Page 69 - Materials Chemistry, Second Edition

56 2 Solid-State Chemistry

Figure 2.32. Photograph of a high-temperature superconductor (HTS) wire used for electrical power
applications. LN refers to the flow direction of the liquid nitrogen cryogenic cooling fluid. Photograph
courtesy of Southwire Company (http://www.southwire.com).

believed to be four times the length of any previously installed HTS cable, will be
designed to provide more power and operate at significantly lower loss levels than
other HTS installations. In order to develop a widespread resilient and ultra-efficient
electric grid, many U.S. government agencies such as the Department of Energy
(DoE) and Homeland Security have recently established priorities to develop HTS
wires and novel cryogenic dielectric materials, as well as associated electrical
[35]
applications such as cables, fault current limiters, and transformers.

2.3.3. Crystal Symmetry and Space Groups
Crystallography employs two terms to describe the symmetry of the crystal lattice:
point groups and space groups. Chemists should be quite familiar with the notion
of point groups, as this designation is used to predict molecular reactivities and
IR/Raman absorption bands. A flowchart for the assignment of a molecular point
group is shown in Figure 2.33. Examples of this notation, given by Schoenflies
3
symbols, are C 3v for ammonia and Os 3 (CO) 9 (C 6 H 6 ), O h for [CoF 6 ] , and D 3h for
(CH 3 ) 8 Si 5 O 6 molecules (Figure 2.34).
By definition, a symmetry operation is an event that results in the transposing of
one item into another that is indistinguishable from the original. This operation may
take place about a point, line, or plane of symmetry. When a set of symmetry
operations is applied to components of the unit cell of a crystal lattice, the resulting
symmetry is designated as a crystallographic point group. Whereas a molecular
point group operation will reproduce an individual molecule, a crystallographic
point group operation must leave the entire crystal unchanged.
Illustrative examples of crystallographic symmetry operations are shown in
Figure 2.35. An integer label, n, indicates the regeneration of an equivalent lattice
point when an object in the crystal lattice is rotated 360 /n about an axis. A rotation-

inversion axis is designated by n, featuring rotation about an axis (360 /n), followed