Surprisingly, β-Fe2O3 was found to transform into a completely new crystal structure following the Rietveld refinement of the synchrotron radiation XRD patterns recorded at pressures above 30 GPa. The analyses were carried out adopting the following scenario. At high pressures (42.9–64.4 GPa), α-Fe2O3 transforms into RO-Fe2O3 and PPV-Fe2O3 with consistent transition pressures compared with the previous reports. On the other hand, β-Fe2O3 transits to a different new phase. By indexing these new peaks, we searched for its space group and found a suitable candidate for the crystal structure. The new peak pattern belonged to a single phase with a monoclinic crystal structure in the I2/a space group with lattice parameters similar to β-Fe2O3. We know that the structure of this new phase is caused by lowering the symmetry from cubic β-Fe2O3 to a monoclinic structure. We designated this new phase as ζ-Fe2O3. ζ-Fe2O3 has thus a monoclinic crystal structure with a space group of I2/a and lattice constants of a = 9.17 Å, b = 9.30 Å, c = 8.50 Å, angle β = 97.6° and a unit volume of V = 718.4 Å3 at 42.9 GPa. While the new iron(III) oxide phase has several structural features that resemble those of its precursor (cubic β-Fe2O3), it also exhibits some unusual pressure-induced changes in its crystal lattice. In particular, the octahedral Fe b-site splits into two non-equivalent Fe sites (Fe1 and Fe4 with a 1:1 ratio), the octahedral Fe d-site splits into four non-equivalent Fe sites (Fe2, Fe3, Fe5, and Fe6 with a 1:1:2:2 ratio) and the β angle between the a- and c-axis increases to 98° compared to 90° for β-Fe2O3. At 42.9–64.4 GPa pressures, the relative abundances of the four phases, i.e., α-Fe2O3, RO-Fe2O3, PPV-Fe2O3, and ζ-Fe2O3, were almost constant. This indicates that the ζ-Fe2O3 polymorph is much more stable at high pressures than γ-Fe2O3, which is transformed into α-Fe2O3 (and then into perovskite or post-perovskite phases) once the pressure exceeds ~37 GPa.
Moreover, each phase's pressure dependence was monotonous, and the cell volume and parameters gradually changed as the pressure increased. When examining the XRD data, it should be stressed that the XRD patterns of β-Fe2O3 and ζ-Fe2O3 are distinct with different numbers of peaks. This implies that the symmetry of the crystal structure became lower (cubic → monoclinic), and the XRD pattern of ζ-Fe2O3 cannot be reproduced by modifying the pattern of β-Fe2O3 considering the effects of strains and defects (peak shift, peak broadening, changing the peak intensities, etc.). If you are looking for high quality, high purity, and cost-effective Iron oxide, or if you require the latest price, please feel free to email contact mis-asia.
