Pressure-induced metallization and electronic-magnetic properties of some mott insulators

Combining the techniques of Mössbauer spectroscopy, synchrotron XRD, and electrical resistivity in conjunction with diamond-anvil cells has enabled us to discover and study new phenomena in magnetism and electronic correlations at high density. It is shown that Hund's rule concerning the high-spin state in transition metal (TM) compounds does not always hold at high pressures, resulting in a spin-crossover and collapse of the magnetic state for even-valence TM ions and for a decline of the magnetic exchange for odd-valence ions. This mechanism is in competition with the breakdown of the d-d electron correlation (Mott transition) in pressure transformation of Mott insulators into normal metals. The experimental issues are described and a few examples of studies of breakdown of the Mott-Hubbard state at very high pressures are presented.