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

Fortes, A.D.

doi:10.1107/S2056989015011354

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Volume 71| Part 7| July 2015| Pages 799-806

doi:10.1107/S2056989015011354

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

A. Dominic Fortes ^a,^b,^c ^*

^aISIS Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QX, England, ^bDepartment of Earth Sciences, University College London, Gower Street, London WC1E 6BT, England, and ^cDepartment of Earth and Planetary Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, England
^*Correspondence e-mail: andrew.fortes@ucl.ac.uk

Edited by M. Weil, Vienna University of Technology, Austria (Received 1 June 2015; accepted 10 June 2015; online 17 June 2015)

Time-of-flight neutron powder diffraction data have been measured from ∼90 mol% deuterated isotopologues of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O 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 NaO₅ and NaO₆ polyhedra that form layers parallel with (010) interleaved with planes of XO₄ (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. E63, i142] is in fact an ordinary two-centred `linear' hydrogen bond.

Keywords: neutron powder diffraction; sodium molybdate dihydrate; sodium tungstate dihydrate.

CCDC references: 1406122; 1406121

1. Chemical context

Na₂MoO₄ and Na₂WO₄ are unusual amongst the alkali metal mono-molybdates and mono-tungstates in being highly soluble in water and forming polyhydrated crystals. Additionally, sodium apparently plays a significant role in the solvation of other alkali metal ions to form a range of double molybdate and tungstate hydrates (Klevtsova et al., 1990 ; Klevtsov et al., 1997 ; Mirzoev et al., 2010 ), for example, Na₃K(MoO₄)₂·9H₂O. Both dihydrate and decahydrate varieties of the two title compounds are known, their solubilities as a function of temperature being well characterised (Funk, 1900 ; Zhilova et al., 2008 ). The structures of the decahydrates have not yet been reported, although I have established that they are not isotypic with the sodium sulfate analogue, Na₂SO₄·10H₂O, as had hitherto been thought.

The dihydrates have been the subject of extensive crystallographic studies, from descriptions of their density, habit and measurements of interfacial angles (Svanberg & Struve, 1848 ; Zenker, 1853 ; Rammelsberg, 1855 ; Marignac, 1863 ; Delafontaine, 1865 ; Ullik, 1867 ; Clarke, 1877 ; Zambonini, 1923 ), through to determination of absolute unit-cell parameters (Pistorius & Sharp, 1961 ), and subsequent solution and refinement of their structures (Mitra & Verma, 1969 ; Okada et al., 1974 ; Matsumoto et al., 1975 ; Atovmyan & D'yachenko, 1969 ; Capitelli et al., 2006; Farrugia, 2007). However, the presence of heavy atoms in these materials makes it impossible to achieve a uniform precision on all structural parameters using X-rays, and even with single-crystal methods that purport to identify hydrogen positions there may be significant inaccuracies. Such problems are minimised using a neutron radiation probe since the coherent neutron scattering lengths of the constituent elements differ by less than a factor of two, being 6.715 fm for Mo, 4.86 fm for W, 3.63 fm for Na, 5.803 fm for O, and 6.67 fm for ²D (Sears, 1992 ). Thus one can locate accurately all of the light atoms and obtain a uniform level of precision on their coordinates and displacement parameters. Since the incoherent neutron scattering cross section of ¹H is large (80.3 barns) it is usual to prepare perdeuterated specimens whenever possible (the incoherent cross section of ²D being only 2.1 barns) as this optimises the coherent Bragg scattering signal above the background, reducing the counting times required for a high-precision structure refinement from many days to a matter of hours on the instrument used for these measurements. These data were therefore measured using Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O samples.

The occurrence of polyhydrated forms of both Na₂MoO₄ and Na₂WO₄ suggests that both would be excellent candidates for the formation of hydrogen-bonded complexes with water-soluble organics, such as amino acids, producing metal-organic crystals with potentially useful optical properties (cf., glycine lithium molybdate; Fleck et al., 2006 ). High-pressure polymorphs of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O are indicated from Raman scattering studies (Luz-Lima et al., 2010 ; Saraiva et al., 2013 ). Characterising the structures and properties of the title compounds provides an essential foundation on which to build future studies of the high-pressure phases, of the as-yet incomplete decahydrate structures and any related organic-bearing hydrates.

2. Structural commentary

Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O are isotypic, crystallizing in the orthorhombic space group Pbca; all atoms occupy general positions (Wyckoff sites 8c). Note that the atom labelling scheme and space-group setting used here follows Farrugia (2007); consequently there are some differences with respect to other literature sources, although equivalent contacts are referred to in Table 1 and Table 2. The X⁶⁺ ions (X = Mo, W) are tetrahedrally coordinated by O²⁻, the Mo—O and W—O bond lengths varying slightly according to the type of coordination adopted by a particular apex: O1 and O4 are each coordinated to Na⁺ and each also accepts two hydrogen bonds; O2 is coordinated to three Na⁺ ions and O3 is coordinated to two Na⁺ ions (Fig. 1). In both title compounds, X–O1 and X–O4 are the longest contacts and X–O3 is the shortest contact in the tetrahedral oxyanion. The mean Mo—O and W—O bond lengths are in good agreement with those found in the anhydrous crystals (Fortes, 2015 ). Furthermore, each of the absolute Mo—O bond lengths are identical (within error) to those found by Capitelli et al. (2006); the agreement in W—O bond lengths with Farrugia (2007) is marginally poorer.

Table 1
Comparison of the X—O (X = Mo, W) and Na—O bond lengths (Å) in Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O with those of the protonated isotopologues reported in the literature

	Na₂MoO₄·2D₂O	Na₂MoO₄·2H₂O	Na₂WO₄·2D₂O	Na₂WO₄·2H₂O
	This work	Capitelli et al. (2006)	This work	Farrugia (2007)
X—O1	1.773 (2)	1.772 (1)	1.785 (2)	1.776 (3)
X—O2	1.764 (1)	1.767 (1)	1.778 (2)	1.778 (3)
X—O3	1.750 (2)	1.751 (1)	1.766 (2)	1.761 (3)
X—O4	1.776 (2)	1.778 (1)	1.783 (2)	1.787 (3)
Mean X—O	1.766	1.767	1.778	1.776

Na1—O2	2.437 (3)	2.446 (2)	2.433 (2)	2.442 (3)
Na1—O2⁽ⁱ⁾	2.417 (3)	2.419 (2)	2.412 (3)	2.416 (3)
Na1—O3⁽ⁱⁱ⁾	2.482 (3)	2.481 (2)	2.479 (3)	2.480 (3)
Na1—O4⁽ⁱⁱⁱ⁾	2.410 (3)	2.395 (2)	2.399 (2)	2.388 (3)
Na1—O5	2.476 (3)	2.456 (2)	2.479 (3)	2.464 (4)
Na1—O6	2.426 (3)	2.423 (2)	2.443 (3)	2.433 (3)
Mean Na1—O	2.441	2.437	2.441	2.437

Na2—O1^iv	2.312 (3)	2.319 (2)	2.320 (2)	2.323 (3)
Na2—O2	2.363 (3)	2.354 (2)	2.355 (2)	2.346 (3)
Na2—O3^v	2.339 (3)	2.341 (2)	2.328 (2)	2.331 (3)
Na2—O5	2.415 (3)	2.403 (2)	2.409 (3)	2.396 (3)
Na2—O6^vi	2.305 (3)	2.300 (2)	2.311 (2)	2.304 (3)
Mean Na2—O	2.347	2.343	2.345	2.340
Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) − $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z; (iii) $[{1\over 2}]$ − x, − $[{1\over 2}]$ + y, z; (iv) $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z; (v) $[{3\over 2}]$ − x, − $[{1\over 2}]$ + y, z; (vi) $[{1\over 2}]$ + x, y, $[{3\over 2}]$ − z.

Table 2
Comparison of the water molecule and hydrogen bond geometry (Å, °) in Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O with the protonated isotopologues as reported in the literature. Note the inclusion of the contact O5–D51···O3, which forms the longer `branch' of Farrugia's proposed bifurcated hydrogen bond

	Na₂MoO₄·2D₂O	Na₂MoO₄·2H₂O	Na₂WO₄·2D₂O	Na₂WO₄·2H₂O
	This work	Capitelli et al. (2006)	This work	Farrugia (2007)
O5—D51	0.977 (2)	0.68 (3)	0.970 (2)	0.86 (3)
O5—D52	0.966 (2)	0.76 (3)	0.959 (2)	0.86 (3)
D51—O5—D52	106.0 (2)	98 (4)	106.0 (2)	100 (5)
D51···O1⁽ⁱ⁾	1.874 (2)	2.16 (3)	1.873 (2)	2.09 (4)
O5—D51···O1⁽ⁱ⁾	167.9 (2)	167 (4)	168.2 (2)	145 (6)
D51···O3⁽ⁱⁱ⁾	–	–	–	2.70 (6)
O5—D51···O3⁽ⁱⁱ⁾	–	–	–	122 (5)
D52···O4⁽ⁱⁱ⁾	1.846 (3)	2.07 (3)	1.863 (2)	1.98 (3)
O5—D52···O4⁽ⁱⁱ⁾	171.2 (2)	176 (3)	170.9 (2)	174 (6)

O6—D61	0.972 (2)	0.83 (3)	0.968 (2)	0.86 (3)
O6—D62	0.972 (2)	0.71 (3)	0.966 (2)	0.86 (3)
D61—O6—D62	103.0 (2)	105 (3)	103.2 (2)	95 (5)
D61···O1	1.816 (2)	2.01 (3)	1.834 (2)	1.95 (3)
O6—D61···O1	167.0 (2)	167 (3)	167.0 (2)	167 (6)
D62···O4⁽ⁱⁱⁱ⁾	1.868 (4)	2.08 (3)	1.876 (2)	2.02 (4)
O6—D62···O4⁽ⁱⁱⁱ⁾	168.7 (2)	170 (3)	168.7 (2)	159 (6)
Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) 1 − x, − $[{1\over 2}]$ + y, $[{3\over 2}]$ − z; (iii) − $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z.

Figure 1
First and second coordination shell of Mo⁶⁺/W⁶⁺ in the title compounds, revealing differences in the environment of each apical O²⁻ that are responsible for the variations in Mo–O and W–O bond lengths. Anisotropic displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z; (iii) − $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z; (iv) $[{1\over 2}]$ − x, $[{1\over 2}]$ + y, z; (v) $[{3\over 2}]$ − x, $[{1\over 2}]$ + y, z; (vi) 1 − x, $[{1\over 2}]$ + y, 1.5 − z.]

The Na⁺ ions occupy two inequivalent sites: in one, Na⁺ is six-fold coordinated by two water molecules and four XO₄²⁻ oxygen atoms, yielding an octahedral arrangement; in the second, Na⁺ is five-fold coordinated by two water molecules and three XO₄²⁻ oxygen atoms, yielding a square-pyramidal arrangement. These two polyhedra share a common edge (O2–O5) and are connected, moreover, with their inversion-centre-related neighbours along three other shared edges to form a cluster (Fig. 2a). The clusters corner-share via O6 to create a `slab' parallel to (010) (Fig. 2b). The mean Na—O bond lengths are statistically identical in Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O being ∼1.6% longer in the NaO₆ octahedra and ∼2.3% shorter in the NaO₅ polyhedra than Na—O bonds in the anhydrous crystals (Fortes, 2015). The agreement in Na—O bond lengths with the X-ray single crystal studies of Capitelli et al. (2006) and Farrugia (2007) is very good. Overall, the agreement in bond lengths and angles for the two independently refined data sets is excellent (Tables 1 and 2).

Figure 2
(a) Arrangement of NaO_x polyhedra into edge-sharing clusters comprised of two Na1O₆ octahedra and two Na2O₅ square pyramids; (b) Arrangement of the clusters shown in (a) by corner sharing to form `slabs' parallel (010). Ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z; (iii) − $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z; (iv) $[{1\over 2}]$ − x, − $[{1\over 2}]$ + y, z; (v) $[{1\over 2}]$ + x, y, $[{3\over 2}]$ − z; (vi) $[{1\over 2}]$ − x, 1 − y, $[{1\over 2}]$ + z; (vii) $[{3\over 2}]$ − x, − $[{1\over 2}]$ + y, z.]

Although it is more usual to find Na⁺ in octahedral coordination, there are abundant examples of Na⁺ in five-fold coordination, including instances where the NaO₅ polyhedron adopts a square-pyramidal arrangement (Beurskens & Jeffrey, 1961 ; Císařová; et al., 2001 ; Sharma et al., 2005 ; Smith & Wermuth, 2014 ; Aksenov et al., 2014 ) or the alternative trigonal-bipyramidal arrangement (Mereiter, 2013 ; Smith, 2013 ). A similar combination of NaO₆ and NaO₅ polyhedra to that found in the title compounds occurs in the closely-related hydrates Na₂CrO₄·1.5H₂O and Na₂SeO₄·1.5H₂O (Kahlenberg, 2012 ; Weil & Bonneau, 2014 ). The two water molecules form hydrogen-bonded chains between the O1 and O4 atoms of the tetrahedral oxyanions; O5-related chains extend along [001] and O6-related chains crosslink them in a staggered fashion along [100]. Fig. 3(a) and 3(b) depict the spatial relationship between this `net' of water linked tetrahedra and the adjacent `slab' of corner-linked Na—O polyhedral clusters. The layers shown in Fig. 3(b) alternate to create the three-dimensional structure and are no doubt responsible for the macro-scale platy habit of the crystals.

Figure 3
(a) View down the b axis of the network of water-linked tetrahedral oxyanions; chains linked by O5 extend along [001] whereas crosslinkages through O6 are staggered along [100]. (b) View of the same structure along the c axis. Ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) 1 − x, $[{1\over 2}]$ + y, $[{3\over 2}]$ − z; (iii) $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, 1 − z; (iv) $[{1\over 2}]$ + x, y, $[{3\over 2}]$ − z; (v) x, $[{3\over 2}]$ − y, − $[{1\over 2}]$ + z; (vi) x, $[{3\over 2}]$ − y, $[{1\over 2}]$ + z.]

There are no significant differences in the hydrogen bond geometries of the molybdate or tungstate crystals. The most recent X-ray single-crystal diffraction study of Na₂WO₄·2H₂O (Farrugia, 2007) implied that one of the water molecules (O5) was involved in a weaker three-centred interaction, although a similarly recent measurement of Na₂MoO₄·2H₂O (Capitelli et al., 2006) identified a `normal' linear two-centred interaction for this bond. This work, using neutrons, has been able to accurately and precisely characterise the hydrogen bond geometry, showing that the latter is true for both structures; there is no bifurcated bond and all hydrogen-bonded interactions are of the linear two-centred variety. Presumably the error in Farrugia's analysis arose due to the substantial absorption correction required (μ = 18.7 mm⁻¹) for an accurate structure refinement from X-ray single-crystal data.

Raman spectra of Na₂MoO₄·2H₂O and Na₂MoO₄·2D₂O were first reported by Mahadevan Pillai et al. (1997 ); subsequently, Luz-Lima et al. (2010) and Saraiva et al. (2013) published the Raman spectra of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O as a function of temperature (13–300 K) and as a function of hydrostatic pressure (to 5 GPa). Both compounds exhibit evidence of a `conformational change' on cooling through 120 K: the molybdate appears to undergo two high-pressure phase transitions, one at 3 GPa and the second at 4 GPa; the tungstate apparently undergoes a high-pressure phase transition at 3.9 GPa. The Raman spectra reported here (Figs. 4 and 5 and Supporting information) agree well with data in the literature (Table 3). The large blue-shifts in the internal vibrational frequencies of the deuterated water molecule are similar to the square root of the D:H mass ratio; the small blue-shifts of most of the internal modes of the tetrahedral oxyanions are consistent with stronger hydrogen bonding in the deuterated species, as expected (cf. Scheiner & Čuma, 1996 ; Soper & Benmore, 2008 ).

Table 3
Comparison of the internal vibrational mode frequencies (cm⁻¹) in fully protonated and 90 mol % deuterated isotopologues of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O with literature data

	Na₂MoO₄·2H₂O			Na₂WO₄·2H₂O
	This work (¹H)	This work (²D)	Busey & Keller (1964 )	This work (¹H)	This work (²D)	Busey & Keller (1964)
ν₂ (XO₄²⁻)	279	271	285	276	269	276
	319	315	325	324	321	325
	335	331		330	331
ν₄ (XO₄²⁻)	359	358		358	355
ν₃ (XO₄²⁻)	804	801	805	804	802	808
	833	826	836	836	831	838
	842	840	843		840
ν₁ (XO₄²⁻)				891	889	893
	894	894	897	929	928	931

Figure 4
Raman spectra of Na₂MoO₄·2H₂O and Na₂MoO₄·2D₂O in the range 200–3900 cm⁻¹. Band positions and vibrational assignments are indicated (see also Table 3

). Vertical scales show intensities relative to ν₁ (XO₄²⁻).

Figure 5
Raman spectra of Na₂WO₄·2H₂O and Na₂WO₄·2D₂O in the range 200–3900 cm⁻¹. Band positions and vibrational assignments are indicated (see also Table 3

). Vertical scales show intensities relative to ν₁ (XO₄²⁻).

3. Synthesis and crystallization

Coarse polycrystalline powders of Na₂MoO₄·2H₂O (Sigma–Aldrich M1003 > 99.5%) and Na₂WO₄·2H₂O (Sigma–Aldrich 14304 > 99%) were dehydrated by drying at 673 K in air. The resulting anhydrous materials were characterised by Raman spectroscopy, X-ray and neutron powder diffraction (Fortes, 2015). This material was dissolved in D₂O (Aldrich 151882, 99.9 atom% D) and twice recrystallized by gentle evaporation at 323 K. The molybdate crystallised with a coarse platy habit whereas the tungstate was deposited as a finer-grained material. Once the supernatant liquid was decanted, the residue was air dried on filter paper and then ground to a fine powder with an agate pestle and mortar. The powders were loaded into standard vanadium sample-holder tubes of internal diameter 11 mm to a depth not less than 20 mm (this being the vertical neutron beam dimension at the sample position). Accurate volumes and masses were determined after the diffraction measurements were complete and used to correct the data for self-shielding. The level of deuteration was determined by Raman spectroscopy (see below) to be ∼91% for both compounds.

Raman spectra were acquired with a B&WTek i-Raman plus portable spectrometer; this device uses a 532 nm laser (37 mW power at the fiber-optic probe tip) to stimulate Raman scattering, which is measured in the range 170–4000 cm⁻¹ with a spectral resolution of 3 cm⁻¹. Data were collected for 600 sec at 17 mW for Na₂MoO₄·2H₂O (as bought), 180 sec at 37 mW for Na₂MoO₄·2D₂O, 300 sec at 17 mW for Na₂WO₄·2H₂O (as bought) and 220 sec at 37 mW for Na₂WO₄·2D₂O; after summation, the background was removed and peaks fitted using Pseudo-Voigt functions in OriginPro (OriginLab, Northampton MA). These data are provided as an electronic supplement in the form of an ASCII file. Small quantities of ordinary hydrogen were found to be present in both specimens, the proportion being determined by the ratio of the areas under the ν₁/ν₃ (H₂O) bands after normalisation relative to the height of the strong ν₁ (XO₄²⁻) band. The molar abundance of ¹H was used to correct the diffraction data for absorption (see below) and to ensure accurate refinement of the structure (see Refinement).

Time-of-flight neutron diffraction patterns were collected at 295 K using the High Resolution Powder Diffractometer, HRPD (Ibberson, 2009 ), at the ISIS spallation neutron source, Harwell Campus, Oxfordshire, UK. Data were acquired in the range of neutron flight times from 30–130 msec (equivalent to neutron wavelengths of 1.24–5.36 Å) for 15.17 hr from the molybdate and 14.40 hr from the tungstate, equivalent to 615 and 590 µAhr of integrated proton beam current, respectively. These data sets were normalized to the incident spectrum and corrected for detector efficiency by reference to a V:Nb null-scattering standard and then subsequently corrected for the sample-specific and wavelength-dependent self-shielding using Mantid (Arnold et al., 2014 : Mantid, 2013 ). In the case of the molybdate, the number density of the specimen was determined to be 3.28 mol nm⁻³, with a scattering cross section, allowing for the water being 9.1 mol % ¹H, σ_scatt = 93.81 b and an absorption cross section, σ_abs = 3.66 b; for the tungstate, the number density was 3.01 mol nm⁻³, the scattering cross section, allowing for the water being 8.6 mol % ¹H, σ_scatt = 94.19 b and σ_abs = 19.48 b. Diffraction data were exported in GSAS format and analysed with the GSAS/Expgui Rietveld package (Larsen & Von Dreele, 2000 : Toby, 2001 ). The fitted diffraction data are shown in Figs. 6 and 7.

Figure 6
Neutron powder diffraction data for Na₂MoO₄·2D₂O; red points are the observations, the green line is the calculated profile and the pink line beneath the diffraction pattern represents Obs−Calc. Vertical black tick marks report the expected positions of the Bragg peaks. The inset shows the data measured at short flight times (i.e. small d-spacings).

Figure 7
Neutron powder diffraction data for Na₂WO₄·2D₂O; red points are the observations, the green line is the calculated profile and the pink line beneath the diffraction pattern represents Obs−Calc. Vertical black tick marks report the expected positions of the Bragg peaks. The inset shows the data measured at short flight times (i.e. small d-spacings).

4. Refinement

Profile refinements were done using GSAS/Expgui (Larsen & Von Dreele, 2000; Toby, 2001) starting from the coordinates reported by Farrugia (2007). Statistically significant anisotropic displacement parameters were refined for all atoms. An assumption was made that ¹H was uniformly distributed on all ²D sites, so the neutron scattering length of ²D was edited in GSAS in accordance with the concentration of ¹H determined by Raman spectroscopy; for the molybdate a value of 5.776 fm was used, and for the tungstate a value of 5.724 fm was adopted. Crystal data, data collection and structure refinement details are summarized in Table 4.

Table 4
Experimental details

	Na₂MoO₄·2D₂O	Na₂WO₄·2D₂O
Crystal data
Chemical formula	Na₂MoO₄·2D₂O	Na₂WO₄·2D₂O
M_r	245.99	333.87
Crystal system, space group	Orthorhombic, Pbca	Orthorhombic, Pbca
Temperature (K)	295	295
a, b, c (Å)	8.482961 (14), 10.566170 (17), 13.83195 (3)	8.482514 (15), 10.595156 (19), 13.85640 (3)
V (Å³)	1239.79 (1)	1245.32 (1)
Z	8	8
Radiation type	Neutron	Neutron
μ (mm⁻¹)	0.03 + 0.0007 * λ	0.03 + 0.0033 * λ
Specimen shape, size (mm)	Cylinder, 38 × 11	Cylinder, 50 × 11

Data collection
Diffractometer	HRPD, High resolution neutron powder	HRPD, High resolution neutron powder
Specimen mounting	Vanadium tube	Vanadium tube
Data collection mode	Transmission	Transmission
Scan method	Time of flight	Time of flight
Absorption correction	Analytical [data were corrected for self shielding using σ_scatt = 93.812 barns and σ_ab(λ) = 3.657 barns at 1.798 Å during the normalization procedure. The linear absorption coefficient is wavelength dependent and is calculated as: μ = 0.0308 + 0.0007 * λ (mm⁻¹)]	analytical [data were corrected for self shielding using σ_scatt = 94.190 barns and σ_ab(λ) = 19.484 barns at 1.798 Å during the normalization procedure. The linear absorption coefficient is wavelength dependent and is calculated as: μ = 0.0284 + 0.0033 * λ (mm⁻¹)]
T_min, T_max	0.685, 0.706	0.700, 0.603
2θ values (°)	2θ_fixed = 168.329	2θ_fixed = 168.329
Distance from source to specimen (mm)	95000	95000
Distance from specimen to detector (mm)	965	965

Refinement
R factors and goodness of fit	R_p = 0.013, R_wp = 0.013, R_exp = 0.007, R(F²) = 0.05255, χ² = 3.534	R_p = 0.014, R_wp = 0.013, R_exp = 0.007, R(F²) = 0.04597, χ² = 3.312
No. of data points	4610	4610
No. of parameters	133	133
Computer programs: HRPD control software, GSAS/Expgui (Larsen & Von Dreele, 2000: Toby, 2001), Mantid (Arnold et al., 2014: Mantid, 2013), DIAMOND (Putz & Brandenburg, 2006 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC references: 1406122; 1406121

Crystal structure: contains datablocks Na2MoO4.2D2O, Na2WO4.2D2O, New_Global_Publ_Block. DOI: 10.1107/S2056989015011354/wm5172sup1.cif

Rietveld powder data: contains datablock Na2MoO4.2D2O. DOI: 10.1107/S2056989015011354/wm5172Na2MoO4.2D2Osup2.rtv

Rietveld powder data: contains datablock Na2WO4.2D2O. DOI: 10.1107/S2056989015011354/wm5172Na2WO4.2D2Osup3.rtv

checkCIF report

Chemical context top

Na₂MoO₄ and Na₂WO₄ are unusual amongst the alkali metal mono-molybdates and mono-tungstates in being highly soluble in water and forming polyhydrated crystals. Additionally, sodium apparently plays a significant role in the solvation of other alkali metal ions to form a range of double molybdate and tungstate hydrates (Klevtsova et al., 1990; Klevtsov et al., 1997; Mirzoev et al., 2010), for example, Na₃K(MoO₄)₂·9H₂O. Both dihydrate and decahydrate varieties of the two title compounds are known, their solubilities as a function of temperature being well characterised (Funk, 1900; Zhilova et al., 2008). The structures of the decahydrates have not yet been reported, although I have established that they are not isotypic with the sodium sulfate analogue, Na₂SO₄·10H₂O, as had hitherto been thought.

The dihydrates have been the subject of extensive crystallographic studies, from descriptions of their density, habit and measurements of interfacial angles (Svanberg & Struve, 1848; Zenker, 1853; Rammelsberg, 1855; Marignac, 1863; Delafontaine, 1865; Ullik, 1867; Clarke, 1877; Zambonini, 1923), through to determination of absolute unit-cell parameters (Pistorius & Sharp, 1961), and subsequent solution and refinement of their structures (Mitra & Verma, 1969; Okada et al., 1974; Matsumoto et al., 1975; Atovmyan & D'yachenko, 1969; Capitelli et al., 2006; Farrugia, 2007). However, the presence of heavy atoms in these materials makes it impossible to achieve a uniform precision on all structural parameters using X-rays, and even with single-crystal methods that purport to identify hydrogen positions there may be significant inaccuracies. Such problems are minimised using a neutron radiation probe since the coherent neutron scattering lengths of the constituent elements differ by less than a factor of two, being 6.715 fm for Mo, 4.86 fm for W, 3.63 fm for Na, 5.803 fm for O, and 6.67 fm for ²D (Sears, 1992). Thus one can locate accurately all of the light atoms and obtain a uniform level of precision on their coordinates and displacement parameters. Since the incoherent neutron scattering cross section of ¹H is large (80.3 barns) it is usual to prepare perdeuterated specimens whenever possible (the incoherent cross section of ²D being only 2.1 barns) since this optimises the coherent Bragg scattering signal above the background, reducing the counting times required for a high-precision structure refinement from many days to a matter of hours on the instrument used for these measurements. These data were therefore measured using Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O samples.

The occurrence of polyhydrated forms of both Na₂MoO₄ and Na₂WO₄ suggests that both would be excellent candidates for the formation of hydrogen-bonded complexes with water-soluble organics, such as amino acids, producing metal-organic crystals with potentially useful optical properties (cf., glycine lithium molybdate; Fleck et al., 2006). High-pressure polymorphs of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O are indicated from Raman scattering studies (Luz-Lima et al., 2010; Saraiva et al., 2013). Characterising the structures and properties of the title compounds provides an essential foundation on which to build future studies of the high-pressure phases, of the as-yet incomplete decahydrate structures and any related organic-bearing hydrates.

Structural commentary top

Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O are isotypic, crystallizing in the orthorhombic space group Pbca; all atoms occupy general positions (Wyckoff sites 8c). Note that the atom labelling scheme and space-group setting used here follows Farrugia (2007); consequently there are some differences with respect to other literature sources, although equivalent contacts are referred to in Table 1 and Table 2. The X⁶⁺ ions (X = Mo, W) are tetrahedrally coordinated by O^2-, the Mo—O and W—O bond lengths varying slightly according to the type of coordination adopted by a particular apex: O1 and O4 are each coordinated to Na⁺ and each also accepts two hydrogen bonds; O2 is coordinated to three Na⁺ ions and O3 is coordinated to two Na⁺ ions (Fig. 1). In both title compounds, X–O1 and X–O4 are the longest contacts and X–O3 is the shortest contact in the tetrahedral oxyanion. The mean Mo—O and W—O bond lengths are in good agreement with those found in the anhydrous crystals (Fortes, 2015). Furthermore, each of the absolute Mo—O bond lengths are identical (within error) to those found by Capitelli et al. (2006); the agreement in W—O bond lengths with Farrugia (2007) is marginally poorer.

The Na⁺ ions occupy two inequivalent sites: in one, Na⁺ is six-fold coordinated by two water molecules and four XO₄^2- oxygen atoms, yielding an octahedral arrangement; in the second, Na⁺ is five-fold coordinated by two water molecules and three XO₄^2- oxygen atoms, yielding a square-pyramidal arrangement. These two polyhedra share a common edge (O2–O5) and are connected, moreover, with their inversion-centre-related neighbours along three other shared edges to form a cluster (Fig. 2a). The clusters corner-share via O6 to create a `slab' parallel to (010) (Fig. 2b). The mean Na—O bond lengths are statistically identical in Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O being ~1.6 % longer in the NaO₆ octahedra and ~2.3 % shorter in the NaO₅ polyhedra than Na—O bonds in the anhydrous crystals (Fortes, 2015). The agreement in Na—O bond lengths with the X-ray single crystal studies of Capitelli et al. (2006) and Farrugia (2007) is very good. Overall, the agreement in bond lengths and angles for the two independently refined data sets is excellent (Tables 1 and 2).

Although it is more usual to find Na⁺ in octahedral coordination, there are abundant examples of Na⁺ in five-fold coordination, including instances where the NaO₅ polyhedron adopts a square-pyramidal arrangement (Beurskens & Jeffrey, 1961; Císarová et al., 2001; Sharma et al., 2005; Smith & Wermuth, 2014; Aksenov et al., 2014) or the alternative trigonal-bipyramidal arrangement (Mereiter, 2013; Smith, 2013). A similar combination of NaO₆ and NaO₅ polyhedra to that found in the title compounds occurs in the closely-related hydrates Na₂CrO₄·1.5H₂O and Na₂SeO₄·1.5H₂O (Kahlenberg, 2012; Weil & Bonneau, 2014). The two water molecules form hydrogen-bonded chains between the O1 and O4 atoms of the tetrahedral oxyanions; O5-related chains extend along [001] and O6-related chains crosslink them in a staggered fashion along [100]. Figs. 3(a) and 3(b) depict the spatial relationship between this `net' of water linked tetrahedra and the adjacent `slab' of corner-linked Na—O polyhedral clusters. The layers shown in Fig. 3(b) alternate to create the three-dimensional structure and are no doubt responsible for the macro-scale platy habit of the crystals.

There are no significant differences in the hydrogen bond geometries of the molybdate or tungstate crystals. The most recent X-ray single-crystal diffraction study of Na₂WO₄·2H₂O (Farrugia, 2007) implied that one of the water molecules (O5) was involved in a weaker three-centred interaction, although a similarly recent measurement of Na₂MoO₄·2H₂O (Capitelli et al., 2006) identified a 'normal' linear two-centred interaction for this bond. This work, using neutrons, has been able to accurately and precisely characterise the hydrogen bond geometry, showing that the latter is true for both structures; there is no bifurcated bond and all hydrogen-bonded interactions are of the linear two-centred variety. Presumably the error in Farrugia's analysis arose due to the substantial absorption correction required (µ = 18.7 mm^-1) for an accurate structure refinement from X-ray single-crystal data.

Raman spectra of Na₂MoO₄·2H₂O and Na₂MoO₄·2D₂O were first reported by Mahadevan Pillai et al. (1997); subsequently, Luz-Lima et al. (2010) and Saraiva et al. (2013) published the Raman spectra of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O as a function of temperature (13–300 K) and as a function of hydrostatic pressure (to 5 GPa). Both compounds exhibit evidence of a 'conformational change' on cooling through 120 K: the molybdate appears to undergo two high-pressure phase transitions, one at 3 GPa and the second at 4 GPa; the tungstate apparently undergoes a high-pressure phase transition at 3.9 GPa. The Raman spectra reported here (Figs. 4 and 5 and Supporting information) agree well with data in the literature (Table 3). The large blue-shifts in the internal vibrational frequencies of the deuterated water molecule are similar to the square root of the D:H mass ratio; the small blue-shifts of most of the internal modes of the tetrahedral oxyanions are consistent with stronger hydrogen bonding in the deuterated species, as expected (cf. Scheiner & Čuma, 1996; Soper & Benmore, 2008).

Synthesis and crystallization top

Coarse polycrystalline powders of Na₂MoO₄·2H₂O (Sigma-Adrich M1003 > 99.5%) and Na₂WO₄·2H₂O (Sigma-Adrich 14304 > 99%) were dehydrated by drying at 673 K in air. The resulting anhydrous materials were characterised by Raman spectroscopy, X-ray and neutron powder diffraction (Fortes, 2015). This material was dissolved in D₂O (Aldrich 151882, 99.9 atom% D) and twice recrystallized by gentle evaporation at 323 K. The molybdate crystallised with a coarse platy habit whereas the tungstate was deposited as a finer-grained material. Once the supernatant liquid was decanted, the residue was air dried on filter paper and then ground to a fine powder with an agate pestle and mortar. The powders were loaded into standard vanadium sample-holder tubes of internal diameter 11 mm to a depth not less than 20 mm (this being the vertical neutron beam dimension at the sample position). Accurate volumes and masses were determined after the diffraction measurements were complete and used to correct the data for self-shielding. The level of deuteration was determined by Raman spectroscopy (see below) to be ~91% for both compounds.

Raman spectra were acquired with a B&WTek i-Raman plus portable spectrometer; this device uses a 532 nm laser (37 mW power at the fiber-optic probe tip) to stimulate Raman scattering, which is measured in the range 170–4000 cm^-1 with a spectral resolution of 3 cm^-1. Data were collected for 600 sec at 17 mW for Na₂MoO₄·2H₂O (as bought), 180 sec at 37 mW for Na₂MoO₄·2D₂O, 300 sec at 17 mW for Na₂WO₄·2H₂O (as bought) and 220 sec at 37 mW for Na₂WO₄·2D₂O; after summation, the background was removed and peaks fitted using Pseudo-Voigt functions in OriginPro (OriginLab, Northampton MA). These data are provided as an electronic supplement in the form of an ASCII file. Small quantities of ordinary hydrogen were found to be present in both specimens, the proportion being determined by the ratio of the areas under the ν₁/ν₃ (H₂O) bands after normalisation relative to the height of the strong ν₁ (XO₄^2-) peak. The molar abundance of ¹H was used to correct the diffraction data for absorption (see below) and to ensure accurate refinement of the structure (see Refinement).

Time-of-flight neutron diffraction patterns were collected at 295 K using the High Resolution Powder Diffractometer, HRPD (Ibberson, 2009), at the ISIS spallation neutron source, Harwell Campus, Oxfordshire, U.K. Data were acquired in the range of neutron flight times from 30–130 msec (equivalent to neutron wavelengths of 1.24–5.36 Å) for 15.17 hr from the molybdate and 14.40 hr from the tungstate, equivalent to 615 and 590 µAhr of integrated proton beam current, respectively. These data sets were normalized to the incident spectrum and corrected for detector efficiency by reference to a V:Nb null-scattering standard and then subsequently corrected for the sample-specific and wavelength-dependent self-shielding using Mantid (Arnold et al., 2014: Mantid, 2013). In the case of the molybdate, the number density of the specimen was determined to be 3.28 mol nm^-3, with a scattering cross section, allowing for the water being 9.1 mol % ¹H, σ_scatt = 93.81 b and an absorption cross section, σ_abs = 3.66 b; for the tungstate, the number density was 3.01 mol nm^-3, the scattering cross section, allowing for the water being 8.6 mol % ¹H, σ_scatt = 94.19 b and σ_abs = 19.48 b. Diffraction data were exported in GSAS format and analysed with the GSAS/Expgui Rietveld package (Larsen & Von Dreele, 2000: Toby, 2001). The fitted diffraction data are shown in Figs. 6 and 7.

Refinement top

Computing details top

For both compounds, data collection: HRPD control software; cell refinement: GSAS/Expgui (Larsen & Von Dreele, 2000: Toby, 2001); data reduction: Mantid (Arnold et al., 2014: Mantid, 2013); program(s) used to solve structure: n/a; program(s) used to refine structure: GSAS/Expgui (Larsen & Von Dreele, 2000: Toby, 2001); molecular graphics: DIAMOND (Putz & Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top

	Fig. 1. First and second coordination shell of Mo⁶⁺/W⁶⁺ in the title compounds, revealing differences in the environment of each apical O^2- that are responsible for the variations in Mo–O and W–O bond lengths. Anisotropic displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 - x, 1 - y, 1 - z; (ii) 1/2 + x, 1.5 - y, 1 - z; (iii) -1/2 + x, 1.5 - y, 1 - z; (iv) 1/2 - x, 1/2 + y, z; (v) 1.5 - x, 1/2 + y, z; (vi) 1 - x, 1/2 + y, 1.5 - z.]
	Fig. 2. (a) Arrangement of NaO_x polyhedra into edge-sharing clusters comprised of two Na1O₆ octahedra and two Na2O₅ square pyramids; (b) Arrangement of the clusters shown in (a) by corner sharing to form `slabs' parallel (010). Ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 - x, 1 - y, 1 - z; (ii) 1/2 + x, 1.5 - y, 1 - z; (iii) -1/2 + x, 1.5 - y, 1 - z; (iv) 0.5 - x, -1/2 + y, z; (v) 1/2 + x, y, 1.5 - z; (vi) 0.5 - x, 1 - y, 1/2 + z; (vii) 1.5 - x, -1/2 + y, z.]
	Fig. 3. (a) View down the b axis of the network of water-linked tetrahedral oxyanions; chains linked by O5 extend along [001] whereas crosslinkages through O6 are staggered along [100]. (b) View of the same structure along the c axis. Ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 - x, 1 - y, 1 - z; (ii) 1 - x, 1/2 + y, 1.5 - z; (iii) 1/2 + x, 1.5 - y, 1 - z; (iv) 1/2 + x, y, 1.5 - z; (v) x, 1.5 - y, -1/2 + z; (vi) x, 1.5 - y, 1/2 + z.]
	Fig. 4. Raman spectra of Na₂MoO₄·2H₂O and Na₂MoO₄·2H₂O in the range 200–3900 cm^-1. Band positions and vibrational assignments are indicated (see also Table 4). Vertical scales show intensities relative to ν₁ (XO₄^2-).
	Fig. 5. Raman spectra of Na₂WO₄·2H₂O and Na₂WO₄·2H₂O in the range 200–3900 cm^-1. Band positions and vibrational assignments are indicated (see also Table 4). Vertical scales show intensities relative to ν₁ (XO₄^2-).
	Fig. 6. Neutron powder diffraction data for Na₂MoO₄·2H₂O; red points are the observations, the green line is the calculated profile and the pink line beneath the diffraction pattern represents Obs-Calc. Vertical black tick marks report the expected positions of the Bragg peaks. The inset shows the data measured at short flight times (i.e. small d-spacings).
	Fig. 7. Neutron powder diffraction data for Na₂WO₄·2H₂O; red points are the observations, the green line is the calculated profile and the pink line beneath the diffraction pattern represents Obs–Calc. Vertical black tick marks report the expected positions of the Bragg peaks. The inset shows the data measured at short flight times (i.e. small d-spacings).

(Na2MoO4.2D2O) Disodium molybdenum(VI) oxide dihydrate top

Crystal data top

Na₂MoO₄·2D₂O	D_x = 2.636 Mg m⁻³
M_r = 245.99	Melting point: 353 K
Orthorhombic, Pbca	Neutron radiation
Hall symbol: -P 2ac 2ab	µ = 0.03+ 0.0007 * λ mm⁻¹
a = 8.482961 (14) Å	T = 295 K
b = 10.566170 (17) Å	white
c = 13.83195 (3) Å	cylinder, 38 × 11 mm
V = 1239.79 (1) Å³	Specimen preparation: Prepared at 323 K and 100 kPa
Z = 8

Data collection top

HRPD, High resolution neutron powder diffractometer	Absorption correction: analytical Data were corrected for self shielding using σ_scatt = 93.812 barns and σ_ab(λ) = 3.657 barns at 1.798 Å during the normalisation procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.0308 + 0.0007 * λ [mm^-1]
Radiation source: ISIS Facility, Neutron spallation source	T_min = 0.685, T_max = 0.706
Specimen mounting: vanadium tube	2θ_fixed = 168.329
Data collection mode: transmission	Distance from source to specimen: 95000 mm
Scan method: time of flight	Distance from specimen to detector: 965 mm

Refinement top

Least-squares matrix: full	Excluded region(s): none
R_p = 0.013	Profile function: TOF profile function #3 (21 terms). Profile coefficients for exp pseudovoigt convolution [Von Dreele, 1990 (unpublished)] (α) = 0.1414, (β₀) = 0.026250, (β₁) = 0.004690, (σ₀) = 0, (σ₁) = 194.5, (σ₂) = 13.5, (γ₀) = 0, (γ₁) = 0, (γ₂) = 0, (γ_2s) = 0, (γ_1e) = 0, (γ_2e) = 0, (ε_i) = 0, (ε_a) = 0, (ε_A) = 0, (γ₁₁) = 0.057, (γ₂₂) = 0, (γ₃₃) = 0.059, (γ₁₂) = -0.087, (γ₁₃) = -0.014, (γ₂₃) = -0.018. Peak tails ignored where intensity <0.0010x peak. Aniso. broadening axis 0.0 0.0 1.0
R_wp = 0.013	133 parameters
R_exp = 0.007	0 restraints
R(F²) = 0.05255	0 constraints
χ² = 3.534	(Δ/σ)_max = 0.03
4610 data points	Background function: GSAS Background function number 1 with 12 terms. Shifted Chebyshev function of 1st kind 1: 4.30598, 2: 1.54022, 3: -0.237828 4: -6.992080x10^-2, 5: -0.113274, 6: -1.736560x10^-2, 7: -1.996810x10^-2, 8: 2.118030x10^-5, 9: -4.698340x10^-3, 10: -2.646770x10^-2, 11: 2.772870x10^-2, 12: -1.690170x10^-3

Crystal data top

Na₂MoO₄·2D₂O	V = 1239.79 (1) Å³
M_r = 245.99	Z = 8
Orthorhombic, Pbca	Neutron radiation
a = 8.482961 (14) Å	µ = 0.03+ 0.0007 * λ mm⁻¹
b = 10.566170 (17) Å	T = 295 K
c = 13.83195 (3) Å	cylinder, 38 × 11 mm

Data collection top

HRPD, High resolution neutron powder diffractometer	T_min = 0.685, T_max = 0.706
Specimen mounting: vanadium tube	2θ_fixed = 168.329
Data collection mode: transmission	Distance from source to specimen: 95000 mm
Scan method: time of flight	Distance from specimen to detector: 965 mm
Absorption correction: analytical Data were corrected for self shielding using σ_scatt = 93.812 barns and σ_ab(λ) = 3.657 barns at 1.798 Å during the normalisation procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.0308 + 0.0007 * λ [mm^-1]

Refinement top

R_p = 0.013	χ² = 3.534
R_wp = 0.013	4610 data points
R_exp = 0.007	133 parameters
R(F²) = 0.05255	0 restraints

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Mo1	0.51477 (10)	0.80193 (8)	0.52313 (8)	0.01137
Na1	0.3438 (3)	0.4964 (2)	0.58515 (17)	0.02371
Na2	0.7433 (2)	0.5509 (2)	0.64802 (16)	0.0216
O1	0.45103 (15)	0.82353 (12)	0.40216 (9)	0.01917
O2	0.55667 (15)	0.64011 (10)	0.54111 (10)	0.01665
O3	0.68676 (15)	0.89087 (11)	0.53936 (11)	0.02282
O4	0.37187 (15)	0.85121 (12)	0.60907 (10)	0.0194
O5	0.53793 (19)	0.40846 (16)	0.70077 (14)	0.0252
O6	0.2281 (2)	0.64176 (17)	0.70081 (11)	0.02505
D51	0.5576 (2)	0.32908 (18)	0.66668 (13)	0.03656
D52	0.5585 (2)	0.39149 (15)	0.76825 (14)	0.03068
D61	0.1235 (2)	0.64696 (14)	0.67235 (12)	0.03034
D62	0.27890 (19)	0.71840 (16)	0.67765 (12)	0.03232

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Mo1	0.0103 (6)	0.0083 (5)	0.0156 (6)	0.0001 (4)	0.0009 (5)	0.0003 (5)
Na1	0.0249 (13)	0.0213 (11)	0.0250 (15)	−0.0011 (10)	0.0026 (10)	0.0012 (9)
Na2	0.0196 (12)	0.0180 (12)	0.0272 (14)	−0.0037 (9)	0.0008 (9)	−0.0006 (10)
O1	0.0188 (7)	0.0191 (7)	0.0196 (8)	0.0025 (6)	0.0004 (6)	0.0017 (6)
O2	0.0161 (6)	0.0086 (6)	0.0252 (8)	−0.0008 (6)	−0.0019 (6)	0.0017 (6)
O3	0.0197 (7)	0.0199 (7)	0.0289 (9)	−0.0079 (6)	−0.0020 (6)	−0.0022 (7)
O4	0.0187 (6)	0.0163 (6)	0.0233 (8)	0.0040 (6)	0.0050 (6)	−0.0011 (6)
O5	0.0272 (9)	0.0207 (9)	0.0277 (10)	−0.0006 (7)	−0.0036 (8)	0.0001 (8)
O6	0.0264 (9)	0.0257 (9)	0.0231 (10)	−0.0018 (8)	−0.0031 (7)	0.0060 (7)
D51	0.0432 (11)	0.0272 (9)	0.0393 (11)	0.0030 (8)	−0.0107 (9)	−0.0006 (9)
D52	0.0399 (9)	0.0292 (8)	0.0229 (8)	0.0008 (8)	−0.0003 (8)	0.0027 (8)
D61	0.0233 (9)	0.0340 (9)	0.0337 (9)	0.0004 (7)	−0.0062 (8)	0.0030 (8)
D62	0.0338 (10)	0.0248 (8)	0.0384 (12)	−0.0056 (8)	−0.0024 (8)	0.0028 (8)

Geometric parameters (Å, º) top

Mo1—O1	1.7732 (17)	Na2—O3^v	2.339 (3)
Mo1—O2	1.7640 (14)	Na2—O5	2.415 (3)
Mo1—O3	1.7499 (16)	Na2—O6^vi	2.305 (3)
Mo1—O4	1.7759 (17)	O5—D51	0.9766 (19)
Na1—O2	2.437 (3)	O5—D52	0.9664 (18)
Na1—O2ⁱ	2.417 (3)	O6—D61	0.9722 (16)
Na1—O3ⁱⁱ	2.482 (3)	O6—D62	0.9719 (18)
Na1—O4ⁱⁱⁱ	2.410 (3)	D51—O1ⁱ	1.874 (2)
Na1—O5	2.476 (2)	D52—O4^vii	1.846 (3)
Na1—O6	2.426 (3)	D61—O1ⁱⁱ	1.816 (2)
Na2—O1^iv	2.312 (3)	D62—O4	1.868 (3)
Na2—O2	2.363 (3)

O1—Mo1—O2	108.62 (8)	O2ⁱ—Na1—O6	174.75 (13)
O1—Mo1—O3	107.83 (8)	O4ⁱⁱⁱ—Na1—O5	100.22 (10)
O1—Mo1—O4	112.69 (8)	O4ⁱⁱⁱ—Na1—O6	90.30 (9)
O2—Mo1—O3	109.54 (7)	O5—Na1—O6	94.63 (10)
O2—Mo1—O4	109.11 (8)	O1^iv—Na2—O2	95.39 (9)
O3—Mo1—O4	109.01 (8)	O1^iv—Na2—O3^v	91.65 (9)
O2—Na1—O2ⁱ	86.12 (8)	O1^iv—Na2—O5	176.34 (12)
O2—Na1—O3ⁱⁱ	85.70 (9)	O1^iv—Na2—O6^vi	94.37 (9)
O2—Na1—O4ⁱⁱⁱ	173.42 (12)	O2—Na2—O3^v	93.22 (9)
O2—Na1—O5	84.40 (9)	O2—Na2—O5	87.38 (8)
O2—Na1—O6	94.02 (10)	O2—Na2—O6^vi	111.35 (10)
O3ⁱⁱ—Na1—O2ⁱ	88.43 (9)	O3^v—Na2—O5	85.79 (9)
O3ⁱⁱ—Na1—O4ⁱⁱⁱ	89.62 (9)	O3^v—Na2—O6^vi	153.97 (12)
O3ⁱⁱ—Na1—O5	170.10 (11)	O5—Na2—O6^vi	86.84 (10)
O3ⁱⁱ—Na1—O6	86.35 (9)	D51—O5—D52	106.0 (2)
O2ⁱ—Na1—O4ⁱⁱⁱ	89.13 (9)	D61—O6—D62	103.0 (2)
O2ⁱ—Na1—O5	90.61 (10)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x−1/2, −y+3/2, −z+1; (iii) −x+1/2, y−1/2, z; (iv) x+1/2, −y+3/2, −z+1; (v) −x+3/2, y−1/2, z; (vi) x+1/2, y, −z+3/2; (vii) −x+1, y−1/2, −z+3/2.

(Na2WO4.2D2O) Disodium tungsten(VI) oxide dihydrate top

Crystal data top

Na₂WO₄·2D₂O	D_x = 3.562 Mg m⁻³
M_r = 333.87	Melting point: 373 K
Orthorhombic, Pbca	Neutron radiation
Hall symbol: -P 2ac 2ab	µ = 0.03+ 0.0033 * λ mm⁻¹
a = 8.482514 (15) Å	T = 295 K
b = 10.595156 (19) Å	white
c = 13.85640 (3) Å	cylinder, 50 × 11 mm
V = 1245.32 (1) Å³	Specimen preparation: Prepared at 323 K and 100 kPa
Z = 8

Data collection top

HRPD, High resolution neutron powder diffractometer	Absorption correction: analytical Data were corrected for self shielding using σ_scatt = 94.190 barns and σ_ab(λ) = 19.484 barns at 1.798 Å during the normalisation procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.0284 + 0.0033 * λ [mm^-1]
Radiation source: ISIS Facility, Neutron spallation source	T_min = 0.603, T_max = 0.700
Specimen mounting: vanadium tube	2θ_fixed = 168.329
Data collection mode: transmission	Distance from source to specimen: 95000 mm
Scan method: time of flight	Distance from specimen to detector: 965 mm

Refinement top

Least-squares matrix: full	Excluded region(s): none
R_p = 0.014	Profile function: TOF profile function #3 (21 terms). Profile coefficients for exp pseudovoigt convolution [Von Dreele, 1990 (unpublished)] (α) = 0.1414, (β₀) = 0.026250, (β₁) = 0.004690, (σ₀) = 0, (σ₁) = 322.9, (σ₂) = 15.7, (γ₀) = 0, (γ₁) = 0, (γ₂) = 0, (γ_2s) = 0, (γ_1e) = 0, (γ_2e) = 0, (ε_i) = 0, (ε_a) = 0, (ε_A) = 0, (γ₁₁) = 0.023, (γ₂₂) = 0, (γ₃₃) = 0.006, (γ₁₂) = 0.050, (γ₁₃) = 0.016, (γ₂₃) = 0.017. Peak tails ignored where intensity <0.0010x peak. Aniso. broadening axis 0.0 0.0 1.0
R_wp = 0.013	133 parameters
R_exp = 0.007	0 restraints
R(F²) = 0.04597	0 constraints
χ² = 3.312	(Δ/σ)_max = 0.04
4610 data points	Background function: GSAS Background function number 1 with 12 terms. Shifted Chebyshev function of 1st kind 1: 3.91163, 2: 1.22805, 3: -0.206144, 4: -8.53351x10^-2, 5: -9.966470x10^-2, 6: -1.847470x10^-2, 7: -1.38195x10^-2, 8: 9.956170x10^-4, 9: 4.49839x10^-3, 10: -2.199010x10^-2, 11: 2.57524x10^-2, 12: -2.00574x10^-3

Crystal data top

Na₂WO₄·2D₂O	V = 1245.32 (1) Å³
M_r = 333.87	Z = 8
Orthorhombic, Pbca	Neutron radiation
a = 8.482514 (15) Å	µ = 0.03+ 0.0033 * λ mm⁻¹
b = 10.595156 (19) Å	T = 295 K
c = 13.85640 (3) Å	cylinder, 50 × 11 mm

Data collection top

HRPD, High resolution neutron powder diffractometer	T_min = 0.603, T_max = 0.700
Specimen mounting: vanadium tube	2θ_fixed = 168.329
Data collection mode: transmission	Distance from source to specimen: 95000 mm
Scan method: time of flight	Distance from specimen to detector: 965 mm
Absorption correction: analytical Data were corrected for self shielding using σ_scatt = 94.190 barns and σ_ab(λ) = 19.484 barns at 1.798 Å during the normalisation procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.0284 + 0.0033 * λ [mm^-1]

Refinement top

R_p = 0.014	χ² = 3.312
R_wp = 0.013	4610 data points
R_exp = 0.007	133 parameters
R(F²) = 0.04597	0 restraints

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
W1	0.51352 (13)	0.80186 (10)	0.52310 (10)	0.01206
Na1	0.3444 (2)	0.4957 (2)	0.58501 (16)	0.02213
Na2	0.7422 (2)	0.54966 (18)	0.64745 (14)	0.02166
O1	0.44940 (14)	0.82253 (11)	0.40144 (8)	0.01858
O2	0.55647 (14)	0.63936 (9)	0.54135 (9)	0.01675
O3	0.68666 (14)	0.89213 (10)	0.53870 (10)	0.02256
O4	0.36916 (14)	0.85058 (11)	0.60895 (9)	0.01972
O5	0.53794 (17)	0.40814 (14)	0.70116 (12)	0.02505
O6	0.2276 (2)	0.64134 (14)	0.70148 (11)	0.02531
D51	0.5576 (2)	0.32926 (17)	0.66767 (12)	0.03829
D52	0.55912 (18)	0.39189 (14)	0.76800 (13)	0.03325
D61	0.1229 (2)	0.64645 (13)	0.67384 (10)	0.03267
D62	0.27774 (17)	0.71764 (15)	0.67874 (12)	0.03524

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
W1	0.0099 (7)	0.0064 (6)	0.0199 (7)	−0.0002 (5)	0.0009 (6)	0.0002 (6)
Na1	0.0209 (11)	0.0167 (9)	0.0289 (13	0.0004 (9)	0.0028 (9)	0.0003 (8)
Na2	0.0192 (10)	0.0183 (11)	0.0275 (13)	0.0001 (8)	−0.0006 (8)	0.0000 (9)
O1	0.0170 (6)	0.0192 (6)	0.0195 (7)	0.0008 (5)	0.0010 (6)	0.0031 (5)
O2	0.0176 (6)	0.0079 (5)	0.0248 (7)	0.0014 (5)	−0.0004 (5)	0.0035 (5)
O3	0.0193 (7)	0.0190 (6)	0.0293 (8)	−0.0083 (5)	−0.0002 (6)	−0.0015 (6)
O4	0.0201 (6)	0.0176 (6)	0.0214 (7)	0.0045 (5)	0.0052 (6)	0.0008 (6)
O5	0.0305 (9)	0.0206 (9)	0.0241 (8)	−0.0027 (7)	−0.0031 (7)	−0.0023 (7)
O6	0.0246 (8)	0.0247 (8)	0.0266 (9)	−0.0004 (7)	−0.0046 (7)	0.0061 (7)
D51	0.0448 (10)	0.0304 (9)	0.0397 (9)	−0.0001 (8)	−0.0104 (8)	−0.0032 (8)
D52	0.0415 (9)	0.0323 (8)	0.0259 (8)	−0.0027 (7)	−0.0001 (8)	0.0004 (7)
D61	0.0259 (9)	0.0354 (9)	0.0367 (9)	−0.0011 (7)	−0.0030 (7)	0.0046 (8)
D62	0.0347 (9)	0.0270 (8)	0.0440 (11)	−0.0059 (7)	−0.0029 (7)	0.0059 (7)

Geometric parameters (Å, º) top

W1—O1	1.7849 (19)	Na2—O3^v	2.328 (2)
W1—O2	1.7779 (15)	Na2—O5	2.409 (3)
W1—O3	1.7659 (17)	Na2—O6^vi	2.311 (2)
W1—O4	1.7834 (18)	O5—D51	0.9702 (18)
Na1—O2	2.433 (2)	O5—D52	0.9591 (16)
Na1—O2ⁱ	2.412 (3)	O6—D61	0.9684 (16)
Na1—O3ⁱⁱ	2.479 (3)	O6—D62	0.9664 (16)
Na1—O4ⁱⁱⁱ	2.399 (2)	D51—O1ⁱ	1.873 (2)
Na1—O5	2.479 (3)	D52—O4^vii	1.863 (2)
Na1—O6	2.443 (3)	D61—O1ⁱⁱ	1.834 (2)
Na2—O1^iv	2.320 (2)	D62—O4	1.876 (2)
Na2—O2	2.355 (2)

O1—W1—O2	108.40 (9)	O2ⁱ—Na1—O6	174.55 (12)
O1—W1—O3	107.61 (8)	O4ⁱⁱⁱ—Na1—O5	99.82 (9)
O1—W1—O4	112.66 (8)	O4ⁱⁱⁱ—Na1—O6	90.40 (8)
O2—W1—O3	109.67 (7)	O5—Na1—O6	94.37 (10)
O2—W1—O4	109.03 (9)	O1^iv—Na2—O2	95.10 (8)
O3—W1—O4	109.43 (9)	O1^iv—Na2—O3^v	91.90 (8)
O2—Na1—O2ⁱ	86.16 (7)	O1^iv—Na2—O5	176.73 (11)
O2—Na1—O3ⁱⁱ	85.82 (8)	O1^iv—Na2—O6^vi	93.43 (9)
O2—Na1—O4ⁱⁱⁱ	173.55 (11)	O2—Na2—O3^v	93.36 (8)
O2—Na1—O5	84.60 (8)	O2—Na2—O5	87.87 (7)
O2—Na1—O6	93.95 (9)	O2—Na2—O6^vi	111.09 (9)
O3ⁱⁱ—Na1—O2ⁱ	88.32 (8)	O3^v—Na2—O5	86.57 (8)
O3ⁱⁱ—Na1—O4ⁱⁱⁱ	89.72 (8)	O3^v—Na2—O6^vi	154.35 (11)
O3ⁱⁱ—Na1—O5	170.42 (10)	O5—Na2—O6^vi	86.74 (9)
O3ⁱⁱ—Na1—O6	86.26 (8)	D51—O5—D52	105.96 (19)
O2ⁱ—Na1—O4ⁱⁱⁱ	89.05 (8)	D61—O6—D62	103.18 (19)
O2ⁱ—Na1—O5	91.07 (9)

Comparison of the X—O (X = Mo, W) and Na—O bond lengths (Å) in Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O with those of the protonated isotopologues reported in the literature. top

	Na₂MoO₄·2D₂O	Na₂MoO₄·2H₂O	Na₂WO₄·2D₂O	Na₂WO₄·2H₂O
	This work	Capitelli et al. (2006)	This work	Farrugia (2007)
X—O1	1.773 (2)	1.772 (1)	1.785 (2)	1.776 (3)
X—O2	1.764 (1)	1.767 (1)	1.778 (2)	1.778 (3)
X—O3	1.750 (2)	1.751 (1)	1.766 (2)	1.761 (3)
X—O4	1.776 (2)	1.778 (1)	1.783 (2)	1.787 (3)
Mean X—O	1.766	1.767	1.778	1.776

Na1—O2	2.437 (3)	2.446 (2)	2.433 (2)	2.442 (3)
Na1—O2⁽ⁱ⁾	2.417 (3)	2.419 (2)	2.412 (3)	2.416 (3)
Na1—O3⁽ⁱⁱ⁾	2.482 (3)	2.481 (2)	2.479 (3)	2.480 (3)
Na1—O4⁽ⁱⁱⁱ⁾	2.410 (3)	2.395 (2)	2.399 (2)	2.388 (3)
Na1—O5	2.476 (3)	2.456 (2)	2.479 (3)	2.464 (4)
Na1—O6	2.426 (3)	2.423 (2)	2.443 (3)	2.433 (3)
Mean Na1—O	2.441	2.437	2.441	2.437

Na2—O1^iv	2.312 (3)	2.319 (2)	2.320 (2)	2.323 (3)
Na2—O2	2.363 (3)	2.354 (2)	2.355 (2)	2.346 (3)
Na2—O3^v	2.339 (3)	2.341 (2)	2.328 (2)	2.331 (3)
Na2—O5	2.415 (3)	2.403 (2)	2.409 (3)	2.396 (3)
Na2—O6^vi	2.305 (3)	2.300 (2)	2.311 (2)	2.304 (3)
Mean Na2—O	2.347	2.343	2.345	2.340

Symmetry codes: (i) 1-x, 1-y, 1-z; (ii) -0.5+x, 1.5-y, 1-z; (iii) 0.5-x, -0.5+y, z; (iv) 0.5+x, 1.5-y, 1-z; (v) 1.5-x, -0.5+y, z; (vi) 0.5+x, y, 1.5-z.

Comparison of the water molecule and hydrogen bond geometry (Å, °) in Na₂MoO₄·2D₂O and Na₂WO₄·2D₂O with the protonated isotopologues as reported in the literature. Note the inclusion of the contact O5–D51···O3, which forms the longer `branch' of Farrugia's proposed bifurcated hydrogen bond. top

	Na₂MoO₄·2D₂O	Na₂MoO₄·2H₂O	Na₂WO₄·2D₂O	Na₂WO₄·2H₂O
	This work	Capitelli et al. (2006)	This work	Farrugia (2007)
O5—D51	0.977 (2)	0.68 (3)	0.970 (2)	0.86 (3)
O5—D52	0.966 (2)	0.76 (3)	0.959 (2)	0.86 (3)
D51—O5—D52	106.0 (2)	98 (4)	106.0 (2)	100 (5)
D51···O1⁽ⁱ⁾	1.874 (2)	2.16 (3)	1.873 (2)	2.09 (4)
O5—D51···O1⁽ⁱ⁾	167.9 (2)	167 (4)	168.2 (2)	145 (6)
D51···O3⁽ⁱⁱ⁾	–	–	–	2.70 (6)
O5—D51···O3⁽ⁱⁱ⁾	–	–	–	122 (5)
D52···O4⁽ⁱⁱ⁾	1.846 (3)	2.07 (3)	1.863 (2)	1.98 (3)
O5—D52···O4⁽ⁱⁱ⁾	171.2 (2)	176 (3)	170.9 (2)	174 (6)

O6—D61	0.972 (2)	0.83 (3)	0.968 (2)	0.86 (3)
O6—D62	0.972 (2)	0.71 (3)	0.966 (2)	0.86 (3)
D61—O6—D62	103.0 (2)	105 (3)	103.2 (2)	95 (5)
D61···O1	1.816 (2)	2.01 (3)	1.834 (2)	1.95 (3)
O6—D61···O1	167.0 (2)	167 (3)	167.0 (2)	167 (6)
D62···O4⁽ⁱⁱⁱ⁾	1.868 (4)	2.08 (3)	1.876 (2)	2.02 (4)
O6—D62···O4⁽ⁱⁱⁱ⁾	168.7 (2)	170 (3)	168.7 (2)	159 (6)

Symmetry codes: (i) 1-x, 1-y, 1-z; (ii) 1-x, -0.5+y, 1.5-z; (iii) -0.5+x, 1.5-y, 1-z.

Comparison of the internal vibrational mode frequencies (cm^-1) in fully protonated and 90 mol % deuterated isotopologues of Na₂MoO₄·2H₂O and Na₂WO₄·2H₂O with literature data. top

	Na₂MoO₄·2H₂O			Na₂WO₄·2H₂O
	This work (¹H)	This work (²D)	Busey & Keller (1964)	This work (¹H)	This work (²D)	Busey & Keller (1964)
ν₂ (XO₄^2-)	279	271	285	276	269	276
	319	315	325	324	321	325
	335	331		330	331
ν₄ (XO₄^2-)	359	358		358	355
ν₃ (XO₄^2-)	804	801	805	804	802	808
	833	826	836	836	831	838
	842	840	843		840
ν₁ (XO₄^2-)				891	889	893
	894	894	897	929	928	931

Experimental details

	(Na2MoO4.2D2O)	(Na2WO4.2D2O)
Crystal data
Chemical formula	Na₂MoO₄·2D₂O	Na₂WO₄·2D₂O
M_r	245.99	333.87
Crystal system, space group	Orthorhombic, Pbca	Orthorhombic, Pbca
Temperature (K)	295	295
a, b, c (Å)	8.482961 (14), 10.566170 (17), 13.83195 (3)	8.482514 (15), 10.595156 (19), 13.85640 (3)
V (Å³)	1239.79 (1)	1245.32 (1)
Z	8	8
Radiation type	Neutron	Neutron
µ (mm⁻¹)	0.03+ 0.0007 * λ	0.03+ 0.0033 * λ
Specimen shape, size (mm)	Cylinder, 38 × 11	Cylinder, 50 × 11

Data collection
Diffractometer	HRPD, High resolution neutron powder diffractometer	HRPD, High resolution neutron powder diffractometer
Specimen mounting	Vanadium tube	Vanadium tube
Data collection mode	Transmission	Transmission
Scan method	Time of flight	Time of flight
Absorption correction	Analytical Data were corrected for self shielding using σ_scatt = 93.812 barns and σ_ab(λ) = 3.657 barns at 1.798 Å during the normalisation procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.0308 + 0.0007 * λ [mm^-1]	–
T_min, T_max	0.685, 0.706	–
2θ values (°)	2θ_fixed = 168.329	2θ_fixed = 168.329
Distance from source to specimen (mm)	95000	95000
Distance from specimen to detector (mm)	965	965

Refinement
R factors and goodness of fit	R_p = 0.013, R_wp = 0.013, R_exp = 0.007, R(F²) = 0.05255, χ² = 3.534	R_p = 0.014, R_wp = 0.013, R_exp = 0.007, R(F²) = 0.04597, χ² = 3.312
No. of data points	4610	4610
No. of parameters	133	133

Computer programs: HRPD control software, GSAS/Expgui (Larsen & Von Dreele, 2000: Toby, 2001), Mantid (Arnold et al., 2014: Mantid, 2013), n/a, DIAMOND (Putz & Brandenburg, 2006), publCIF (Westrip, 2010).

Acknowledgements

The author thanks the STFC ISIS facility for beam-time access and acknowledges financial support from STFC (grant No. ST/K000934/1).

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Volume 71| Part 7| July 2015| Pages 799-806

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research communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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

1. Chemical context

2. Structural commentary

3. Synthesis and crystallization

4. Refinement

Supporting information

Acknowledgements

References

research communications