![]() ![]() The evolution of the volume represented in the inset of Fig. The overall result is consistent with previous diffraction experiment reported 12 up to 45 GPa. The structural transition found at \(P_c \approx 30\) GPa is characterized by a sizable contraction of the a parameter while the c parameter exhibits only a slope change marking a variation of its compressibility. Their evolution as function of pressure is shown in Fig. Using a Le Bail fit on the 19 first Bragg reflections, we extracted the values of the a and c lattice parameters. The change in structure is clearly visible around 30 GPa. The intensity is displayed in logarithmic scale to enhance the weak intensity features. The diffractograms collected between 13 and 111 GPa are presented as a color map in Fig. The results demonstrate the outstanding stability of SrFeO \(_2\) up to 110 GPa. In this article, 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 Fe K-edge. On the other hand, DFT calculations 16 predict that magnetism will survive well within the Mbar range within the same structure but data are lacking. Resistivity measurements reveal an anomaly between 65 and 90 GPa which is interpreted as a resurgence of metal-insulator transition 15. Additional phenomena are expected at higher pressure from the details of the molecular orbitals 14 but data are scarce in this pressure range. The main change occurs around 40 GPa with a sudden contraction of the lattice within the same space group, a decrease of the magnetic moment and drop of the resistivity, which marks a transition from an antiferromagnetic, insulating high spin state (AFM-I-HS) to ferromagnetic, metallic intermediate spin state (FM-M-IS). In this pressure range, SrFeO \(_2\) is shown to undergo a series of well-identified electronic, magnetic and structural transitions. The magnetic and structural properties of SrFeO \(_2\) under pressure were previously studied by Mössbauer spectroscopy, resistivity and X-ray diffraction 12, and subsequently by X-ray emission spectroscopy 13 up to 40–50 GPa. SrFeO \(_2\) crystallized in the P4/ mmm space group with Fe-O forming planar layers sandwiched by Sr atoms. Recently a new system with iron in a square planar oxygen environment, SrFeO \(_2\), was synthesized 11, offering a new playground to investigate the magnetic and structural stability of Fe in a different local symmetry. In all these compounds, Fe atoms occupy exclusively octahedral or tetrahedral sites. Finally, FeO is expected to have a zero spin configuration above 70 GPa 10. ![]() Other less common iron oxides like Fe \(_4\)O \(_5\) 8 and Fe \(_5\)O \(_6\) were recently discovered to be stable under pressure, but decompose into FeO and Fe \(_3\)O \(_4\) above 40 GPa 9. In Fe \(_3\)O \(_4\), another very common form of iron oxide, a structural transition occurs at 8 GPa 6 while the ferromagnetic state progressively disappears under pressure, vanishing at 70 GPa 7. Above the last transition pressure at 50 GPa, no long range order is detected 5. 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 region 4. The crystalline structures of iron oxides and their stability under high pressure is especially important to understand the formation and dynamics of Earth-like planet interiors which are rich in Fe-bearing minerals 2, 3. Understanding how iron behaves at extreme conditions, in particular its valence and spin state, can help to explain the stability of various Fe phases, guide the synthesis of new resistant materials and test theoretical approaches of Fe electronic structure at extreme conditions 1. The investigation of the stability of iron in different oxygen environments under high pressure has attracted strong interest because of its broad impact for research in chemistry to geophysics. ![]()
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