Pressure-induced breakdown of a correlated system: The progressive collapse of the Mott-Hubbard state in RFeO3

Moessbauer spectroscopy, resistance, and synchrotron x-ray-diffraction (XRD) methods were combined for detailed studies of the pressure-induced breakdown of the strongly correlated perovskite RFe3+O3 (R=La, 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 ∼3% volume contraction. The HP phases at P<50 GPa are characterized by the coexistence, with equal abundance, of high (S=(5/2), 6A1g) and low-spin (S=(1/2), 2T2g) Fe3+ sublattices. With further pressure increase a gradual high- to low-spin transition occurs, fully converting to an S=(1/2) state at ∼65 GPa for both La and Pr. For PrFeO3 up to 90 GPa, the highest pressure reached with MS in this compound, and for LaFeO3 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 LaFeO3 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 LaFeO3 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