Pressure Tuning of Electronic Energy Levels

Over the past three decades it has been demonstrated that pressure is a very useful tool for investigating electronic phenomena in condensed systems. Compression results in different degrees of perturbation of different types of orbitals. This ‘pressure tuning’ can induce electronic transitions to new ground states. It can also be used for critical tests of theories or to uncover hitherto unforeseen correlations involving electronic properties. We discuss three types of electronic transitions: insulator-conductor transitions with emphasis on iodine; transitions between photochromic, thermochromic, and piezochromic ground states; and changes of spin state of magnetic ions. Tests of theories include Bethe's R−5 rule for magnetic ions, Van Vleck's prediction of spin pairing, the Forster-Dexter theory of energy transfer in phosphors, theories of emission efficiency in doped zincblende phosphors, and energy transfer between excited states of electron donor-acceptor complexes. Finally, as a challenge to further theoretical work, we demonstrate a relationship between the rate of phosphor emission kr and the low-frequency dielectric constant ∊.