We analyze a series of recent experiments in which it has been found that pressure induces changes in charge or spin of iron [i.e., Fe(III) reduces to Fe(II) and in Fe(II) there are spin changes involving either decrease or increase of multiplicity]. In addition to considering the volume difference of the two states at zero pressure, we find it important to include effects both of the difference in compressibility of the two states, and the change in compressibility of a given state with pressure. The theory casts the problem in terms of the changes in Coulomb energy, closed shell repulsions, and both covalent bonding energy and crystal field energy accompanying the change in electronic state. In addition, interactions between converted iron atoms are included by a form of mean field theory and the effects are shown to be significant. Not only is the theory discussed analytically, but also a simple graphical solution is shown which makes it possible to examine readily the qualitative effects of the various parameters. Repulsive interactions spread the conversion over a larger pressure range and may thus explain why so many compounds exhibit rather broad transitions. Attractive and repulsive interactions can lead to cooperative effects. They should account for the reversible yet discontinuous jumps in conversion as a function of temperature previously observed in several phenanthroline and dipyridyl compounds. The possibility of hysteresis is also indicated. Major anomalies in the combined temperature and pressure variation of the conversion are accounted for by including the temperature dependence of the free energy of interaction.

We shall not attempt here to enumerate the results or review in a systematic way the significant literature dealing with the use of high pressure in studies of proteins and other molecules of biological interest. Two recent reviews on this subject, one by MOrild (1981) and another by Heremans (1982), and a further article by Jaenicke (1981) on enzymes under extreme environmental conditions contain expositions and references that would render redundatn such a task. Rather we concentrate here on the examination of othe conceptual framework employed in the interpretation of high pressure experiments and in the critical discussion of our knowledge of selected areas of present interest and likely future significance.

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The effect of pressure to several hundred kilobars has been measured on the lattice parameters of diamond, graphite, and hexagonal boron nitride. The linear compressibility of diamond is independent of pressure. The a axis of graphite (characteristic distance in the plane) is considerably more compressible than diamond at low pressure but above about 160 kbar is much less compressible than diamond. The c axis of graphite is much more compressible than the a axis. Assuming van der Waals forces between the graphite planes and using the lattice sums of Girifalco and Lad, it is possible to describe the compressibility of graphite to very high pressure from low-pressure data. Hexagonal BN is much like graphite, with both a and c axes somewhat more compressible than the corresponding parameters of graphite.

The electrical resistance of single crystal graphite shows a very sharp increase at above 150 kilobars, accompanied by a drifting upward with time. The behavior is typical of a first-order phase transition, and is irreversible. X-rays on the material after removal from the cell show lines of a new material with a structure which can be indexed as a cubic lattice with a unit cell edge of 5.545 angstroms. The density of the new phase is estimated at 2.80 grams per cubic centimeter.