A Rietveld refinement using neutron powder diffraction data of a fully deuterated topaz, Al₂SiO₄(OD)₂

The structure of topaz-OD, dialuminium orthosilicate di­hydro­xide, Al₂SiO₄(OD)₂, was refined in the space group Pbnm by Rietveld analysis of constant wavelength neutron powder diffraction data. Two non-equivalent half-occupied deuterium positions were located. Each D atom is characterized by an irregular trifurcated hydrogen-bond geometry. The refined hydrogen-bond distances are in the ranges 2.038 (5)-2.281 (6) and 2.280 (5)-2.524 (6) Å for the two D atoms. Hydro­gen-bond angles range from 83.6 (4) to 151.9 (4)°. Results indicate that it is feasible to characterize the hydrogen bonding in small-volume samples (25 mg) synthesized at high pressure and temperature.