Chemistry Reference
In-Depth Information
Figure 14.12 Anion cuboctahedron coordinated by cations
The supercluster model [42,43] as a development of the cluster model considers not only
anion cuboctahedral clusters but surrounding cations as well.
A supercluster is an assembly of all cationic polyhedra having joint faces with the central
anionic cubooctahedron. A supercluster has the [M
14
F
68-69
] formula because 8 M
2
þ
cations
located in cube corners with their closest anion surroundings from the initial fluorite matrix
are added to the central anion cuboctahedron surrounded by 6 R
3
þ
cations. In this formula,
F
69
arises if the position in the centre of cuboctahedron is filled (i.e. 8:12:1). According to
[7,43], all RE ions and all types of structural defects are gathered in superclusters, for
example in the form of a 'rare-earth' supercluster [M
8
R
6
F
68-69
] (Figure 14.13).
Figure 14.13 [M
8
R
6
F
6869
] supercluster and distorted cationic polyhedra around the anionic
cluster: white colored - M
2þ
polyhedra, grey colored - R
3þ
polyhedra. (Reprinted with
permission from [7] Copyright (2001) Bovis P. Sobolev.)
