High-pressure polymorphism of the iron oxides

摘要: Summary. High-pressure shock-wave data for hematite (Fe2O3) and magnetite (Fe3O4) are remarkable the very large density increases (∼ 10 per cent at 100 GPa) associated with shock-induced transformation from relatively close-packed corundum spinel structures. The recently reported of Jeanloz & Ahrens wustite (Fe0.94O) also reveal a major phase change which they interpret as B1 B2 polymorphism. Reassessment data, however, indicates that zero-pressure increase, polymorphism, is least 10—16 possibly great 18–28 cent. all three iron oxides thus appear to be too explained in terms geometric rearrangement (e.g. B2) usual Fe2+, Fe3+ O2- ions, unless it assumed these ions possess substantially smaller effective radii high-pressure phases. In this connection, proposed more covalently bonded nickel arsenide (NiAs) derivative structures might accommodate dense oxide phases stoichiometries high pressures. Systematics, involving volume free energy changes NaCl NiAs polymorphism various transition metal chalcogenides, suggest pressure ∼ 31 GPa FeO an estimated increase 11 An alternative, or additional, mechanism by Fe-O bonds shortened significantly involves spin-pairing 3d electrons. Calculations, based on structural Co3+ oxides, provide 50 9 transition. While occurrence substantial ‘mixed-phase’ regimes along Fe2O3 Hugoniots seems preclude exclusively electronic interpretation observed changes, quite possible augments due Comparative studies behaviour MnO, FeO, CoO NiO should resolve relative roles transitions. stability type would enhance their solubility molten Fe, attractive scenario incorporation oxygen into Earth's core.