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

X-ray diffraction studies to $\ensuremath{\sim}90\phantom{\rule{4.pt}{0ex}}\text{GPa}$ at room temperature show that $\mathrm{F}{\mathrm{e}}₂\mathrm{Ti}{\mathrm{O}}₄$ ferrous inverse spinel undergoes the following sequence of structural transitions: $\mathrm{cubic}\phantom{\rule{4pt}{0ex}}(Fd\overline{3}m)\stackrel{\ensuremath{\sim}8\phantom{\rule{4.pt}{0ex}}\text{GPa}}{\ensuremath{\rightarrow}}\mathrm{tetragonal}\phantom{\rule{4pt}{0ex}}(I{4}₁/amd)\stackrel{\ensuremath{\sim}16\phantom{\rule{4.pt}{0ex}}\text{GPa}}{\ensuremath{\rightarrow}}\mathrm{orthorhombic}\phantom{\rule{4pt}{0ex}}(Cmcm)\stackrel{\ensuremath{\sim}55\phantom{\rule{4.pt}{0ex}}\text{GPa}}{\ensuremath{\rightarrow}}\mathrm{orthorhombic}\phantom{\rule{4pt}{0ex}}(Pmma),$ at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting $\ensuremath{\sim}25%$ of all Fe locations. This is manifest as a steeper volume decrease of $\mathrm{\ensuremath{\Delta}}V/{V}₀\ensuremath{\sim}3.5%$ beyond $\ensuremath{\sim}40\phantom{\rule{4.pt}{0ex}}\text{GPa}$ and an emergent diamagnetic component discerned in $⁵⁷\mathrm{Fe}$ M\"ossbauer spectroscopy at variable cryogenic temperatures. A subsequent $Cmcm\ensuremath{\rightarrow}Pmma$ 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 ($\ensuremath{\sim}75%$ abundance) in the Pmma phase exhibit average saturation internal magnetic fields of ${H}_hf\ensuremath{\sim}42\phantom{\rule{4.pt}{0ex}}\text{T}$ to $\ensuremath{\sim}90\phantom{\rule{4.pt}{0ex}}\text{GPa}$, typical of spin-only (orbitally quenched) Fermi-contact values. By contrast, average ${H}_hf\ensuremath{\sim}20\phantom{\rule{4.pt}{0ex}}\text{T}$ values, signifying unquenched orbital moments, occur below the $40--45\phantom{\rule{4.pt}{0ex}}\text{GPa}$ 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 $3d$ level (sub-band) changes and consequential orbital moment quenching. Near $\ensuremath{\sim}90\phantom{\rule{4.pt}{0ex}}\text{GPa}, \mathrm{F}{\mathrm{e}}₂\mathrm{Ti}{\mathrm{O}}₄$ is a partially spin-converted chemically ordered Pmma post-spinel having a persistent charge gap of $\ensuremath{\sim}100\phantom{\rule{4.pt}{0ex}}\text{meV}$. Despite structural symmetry changes, partial spin crossover and lattice compressibility, resulting in a $\ensuremath{\sim}33%$ total reduction in unit-cell volume and corresponding $3d$ bandwidth broadening, strong electron correlations persist at high densification.