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Inorganic Polymers 443

powders, including the sol-gel synthesis where processing involves a stable liquid medium, copre-
cipitation where two or more ions are precipitated simultaneously. More recently, Carraher and Xu
have used the thermal degradation of metal containing polymers to deposit metal atoms and oxides
on a molecular level.

12.24 HIGH-TEMPERATURE SUPERCONDUCTORS
12.24.1 DISCOVERY OF THE 123-COMPOUND
In early 1986, George Bedorz and K. Alex Muller reported a startling discovery—a ceramic mate-
rial, La–Ba–Cu–O, lost its resistance to electrical current at abut 30 K. This was the fi rst report
of a so-called high-T superconductor. Intensive efforts were then concentrated on substituting the
c
component ions with similar elements on both the La and Ba sites. The first success was reported

by Kishio et al. with an (La Sr ) CuO system that exhibited a higher T to about 37 K. Then the
1-x x 2 4 c

substitution on the La sites led Wu and coworkers to find another superconductor, the Y–Ba–Cu–O

system with a T of 93 K in February 1987. This finally broke the technological barrier that would
c
allow superconductivity at temperatures above liquid nitrogen. The superconducting phase was

identified as Y Ba Cu O (commonly referred to as the 123-compound). The 123-compound was
1 2 3 7

the first of the 90 K plus superconductors to be discovered and it has been the most thoroughly
studied.
12.24.2 STRUCTURE OF THE 123-COMPOUND
The structure of the 123-compound is related to that of an important class of minerals called per-
ovskites. These minerals contain three oxygen atoms for every two metal atoms. It has six metal
atoms in its unit cell and would be expected to have nine oxygens if it were an ideal perovskite. In
fact, it has, in most samples, between 6.5 and 7 oxygens. In other words, by comparison to an ideal
perovskite, about one-quarter of the oxygens are missing. The unit cell can be thought of as a pile
of three cubes. Each cube has a metal atom at its center: barium in the bottom cube, yttrium in the
middle one, and barium in the top one. At the corners of each cube a copper would be surrounded
by six oxygens in an octahedral arrangement linked at each oxygen in an ideal perovskite. Each
barium and yttrium would then be surrounded by 12 oxygens. But X-ray and neutron diffraction
studies have shown that the unit cell does not conform to this simple picture because certain oxygen
positions are vacant. All oxygen positions in the horizontal plane containing yttrium are vacant. The
other vacancies are located in the top and bottom Cu-O planes.
The two copper oxide layers can be considered as polymeric since the covalent character is in the
same range as for the carbon fluoride bond in Teflon. Thus, the 123-superconductors consist of two

types of polymeric copper oxide layers held together by ionic bonding metals such as barium and
yttrium. This theme of polymeric layers held together by ionic bonding to metals is common in the
silicates and other minerals.

12.25 ZEOLITES

At least three major themes are helping drive polymer synthesis and use of polymers today. These
involve synthesis and assembling on an individual scale (nanolevel); synthesis in confi ned spaces
(selected inorganic zeolites and biological syntheses); and single-site catalysis (both selected biolog-
ical and synthetic polymer synthesis). Superimposed on this is the applications aspects, including
the human genome/biomedical, electronic/communications, and so on.
Zeolites are three-dimensional microporous crystalline solids. Zeolites include a whole group of
aluminosilicates with an approximate formula of SiAlO . With respect to type of bonding, zeolites
4
can be divided into three groups. The natrolite group (mesolite, thomsonite, edingtonite, natrolite)

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