Electronic and magnetic phase diagram of Sr2FeO4 at high pressure: A synchrotron Mössbauer study

Transition metal (TM) oxides with high oxidation state TM ions exhibit a variety of unconventional electronic and magnetic states owing to electron correlations effects combined with highly covalent TM-O bonding. Here, we have studied the pressure dependence of electronic state and magnetism of the K2NiF4-type iron(IV) oxide Sr2FeO4 up to 89 GPa by temperature and magnetic field dependent energy-domain synchrotron M & ouml;ssbauer spectroscopy and derived a (P,T) magnetic phase diagram. Considering also previous resistance studies [Rozenberg et al., Phys. Rev. B 58, 10283 (1998)] several magnetic and electronic regimes with increasing pressure can be identified. Near 7 GPa, the insulating cycloidal antiferromagnetic low-P state is transformed into a semiconducting ferromagnetic state and the magnetic ordering temperature Tm increases from 55 K at ambient pressure to about 100 K at 13 GPa. Between 18 and about 50 GPa the system is ferromagnetic and metallic (FMM) with a strong rise of Tm to above room temperature (RT). Contrary to a recent theoretical study [Kazemi-Moridani et al., Phys. Rev. B 109, 165146 (2024)], the FMM state is attributed to a high-spin t32ge1g electronic state with itinerant eg coupled to more localized t2g electrons. Between 50 and 89 GPa a doublet with large quadrupole splitting in the RT M & ouml;ssbauer spectra indicates a partial high-spin to low-spin (t42g) transition leading to a decrease in Tm again. The general features of the (P,T) phase diagram of Sr2FeO4 are comparable to those of other simple and A-site ordered iron(IV) perovskite-related oxides with the peculiarity that Sr2FeO4 adopts an insulating state without charge disproportionation of Fe4+ in the low-P region. The high-pressure behavior of Sr2FeO4 and other iron(IV) oxides may be relevant for exploring the role of Hund's physics in multiorbital systems and contributing to the understanding of the electronic situation in unconventional superconductors such as La3Ni2O7 and Sr2RuO4.