Neutron powder diffraction studies of small-volume samples synthesized at high P-T

The location of the hydrogen atom and the nature of hydrogen bonding in hydrous high-pressure phases are fundamental to the understanding of their thermodynamic, rheological and physical properties. However, neutron powder diffraction data have not been collected for many high-pressure phases because the amount of sample ( ∼10-25 mg) that can be synthesized at extreme P-T conditions (e.g., 1300 ° C and 16.5 GPa for wadsleyite) is generally insufficient for most neutron experiments. Multiple-run samples cannot be used for two reasons: 1) chemical variations between individual sample runs can be significant; and 2) synthesis of larger samples ( ∼100 mg) is highly impractical due to the very high demand for experimental time on the multi-anvil presses. To determine the feasibility of using neutron diffraction to characterize the H bonding in phases with small sample volumes, powder data were obtained for topaz-OD [Al₂SiO₄(OD)₂] and orthorhombic, Fe-free OD-wadsleyite [β - Mg₂SiO₄] with the high-flux diffractometer (D20) at the ILL. Topazes found in ultrahigh-pressure metamorphic terrains are relatively OH-rich.Wadsleyite has been proposed as the major storage site for water in the transition zone. High-pressure Raman spectra measured at room temperature for orthorhombic, Fe-free OD-wadsleyite suggest that there are at least three hydrogen sites. Data collection times for topaz-OD were 1, 2, 3 and 9 h for samples that varied in mass from 17-26 mg. The OD-wadsleyite data were collected on a 10-mg sample for a period of 24 h. Rietveld refinement was carried out using GSAS (wavelength 1.3714 Å; 2Θ range 15-154° ). For topaz-OD, the best fit was obtained from data collected for 2 h on a 25-mg sample [750 ° C 7.5 GPa 48 h] [wRp = 0.0538, CHI**2 = 2.348 with 37 variables]. Results confirm the X-ray model with deuterium disordered over two half-occupied sites. O-D bond lengths are approximately equal [0.977(6) and 0.978(6) Å]. A trifurcated hydrogen bond is observed for both D1 and D2 with bond lengths of 2.263(6), 2.257(6), 2.038(5) and 2.541(7), 2.392(6), 2.253(5), respectively. For OD-wadsleyite, the Rietveld fit was poor and the deuterium position could not be located in difference Fourier maps, suggesting that the deuterium atom was disordered over several sites. This is consistent with the IR spectra of the sample, which showed broadening of the O-D bands at 2500 and 2700 cm^-1.