|Abstract:||During the past decade there has been a considerable expansion in the amount and variety of high pressure research. In part this has been caused by an increased interest in experimental geophysics. Much of the industrial high pressure effort has been triggered by the successful synthesis of artificial diamonds at the General Electric Company. In large part, however, this research is a response to the increased awareness of the significance of interatomic distance as a prime parameter in physical science, and particularly in solid state research. In the pressure range to 12 kilobars, experiments of a variety and sophistication comparable to that at one atmosphere are possible. In the range to 30 kilobars liquid media can still be used. Relatively accurate and direct measurements of pressure are possible to perhaps 70 kilobars. In the range from 100-600 kilobars static measurements become more difficult. There are available two recent books surveying the field of high pressure research in an extensive way (1, 2). In this paper we shall review the experimental activities in our laboratory on the electronic structure of solids at pressure to perhaps 600 kilobars.
Experiments in the range beyond 100 kilobar encounter limitations as to the possible variety of measurements, as to the accuracy with which the pressure is known, and as regards hydrostaticity of the pressure. There are, however, important compensations. The volume decreases are large (20-50% in metals and frequently more in non-metals) and one can observe second and third order effects beyond the direction and magnitude of the first order pressure shift of a variable. Many qualitatively new events are observed--continuous and
discontinuous transitions from non-metal to metal, electronic transitions, and changes from ferromagnetic to non-ferromagnetic states. Included in this discussion are optical absorption measurements to 160 kilobars, phosphor emission and decay to 50 kilobars, electrical resistance measurements to about 600 kilobars, X-ray diffraction studies to 500 kilobars, and Mossbauer measurements to 250 kilobars. The experimental methods are not covered in this paper, but can be found in the literature (3 - 7).
The plan of the paper is as follows. In the first two sections optical absorption studies involving localized electronic energy levels we discussed. In the next two sections optical absorption phosphor emission and electrical resistance measurements involving insulators, semiconductors, and the transition to the metallic state are covered. The last sections discuss the effect of pressure on the structure of metals including electronic transitions, the interaction of the Fermi surface with Brillouin zone boundaries in non-cubic metals, and the electronic structure of transition metals at high pressure.