Synthesis and characterisation of transition metal substituted barium hollandite ceramics
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Synthesis and characterisation of transition metal substituted barium hollandite ceramics Neil C. Hyatt1*, Martin C. Stennett,1 Steven G. Fiddy,2 Jayne S. Wellings,1 Sian S. Dutton,1 Ewan R. Maddrell3, Andrew J. Connelly1 and William E. Lee1. 1
Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD. UK.
2
CCLRC Daresbury Laboratory, Warrington, WA4 4AD. UK.
3
Nexia Solutions Ltd., Sellafield, Seascale, Cumbria, CA20 1PG. UK.
ABSTRACT A range of transition metal bearing hollandite phases, formulated Ba1.2B1.2Ti6.8O16 (B2+ = Mg, Co, Ni, Zn, Mn) and Ba1.2B2.4Ti5.6O16 (B3+ = Al, Cr, Fe) were prepared using an alkoxide - nitrate route. X-ray powder diffraction demonstrated the synthesis of single phase materials for all compositions except B = Mn. The processing conditions required to produce > 95 % dense ceramics were determined for all compositions, except B = Mg for which the maximum density obtained was > 93 %. Analysis of transition metal K-edge XANES data confirmed the presence of the targeted transition metal oxidation state for all compositions except B = Mn, where the overall oxidation state was found to be Mn3+. The K-edge EXAFS data of Ba1.2B1.2Ti6.8O16 (B = Ni and Co) were successfully analysed using a crystallographic model of the hollandite structure, with six oxygen atoms present in the first co-ordination shell at a distance of ca. 2.02Å. Analysis of Fe K-edge EXAFS data of Ba1.2B2.4Ti5.4O16 revealed a reduced co-ordination shell of five oxygens at ca. 1.99Å. INTRODUCTION Several Synroc formulations have been developed for the immobilisation of the High Level nuclear Waste (HLW) stream, arising from reprocessing of spent nuclear fuel, comprising the fission products and minor actinides, together with corrosion products and fuel cladding elements [1]. The reference formulation of Synroc comprises an assemblage of four titantate minerals, zirconolite (CaZrTi2O7), hollandite (Ba1.2(Al,Ti)8O16), perovskite (CaTiO3) and titanium oxide(s) (TinO2n-1). In this multi-phase assemblage, hollandite functions as the host phase for large fission product cations such as Rb+, Cs+ and Ba2+ [1]. Barium titanate hollandites have also attracted interest as single phase hosts for the conditioning of 134Cs and 137Cs isotopes arising from chemical separation of minor actinides (Np, Am, Cm) and long lived fission products (Cs, I, Tc) from HLW [2]. The crystal structure of hollandite, general formula AxB’yB”8-yO16, comprises corner and edge shared (B’,B”)O6 octahedra that link to form a system of tunnels containing the A cations in distorted 8-fold co-ordination, where A is a large mono- or divalent cation and B is a small di-, tri-, tetra- or pentavalent cation, see Figure 1. In this study, A = Ba, B” = Ti and B’ = Mg, Al, Cr, Mn, Fe, Co or Ni. The archetype hollandite structure is tetragonal (space group I4/m) and is generally adopted by compositions with large A cations and
small B cations, typically with a radius ratio rB / rA < 0.48 Å [3].
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