Guyanaite, β-CrOOH, is a structural analogue of the high-pressure oxyhydroxide phases δ-AlOOH and ɛ-FeOOH. Here, it was synthesized in piston-cylinder and multi-anvil press experiments at 4–13.5 GPa and 900–1100 °C. The deuterated phase β-CrOOD was synthesized at 4 GPa and 600 °C. The samples were characterized at ambient conditions by X-ray diffraction, diffuse optical reflectance spectroscopy and infrared absorption (IR) spectroscopy. In addition, the structural evolution of β-CrOOH and β-CrOOD with increasing pressure up to about 20 GPa was studied by in situ IR spectroscopy in a diamond anvil cell (DAC). This investigation was complemented by first-principles calculations in the framework of the density-functional theory (DFT). A pronounced geometric isotope effect and very short O-H. . .O bond lengths of 2.497(3) Å for β-CrOOH and 2.541(3) Å for β-CrOOD are observed at ambient pressure. In the IR spectra, no bands show up above 2000 cm−1, which indicates strong hydrogen bonding. The evolution of OH- and OD-related vibrational bands with pressure studied by IR spectroscopy shows a discontinuity at about 5 GPa. The DFT calculations suggest that this change in compression mechanism is related to a second-order phase transition from the low-pressure phase with asymmetric hydrogen positions (space group P21nm or Pnnm) to a high-pressure phase with space group Pnnm that is characterized by symmetric hydrogen bonds with two identical OH bond lengths of 1.20(1) Å. Using density functional perturbation theory, the most prominent high-frequency modes observed in the IR spectra are assigned to O-H-O bending vibrations. The transition pressure for hydrogen bond symmetrisation in β-CrOOH is considerably lower than in other hydrous phases of recent interest, such as δ-AlOOH or phase D.

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