High-pressure Mossbauer spectroscopy, a unique tool for unraveling the nature of electron correlations in Mott insulators

Combining the methods of Mossbauer spectroscopy, synchrotron XRD, and resis tivity and using diamond-anvil cells enables the discovery and studies of n ew phenomena in magnetism and electronic correlation at high density. It is shown that Hund's rule concerning the high-spin state in TM-compounds does not hold in this regime resulting in spin-crossover and collapse of the ma gnetic state for even-valence TM ions and for the decline of magnetic excha nge in the odd-valence species. This mechanism competes with the breakdown of the d-d electron correlation (Mott transition) in transforming the Mott insulators into normal metals. The experimental issues are described and ex amples of magnetic studies at very high-pressures are portrayed.