Zeta-Fe(2)O(3) – A new stable polymorph in iron(III) oxide family

Iron(III) oxide shows a polymorphism, characteristic of existence of phases with the same chemical composition but distinct crystal structures and, hence, physical properties. Four crystalline phases of iron(III) oxide have previously been identified: α-Fe(2)O(3) (hematite), β-Fe(2)O(3), γ-Fe(2)O(3) (maghemite), and ε-Fe(2)O(3). All four iron(III) oxide phases easily undergo various phase transformations in response to heating or pressure treatment, usually forming hexagonal α-Fe(2)O(3), which is the most thermodynamically stable Fe(2)O(3) polymorph under ambient conditions. Here, from synchrotron X-ray diffraction experiments, we report the formation of a new iron(III) oxide polymorph that we have termed ζ-Fe(2)O(3) and which evolved during pressure treatment of cubic β-Fe(2)O(3) ([Image: see text] space group) at pressures above 30 GPa. Importantly, ζ-Fe(2)O(3) is maintained after pressure release and represents the first monoclinic Fe(2)O(3) polymorph (I2/a space group) that is stable at atmospheric pressure and room temperature. ζ-Fe(2)O(3) behaves as an antiferromagnet with a Néel transition temperature of ~69 K. The complex mechanism of pressure-induced transformation of β-Fe(2)O(3), involving also the formation of Rh(2)O(3)-II-type Fe(2)O(3) and post-perovskite-Fe(2)O(3) structure, is suggested and discussed with respect to a bimodal size distribution of precursor nanoparticles.