Crystallography at High Pressure

Abstract

Crystallography (defined in Webster's New World Dictionary as science of the form, structure, properties and classification of crystals) obviously covers more than just the effect of pressure on crystalline materials. In this chapter I confine my attention to very recent develop­ ments that promise to make simple techniques available to the majority of active crystallographers. These are (a) the development of truly hydrostatic ( l 00 kb) pressure1 systems, (b) the ruby R-line internal pressure gage, and (c) the extension of nonhydrostatic diffraction mea­ surements to over 1 megabar ( = 100 gigapascals). The earlier work, mostly powder diffraction on samples in nonhydrostatic environments, is reviewed elsewhere; elements by Jamieson ( 1) in 1964 and Cannon (2) in 1974, and AB-type compounds by Rooymans (3) in 1969 and Merrill (4) in 1977. The extensive work done in Russia at the Institute for High Pressure Physics is reviewed in the book by Vereschagin (deceased) & Kabalkina (5) in 1979 (in Russian). This latter contains many references to non-Soviet sources as well and descriptions of apparatus. A general review (6) of the earlier diamond cell work appeared in 1974. RX2 compounds based on rutile and fluorite structures have been heavily investigated by in situ studies (5, 7, 8) and by quench studies with emphasis on oxides, silicates, germanates, etc. (9-11). All these reviews emphasize changes in crystal structures at high pressure (and occasion­ ally, but rarely, at combined high pressures and temperatures). As indicated earlier, I shall not emphasize the area of crystal structures found only at high pressure or those already known under I-bar condi­ tions but having a stability field at high pressure.