FeCr2O4 spinel to near megabar pressures: Orbital moment collapse and site-inversion facilitated spin crossover

The interplay between lattice, orbital, and spin degrees of freedom in iron chromite (spinel $[\mathrm{Fe}]{\mathrm{C}{\mathrm{r}}_{2}}{\mathrm{O}}_{4})$ has been investigated to near megabar pressures. The cubic-to-tetragonal transition, from static Jahn-Teller distortions at the Fe locality, rises from ${T}_{\mathrm{J}\text{\ensuremath{-}}\mathrm{T}}\ensuremath{\sim}135\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure to 300 K by $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The tetragonal distortion progressively increases and orbital moment quenching is triggered beyond $\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, as monitored by the magnetic hyperfine field ${H}_{hf}$ at Fe sites. In the range 30--60 GPa, original $\mathrm{F}{\mathrm{e}}^{2+}$ tetrahedral sites with an unquenched orbital moment $({H}_{hf}\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ coexist with newly evolved $\mathrm{F}{\mathrm{e}}^{2+}$ at tetrahedral sites having the orbital moment quenched and a resultant large ${H}_{hf}\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Additionally, new $\mathrm{F}{\mathrm{e}}^{2+}$ sites having distinguishable orbital moment quenching signatures $({H}_{hf}\ensuremath{\sim}42\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ are discerned. Those sites also have other Fe nuclear hyperfine interaction parameter values typical of $\mathrm{F}{\mathrm{e}}^{2+}$ in octahedral coordination. There is a concurrent change to a steeper decrease of unit-cell volume as pressure rises above $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. These electronic and lattice responses are interpreted as signatures of progressive partial spinel inversion from high-spin $\mathrm{Fe}\ensuremath{\leftrightarrow}\mathrm{Cr}$ tetrahedral/octahedral site exchange, triggered near $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Beyond 60 GPa a new diamagnetic low-spin M\"ossbauer 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 $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, 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 ${H}_{hf}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The charge gap from electron correlations does not close, despite an anticipated appreciable band broadening from a $\ensuremath{\sim}30%$ unit-cell volume reduction upon pressurization to $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. This is attributable to an increase in effective on-site repulsion (Hubbard ${U}_{\mathrm{eff}}$) with increasing pressure or at spin crossover, specific to the $\mathrm{C}{\mathrm{r}}^{3+}({d}^{3})$ and $\mathrm{F}{\mathrm{e}}^{2+}({d}^{6})$ electronic configurations, respectively. Thus a $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ ``Mott'' gap prevails in the mixed spin-state of the partially inverted tetragonal phase $[\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{r}}_{x}]{\mathrm{F}{\mathrm{e}}_{x}\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}}{\mathrm{O}}_{4}\phantom{\rule{0.28em}{0ex}}(x\ensuremath{\sim}0.5)$ up to near a megabar (100 GPa) densification.