Page 68 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

symmetry from tetragonal to orthorhombic. The additional electronegative O atoms
in the lattice serve to inject excess “holes” via oxidation of some Cu 2þ centers to
3þ
Cu ; hence, at T < T c , the system is perfectly diamagnetic as the unpaired elec-
8
trons condense into Cooper pairs and Cu 3þ ions are formally d (low-spin, diamag-
netic). In these structures, the holes are also found to segregate themselves into
stripes that alternate with antiferromagnetic regions in the material, widely referred
to as a “stripe phase” – thought to be important in the mechanism for high-T c
superconductivity. [28] Others have shown that lattice vibrations (phonons) play an
unconventional role in superconductivity – in particular, electron–phonon coupling
interactions. [29] As you can see, there is no unifying theory that has yet been adopted
to explain the superconductivity of high-T c materials.
In terms of formal charges on the ions, p-type (or hole-doped) YBCO may be
3þ
2
2þ
3þ
2þ
written as: Y (Ba ) 2 (Cu ) 2 Cu (O ) 7 d . YBCO becomes superconductive
at d 0.4, with its most pronounced superconductivity at d ¼ 0.05. [30] It
should be noted that there are other examples of p-type superconductors that
involve metal doping rather than varying oxygen content, such as La 2 x Sr x CuO 4
(T c ¼ 34 K at x ¼ 0.15). [31] Similarly, electron-doped (n-type) superconductors
may be synthesized such as Nd 2 x Ce x CuO 4 (T c ¼ 20 K), written formally as
4þ
2þ

3þ
Nd 2 x Ce x (e ) x Cu O 4 .
A new class of superconductors that contain iron have been synthesized only
[32]
within the last few years. The compound LaFePO was discovered in 2006, with a
critical temperature of 4 K; fluorine doping to yield LaFe[O 1 x F x ] increases the T c
to 26 K. Since 2008, analogous compounds of general formula (Ln)FeAs(O, F)
(Ln ¼ Ce, Sm, Nd, Pr) have been prepared that exhibit superconductivity at tem-
peratures up to 56 K. Other compounds such as (Ba, K)Fe 2 As 2 have T c values up to
38 K, and MFeAs (M ¼ Li, Na) have a T c around 20 K. [33] What is most intriguing
about iron-based superconductors is that ferromagnetism (see Chapter 3) directly
competes against Cooper pair formation. Interestingly, these structures exhibit
tetragonal-orthorhombic transitions, analogous to cuprate superconductors; how-
ever, there appears to be participation of all five 3d orbitals in the Fermi level,
relative to just the d x2 y2 in the cuprates. Not only will further discoveries in this
field be important in developing a unifying theory for HTS, but the physical proper-
ties of alternative HTS materials may be more conducive for applications; that is,
cuprates suffer from a high degree of brittleness that precludes the facile production
of superconductive power lines. [34]
There are already commercial applications that employ superconductive materi-
als; for example, MAGLEV trains have been operable for many years in Japan and
England. However, reports of deleterious effects of radio waves may slow the
widespread use of this technology. In 2001, three 400-foot HTS cables (Figure 2.32)
were installed at the Frisbie Substation of Detroit Edison, capable of delivering
100 million watts of power. This marked the first time commercial power has
been delivered to customers of a U.S. power utility through superconducting wire.
Similar plans are underway to install an underground HTS power cable in Albany,
New York, in Niagara Mohawk Power Corporation’s power grid. The 350-m cable,