The hydrogen bond plays a crucial role in a wide variety of chemical and physical processes. Under high pressures the hydrogen bonds were predicted [1] to transform from a highly asymmetric soft O–H···O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on water- and hydroxyl containing compounds, pressure induced hydrogen bond symmetrization has been unambiguously established [2] only in H₂O (Ice X) at P > 60 GPa. Following single crystal X-ray diffraction, Mössbauer (MS) and Raman spectroscopy measurements, we report the discovery of the H-bonds symmetrization in goethite, α-FeOOH, resulting from the Fe³⁺ high-to-low spin crossover at P > 45 GPa. The isosymmetric phase transition with a discontinuous volume reduction of ~11%, induced by the spin crossover, results in a symmetrization of the FeO₆ octahedra and an unexpected shortening of the O–H···O distances to ~2.2 Å at 50 GPa. Both of these features are clear characteristics of the hydrogen-bond symmetrization. These results are confirmed by ab initio calculations and calculations of the interatomic distances of the hydroxyl and H···O species based on the valence bond rule and the experimentally measured atomic positions of iron and oxygen atoms. Our findings suggest that hydrogen bond symmetrization may occur in crystalline materials at relatively low pressures if electronic transformations result in significant volume reduction, such as spin crossover or pressure-induced oxidation of the TM ion.

[1] W.B. Holzapfel, , J. Chem. Phys. 56, 712–715 (1972).

[2] P. Loubeyre et al., Nature 397, 503-506 (1999).