Chemistry Reference
In-Depth Information
-Li
7
VN
4
contains
an identical substructure comprising a body-centred cube of composition [V(1)]
2
with an inscribed icosahedron of composition Li(2)
6
. However, in this case there is
also an alternative inscribed icosahedron of composition V(2)
6
which is arranged at
90
to the first. Taken together, these two icosahedra combine with the body-
centred cube to form a body-centred cuboctahedron of composition V(1)
2
[V(2)
6
]
[Li(2)
6
]. Not surprisingly, these structural patterns can be identified in all the
substructures to be discussed below.
These discrepancies can be resolved very satisfactorily. In the case of
g
-Li
7
VN
4
,
the moieties comparable with the Si and Cr atom sites in Cr
3
Si are the V(1)N
4
and
Li(2)N
4
tetrahedra act in place of the single Si and Cr atoms, respectively, but have
a much larger volume, which accounts for the difference in the cell sizes. We are
equating Si atoms with V(1)N(1)
4
tetrahedra and Cr atoms with Li(2)N(2)
4
tetra-
hedra, these tetrahedra acting as pseudo-atoms. The composition of the substructure
in the
g
-Li
7
VN
4
cell [Li
56
V
8
N
32
] is then simply [Li(2)N
4
]
6
[V(1)N
4
]
2
, leaving the
remaining 50 Li atoms and 6 V atoms as the “stuffing”.
We are reminded of the well-known situation with the compound K
2
PtCl
6
-
there is a similar discrepancy between the cubic cell parameters for this compound
and K
2
O. If the PtCl
6
octahedron is considered as a single entity, we have the
anti-fluorite-typ
e
structure with the larger PtCl
6
octahedron centred on the 4
a
site of
space group
g
m
3
m
(225) and the K atoms in the 8
c
site.
As an aside, we know that the Cr
3
Si alloy structure has been described previ-
ously as rod packing, i.e. rods of interweaving, strongly bonded Cr atoms in all
This is shown in Fig.
2a
, drawn in the [111] projection, with different colours used
to differentiate the three 100
F
cubic directions. Figure
2b
shows the comparable
drawing of rods of Li(2)N
4
tetrahedra in
hi
-Li
7
VN
4
.
We have explored further this rod relationship between the structures of Li
7
VN
4
and Cr
3
Si by considering only the cations of the former, bearing in mind that every
g
Fig. 2 (a) Rod packing in Cr
3
Si. Cr-Cr rods interweave in all three cubic directions. Si atoms
(
grey
) occupy the interstices. (b) Rod packing of Li(2)N
4
tetrahedra. Isolated V(1)N
4
tetrahedra -
grey
