Exceptionally strong magnetism and structure of SrFeO $$_2$$ above 100 GPa

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The study of the stability of iron in different oxygen environments under high pressure has aroused great interest because of its wide impact for research from chemistry to geophysics. Understanding the behavior of iron under extreme conditions, in particular its valence and spin state, can help explain the stability of different phases of Fe, guide the synthesis of new resistant materials, and test theoretical approaches to the electronic structure of Fe. in extreme conditions.1. The crystal structures of iron oxides and their stability under high pressure are particularly important for understanding the formation and dynamics of Earth-like planet interiors rich in iron-containing minerals.2.3. For example Fe(_2)O(_3) undergoes a series of transitions in the 0-100 GPa range with 5 different structures in the 40-50 GPa region4. Above the last transition pressure at 50 GPa, no long range order is detected5. In Fe(_3)O(_4)another very common form of iron oxide, a structural transition occurs at 8 GPa6 while the ferromagnetic state gradually disappears under pressure, vanishing at 70 GPaseven. Other less common iron oxides like Fe(_4)O(_5)8 and Fe(_5)O(_6) have recently been found to be stable under pressure, but decompose to FeO and Fe(_3)O(_4) above 40 GPa9. Finally, FeO should have a zero spin configuration above 70 GPaten.

In all these compounds, the Fe atoms exclusively occupy octahedral or tetrahedral sites. Recently, a new system with iron in a square planar oxygen environment, SrFeO(_2)has been synthesized11, providing a new playground to study the magnetic and structural stability of Fe in a different local symmetry. SrFeO(_2) crystallized in the P4/mmm space group with Fe-O forming planar layers sandwiched by Sr atoms. The magnetic and structural properties of SrFeO(_2) under pressure have been previously studied by Mössbauer spectroscopy, resistivity and X-ray diffraction12then by X-ray emission spectroscopy13 up to 40–50 GPa. In this pressure range, SrFeO(_2) undergoes a series of well-identified electronic, magnetic and structural transitions. The main change occurs around 40 GPa with an abrupt contraction of the lattice within the same space group, a decrease in the magnetic moment and a drop in the resistivity, which marks a transition from an insulating antiferromagnetic state to high spin (AFM-I-HS) to the metallic ferromagnetic intermediate spin state (FM-M-IS). Additional phenomena are expected at higher pressure from molecular orbital details14 but data are sparse in this pressure range. Resistivity measurements reveal an anomaly between 65 and 90 GPa which is interpreted as a resurgence of the metal-insulator transition15. On the other hand, the DFT calculations16 predict that magnetism will survive well in the Mbar range within the same structure, but the data is lacking. In this paper, we explore the structural and magnetic stability of SrFeO(_2) above 100 GPa by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray circular magnetic dichroism (XMCD) at the FeK edge. The results demonstrate the exceptional stability of SrFeO(_2) up to 110 GPa.

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