We pressure tune the hydrogen bond in Fe-O–H⋅⋅⋅O-P structural segments of mixed-valence barbosalite $\left({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_2^{3+}\right){\left(\mathrm{P}{\mathrm{O}}_4\right)}_2{\left(\mathrm{OH}\right)}_2$. 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 ${\mathrm{Fe}}^{2+}\iff {\mathrm{Fe}}^{3+}$ 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 ${\mathrm{Fe}}^{2+}\to L\to {\mathrm{Fe}}^{3+}$ pathways [ligand $L=\mathrm{O}$ or ${\left(\mathrm{OH}\right)}^{-}$ of shared octahedral faces]. Thus, $\left({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_2^{3+}\right){\left(\mathrm{P}{\mathrm{O}}_4\right)}_2{\left(\mathrm{OH}\right)}_2$ under pressure exemplifies the interplay between proton (THz) and electron (MHz) dynamics on two disparate timescales in the same condensed phase.

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• High-pressure studies
• Hydrogen bonds
• Density functional theory
• Infrared spectroscopy
• Mössbauer spectroscopy
• X-ray diffraction