Chemistry Reference
In-Depth Information
4 Experimental Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.1 Incommensurate Te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
4.2 Incommensurate Sc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.3 Single-Crystal Studies of Na Above 100 GPa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5 Twenty-First Century High-Pressure Crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
1
Introduction
Pressure is a sadly underused thermodynamic variable in the study of chemical
systems, despite the effects of pressure being very much more dramatic than those
of temperature. The ability of pressure to change inter-atomic distances strongly
leads to even the simplest chemical systems undergoing a variety of pressure-
induced structural and electronic phase transitions that can change insulating solids
into superconducting metals [
1
-
3
], gases into exotic coloured crystals with unusual
inter-molecular bonding [
4
] and alkali metals into transparent insulators [
5
].
Key to understanding the effects of compression on materials is the determina-
tion of the crystal structure. Crystallography at high pressure stretches back to the
late 1950s and early 1960s, but has undergone a revolution in the last 20 years as the
result of both breakthroughs in high-pressure techniques and the development of
new X-ray and neutron sources. In this chapter, I will briefly review the history of
high-pressure crystallography, describe the experimental methods used to study
materials at high pressures, and then illustrate the effects of high pressure on
a number of different elemental systems.
2 A Brief History of High-Pressure Crystallography
Before reviewing the history of high-pressure crystallography, it is perhaps first
necessary to define what is meant by “crystallography”. The Oxford English
Dictionary describes it as “
That branch of physical science which treats of the
structure of crystals, and their systematic classification
”. By “structure” I will mean
the determination of the fractional coordinates of atoms, and their displacement
parameters, in a crystalline material. At high pressures, this typically involves the
measurement of the intensities of Bragg reflections using X-ray or neutron diffrac-
tion, followed by structural solution and/or least-squares refinement.
The history of high-pressure science is described extremely well in two topics by
R.M. Hazen, and the reader is directed to these for an excellent introduction to the
field [
6
,
7
]. While much high-pressure science was conducted prior to the 1950s,
most notably by P.W. Bridgeman [
8
], who received the 1946 Nobel Prize in physics
for “
the invention of an apparatus to produce extremely high pressures
,
and for the
discoveries he made therewith in the field of high pressure physics
”[
9
], it was the
