FeC r2O4 spinel to near megabar pressures: Orbital moment collapse and site-inversion facilitated spin crossover
Abstract
The interplay between lattice, orbital, and spin degrees of freedom in iron chromite (spinel [Fe ] {C r2 } O4) has been investigated to near megabar pressures. The cubic-to-tetragonal transition, from static Jahn-Teller distortions at the Fe locality, rises from TJ -T∼135 K at ambient pressure to 300 K by ∼12 GPa . The tetragonal distortion progressively increases and orbital moment quenching is triggered beyond ∼24 GPa , as monitored by the magnetic hyperfine field Hh f at Fe sites. In the range 30-60 GPa, original F e2 + tetrahedral sites with an unquenched orbital moment (Hh f∼20 T ) coexist with newly evolved F e2 + at tetrahedral sites having the orbital moment quenched and a resultant large Hh f∼35 T . Additionally, new F e2 + sites having distinguishable orbital moment quenching signatures (Hh f∼42 T ) are discerned. Those sites also have other Fe nuclear hyperfine interaction parameter values typical of F e2 + in octahedral coordination. There is a concurrent change to a steeper decrease of unit-cell volume as pressure rises above ∼30 GPa . These electronic and lattice responses are interpreted as signatures of progressive partial spinel inversion from high-spin Fe ↔Cr tetrahedral/octahedral site exchange, triggered near ∼30 GPa . Beyond 60 GPa a new diamagnetic low-spin Mössbauer spectral component emerges. This is preceded by an inflection and discontinuity in the pressure dependence of the resistance and tetragonal unit-cell volume, respectively. By ∼93 GPa , half of the iron is low spin in octahedral sites from evolved tetragonal-spinel inversion processes. The remainder reside in tetrahedral high-spin sites with Hh f∼30 T . The charge gap from electron correlations does not close, despite an anticipated appreciable band broadening from a ∼30 % unit-cell volume reduction upon pressurization to ∼93 GPa . This is attributable to an increase in effective on-site repulsion (Hubbard Ueff) with increasing pressure or at spin crossover, specific to the C r3 +(d3) and F e2 +(d6) electronic configurations, respectively. Thus a ∼200 meV "Mott" gap prevails in the mixed spin-state of the partially inverted tetragonal phase [F e1 -xC rx] {F exC r2 -x } O4(x ∼0.5 ) up to near a megabar (100 GPa) densification.
- Publication:
-
Physical Review B
- Pub Date:
- January 2017
- DOI:
- 10.1103/PhysRevB.95.045110
- Bibcode:
- 2017PhRvB..95d5110X