Local structure and spin transition in Fe2O3 hematite at high pressure

The pressure evolution of the local structure of Fe2O3 hematite has been determined by extended x-ray absorption fine structure up to ∼79 GPa. Below the phase-transition pressure at ∼50 GPa, no increasing of FeO6 octahedra distortion is observed as pressure is applied. Above the phase transition, an abrupt decrease of the nearest-neighbor Fe-O distance is observed concomitantly with a strong reduction in the FeO6 distortion. This information on the local structure, used as a test-bench for the different high-pressure forms proposed in the literature, suggests that the orthorhombic structure with space group Aba2, recently proposed by Bykova et al. [Nat. Commun. 7, 10661 (2016)], is the most probable, but puts into question the presence of the P21/n form in the pressure range 54–67 GPa. Finally, the crossover from Fe high-spin to low-spin states with pressure increase has been monitored from the pre-edge region of the Fe K-edge absorption spectra. Its “simultaneous” comparison with the local structural changes allows us to conclude that it is the electronic transition that drives the structural transition and not vice versa.