A-site compositional effects in Ga-doped hollandite materials of the form BaxCsyGa2x+yTi8−2x−yO16: implications for Cs immobilization in crystalline ceramic waste forms

Abstract

The hollandite structure is a promising crystalline host for Cs immobilization. A series of Ga-doped hollandite BaxCsyGa2x+yTi8−2x−yO16 (x = 0, 0.667, 1.04, 1.33; y = 1.33, 0.667, 0.24, 0) was synthesized through a solid oxide reaction method resulting in a tetragonal hollandite structure (space group I4/m). The lattice parameter associated with the tunnel dimension was found to increases as Cs substitution in the tunnel increased. A direct investigation of cation mobility in tunnels using electrochemical impedance spectroscopy was conducted to evaluate the ability of the hollandite structure to immobilize cations over a wide compositional range. Hollandite with the largest tunnel size and highest aspect ratio grain morphology resulting in rod-like microstructural features exhibited the highest ionic conductivity. The results indicate that grain size and optimized Cs stoichiometry control cation motion and by extension, the propensity for Cs release from hollandite.

An Interdisciplinary View of Interfaces: Perspectives Regarding Emergent Phase Formation

“…Hollandite-type structures can exist in both tetragonal (I4/m) and monoclinic (I2/m) phases, exhibiting tunnels formed by linked oxygen octahedra. The size and geometry of these tunnels serve to facilitate or impede the mobile alkali and alkaline-earth ions that reside on the A-site in these tunnels [11,51]. The polycrystalline hollandites were originally investigated as sodium and potassium ion conductors exhibiting ion conductivity on the order of magnitude of 10 À4 (S/cm) at 300 C. Later studies evaluated lithium ion insertion into hollandites with multivalent M site dopants such as Fe, Mn for use as cathode materials in lithium ion batteries [52].…”

“…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.…”

“…High Cs-content compositions revealed large rod-like grains and the largest measured total conductivity by impedance methods. Using equivalent circuit modeling to separate bulk versus grain boundary effects in these materials indicated bulk conductivity was similar at high and low Cs loadings; while interfacial resistance was greatest at fine grained (low Cs compositions) [11]. The behavior of the atomistic level tunnel size and the microstructure dependence on cesium concentration is depicted in Fig.…”

“…Garnet structures currently being evaluated for solid-state lithium battery electrolytes such as Li 7 La 3 Zr 2 O 12 (LLZO) are similar to based garnets being evaluated for nuclear waste immobilization of lanthanide fission products [8,9]. Finally, tunnel structured materials such as hollandites which incorporate mobile alkali ions such as Li, K, and Na in tunnels have been looked at as candidate battery electrode materials [10]; current work in solid state materials synthesis in the nuclear materials realm seeks to engineer the tunnels to block the motion of larger alkali ions such as Cs for immobilization applications [11]. These examples highlight the importance of an interdisciplinary approach; learning how researchers in one application area "promote" transport, may be used in other disciplines interested in "blocking"transport.…”

Temperature‐dependent electrical transport behavior and structural evolution in hollandite‐type titanium‐based oxide

“…[18][19][20] Moreover, hollandite-type oxides also exhibit good performance in the catalysis field. 25 The performance of a secondary battery is highly dependent on the ionic and electronic conductivities of the electrode. For instance, the Cs ionic conductivity is an important indicator of the suitability of hollandites to immobilize reprocessed nuclear waste.…”