Crystal structures of spinel-type Na2MoO4 and Na2WO4 revisited using neutron powder diffraction

High-precision structural parameters for cubic Na2MoO4 and Na2WO4 are reported based on refinement of high-resolution time-of-flight neutron powder diffraction data. Complementary Raman spectra are also provided.


Chemical context
Both Na 2 MoO 4 and Na 2 WO 4 have rich phase diagrams in pressure and temperature space (Pistorius, 1966). The stable form at room temperature is the -Ag 2 MoO 4 cubic spinel structure type, space group Fd3m, which has been known for almost a century (Wyckoff, 1922). Among the alkali metal sulfates, chromates, molybdates and tungstates, only Na 2 MoO 4 and Na 2 WO 4 adopt the normal spinel structure at ambient pressure. Li 2 MoO 4 forms a cubic spinel structure at high pressure (Liebertz & Rooymans, 1967). Li 2 WO 4 forms a 'spinel-like' phase at high pressure (Pistorius, 1975;Horiuchi et al., 1979). Cubic sodium molybdate and sodium tungstate have been examined intermittently over subsequent decades using a variety of crystallographic techniques (Lindqvist, 1950;Becka & Poljak, 1958;Swanson et al., 1957Swanson et al., , 1962Singh Mudher et al., 2005) and vibrational spectroscopic methods (Busey & Keller, 1964;Preudhomme & Tarte, 1972;Breitinger et al., 1981;Luz Lima et al., 2010, 2011, or by nuclear magnetic resonance and quadrupole coupling (Lynch & Segel, 1972). However, the extant structural information on both phases is derived from X-ray diffraction data of low to modest precision. The first published structure refinement of Na 2 MoO 4 was only reported recently (Bramnik & Ehrenberg, 2004) from X-ray powder diffraction data measured to sin ()/ = 0.71 Å À1 ; the last structure refinement of Na 2 WO 4 was reported by Okada et al. (1974) from X-ray single-crystal diffraction data to sin ()/ = 0.81 Å À1 . Both compounds are highly soluble in water, crystallizing at room temperature as orthorhombic dihydrates (space group Pbca, Atovmyan & D'yachenko, 1969;Farrugia, 2007). Below 283.5 K for the molybdate and 279.2 K for the tungstate, crystals grow with ten water molecules per formula unit (Funk, 1900;Cadbury, 1955;Zhilova et al., 2008). The high solubility in water and propensity towards forming hydrogen-bonded hydrates (unlike the heavier alkali metal molybdates and tungstates) suggests that both compounds would be excellent candidates for formation of hydrogen-bonded complexes with water soluble organics, such as amino acids, producing metalorganic crystals with potentially useful optical properties (cf., glycine lithium molybdate; Fleck et al., 2006).
In the course of preparing deuterated specimens of the dihydrated and decahydrated forms of Na 2 MoO 4 and Na 2 WO 4 for neutron diffraction analysis, the anhydrous phases were synthesised and an opportunity arose to acquire neutron powder diffraction data. The advantage of using a neutron radiation probe is that the scattering lengths of the atoms concerned are fairly similar, coherent scattering lengths being 6.715 fm for Mo,4.86 fm for W, 3.63 fm for Na and 5.803 fm for O (Sears, 2006). Secondly, with the time-of-flight method, particularly with a very long primary flight path and highangle backscattering detectors, one can acquire unparalleled resolution at very short flight times (i.e., small d-spacings), ensuring an order of magnitude improvement in parameter precision over the previous studies. In this work, usable data were obtained at a resolution of sin ()/ = 1.25 Å À1 , roughly tripling the number of measured reflections with respect to Okada et al. (1974) and Bramnik & Ehrenberg (2004). This work provides the most accurate and precise foundation on which to build future discussion of the hydrated forms of Na 2 MoO 4 and Na 2 WO 4 . Neutron powder diffraction data for Na 2 MoO 4 and Na 2 WO 4 are given in Figs. 1 and 2.

Structural commentary
The structure of both compounds is the normal spinel type with Na + ions on the 16c sites in octahedral coordination and Mo 6+ /W 6+ ions on 8b sites in tetrahedral coordination. The coordinating oxygen atoms occupy the 32e general positions, their location being defined by a single variable parameter u. For ideal cubic close packing, the u coordinate adopts a value of 0.25 although for various spinels is found in the range 0.24 to 0.275. In Na 2 MoO 4 the u parameter has a value of 0.262710 (15) and in Na 2 WO 4 it has a value of 0.262246 (15). The practical consequence of this compared with the 'ideal' value of u = 0.25 is that the shared edges of the NaO 6 octahedra are shorter than the unshared edges ( Neutron powder diffraction data for Na 2 MoO 4 ; red points are the observations, the green line is the calculated profile and the pink line beneath the diffraction pattern represents Obs À Calc. Vertical black tick marks report the expected positions of the Bragg peaks. The inset shows the data measured at very short flight times (i.e., small d-spacing).

Figure 2
Neutron powder diffraction data for Na 2 WO 4 ; red points are the observations, the green line is the calculated profile and the pink line beneath the diffraction pattern represents Obs À Calc. Vertical black tick marks report the expected positions of the Bragg peaks. The inset shows the data measured at very short flight times (i.e., small d-spacing).

Figure 3
(a) Arrangement of molybdate ions in the unit cell of Na 2 MoO 4 ; anisotropic displacement ellipsoids are drawn at the 75% probability level. (b) Connectivity of the NaO 6 octahedra, with shorter shared edges and longer unshared edges, to the MoO 4 tetrahedra in Na 2 MoO 4 ; as in (a), the ellipsoids are drawn at the 75% probability level. molybdate, these lengths are 3.2288 (2) and 3.5479 (2) Å , the ratio being 1.0988 (1); in the tungstate, the lengths of the two inequivalent octahedral edges are 3.2356 (2) Å and 3.5441 (2) Å , their ratio being 1.0953 (1). The MoO 4 2À and WO 4 2À tetrahedra have perfect T d symmetry with Mo-O and W-O bond lengths of 1.7716 (3) and 1.7830 (2) Å , respectively. The unit-cell parameters for both compounds are in excellent agreement with those of Swanson et al. (1962) and the structural parameters for the molybdate agree very well with those of Bramnik & Ehrenberg (2004). However, the Na 2 WO 4 structure refinement of Okada et al. (1974) stands apart as being conspicuously inaccurate, giving significantly longer W-O distances, 1.819 (8) Å , and shorter Na-O distances, 2.378 (8) Å , than are reported here or in many other simple tungstates. Indeed the ionic radii of four-coordinated Mo 6+ and W 6+ obtained from analysis of a large range of crystal structures are nearly identical, being 0.41 and 0.42 Å , respectively (Shannon, 1976). The values reported here agree very well with the majority of Mo-O and W-O bond lengths in isolated MoO 4 2À and WO 4 2À tetrahedral oxyanions from a range of alkali metal and alkaline earth compounds tabulated in the literature (e.g., Zachariasen & Plettinger, 1961;Gatehouse & Leverett, 1969;Koster et al., 1969;Gü rmen et al., 1971;Wandahl & Christensen, 1987;Farrugia, 2007; van den Berg & Juffermans, 1982). As such, this work represents an improvement in accuracy for sodium molybdate and an improvement in both accuracy and precision for sodium tungstate.

Synthesis and crystallization
Na 2 MoO 4 Á2H 2 O (Sigma Aldrich M1003, > 99.5%) and Na 2 WO 4 Á2H 2 O (Sigma Aldrich 14304, > 99.0%) were heated to 673 K in ceramic crucibles for 24 hr. Loss of water was confirmed by Raman spectroscopy; X-ray powder diffraction confirmed the phase identity and purity of the two anhydrous products, Na 2 MoO 4 and Na 2 WO 4 .
Raman spectra were acquired using 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 À1 with a spectral resolution of 3 cm À1 . Data were collected in a series of 20 x 9 sec integrations for Na 2 MoO 4 and 20 x 7 sec integrations for Na 2 WO 4 ; after summation, the background was removed and peaks fitted using Pseudo-Voigt functions in OriginPro (OriginLab, Northampton, MA) (Fig. 4). These data are provided as an electronic supplement in the form of an ASCII file.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. For the neutron scattering experiments, each specimen was loaded into a vanadium tube of 11 mm internal diameter to a depth of approximately 25 mm. The exact sample volume and mass were measured in order to determine the number density for correction of the specimen self-shielding. The samples were mounted on the HRPD beamline (Ibberson, 2009) at the ISIS neutron spallation source and data were collected in the 10-110 ms timeof-flight window for 2.5 h (Na 2 MoO 4 ) and 3.5 h (Na 2 WO 4 ). Data were corrected for self-shielding, focussed to a common scattering angle and normalized to the incident spectrum by reference to a V:Nb null-scattering standard before being output in a format suitable for Rietveld refinement with GSAS/Expgui ( Raman spectra of Na 2 MoO 4 (left) and Na 2 WO 4 (right) in the range 0-1200 cm À1 (the full range of data to 4000 cm À1 is given in the electronic supplement). Band positions and vibrational assignments are indicated. For the tungstate these agree very well with literature values (e.g., Busey & Keller, 1964) whereas for the molybdate, these data show a systematic shift to lower frequencies by 3-4 wavenumbers with respect to published values (Luz Lima et al., 2010Lima et al., , 2011. 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 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: coordinates taken from a previous refinement. Program(s) used to refine structure: GSAS/Expgui (Larsen & Von Dreele, 2000, Toby, 2001 for Na2MoO4; GSAS/Expgui (Larsen & Von Dreele, 2000;Toby, 2001) for Na2WO4. For both compounds, molecular graphics: DIAMOND (Putz & Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Refinement
Least-squares matrix: full R p = 0.037 R wp = 0.043 R exp = 0.022 R(F 2 ) = 0.06364 χ 2 = 3.842 7716 data points Excluded region(s): Data at d-spacings smaller than 0.4 Å were excluded since the counting statistics became progressively poorer at very short flight times due to the lower neutron flux at the shortest wavelengths.

Refinement
Least-squares matrix: full R p = 0.037 R wp = 0.044 R exp = 0.024 R(F 2 ) = 0.06245 χ 2 = 3.423 7716 data points Excluded region(s): Data at d-spacings smaller than 0.4 Å were excluded since the counting statistics became progressively poorer at very short flight times due to the lower neutron flux at the shortest wavelengths.