Site-specific spin crossover in F e2Ti O4 post-spinel under high pressure up to nearly a megabar
Abstract
X-ray diffraction studies to ∼90 GPa at room temperature show that F e2Ti O4 ferrous inverse spinel undergoes the following sequence of structural transitions: cubic (F d 3 ¯m ) →∼8 GPa tetragonal (I 41/a m d ) →∼16 GPa orthorhombic (C m c m ) →∼55 GPa orthorhombic (P m m a ) , at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting ∼25 % of all Fe locations. This is manifest as a steeper volume decrease of Δ V /V0∼3.5 % beyond ∼40 GPa and an emergent diamagnetic component discerned in 57Fe Mössbauer spectroscopy at variable cryogenic temperatures. A subsequent C m c m →P m m a Fe/Ti disorder-order reconfiguration is facilitated at sixfold coordinated (octahedral) sites. The rest of the high-spin Fe in sixfold and eightfold coordinated sites (∼75 % abundance) in the Pmma phase exhibit average saturation internal magnetic fields of Hh f∼42 T to ∼90 GPa , typical of spin-only (orbitally quenched) Fermi-contact values. By contrast, average Hh f∼20 T values, signifying unquenched orbital moments, occur below the 40 -45 GPa spin-crossover initiation regime in the Cmcm phase. Therefore, site-specific spin crossover invokes a cooperative lattice response and polyhedral distortions at the rest of the high-spin Fe sites, translating to 3 d level (sub-band) changes and consequential orbital moment quenching. Near ∼90 GPa , F e2Ti O4 is a partially spin-converted chemically ordered Pmma post-spinel having a persistent charge gap of ∼100 meV . Despite structural symmetry changes, partial spin crossover and lattice compressibility, resulting in a ∼33 % total reduction in unit-cell volume and corresponding 3 d bandwidth broadening, strong electron correlations persist at high densification.
- Publication:
-
Physical Review B
- Pub Date:
- July 2017
- DOI:
- 10.1103/PhysRevB.96.045108
- arXiv:
- arXiv:1809.09056
- Bibcode:
- 2017PhRvB..96d5108X
- Keywords:
-
- Condensed Matter - Strongly Correlated Electrons;
- Condensed Matter - Materials Science
- E-Print:
- 35 Pages and 7 Figures