Pressure‐Induced Electronic Changes in Compounds of Iron

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.