Pressure-Induced Hydrogen Bond Symmetrization in Babingtonite.

Hydrogen bond symmetrization and electronic spin transition of iron under pressure are key factors in understanding the physical properties and behaviors of Fe- and H-bearing minerals. However, the coexistence of these two effects may often result in inconsistent interpretations. Babingtonite is characterized by prominent hydrogen bonds and diverse iron oxidation states within its crystal structure. These features make it an ideal subject for studying the interactions between hydrogen bond symmetrization and spin transitions of iron. Here, we conducted high-pressure infrared absorption spectroscopy, single-crystal X-ray diffraction, and Mössbauer spectroscopy experiments on babingtonite to investigate the behaviors of hydrogen bonds and spin states of iron. Pressure-induced hydrogen bond symmetrization occurs at approximately 22 GPa in babingtonite, while no spin transition of iron is detected up to 40 GPa. Our results show that the notable deformation of the unit cell and the kink in the trend of the hydrogen-bonded O-O distance with increasing pressure provide insightful evidence for hydrogen bond symmetrization. The symmetrical rigid configuration of the hydrogen bond leads to increased distortion in the neighboring Fe-O octahedron. These findings will contribute to the determination of hydrogen bond symmetrization through crystallographic structural analysis, partly overcoming the challenge of accurately locating hydrogen positions using X-ray diffraction.