Electronic, magnetic and structural properties of the RFeO₃ antiferromagnetic-perovskites at very high pressures

At ambient pressure the orthorhombic perovskites R-orthoferrites (R ≡ Lu, Eu, Y, Pr, and La) exhibit very large optical gaps. These large- gap Mott insulators in which the 3d⁵ high-spin ferric ions carry large local moments and magnetically order at T_N > 600 K, undergo a sluggish structural first-order phase transition in the 30-50 GPa range, with the exception of the LuFeO₃ which undergoes an isostructural volume reduction resulting from a high to low-spin crossover. High-pressure methods to 170 GPa using Mössbauer spectroscopy, resistance, and synchrotron-based XRD in diamond anvil cells were applied. Following the quasi-isostructural volume reduction (3-5%) the new phase the magnetic-ordering temperature is drastically reduced, to ∼ 100 K, the direct and super-exchange interactions are drastically weakened, and the charge-transfer gap is substantially reduced. The high-pressure (HP) phases of the La and Pr oxides, at their inception, are composed of high- and low-spin Fe³⁺ magnetic sublattices, the abundance of the latter increasing with pressure but HP phases of the Eu, Y, and Lu oxides consist solely of low-spin Fe³⁺. Resistance and Mössbauer studies in La and Pr orthoferrites reveal the onset or a metallic state with moments starting at P > 120 GPa. Based on the magnetic and electrical data of the latter species, a Mott phase diagram was established.