  X-ray diffraction studies to ~90 GPa at room temperature show that
Fe$_{2}$TiO$_{4}$ ferrous inverse spinel undergoes the following sequence of
structural transitions : cubic (Fd3m) to tetragonal (I41/amd)to
orthorhombic(Cmcm) to orthorhombic(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 dV/V0 ~ 3.5% beyond
~40 GPa and an emergent diamagnetic component discerned in 57Fe M\"ossbauer
spectroscopy at variable cryogenic temperatures. A subsequent Cmcm to Pmma
Fe/Ti disorder-order reconfiguration is facilitated at 6-fold coordinated
(octahedral) sites. The rest of the high-spin Fe in 6-fold and 8-fold
coordinated sites (~75% abundance) in the Pmma phase exhibit average saturation
internal magnetic fields of H$_{hf}$ ~ 42 T to ~90 GPa, typical of spin-only
(orbitally quenched) Fermi-contact values. By contrast average H$_{hf}$ ~ 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 3d level
(sub-band) changes and consequential orbital moment quenching. Near ~90 GPa
Fe$_{2}$TiO$_{4}$ is a partially spin-converted chemically ordered Pmma
post-spinel structure 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 3d
bandwidth broadening, strong electron correlations persist at high
densification.
