Site-specific spin crossover in Fe2TiO4 post-spinel under high pressure up to nearly a megabar

• Published in: Physical review. B 2017-07, Vol.96 (4)
• Main Authors: Xu, W M, Hearne, G R, Layek, S, Levy, D, Itié, J-P, Pasternak, M P, Rozenberg, G Kh, Greenberg, E
• Format: Article
• Language: English
• Subjects: Compressibility; Crossovers; Cryogenic temperature; Densification; Diamagnetism; Mossbauer spectroscopy; Organic chemistry; Polymethyl methacrylate; Reconfiguration; Spinel; Unit cell; X-ray diffraction
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.96.045108

X-ray diffraction studies to ∼90GPa at room temperature show that Fe2TiO4 ferrous inverse spinel undergoes the following sequence of structural transitions: cubic(Fd3¯m)→∼8GPatetragonal(I41/amd)→∼16GPaorthorhombic(Cmcm)→∼55GPaorthorhombic(Pmma), 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 ΔV/V0∼3.5% beyond ∼40GPa and an emergent diamagnetic component discerned in Fe57 Mössbauer spectroscopy at variable cryogenic temperatures. A subsequent Cmcm→Pmma 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 Hhf∼42T to ∼90GPa, typical of spin-only (orbitally quenched) Fermi-contact values. By contrast, average Hhf∼20T values, signifying unquenched orbital moments, occur below the 40–45GPa 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 3d level (sub-band) changes and consequential orbital moment quenching. Near ∼90GPa, Fe2TiO4 is a partially spin-converted chemically ordered Pmma post-spinel having a persistent charge gap of ∼100meV. Despite structural symmetry changes, partial spin crossover and lattice compressibility, resulting in a ∼33% total reduction in unit-cell volume and corresponding 3d bandwidth broadening, strong electron correlations persist at high densification.