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K.R. Whittle, S.E. Ashbrook, S.A.T. Redfern, G.R. Lumpkin, J.P. Attfield
1
, 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
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Hollandites with compositions Ba
1.2-x
Cs
x
Mg
1.2-x/2
Ti
6.8+x/2
O
16
, and Ba
1.2-x
Cs
x
Al
2.4-x
Ti
5.6+x
O
16
(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,
A
x
B
8
O
16,
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. Ba
1.2
Mg
1.2
Ti
6.8
O
16
, or tetragonal[4] in nature e.g.
Ba
1.121
Al
2.24
Ti
5.76
O
16
, 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.Ba
1.2
Mg
1.2
Ti
6.8
O
16
. The use of Ti is important because, as Cs
+
undergoes β-decay forming
Ba
2+
a charge imbalance results, in order to compensate for this a Ti
4+
cation in the lattice
undergoes reduction to Ti
3+
conserving charge balance.
Cs
+
can be immobilized in hollandites that contain Al
3+
and Mg
2+
[9], in these systems the
Al
3+
and Mg
2+
are present to ensure charge balance is maintained during formation, preventing
the premature formation of Ti
3+
. 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 8-
fold 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 Ba
2+
) 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 ordering and see whether it
contributes to the stability of Cs-containing hollandites under leaching conditions.
Mat. Res. Soc. Symp. Proc. Vol. 807 © 2004 Materials Research Society
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