The Effect of Cs on the Structural Properties of Barium Titanate Hollandites
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THE EFFECT OF CS ON THE STRUCTURAL PROPERTIES OF BARIUM TITANATE HOLLANDITES K.R. Whittle, G.R. Lumpkin and S.E. Ashbrook Cambridge Centre for Ceramic Immobilisation – C3i, Department Of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, United Kingdom. ABSTRACT Hollandite, based on Ba and Ti, has been shown to be an ideal medium by which active Cs and 137Cs can be immobilised. Diffraction measurements have shown that Cs modifies the symmetry of the material, from monoclinic to tetragonal. Experiments carried out here show the effects of Cs on two hollandite systems based on Ba-Al-Ti and Ba-Mg-Ti. Structural measurements are reported for neutron diffraction, and MAS NMR. The results show that Cs modifies the overall crystal symmetry while having little effect on the order/disorder of Al-Ti and Mg-Ti octahedra. 135
INTRODUCTION The safe immobilisation of radioactive Cs waste has been studied for a number of years; the medium of choice in crystalline wasteforms are based on hollandite-type (AxB8O16) systems, as these show high durability under aqueous conditions. This type of system has also been chosen to be the basis by which Cs is immobilised in the Synroc [1] family of wasteforms. 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, e.g. Ba1.2Mg1.2Ti6.8O16, or tetragonal in nature, e.g. Ba1.121Al2.24Ti5.76O16. Essentially the difference is due to variations in A/B cation radius ratio, causing a shear-type collapse of the tunnel and a reduction in symmetry (I4/m @ C2/m)
Figure 1 - Image of tetragonal Ba-hollandite crystal structure, showing the tunnels and Ba2+/Cs+ ions large spheres, located within – view looking down the c-axis. The hollandite structure can accommodate a variety of atoms on both the A and B sites [29], e.g. on the A site Ba, Na, Rb, Pb, Cs and K; on the B site it is possible to mix cations such as Mo, Sn, Mg, Ti, Al, and Zr. In the area of nuclear waste immobilization it is routine to base the
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hollandite on Ti, e.g. Ba1.2Mg1.2Ti6.8O16. The use of Ti is important because, as Cs+ undergoes 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+, as in these systems the Al3+ 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 in 8-fold co-ordination [10,11]. The aim of this work is to understand the dynamical motion of Cs+ cations through the lattice and relate this to the stability under leaching conditions. In order to understand the dynamical motion through the lattice, the material must be carefully analysed using te
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