Interplay between structural and magnetic-electronic responses of FeAl$_{2}$O$_{4}$ to a megabar: site inversion and spin crossover

TL;DR

This study investigates how high pressure induces structural and magnetic-electronic changes in FeAl₂O₄, revealing phase transitions, site inversion, and spin crossover phenomena up to 106 GPa, with implications for magnetic behavior under extreme conditions.

Contribution

It provides the first detailed analysis of pressure-induced site inversion and spin crossover in FeAl₂O₄, combining X-ray diffraction and Mössbauer spectroscopy data.

Findings

FeAl₂O₄ transforms to tetragonal phase at ~18 GPa

Spin crossover occurs at 45-50 GPa during structural transition

Magnetic properties persist at high densities with 50% low-spin Fe

Abstract

X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl$_{2}$O$_{4}$ transforms to a tetragonal phase at ~18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ~45 GPa, after which a transformation to a Cmcm post-spinel phase is onset. This is attributable to pressure driven Fe-Al site inversion at room temperature, corroborated by signatures in the 57Fe M\"ossbauer spectroscopy pressure data. At the tetragonal to post-spinel transition, onset in the range 45-50 GPa, there is a concurrent emergence of a non-magnetic spectral component in the M\"ossbauer data at variable cryogenic temperatures. This is interpreted as spin crossover at sixfold coordinated Fe locations emanated from site inversion. Spin crossover commences at the end of the pressure range of the tetragonal phase and progresses in the post-spinel structure. There is also a much steeper volume change dV/V ~ 10% in the range 45-50 GPa compared to the preceding pressure regime, from the combined effects of the structural transition and spin crossover electronic change. At the highest pressure attained, ~106 GPa, the M\"ossbauer data evidences a diamagnetic Fe low-spin abundance of ~50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of ~33 T. This is indicative of a magnetic ground state associated with strong covalency, as well as substantive disorder from site inversion and the mixed spin-state configuration. Intriguingly magnetism survives in such a spin-diluted post-spinel lattice at high densities. The R(300 K) data decreases by only two orders of magnitude from ambient pressure to the vicinity of ~100 GPa. Despite a ~26% unit-cell volume densification from the lattice compressibility, structural transitions and spin crossover.