Structural Studies of Hollandite-Based Radioactive Waste Forms
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6WUXFWXUDO6WXGLHVRI+ROODQGLWH%DVHG5DGLRDFWLYH:DVWH)RUPV K.R. Whittle, S.E. Ashbrook, S.A.T. Redfern, G.R. Lumpkin, J.P. Attfield1, M. Dove, I. Farnan Cambridge Centre for Ceramic Immobilisation – C3L, Dept Of Earth Sciences, University of Cambridge, Cambridge, United Kingdom. 1 Dept of Chemistry, University of Cambridge, Cambridge, United Kingdom $%675$&7 Hollandites with compositions Ba1.2-xCsxMg1.2-x/2Ti6.8+x/2 O16, and Ba1.2-xCsxAl2.4-xTi5.6+xO16 (x=0, 0.1, 0.25) have been synthesised using a modified alkoxide/acetate precursor route. The samples have been sintered using two procedures; hot isostatic pressing and sintering at ambient pressure. X-ray powder diffraction has shown samples from both systems to form tetragonal hollandites, with little change when pressed by both methods. Cs-133 MAS NMR spectra have been recorded showing the chemical shift in Al containing samples to be ~250ppm, and in Mg hollandites ~175ppm and 200ppm, with little change when prepared by both methods. ,1752'8&7,21 In the field of nuclear waste immobilisation the safe storage of active Cs-135 and Cs-137 is essential, as the Cs+ tends to be soluble under most conditions [1]. Systems based on hollandite, AxB8O16, have been selected as the ‘wasteform of choice’ and are part of the Synroc assemblage [1,2]. The hollandite type structure is based on octahedra, in these samples Ti-O, which share edges and corners forming tunnels. The A cation (Ba,Cs) is located within the tunnels, (Figure 1). The structure can either be monoclinic[3], e.g. Ba1.2Mg1.2Ti6.8O16, or tetragonal[4] in nature e.g. Ba1.121Al2.24Ti5.76O16, essentially the difference is due to variations in A/B cations radius ration, causing a shear-type collapse of the tunnel and a reduction in symmetry (I4/m &P The hollandite structure can accommodate a variety of atoms on both the A and B sites, e.g. on the A site Ba, Na, and K; on the B site it is possible to mix cations such as Mg, Ti, Al, and Zr. In the area of nuclear waste immobilization it is routine to base the hollandite on Ti[1,5-8], e.g.Ba1.2Mg1.2Ti6.8O16. The use of Ti is important because, as Cs+ undergoes β-decay forming Ba2+ a charge imbalance results, in order to compensate for this a Ti4+ cation in the lattice undergoes reduction to Ti3+ conserving charge balance. Cs+ can be immobilized in hollandites that contain Al3+ and Mg2+[9], in these systems the 3+ Al and Mg2+ are present to ensure charge balance is maintained during formation, preventing the premature formation of Ti3+. Such components are also used as they modify the tunnel size allowing larger atoms to be accommodated e.g. Cs+ ~1.7Å and Ba~ 1.4Å – both ions are in 8fold co-ordination[10]. Although there is a broad understanding of tunnel site ordering for individual elements (e.g. K, Cs or Ba) within in the hollandite structure it is unknown how mixed large cations (e.g. Cs+ and Ba2+) order within the tunnels and how their location is related to the atomic constituents on the B-site, e.g. Mg, Al, and Ti. The aim of this work is to study such or
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