Electronic and magnetic phase diagram of $S{r}_2\mathrm{Fe}{O}_4$ 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 $K_2\mathrm{Ni}{F}_4$-type iron(IV) oxide $S{r}_2\mathrm{Fe}{O}_4$ up to 89 GPa by temperature and magnetic field dependent energy-domain synchrotron Mössbauer 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 $T_m$ 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 $T_m$ 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 $t_{2g}^3{e}_g^1$ electronic state with itinerant $e_g$ coupled to more localized $t_{2g}$ electrons. Between 50 and 89 GPa a doublet with large quadrupole splitting in the RT Mössbauer spectra indicates a partial high-spin to low-spin ($t_{2g}^4$) transition leading to a decrease in $T_m$ again. The general features of the ($P,T$) phase diagram of $S{r}_2\mathrm{Fe}{O}_4$ are comparable to those of other simple and $A$-site ordered iron(IV) perovskite-related oxides with the peculiarity that $S{r}_2\mathrm{Fe}{O}_4$ adopts an insulating state without charge disproportionation of $F{e}^{4+}$ in the low-$P$ region. The high-pressure behavior of $S{r}_2\mathrm{Fe}{O}_4$ 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 $L{a}_{3}N{i}_2{O}_7$ and $S{r}_2\mathrm{Ru}{O}_4$.