Chemistry Reference
In-Depth Information
c
m
a
m
b
m
F
F
F
F
Figure 13.13 The crystal structure of La
2
CuO
3.6
F
0.8
and scheme of cooperative rotation of the
Cu coordination polyhedra. The Cu cations are situated in octahedra, the La cations are shown
as large spheres, the F anions are shown as small spheres.
the (LaO) layers and transforming part of the CuO
6
octahedra into CuO
5
square pyramids
(Figure 13.13). The remaining octahedra are cooperatively rotated maximizing the
distance between the apical oxygen atoms and interstitial fluorines. Thus, the blocks
with filled tetrahedral interstices and empty octahedral interstices (fluorite fragments)
are present in the structure along with the NaCl-like fragments of the initial structure, but
in contrast to the T* structure, these blocks alternate not only along [001]
K2NiF4
, but also
along [110]
K2NiF4
.
La
2
CuO
4
can be also considered as the n
¼
1 member of the Ruddlesden-Popper
homologous series with general (R,A)
n
þ
1
Cu
n
O
2n
þ
2
þ
(R
¼
rare-earth cation, A
¼
Ca, Sr)
composition. In the structure of the n
¼
2 member R
2
x
A
1
þ
x
Cu
2
O
6
þ
the (CuO
2
) planes
are separated by the (A
N
) layer (Figure 13.14):
ð
CuO
2
Þð
O
ð
R
;
A
ÞÞðð
R
;
A
Þ
O
Þð
CuO
2
Þð
A
N
Þð
CuO
2
Þðð
R
;
A
Þ
O
Þ
ð
O
ð
R
;
A
ÞÞ ð
CuO
2
Þ
-
:
The cation position in the (A
N
) layers can be jointly occupied by rare-earth and alkali-
earth cations if A
¼
Sr. The Cu cations have tetragonal pyramidal oxygen coordination. In
contrast to the n
¼
1 member, in the structure of the second member there are two possible
positions for extra anions: in the tetrahedral interstices between the (RO) and (OR) layers
and in the (A
N
) layer, that will complete the Cu coordination to octahedron. The behaviour
