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

<p>The mutual influence of F and OH groups in neighboring sites in topaz (Al<span class="inline-formula">₂</span>SiO<span class="inline-formula">₄</span>(F,OH)<span class="inline-formula">₂</span>) was investigated using magic angle spinning nuclear magnetic resonance (MAS NMR) and Raman spectroscopy. The splitting of <span class="inline-formula">¹⁹</span>F and <span class="inline-formula">¹</span>H 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<span class="inline-formula">⋯</span>O<span class="inline-formula">⋯</span>HO or F<span class="inline-formula">⋯</span>H<span class="inline-formula">⋯</span>O. The proton accepting oxygen was determined to be part of the SiO<span class="inline-formula">₄</span> tetrahedron using <span class="inline-formula">²⁹</span>Si MAS NMR. Comparison of the MAS NMR data between an OH-bearing and an OH-free topaz sample confirms that the <span class="inline-formula">¹⁹</span>F signal at <span class="inline-formula">−</span>130 ppm stems from F<span class="inline-formula">⁻</span> ions that take part in H<span class="inline-formula">⋯</span>F bonds with a distance of <span class="inline-formula">∼</span> 2.4 Å, whereas the main signal at <span class="inline-formula">−</span>135 ppm belongs to fluoride ions with no immediate OH group neighbors. The Raman OH sub-band at 3644 cm<span class="inline-formula">⁻¹</span> stems from OH groups neighboring other OH groups, whereas the sub-band at 3650 cm<span class="inline-formula">⁻¹</span> stems from OH groups with fluoride neighbors, which are affected by H<span class="inline-formula">⋯</span>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 <span class="inline-formula">²⁷</span>Al MAS NMR, suggesting that the investigated samples likely have nearly ideal <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Al</mi><mo>/</mo><mi mathvariant="normal">Si</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="28pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="d0bd312d27fa3264f4721ed1a8b84140"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-34-507-2022-ie00004.svg" width="28pt" height="14pt" src="ejm-34-507-2022-ie00004.png"/></svg:svg></span></span> ratios, making them potentially useful as high-density electron microprobe reference materials for Al and Si, as well as for F.</p>