Crystal structures of deuterated sodium molybdate dihydrate and sodium tungstate dihydrate from time-of-flight neutron powder diffraction

Abstract

High-precision structural parameters for Na ₂MoO ₄·2D ₂O and Na ₂WO ₄·2D ₂O are reported based on refinement of high-resolution time-of-flight neutron powder diffraction data. Complementary Raman spectra are also provided.

Abstract

Time-of-flight neutron powder diffraction data have been measured from ∼90 mol% deuterated isotopologues of Na ₂MoO ₄·2H ₂O and Na ₂WO ₄·2H ₂O at 295 K to a resolution of sin (θ)/λ = 0.77 Å ⁻¹. The use of neutrons has allowed refinement of structural parameters with a precision that varies by a factor of two from the heaviest to the lightest atoms; this contrasts with the X-ray based refinements where precision may be > 20× poorer for O atoms in the presence of atoms such as Mo and W. The accuracy and precision of inter­atomic distances and angles are in excellent agreement with recent X-ray single-crystal structure refinements whilst also completing our view of the hydrogen-bond geometry to the same degree of statistical certainty. The two structures are isotypic, space-group Pbca, with all atoms occupying general positions, being comprised of edge- and corner-sharing NaO ₅ and NaO ₆ polyhedra that form layers parallel with (010) inter­leaved with planes of XO ₄ ( X = Mo, W) tetra­hedra that are linked by chains of water mol­ecules along [100] and [001]. The complete structure is identical with the previously described molybdate [Capitelli et al. (2006 ▸). Asian J. Chem. 18, 2856–2860] but shows that the purported three-centred inter­action involving one of the water mol­ecules in the tungstate [Farrugia (2007 ▸). Acta Cryst. E 63, i142] is in fact an ordinary two-centred ‘linear’ hydrogen bond.