Interplay Between Structural And Magnetic-Electronic Responses Of Feal2o4 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 similar to 18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to similar 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 Fe-57 Mossbauer 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 Mossbauer 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 Delta V/V similar to 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, similar to 106 GPa, the Mossbauer data evidence a diamagnetic Fe low-spin abundance of similar to 50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of similar to 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 similar to 100 GPa. Despite a similar to 26% unit-cell volume densification from the lattice compressibility, structural transitions, and spin crossover, FeAl2O4 is definitively nonmetallic with an estimated gap of similar to 400 meV at similar to 100 GPa. At such high densification appreciable bandwidth broadening and gap closure would be anticipated. Reasons for the resilient nonmetallic behavior are briefly discussed.