Studies on the local structure of the F/OH site in topaz by magic angle spinning nuclear magnetic resonance and Raman spectroscopy

The mutual influence of F and OH groups in neighboring sites in topaz (Al2SiO4(F,OH)(2)) was investigated using magic angle spinning nuclear magnetic resonance (MAS NMR) and Raman spectroscopy. The splitting of F-19 and H-1 NMR signals, as well as the OH Raman band, provides evidence for hydrogen bond formation within the crystal structure. Depending on whether a given OH group has another OH group or fluoride as its neighbor, two different hydrogen bond constellations may form: either OH center dot center dot center dot O center dot center dot center dot HO or F center dot center dot center dot H center dot center dot center dot O. The proton accepting oxygen was determined to be part of the Siat tetrahedron using Si-29 MAS NMR. Comparison of the MAS NMR data between an OH-bearing and an OH-free topaz sample confirms that the F-19 signal at -130 ppm stems from F- ions that take part in H center dot center dot center dot F bonds with a distance of similar to 2.4 angstrom, whereas the main signal at -135 ppm belongs to fluoride ions with no immediate OH group neighbors. The Raman OH sub-band at 3644 cm(-1) stems from OH groups neighboring other OH groups, whereas the sub-band at 3650 cm(-1 )stems from OH groups with fluoride neighbors, which are affected by H center dot center dot center dot F bridging. The integrated intensities of these two sub-bands do not conform to the expected ratios based on probabilistic calculations from the total OH concentration. This can be explained by (1) a difference in the polarizability of the OH bond between the different hydrogen bond constellations or (2) partial order or unmixing of F and OH, or a combination of both. This has implications for the quantitative interpretation of Raman data on OH bonds in general and their potential use as a probe for structural (dis-)order. No indication of tetrahedrally coordinated Al was found with Al-27 MAS NMR, suggesting that the investigated samples likely have nearly ideal Al/Si ratios, making them potentially useful as high-density electron microprobe reference materials for Al and Si, as well as for F.