Observation of the second percolation threshold during recrystallization in amorphous pyrochlore

Abstract

The behaviour of magnetite Fe3O4 at high pressure has been tackled quite extensively in recent years in an effort to eluci-date the magnetic-electronic state and structural characteristics of the high-pressure phase [1]. Maghemite (γ-Fe2O3), the fully oxidised structural analogue of magnetite Fe3O4, is both of geophysical significance and of crucial importance to the magnetic read-write recording industries. In this study the behaviour of a well-characterised sample ofmaghemite, in which there is evidence of vacancy ordering [2], has been investigated under quasi-hydrostatic conditions in a diamond-anvil cell at pressures up to ∼30 GPa using synchrotron X-ray diffraction (beamline XRD1, ELETTRA). Inmaghemite iron is trivalent in the spinel lattice and a preponderance of ordered or disordered vacancies (perhaps partially occupied by impurity atoms) occurs, to ensure charge compensation. Our sample has vacancies in an ordered array as deduced from the superstructure peaks in the diffraction pattern [2]. The original ordered vacancy structure is main-tained upon pressurising to ∼17 GPa. This is also the onset transformation pressure from maghemite to a new structural phase, ascribed to hematite (α-Fe2O3). Maghemite (and vacancy superstructure) phase signatures coexist with the new structural phase up to ∼25GPa. Radical changes occur to the original maghemite vacancy superstructure in this regime of phase coexistence. The transformation to the new structural phase of Fe2O3 is completed at ∼27 GPa. This is similar to the transition pressures reported in a previous Fe Mossbauer pressure study of well crystallized maghemite [3] and for annealed bulk γ-Fe2O3 [4]. The pressure-induced transfor-mation is irreversible and upon decompression the high pressure phase is preserved to ambient conditions. The bulk modulus for the maghemite and the hematite highpressure phase have been extracted from the P-V data.