Pressure-Induced Hydrogen Bond Symmetrization in Iron Oxyhydroxide
Weiming Xu, Eran Greenberg, Gregory Kh. Rozenberg, Moshe P. Pasternak,Elena Bykova, Tiziana Boffa-Ballaran, Leonid Dubrovinsky, Vitali Prakapenka, Michael Hanfland, Olga Yu. Vekilova, Sergei I. Simak, and Igor A. Abrikosov
PHYSICAL REVIEW LETTERS 111, 175501 (2013)
The hydrogen bond in oxyhydroxides and hydroxides is an attractive interaction between a hydrogen atom from a hydroxyl (O-H) group and a near neighbor oxygen atom or a group of atoms. In contrast to other interacting atoms, H bonds undergo large variations of their energetic and geometrical parameters under pressure. At ambient pressure the O-H…O configuration in goethite FeOOH compound was found to be highly asymmetric. However, in this work the hydrogen bonds were predicted to transform from asymmetric soft O-H…O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms under high pressures. Ab initio calculations were employed in order to investigate the stability and structural properties of FeOOH. The antiferromagnetic (AFM) high spin (HS) phase was found to have the lowest total energy, i.e. to be the most stable configuration in the 0–57 GPa pressure range (Fig. 1). Above 57 GPa (inset in Fig. 1) the stable configuration becomes the low spin (LS) phase, in excellent agreement with the present experimental results. The theoretical analysis also implies that the spin crossover results in nearly symmetric hydrogen bonds. Thus the two phenomena—electronic transition in Fe3+ and modification of the hydrogen bond, resulting in the dissociation of the hydroxyl, are closely interlinked. The experimental studies combined with the ab initio calculations suggest that hydrogen bond symmetrization may occur in other inorganic oxy-hydroxide transition metal (TM) species at relatively low pressures in cases of pressure-induced electronic processes, such as spin crossover or pressure-induced oxidation of the TM ion which eventually leads to substantial volume reduction and change of electronic state. Such an effect may be common for crystalline materials and minerals containing water and transition metals, particularly for components of Earth and planetary mantles. Indeed, water is expected to be carried into Earth’s interiors by ferric iron bearing oxides and silicates and, induced by spin transition in iron, at conditions of the middle lower mantle, changes in hydrogen bonding may significantly affect water balance and dynamics.
Fig. 1 Dependence of the total energy as a function of the unit cell volume of orthorhombic FeOOH. The afm HS solution (black symbols) is the most stable at pressures from 0 to ~57 GPa. At ~57 GPa (see the inset) it transforms to the afm LS phase (red symbols). The non-magnetic (nm) configuration of FeOOH (green symbols) is higher in energy than the afm phases in the whole range of considered volumes. The inset showing the difference between enthalpies of the anti-ferromagnetic (afm) LS and HS states as a function of pressure, clearly demonstrates that the isostructural phase transition might take place, in agreement with the experiment.
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