Pressure-induced breakdown of a correlated system: The progressive collapse of the Mott-Hubbard state in (formula presented)

Mössbauer spectroscopy, resistance, and synchrotron x-ray-diffraction (XRD) methods were combined for detailed studies of the pressure-induced breakdown of the strongly correlated perovskite (formula presented) (formula presented) systems. The XRD studies have shown that in the range 30–50 GPa both orthorhombic perovskites undergo a first-order phase transition to a new high-pressure (HP) phase accompanied by a ∼3% volume contraction. The HP phases at (formula presented) are characterized by the coexistence, with equal abundance, of high (formula presented) and low-spin (formula presented) (formula presented) sublattices. With further pressure increase a gradual high- to low-spin transition occurs, fully converting to an (formula presented) state at ∼65 GPa for both La and Pr. For (formula presented) up to 90 GPa, the highest pressure reached with MS in this compound, and for (formula presented) between 70–120 GPa, magnetic spin-spin relaxation spectra are observed suggesting the presence of a weak magnetic exchange. This coincides with a drastic decrease in the resistance. The observation of spin-lattice paramagnetic relaxation in spectra in the 120- to 170-GPa range for (formula presented) concurs with the onset of a metallic state with noninteracting moments as evidenced by (formula presented) studies. It is predicted that a normal metal, with no moments, will be established in (formula presented) at ∼240 GPa. A detailed analysis of the magnetic interactions in an antiferromagnetic insulator at very high pressures and a Mott-Hubbard phase diagram are presented in terms of the pressure versus the magnetic moment.