Optical versus thermal transitions in solids at high pressure.

Pressure-induced electronic transitions have been observed in a wide variety of materials. In particular, Mössbauer resonance studies under high pressure have revealed changes of oxidation state and spin state of iron as a function of pressure. These processes involve the thermal transfer of an electron to a new ground state of the system. The difference in energy between states is frequently measured by optical absorption. In this paper, we relate the energy of the optical absorption peak and its half-width to the difference in thermal energy between the two states as a function of pressure. We show that the optical peak will broaden with pressure only if there is a difference between the force constants of the ground and excited states. These relationships permit the prediction from optical absorption data of the pressure at which a new ground state will be obtained. We demonstrate that the predictions are qualitatively correct for the reduction of Fe(III) in a series of ferric hydroxamates, and for the low-spin to high-spin transition of Fe(II) in ferrous phenanthroline complexes.