The effect of pressure treatment on the crystal structure of β-Fe2O3 was investigated using high-pressure synchrotron radiation XRD measurements. Representative high-pressure synchrotron XRD spectra and the detailed Rietveld analyses of all the measured synchrotron radiation XRD patterns (including the values of the Rwp-factor) are depicted in Supplementary Figures S1–S7 in the Supplementary Material. At up to 10 GPa pressures, the sample consists of β-Fe2O3 and α-Fe2O3 in approximately the same ratio as in the original sample (93/7 wt.%). This reflects the pressure stability of β-Fe2O3 up to 10 GPa. The crystal structure of β-Fe2O3 was determined, and that of the increasing α-Fe2O3 phase was refined sequentially between 10 and 30 GPa. Some of the β-Fe2O3 nanoparticles undergo polymorphous transformation to α-Fe2O3, but no other iron(III) polymorphs are observed. At 29.6 GPa, the fractions of β-Fe2O3 and α-Fe2O3 were 35.1(2)% and 64.9(2)%, respectively.
Given the relative volume (mass) ratio of the two β-Fe2O3 particle size fractions in the starting material (as determined from TEM/HRTEM analysis, see above), the smaller nanoparticle assembly remains untransformed. Still, the larger nanoparticle assembly readily converts into α-Fe2O3. One might expect this trend for the conversion of β-Fe2O3 into α-Fe2O3 to continue as the pressure increases. However, there was a dramatic shift in the mechanism of the pressure-induced transformation when the applied pressure was raised above 30 GPa, with both the α-Fe2O3 and β-Fe2O3 phases undergoing new structural transformations. Specifically, α-Fe2O3 was converted into Rh2O3-II-type Fe2O3 (RO-Fe2O3, orthorhombic, Pbcn space group) and post-perovskite Fe2O3 (PPV-Fe2O3, orthorhombic, Cmcm space group) structures. Both these phases have previously been observed during high-pressure treatment of α-Fe2O3. The simultaneous formation of perovskite and post-perovskite structures can be understood by considering the particle size (and, hence, volume) distribution within the larger nanoparticle assembly. The applied pressure forces the crystal structure of α-Fe2O3 to change, but the magnitude of the change depends on the level of strain inside the nanoparticles, which tends to resist structural alteration. This strain, in turn, varies considerably with particle size. The transformation process is associated with the lowest overall Gibbs free energy, influenced by the strain and associated stress. In addition to these (post)-perovskite structures, some α-Fe2O3 nanoparticles remain untransformed because their size (and, hence, strain) is such that they can resist the effect of the applied pressure. 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.
