Pressure-induced hydrogen bond symmetrisation in guyanaite, beta -CrOOH; evidence from spectroscopy and ab initio simulations
Pressure-induced hydrogen bond symmetrisation in guyanaite, beta -CrOOH; evidence from spectroscopy and ab initio simulations (in The 7th European conference on Mineralogy and spectroscopy, Monika Koch-mueller (editor), Michael Fechtelkord (editor), Vladimir Khomenko (editor) and Sergio Speziale (editor))
European Journal of Mineralogy (September 2012) 24 (5): 839-850
- anvil cells
- bonding
- cell dimensions
- compression
- crystal structure
- density functional theory
- enthalpy
- experimental studies
- high pressure
- hydroxides
- in situ
- infrared spectra
- optical spectra
- oxides
- oxyhydroxides
- pressure
- reflectance
- Rietveld refinement
- simulation
- space groups
- spectra
- symmetry
- synthesis
- transformations
- X-ray diffraction data
- hydrogen bonding
- guyanaite
Guyanaite, beta -CrOOH, is a structural analogue of the high-pressure oxyhydroxide phases delta -AlOOH and epsilon -FeOOH. Here, it was synthesized in piston-cylinder and multi-anvil press experiments at 4-13.5 GPa and 900-1100 degrees C. The deuterated phase beta -CrOOD was synthesized at 4 GPa and 600 degrees 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 beta -CrOOH and beta -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) Aa for beta -CrOOH and 2.541(3) Aa for beta -CrOOD are observed at ambient pressure. In the IR spectra, no bands show up above 2000 cm (super -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 P2 (sub 1) nm 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) Aa. 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 beta -CrOOH is considerably lower than in other hydrous phases of recent interest, such as delta -AlOOH or phase D.