Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3

Abstract

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO₃ and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a "mixed" HS/LS state with a stable abundance ratio realized in the [Formula: see text] crystal structure with a single Fe site within a wide pressure range of ~ 50-106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.

Figure 1

X-ray single crystal (a) and powder (b) diffraction patterns of FeBO₃ at RT at various pressures (λ = 0.2898 Å and 0.3738 Å, respectively). Note a splitting of the $(10\overline{2})$ reflection in the SC and powder XRD pattern at 129 and 145.8 GPa, respectively, signifying lowering of the original symmetry. * marks an unidentified peak, which disappears at higher pressures.

Figure 2

Pressure dependencies of the unit-cell volume divided by Z unit formulas (Z = 6 and 4 for the $R\overline{3}\mathrm{c}$ and C2/c phases, respectively) (a), determined in the powder and single crystal XRD studies; FeO₆ octahedral volume (b) and average Fe–O, B-O distances (c) for FeBO₃. The solid, dashed, dash-dot and short-dash lines in (a) are fits with the Birch-Murnaghan equation of state (see text). The panel (b) insets show the $R\overline{3}\mathrm{c}$ and C2/c crystal structures, respectively. The red and green spheres correspond to the oxygen and boron atoms.

Figure 3

(a) Mössbauer spectra of FeBO₃ at various pressures and room temperature. Empty circles represent experimental data points whereas the black solid line through the data points represents the overall fit to the data from the sum of sub-components shown. The blue and orange shaded sub-components refer to LP-HS-and HP-HS states, whereas the green one refers to HP-LS. (b) Pressure-dependence of the isomer shift (IS), hyperfine field (H_hf) and abundances (or area percentage) extracted from best fits to the Mössbauer spectra. Solid symbols indicate the values extracted from low-temperature measurements (3–10 K); IS values at 140 GPa correspond to T = 150 K.

Figure 4

Mössbauer spectra of FeBO₃ at different temperatures at 85 GPa (a), 140 GPa (b) and 115 GPa (c). Spectra at 115 and 140 GPa were collected using synchrotron MS. Spectra collected at 115 GPa allow us to define the Néel temperature of ~ 60(10) K.

Figure 5

Temperature dependence of the relative unit-cell volume for FeBO₃ at ~ 78 GPa. For comparison we show also the temperature dependence of the relative unit-cell volume for HS Fe₂O₃, which belong to the same FeXO₃ family, at ambient pressure (solid line) calculated from Ref.. The dashed line is to guide the eye for a conventional thermal expansion behavior of FeBO₃ at the range 10–180 K.