M A T T E R A T E X T R E M E S
S C I E N T I F I C H I G H L I G H T S
1 4 H I G H L I G H T S 2 0 2 1 I
High pressure tweaks the chemical behaviour of oxygen: unique Fe-O bonding at extreme conditions
Combining in-situ single-crystal X-ray diffraction, XANES and Mössbauer spectroscopy, the reduction of oxygen s formal valence down to 1.5- was demonstrated in cubic high-pressure FeO2 and FeO2Hx phases with a high-pressure HP-PdF2 type structure. Such an unexpected feature in Fe-O bonding at extreme conditions is important for understanding the oxygen cycle in the deep Earth interior.
Iron and oxygen are the most abundant elements by mass in the bulk Earth. Until recently, iron was considered to be the only geochemically significant element in Earth s interior with a variable oxidation state. While oxygen is also known to possess different oxidation states in different species (for example, molecular O2 in the atmosphere, O2- in oxides and silicates, O2
2- in peroxides, O2 3- in electrochemically
oxidised transition metal oxides), so far only oxidation state O2- was considered to exist in the materials constituting the Earth s mantle. Synthesis at high pressures and high temperatures (HPHT) yields numerous iron oxides with unexpected compositions, unusual crystal structures and intriguing physical properties. The existence of such iron oxides as Fe4O5, Fe5O6, Fe7O9, Fe5O7, Fe25O32  and especially recently discovered cubic FeO2  indicate that the chemistry of the Fe-O system might be very peculiar at the conditions of the deep Earth interior. Initially, it was proposed that in cubic high-pressure FeO2 and FeO2Hx (x=0 to 1) phases, oxygen forms peroxide (O2-) ions and iron remains ferrous, even at strongly oxidising conditions. However, due to the lack of experimental data, the chemical nature of FeO2 (and FeO2Hx) was highly uncertain.
The focus of the present work was to investigate the properties of high-pressure cubic FeO2 and FeO2Hx iron oxides: the crystal structures, the oxidation state of iron and oxygen, and the features of chemical bonding between them. Samples of cubic FeO2 and FeO2Hx were synthesised at BGI and at beamline ID27 in laser-heated diamond anvil cells by the direct interaction between iron and oxygen, or by decomposition of FeOOH at pressures between 40 and 80 GPa and temperatures above 1200 K. The crystal structures of both FeO2 and FeO2Hx were established against the single-crystal X-ray diffraction data collected in situ at beamlines ID15B and ID27. Accurate determination of the atomic coordinates of iron and oxygen prove that the FeO2 and FeO2Hx phases are isostructural and belong to the HP-PdF2 type structure, rather to the pyrite-type one, as was previously assumed . Structural data unambiguously exclude peroxide (O2-) ions as a structural component in cubic FeO2 and FeO2Hx.
In order to determine the oxidation state of iron, samples of cubic FeO2 and FeO2Hx were studied using Mössbauer and XANES spectroscopies at beamlines ID18 and ID24 (Figure 1). Both techniques suggest the 3+ oxidation of state of iron in cubic FeO2 and FeO2Hx phases. That led to unambiguous conclusion that the oxygen oxidation state
Fig. 2: Crystal structure of the FeO2 phase refined against in-situ SC-XRD data with a calculated electron density (DFT + DMFT). Both experimental and theoretical results suggest no bonding in between closest oxygen atoms.
Fig. 1: a) Mössbauer spectrum collected for the FeO2 phase. b) XANES spectra of isostructural hydrogen-containing FeO2H0.5 compared to the starting material. Both techniques are consistent with a Fe3+ oxidation state in both phases.