Chemistry Reference
In-Depth Information
La, Ba
Cu
O
Figure 8.17
The crystal structure of La
1
−
x
Ba
x
CuO
4
, an archetypal high-
T
c
super-
conductor. Each copper atom is bonded to four neighbouring oxygen
atoms within the plane. The copper oxide planes (the bottom, middle,
and top layers in the structure shown) are separated from each other by
layers containing La, Ba, and further oxygen atoms. Some high-
T
c
super-
conductors with more complex crystal structures have two or three
neighbouring copper oxide layers, with each group of copper oxide lay-
ers again separated from the next group of copper oxide layers by other
metal oxide layers. (From H. P. Myers (1997)
Introductory Solid State Physics,
2nd edn.)
high-
T
c
superconductivity has still not been resolved, although a wide
range of measurements have established some of the key experimental
features of these materials.
Figure 8.17 shows the crystal structure of La
2
−
x
Ba
x
CuO
4
, whose struc-
ture and properties are typical of all high-temperature superconductors.
Each copper atom lies in a 2D layer, bonded to four neighbouring oxygen
atoms within the layer. The copper layers are then separated from each
other by layers of lanthanum, barium, and oxygen atoms.
The electronic properties of La
2
−
x
Ba
x
CuO
4
varydramatically as the alloy
composition,
x
, is varied. When
x
0, La
2
CuO
4
is an antiferromagnetic
insulator, with a Néel temperature,
T
N
=
240 K. The electronic structure
can be described in terms of three electrons transferring from each lan-
thanum ion to the oxygen ions, with two electrons transferring from each
of the copper ions, to give
=
La
3
+
)
2
Cu
2
+
(
O
2
−
)
4
. The only ion which then
possesses an incomplete shell of electrons is the Cu
2
+
ion, which has nine
3d electrons, just one electron short of a filled 3d shell. For further dis-
cussion, it is considerably more convenient to describe the Cu
2
+
ion as
having one 3d hole state in an otherwise filled 3d band. This 3d hole
state has a magnetic moment. Each hole is localised on a single copper
(
