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

Combining the techniques of Mossbauer spectroscopy, synchrotron XRD, and el ectrical resistivity in conjunction with diamond-anvil cells has enabled us to discover and study new phenomena in magnetism and electronic correlatio ns at high density. It is shown that Hund's rule concerning the high-spin s tate in transition metal (TM) compounds does not always hold at high pressu res, resulting in a spin-crossover and collapse of the magnetic state for e ven-valence TM ions and for a decline of the magnetic exchange for odd-vale nce ions. This mechanism is in competition with the breakdown of the d-d el ectron correlation (Mott transition) in pressure transformation of Mott ins ulators into normal metals. The experimental issues are described and a few examples of studies of breakdown of the Mott-Hubbard state at very high pr essures are presented.