Consequences of pressure-instigated spin crossover in RFeO₃ perovskites; a volume collapse with no symmetry modification

High-pressure X-ray diffraction, Mössbauer, and Raman studies in the antiferromagnetic insulators RFeO₃ orthorhombic perovskites (R = Pr, Eu, Lu) disclose an unusual phenomena of a reversible first-order isostructural phase transition around 50 GPa concurring with an abrupt ∼ 5% volume decrease. It is shown experimentally that this transformation concurs with, and is driven by, a high-to-low-spin transition taking place in Fe³⁺; a manifestation of a new kind of isostructural phase transition. These studies suggest that the RFeO₃ perovskite is a rather sturdy oxide-structure, maintaining its original structural symmetry beyond 125 GPa.