Pressure-induced breakdown of a correlated system: The progressive collapse of the Mott-Hubbard state inRFeO3

M\"ossbauer spectroscopy, resistance, and synchrotron x-ray-diffraction (XRD) methods were combined for detailed studies of the pressure-induced breakdown of the strongly correlated perovskite $R{\mathrm{Fe}}^{3+}{\mathrm{O}}_{3}$ $(R=\mathrm{L}\mathrm{a},\mathrm{}\mathrm{Pr})$ 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 \ensuremath{\sim}3% volume contraction. The HP phases at $P<50\mathrm{GPa}$ are characterized by the coexistence, with equal abundance, of high $(S=\frac{5}{2},{}^{6}{A}_{1g})$ and low-spin $(S=\frac{1}{2},{}^{2}{T}_{2g})$ ${\mathrm{Fe}}^{3+}$ sublattices. With further pressure increase a gradual high- to low-spin transition occurs, fully converting to an $S=\frac{1}{2}$ state at \ensuremath{\sim}65 GPa for both La and Pr. For ${\mathrm{PrFeO}}_{3}$ up to 90 GPa, the highest pressure reached with MS in this compound, and for ${\mathrm{LaFeO}}_{3}$ 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 ${\mathrm{LaFeO}}_{3}$ concurs with the onset of a metallic state with noninteracting moments as evidenced by $R(P,T)$ studies. It is predicted that a normal metal, with no moments, will be established in ${\mathrm{LaFeO}}_{3}$ at \ensuremath{\sim}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.