Interplay between H-bonding proton dynamics and Fe valence fluctuations in Fe3(P O4)2(OH)2 at high pressure

We pressure tune the hydrogen bond in Fe-O-H⋅⋅⋅O-P structural segments of mixed-valence barbosalite (Fe2+Fe23+)(PO4)2(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 10-30 GPa, Fe Mössbauer spectroscopy discerns Fe2+⇔Fe3+ valence fluctuations at proximate cations of the hydrogen bonds. When the original crystal potential at an Fe2+ site is perturbed by proton delocalization at a ligand, electron exchange is induced along Fe2+→L→Fe3+ pathways [ligand L=O or (OH)- of shared octahedral faces]. Thus, (Fe2+Fe23+)(PO4)2(OH)2 under pressure exemplifies the interplay between proton (THz) and electron (MHz) dynamics on two disparate timescales in the same condensed phase.