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.
Highlights
One of the major challenges confronting the nuclear power industry is to provide an enduring solution to the problem of high-level waste disposal[1]
Ba- or Cs-hollandites were found to have low tunnel cation mobility and have the potential to be a stable waste form[21,22]. Those impedance studies were limited to baseline compositions with a fixed A-site stoichiometry of Ba1.04Cs0.24, which has been the established SYNROC composition used in ceramic waste forms for several decades
This peak is presumed to be associated with a Ti-rich phase devoid of Cs that was identified by detailed scanning electron microscopy (SEM)-energy dispersive X-ray analysis (EDX) analysis
Summary
One of the major challenges confronting the nuclear power industry is to provide an enduring solution to the problem of high-level waste disposal[1]. A number of studies have been conducted on how the M-site dopant affects microstructure and Cs incorporation[14] Among those cations studied (Al3+, Cr3+, Ga3+, Fe3+), Ga3+ demonstrates the ability to lower the melting point of synthetic hollandite and provide redox stability, which is of interest to current US DOE efforts in waste forms aimed at melt processing[15,16,17,18]. Hollandite-type structures have been widely studied for their potential as fast-ionic conductors due to the high mobility of A-site cations in tunnels[19,20]. Ba- or Cs-hollandites were found to have low tunnel cation mobility and have the potential to be a stable waste form[21,22] Those impedance studies were limited to baseline compositions with a fixed A-site stoichiometry of Ba1.04Cs0.24, which has been the established SYNROC composition used in ceramic waste forms for several decades. A baseline hollandite composition Ba1.04Cs0.24 was included to compare the results with previous studies
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