Interplay between structural and magnetic-electronic responses of FeA l2 O4 to a megabar: Site inversion and spin crossover

X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl2O4 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 postspinel phase is onset. This is attributable to pressure driven Fe↔Al site inversion at room temperature, corroborated by signatures in the Fe57 Mössbauer spectroscopy pressure data. At the tetragonal→postspinel transition, onset in the range 45-50 GPa, there is a concurrent emergence of a nonmagnetic spectral component in the Mössbauer 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 postspinel structure. There is also a much steeper volume change ΔV/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 Mössbauer data evidence 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 postspinel lattice at high densities. The R (300 K) data decrease 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, FeAl2O4 is definitively nonmetallic with an estimated gap of ∼400 meV at ∼100 GPa. At such high densification appreciable bandwidth broadening and gap closure would be anticipated. Reasons for the resilient nonmetallic behavior are briefly discussed.