Synchrotron-based diffraction and spectroscopy experiments combined with ab-initio calculations unveiled the fate of e-Fe2O3 under isostatic compression: transforming to a new polymorph through a spin crossover transition
Ferric oxide (Fe2O3) has been widely studied in many fields from geophysics and biomedicine to technological applications. At ambient conditions it is found in form of five polymorphs with quite different properties. Among them, e-Fe2O3 presents giant coercivity and has been lately found as a nanomineral in basaltic rocks, opening the possibility of its relevant presence in the Earth’s interior. However, an important prerequisite is the stability of e-Fe2O3 at extreme conditions, which has motivated the high pressure study of this oxide.
e-Fe2O3 presents an orthorhombic structure with four polyhedral units: a regular octahedron, two distorted octahedra and a regular tetrahedron (Figure). By X-ray absorption fine structure (EXAFS) the deformation of polyhedral units were monitored under increasing pressures and revealed a remarkable stability, altered by a sudden change in the average interatomic distances at 27 GPa, the limiting pressure to access the upper Earth’s mantle. This structural anomaly is also reflected by a collapse of the unit cell volume revealed by pressure dependent X-ray diffraction. The analysis of the diffraction patterns at different pressures allowed obtaining the pressure-volume equation of state which was reproduced, including the volume collapse, by ab-initio theoretical simulations. These calculations also indicated the phase stability up to 1800 K and provided its structural characteristics above the volume collapse, which were found to be compatible with the experimental diffraction data.
In particular, in the new high pressure e’-phase, one of the irregular octahedrons and the regular tetrahedron become very distorted octahedral units, closer to a 5+1 coordination. Synchrotron-based Mössbauer spectroscopy measurements indicated that the ε to ε’ transformation is a spin crossover transition.
Juan Ángel Sans,1 Virginia Monteseguro,2,3 Gaston Garbarino,2 Martí Gich,4 Valerio Cerantola,2 Vera Cuartero,2,5 Manuel Monte,2 Tetsuo Irifune,6,7 Alfonso Muñoz,8 and Catalin Popescu9
1 Instituto de Diseño para la Fabricación y Producción Automatizada, MALTA Consolider Team, Universitat Politècnica de València, Spain
2 European Radiation Synchrotron Facility 38043, France
3 ICMUV. MALTA Consolider Team, Universitat de València, Spain
4 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
5 Centro Universitario de la Defensa de Zaragoza, Spain.
6 Ehime University, Japan
7 Earth-Life Science Institute, Tokyo Institute of Technology, Japan
8 Departamento de Física, Instituto de Materiales y Nanotecnología, MALTA Consolider Team, Universidad de La Laguna, Spain
9 ALBA Synchrotron, Spain
 Xu, H., Lee, S., and Xu, H. Luogufengite: A new nano-mineral of Fe2O3 polymorph with giant coercive field. Am. Mineral. 102, 711–719 (2017).
Stability and nature of the volume collapse of ε-Fe2O3 under extreme conditions
Nature Communications, 9, 4554 (2018)
Evolution of the unit cell volume of ε–Fe2O3 under compression. Experimental data (black circles), theoretically simulated data (blue squares) and the fits to 3rd order Birch-Murnaghan equation of state (dashed lines). The structure of ε–Fe2O3 just below the volume collapse and above it (ε’–phase) as obtained by ab-initio calculations are presented on the right side.