Investigation of hydrogen sites of wadsleyite: A neutron diffraction study

Abstract

A neutron powder diffraction experiment was conducted to refine the hydrogen position of Mg-endmember deuterated wadsleyite. Preliminary refinement using the dry-structure determined by single crystal X-ray diffraction reveals a maximum peak Q1 of nuclear density in the difference Fourier map at the M3 octahedral edge, between the O1 and O4 atoms (3.071(3) Å). Full Rietveld refinement was conducted assuming that the maximum peak corresponds to deuterium atom. The deuterium position was determined as (0.096, 0.289, 0.315) with occupancy of 8.2%. The O1–D1 vector lies along M3 octahedral edge pointing O4 atom, and the bond length is determined as 1.037(15) Å for O1–D1, and 2.041(15) Å for D1⋯O4. The O1–D1⋯O4 geometry is almost linear with angle of 171.7(5) °. The residual peak Q2 was found in the map at (0, 0.172, 0.268), suggesting the existence of a bent hydrogen bond between O1⋯O3 (3.035 Å) of the M3 octahedral edge. The respective distances from the surrounding oxygen atoms are 0.97 and 2.13 Å for O1–Q2 and Q2⋯O3. The O1–Q2⋯O3 configuration is nonlinear, with angle of 154.4 °. These results reveal that the dominant site of protonation in wadsleyite is the O1 site, which is consistent with the previous prediction.