Interplay between H-bonding proton dynamics and Fe valence fluctuations in Fe3(PO4)2(OH)2 at high pressure

Abstract

We pressure tune the hydrogen bond in Fe-O--H\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}O-P structural segments of mixed-valence barbosalite $({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_{2}^{3+}){(\mathrm{P}{\mathrm{O}}_{4})}_{2}{(\mathrm{OH})}_{2}$. Infrared spectroscopy evidences changes in softening of O--H stretch modes and excessive profile broadening onset below 10 GPa. Single-crystal x-ray diffraction shows pseudosymmetrization of the original monoclinic unit cell concurs with these changes in the O--H vibrational mode. These are considered compelling indicators of proton delocalization onset below 10 GPa as hydrogen bonds are strengthened under pressure. Subsequently in the range $10\ensuremath{-}30$ GPa, Fe M\"ossbauer spectroscopy discerns ${\mathrm{Fe}}^{2+}\ensuremath{\Leftrightarrow}{\mathrm{Fe}}^{3+}$ valence fluctuations at proximate cations of the hydrogen bonds. When the original crystal potential at an ${\mathrm{Fe}}^{2+}$ site is perturbed by proton delocalization at a ligand, electron exchange is induced along ${\mathrm{Fe}}^{2+}\ensuremath{\rightarrow}L\ensuremath{\rightarrow}{\mathrm{Fe}}^{3+}$ pathways [ligand $L=\mathrm{O}$ or ${(\mathrm{OH})}^{\ensuremath{-}}$ of shared octahedral faces]. Thus, $({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_{2}^{3+}){(\mathrm{P}{\mathrm{O}}_{4})}_{2}{(\mathrm{OH})}_{2}$ under pressure exemplifies the interplay between proton (THz) and electron (MHz) dynamics on two disparate timescales in the same condensed phase.