Wigner-Mott insulator-to-insulator transition at pressure in charge-ordered Fe2OBO3

Magnetic-electronic studies of mixed-valence Fe${}_{2}$OBO${}_{3}$ have shown that ionic charge order (CO) is disrupted at \ensuremath{\sim}16 GPa. The pertinent minority-spin carrier exhibits persistent intersite electron exchange Fe${}^{2+}$ $\ensuremath{\Leftrightarrow}$ Fe${}^{3+}$ to well beyond this pressure. Temperature-dependent electrical transport measurements over an extended pressure range presented here demonstrate that the electronic structure remains gapped to well beyond 16 GPa. Extrapolation of data to higher pressure suggests that metallization will only prevail at $Pg50$ GPa. Both the persistent gapped electronic state across the CO instability and signature of carrier confinement to Fe-Fe dimers in the Fe${}^{2+}$ $\ensuremath{\Leftrightarrow}$ Fe${}^{3+}$ electron exchange are rationalized as crossover from a Wigner crystal (site centered) insulator to a dimer Mott (bond centered type) insulator---``Wigner-Mott transition'' at \ensuremath{\sim}16 GPa. The dimer insulating state is a consequence of modulation of the relevant hopping parameter $t$ in quasi-low-dimensional features in the structure (ribbons and chains). Complementary structural studies suggest that the $a$ axis is appreciably more compressible than other crystallographic directions of the original monoclinic unit cell. Therefore, such a modulation in $t$ may arise from Peierls type distortions along the $a$ axis or else stems from intrinsic modulation in the $c$ axis direction of the unit cell. This is aided by a monoclinic ($P$2${}_{1}/c$) \ensuremath{\rightarrow} orthorhombic (Pmcn) structural adjustment that is concurrent across the electronic transition. Pressure tuning of relative values of on-site $U$/$t$ and intersite $V$/$t$ Coulomb interaction parameters of the quasi-low-dimensional features evolve the system from site-centered to dimer-centered electron localization.