Chemistry Reference
In-Depth Information
not in question. Let us conclude this description of relevant superconductors with
the series of ambient-pressure superconductors
SbF
6
and AsF
6
, whose discovery has been crucial since it demonstrates that the TTF
core is not essential for the production of organic superconductors (Yamada
et al.
,
2001).
Those readers not familiar with superconductivity in organic materials may find
the
T
c
values rather low. However, they are comparable to values for inorganic
metallic elements. Here is a list of some selected examples:
T
c
(Nb)
β
-(BDA-TTP)
2
X, with X
=
=
9.25 K,
=
0.40 K, etc. It is interesting to note that copper does not exhibit a superconducting
transition. The highest known
T
c
values of any material correspond to the copper-
oxide series with
T
c
T
c
(Pb)
=
7.20 K,
T
c
(
α
-Hg)
=
4.15 K,
T
c
(Sn)
=
3.72 K,
T
c
(Al)
=
1.17 K,
T
c
(Ti)
138K as the absolute record for the thallium-dopedmercury-
cuprate compound.
These copper-oxide compounds crystallize in the perovskite structure and super-
conductivity is based on the (hole or electron) doping in the copper-oxide planes.
This is the reason why these materials can be regarded as being 2D. The first
compound of the family was La
2
−
x
Sr
x
CuO
4
with
T
c
38 K, which soon led to
YBa
2
Cu
3
O
7
−
δ
with
T
c
1 (Burns, 1993). The non-copper oxide
electron-doped perovskite Ba
1
−
x
K
x
BiO
3
exhibits superconductivity near 30 K for
0
92 K for
δ<
5 (Cava
et al.
, 1988).
Solid C
60
is a semiconductor, as previously discussed, and becomes metallic
under doping. Electron-doped Cs
x
Rb
y
C
60
has
T
c
as high as 33 K (Tanigaki
et al.
,
1991). The recently discovered MgB
2
superconductor, with
T
c
.
3
<
x
<
0
.
39 K, deserves
special attention (Nagamatsu
et al.
, 2001). MgB
2
is structurally and electronically
related to graphite. The crystal structure of MgB
2
is shown in Figs. 1.22(a) and
(b) and consists of alternating sheets of honeycomb boron layers and hexagonal
magnesium layers.
Magnesium atoms donate electrons to the conduction band where no
d
-electrons
are involved. The honeycomb planes of boron determine the electronic process. We
thus have the formation of
bands, as in graphite. The structure of graphite
is depicted in Fig. 1.22(c) and consists of stacked planar sheets, in which the carbon
atoms are covalently bound (
σ
and
π
,
sp
2
-hybridization) into a honeycomb lattice and the
sheets are weakly bound through van der Waals interactions (the interlayer distance
is
c
σ
335 nm). Perpendicular to the basal planes
p
π
-electrons are delocalized
forming a conducting 2D system. As a result of its electronic structure, graphite
is a semimetal exhibiting highly anisotropic conductivity. The high
T
c
value of
MgB
2
is ascribed to the strong coupling between electrons in the 2D
/
2
=
0
.
band and
the optical
E
2g
phonon associated to in-plane motion of boron atoms. The energy
of this phonon is
h
σ
ω
∝
ω
570 meV and hence the high
T
c
value, since
T
c
h
D
.
D
