure . Notably, natural analogs of hollandite including ankagite are present in dolomitic marble in the Apuan Alps in Tuscany, Italy, which demonstrates the stability of the hollandite phase over geologic timescales of interest for nuclear waste immobilization.…”

“…Studies have been conducted to analyze the effect of M‐site substitution on the crystallographic structure of hollandite and Cs incorporation . Costa et al, for instance, determined that hollandite thermodynamic stability generally increased with decreasing average M‐site cation radius while Aubin‐Chevaldonnet et al demonstrated that various M‐site substitutions for Ti +4 affects the fraction of Cs incorporated into the hollandite tunnel sites.…”

“…Similarly, the reported 1473 K temperature at which the cooling rate of melt processed samples started slowing was adopted as defining the equilibrium state and, as such, calculations for comparison with melt processed sample results were conducted at this temperature. Xu et al, Aubin‐Chevaldonnet et al, and Costa et al synthesized hollandites with the additives Ga, Al, Cr, and Fe, hence the database was developed to include the oxides of these additives.…”

An Interdisciplinary View of Interfaces: Perspectives Regarding Emergent Phase Formation

“…More recent work has demonstrated the stability of high Cs containing compositions. Structural studies demonstrated the higher Cs loading resulting in a framework expansion in the direction perpendicular to the tunnel axis due to the incorporation of the larger Cs ions, resulting in less distortion in the oxygen octahedra and a more symmetric structure [11,56]. Higher symmetry structures resulted in a more stable hollandite in terms of formation enthalpy with respect to their constituent oxides.…”