Séminaire

Vendredi 2 Septembre 2022 à 11h00.

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

''Hydrogen bonding, O–H⋅⋅⋅O, has widespread relevance in condensed phases, e.g., charge transport in biomolecular complexes and implications of water-ice locked in planetary interiors. Under pressure, weak hydrogen bonding (involving proton localization) is sequentially tuned to intermediate strength (involving proton delocalization via quantum tunneling), and then to strong bonds with symmetric O–H–O configurations (where there is localization again). This affects the physical properties of the host lattice involving hydrogen bonded networks, e.g., charge dynamics or thermoelasticity. An example of this is demonstrated when we pressure tune the hydrogen bond in Fe-O–H⋅⋅⋅O-P structural segments of mixed-valence barbosalite (Fe2+Fe3+2)(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 symmetrization of the 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+Fe3+2)(PO4)2(OH)2 under pressure exemplifies the interplay between proton (THz) and electron (MHz) dynamics on two disparate time scales in the same condensed phase.''