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

Fortes, A.D.

doi:10.1107/S2056989015008774

research communications

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 71| Part 6| June 2015| Pages 592-596

doi:10.1107/S2056989015008774

Open

access

Crystal structures of spinel-type Na₂MoO₄ and Na₂WO₄ revisited using 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 24 April 2015; accepted 5 May 2015; online 9 May 2015)

Time-of-flight neutron powder diffraction data have been collected from Na₂MoO₄ and Na₂WO₄ to a resolution of sin (θ)/λ = 1.25 Å⁻¹, which is substantially better than the previous analyses using Mo Kα X-rays, providing roughly triple the number of measured reflections with respect to the previous studies [Okada et al. (1974 ). Acta Cryst. B30, 1872–1873; Bramnik & Ehrenberg (2004 ). Z. Anorg. Allg. Chem. 630, 1336–1341]. The unit-cell parameters are in excellent agreement with literature data [Swanson et al. (1962 ). NBS Monograph No. 25, sect. 1, pp. 46–47] and the structural parameters for the molybdate agree very well with those of Bramnik & Ehrenberg (2004). However, the tungstate 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 other simple tungstates. As such, this work represents an order-of-magnitude improvement in precision for sodium molybdate and an equally substantial improvement in both accuracy and precision for sodium tungstate. Both compounds adopt the spinel structure type. The Na⁺ ions have site symmetry .-3m and are in octahedral coordination while the transition metal atoms have site symmetry -43m and are in tetrahedral coordination.

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

CCDC references: 1063434; 1063433

1. Chemical context

Both Na₂MoO₄ and Na₂WO₄ have rich phase diagrams in pressure and temperature space (Pistorius, 1966 ). The stable form at room temperature is the β-Ag₂MoO₄ cubic spinel structure type, space group Fd $[\overline{3}]$ m, which has been known for almost a century (Wyckoff, 1922 ). Among the alkali metal sulfates, chromates, molybdates and tungstates, only Na₂MoO₄ and Na₂WO₄ adopt the normal spinel structure at ambient pressure. Li₂MoO₄ forms a cubic spinel structure at high pressure (Liebertz & Rooymans, 1967 ). Li₂WO₄ 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., 1957 , 1962; Singh 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 Na₂MoO₄ was only reported recently (Bramnik & Ehrenberg, 2004) from X-ray powder diffraction data measured to sin (θ)/λ = 0.71 Å⁻¹; the last structure refinement of Na₂WO₄ was reported by Okada et al. (1974) from X-ray single-crystal diffraction data to sin (θ)/λ = 0.81 Å⁻¹. 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 metal–organic 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 Na₂MoO₄ and Na₂WO₄ 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 high-angle 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 (θ)/λ = 1.25 Å⁻¹, 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 Na₂MoO₄ and Na₂WO₄. Neutron powder diffraction data for Na₂MoO₄ and Na₂WO₄ are given in Figs. 1 and 2.

Figure 1
Neutron powder diffraction data for Na₂MoO₄; 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 very short flight times (i.e., small d-spacing).

Figure 2
Neutron powder diffraction data for Na₂WO₄; 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 very short flight times (i.e., small d-spacing).

2. Structural commentary

The structure of both compounds is the normal spinel type with Na⁺ ions on the 16c sites in octahedral coordination and Mo⁶⁺/W⁶⁺ 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 Na₂MoO₄ the u parameter has a value of 0.262710 (15) and in Na₂WO₄ 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 NaO₆ octahedra are shorter than the unshared edges (Fig. 3b). In the 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 MoO₄²⁻ and WO₄²⁻ 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 Na₂WO₄ 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 Mo⁶⁺ and W⁶⁺ 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 MoO₄²⁻ and WO₄²⁻ 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 ; Gürmen 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.

Figure 3
(a) Arrangement of molybdate ions in the unit cell of Na₂MoO₄; anisotropic displacement ellipsoids are drawn at the 75% probability level. (b) Connectivity of the NaO₆ octahedra, with shorter shared edges and longer unshared edges, to the MoO₄ tetrahedra in Na₂MoO₄; as in (a), the ellipsoids are drawn at the 75% probability level.

3. Synthesis and crystallization

Na₂MoO₄·2H₂O (Sigma Aldrich M1003, > 99.5%) and Na₂WO₄·2H₂O (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, Na₂MoO₄ and Na₂WO₄.

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⁻¹ with a spectral resolution of 3 cm⁻¹. Data were collected in a series of 20 x 9 sec integrations for Na₂MoO₄ and 20 x 7 sec integrations for Na₂WO₄; 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.

Figure 4
Raman spectra of Na₂MoO₄ (left) and Na₂WO₄ (right) in the range 0–1200 cm⁻¹ (the full range of data to 4000 cm⁻¹ 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., 2010

, 2011

4. 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 time-of-flight window for 2.5 h (Na₂MoO₄) and 3.5 h (Na₂WO₄). 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 (Larsen & Von Dreele, 2000 : Toby, 2001 ).

Table 1
Experimental details

	Na₂MoO₄	Na₂WO₄
Crystal data
Chemical formula	Na₂MoO₄	Na₂WO₄
M_r	205.92	293.83
Crystal system, space group	Cubic, Fd $[\overline{3}]$ m	Cubic, Fd $[\overline{3}]$ m
Temperature (K)	298	298
a (Å)	9.10888 (3)	9.12974 (4)
V (Å³)	755.78 (1)	760.98 (1)
Z	8	8
Radiation type	Neutron	Neutron
μ (mm⁻¹)	0.014 + 0.0018 * λ	0.014 + 0.0097 * λ
Specimen shape, size (mm)	Cylinder, 25 × 11	Cylinder, 27 × 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	Analytical
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 = 0.037, R_wp = 0.043, R_exp = 0.022, R(F²) = 0.06364, χ² = 3.842	R_p = 0.037, R_wp = 0.044, R_exp = 0.024, R(F²) = 0.06245, χ² = 3.423
No. of data points	7716	7716
No. of parameters	24	24
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: 1063434; 1063433

Crystal structure: contains datablocks Na~2~MoO~4~, Na~2~WO~4~, New_Global_Publ_Block. DOI: 10.1107/S2056989015008774/wm5152sup1.cif

Rietveld powder data: contains datablock Na2MoO4. DOI: 10.1107/S2056989015008774/wm5152Na2MoO4sup2.rtv

Rietveld powder data: contains datablock Na2WO4. DOI: 10.1107/S2056989015008774/wm5152Na2WO4sup3.rtv

Supporting information file. DOI: 10.1107/S2056989015008774/wm5152sup4.txt

checkCIF report

Chemical context top

Both Na₂MoO₄ and Na₂WO₄ have rich phase diagrams in pressure and temperature space (Pistorius, 1966). The stable form at room temperature is the β-Ag₂MoO₄ 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 Na₂MoO₄ and Na₂WO₄ adopt the normal spinel structure at ambient pressure. Li₂MoO₄ forms a cubic spinel structure at high pressure (Liebertz & Rooymans, 1967). Li₂WO₄ 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., 1957, 1962; Singh 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 Na₂MoO₄ was only reported recently (Bramnik & Ehrenberg, 2004) from X-ray powder diffraction data measured to sin(θ)/λ = 0.71 Å^-1; the last structure refinement of Na₂WO₄ was reported by Okada et al. (1974) from X-ray single-crystal diffraction data to sin(θ)/λ = 0.81 Å^-1. 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 metal–organic 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 Na₂MoO₄ and Na₂WO₄ 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, 206). Secondly, with the time-of-flight method, particularly with a very long primary flight path and high-angle 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(θ)/λ = 1.25 Å^-1, 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 Na₂MoO₄ and Na₂WO₄. Neutron powder diffraction data for Na₂MoO₄ and Na₂WO₄ are given in Figs. 1 and 2.

Structural commentary top

The structure of both compounds is the normal spinel type with Na⁺ ions on the 16c sites in octahedral coordination and Mo⁶⁺/W⁶⁺ 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 Na₂MoO₄ the u parameter has a value of 0.262710 (15) and in Na₂WO₄ 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 NaO₆ octahedra are shorter than the unshared edges (Fig. 3). In the 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 MoO₄^2- and WO₄^2- 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 Na₂WO₄ 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 Mo⁶⁺ and W⁶⁺ 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 MoO₄^2- and WO₄^2- 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; Gürmen 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 top

Na₂MoO₄·2H₂O (Sigma Aldrich M1003, > 99.5 %) and Na₂WO₄·2H₂O (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, Na₂MoO₄ and Na₂WO₄.

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^-1 with a spectral resolution of 3 cm^-1. Data were collected in a series of 20 x 9 sec integrations for Na₂MoO₄ and 20 x 7 sec integrations for Na₂WO₄; after summation, the background was removed and peaks fitted using Pseudo-Voigt functions in OriginPro (Fig. 4). These data are provided as an electronic supplement in the form of an ASCII file.

Refinement top

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 diameter to a depth of approximately 25 mm. The exact sample height 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 time-of-flight window for 2.5 h (Na₂MoO₄) and 3.5 h (Na₂WO₄). 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 (Larsen & Von Dreele, 2000: Toby, 2001).

Related literature top

For related literature, see: Atovmyan & D'yachenko (1969); Becka & Poljak (1958); Bramnik & Ehrenberg (2004); Breitinger et al. (1981); Busey & Keller (1964); Cadbury (1955); Farrugia (2007); Fleck et al. (2006); Funk (1900); Gürmen et al. (1971); Gatehouse & Leverett (1969); Horiuchi et al. (1979); Ibberson (2009); Koster et al. (1969); Larsen & Von Dreele (2000); Liebertz & Rooymans (1967); Lindqvist (1950); Luz Lima, Saraiva, Freire, Maczka, Paraguassu, de Sousa & Mendes Filho (2011); Luz Lima, Saraiva, Souza Filho, Paraguassu, Freire & Mendes Filho (2010); Okada et al. (1974); Pistorius (1966, 1975); Preudhomme & Tarte (1972); Sears (2006); Shannon (1976); Singh Mudher, Keskar, Krishnan & Venugopal (2005); Swanson et al. (1957, 1962); Toby (2001); van den Berg & Juffermans (1982); Wandahl & Christensen (1987); Wyckoff (1922); Zachariasen & Plettinger (1961); Zhilova et al. (2008).

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: 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).

Figures top

	Fig. 1. Neutron powder diffraction data for Na₂MoO₄; 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 very short flight times (i.e., small d-spacing).
	Fig. 2. Neutron powder diffraction data for Na₂WO₄; 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 very short flight times (i.e., small d-spacing).
	Fig. 3. (a) Arrangement of molybdate ions in the unit cell of Na₂MoO₄; anisotropic displacement ellipsoids are drawn at the 75% probability level. (b) Connectivity of the NaO₆ octahedra, with shorter shared edges and longer unshared edges, to the MoO₄ tetrahedra in Na₂MoO₄; as in (a), the ellipsoids are drawn at the 75% probability level.
	Fig. 4. Raman spectra of Na₂MoO₄ (left) and Na₂WO₄ (right) in the range 0–1200 cm^-1 (the full range of data to 4000 cm^-1 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., 2010, 2011).

(Na2MoO4) Disodium molybdenum(VI) oxide top

Crystal data top

Na₂MoO₄	Melting point: 961 K
M_r = 205.92	Neutron radiation
Cubic, Fd3m	µ = 0.01+ 0.0018 * λ mm⁻¹
Hall symbol: -F 4vw 2vw 3	T = 298 K
a = 9.10888 (3) Å	white
V = 755.78 (1) Å³	cylinder, 25 × 11 mm
Z = 8	Specimen preparation: Prepared at 673 K and 100 kPa
D_x = 3.619 Mg m⁻³

Data collection top

HRPD, High resolution neutron powder diffractometer	Absorption correction: analytical Data were corrected for self shielding using σ_scatt = 29.198 barns and σ_ab(λ) = 3.541 barns at 1.798 Å during the normalization procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.014 + 0.0018 * λ (mm^-1).
Radiation source: ISIS Facility, Neutron spallation source	T_min = 1.000, T_max = 1.000
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): 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.
R_p = 0.037	Profile function: TOF profile function #3 (21 terms). Profile coefficients for exp pseudovoigt convolution [Von Dreele, 1990 (unpublished)] (α) = 0.1919, (β₀) = 0.025953, (β₁) = 0.005213, (σ₀) = 0, (σ₁) = 196.3, (σ₂) = 23.5, (γ₀) = 0, (γ₁) = 14.91, (γ₂) = 0, (γ_2s) = 0, (γ_1e) = 0, (γ_2e) = 0, (ε_i) = 0, (ε_a) = 0, (ε_A) = 0, (γ₁₁) = 0, (γ₂₂) = 0, (γ₃₃) = 0, (γ₁₂) = 0, (γ₁₃) = 0, (γ₂₃) = 0. Peak tails ignored where intensity <0.0005x peak. Aniso. broadening axis 0.0 0.0 1.0
R_wp = 0.043	24 parameters
R_exp = 0.022	0 restraints
R(F²) = 0.06364	0 constraints
χ² = 3.842	(Δ/σ)_max = 0.03
7716 data points	Background function: GSAS Background function #1 (10 terms). Shifted Chebyshev function of 1st kind 1: 1.18715, 2: -7.466630x10^-3, 3:8.117230x10^-2, 4: -5.411800x10^-2, 5: -1.714140x10^-2, 6: -1.882400x10^-2, 7: -1.930110x10^-2, 8: -6.255180x10^-3, 9: 6.598230x10^-3, 10: 8.478560x10^-3

Crystal data top

Na₂MoO₄	Z = 8
M_r = 205.92	Neutron radiation
Cubic, Fd3m	µ = 0.01+ 0.0018 * λ mm⁻¹
a = 9.10888 (3) Å	T = 298 K
V = 755.78 (1) Å³	cylinder, 25 × 11 mm

Data collection top

HRPD, High resolution neutron powder diffractometer	T_min = 1.000, T_max = 1.000
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 = 29.198 barns and σ_ab(λ) = 3.541 barns at 1.798 Å during the normalization procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.014 + 0.0018 * λ (mm^-1).

Refinement top

R_p = 0.037	χ² = 3.842
R_wp = 0.043	7716 data points
R_exp = 0.022	24 parameters
R(F²) = 0.06364	0 restraints

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

	x	y	z	U_iso*/U_eq
O	0.262710 (16)	0.262710 (16)	0.262710 (16)	0.01182
Mo	0.375	0.375	0.375	0.00740
Na	0.0	0.0	0.0	0.01381

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O	0.01182 (6)	0.01182 (6)	0.01182 (6)	−0.00156 (6)	−0.00156 (6)	−0.00156 (6)
Mo	0.00740 (8)	0.00740 (8)	0.00740 (8)	0.0	0.0	0.0
Na	0.01381 (11)	0.01381 (11)	0.01381 (11)	−0.00068 (12)	−0.00068 (12)	−0.00068 (12)

Geometric parameters (Å, º) top

Mo—O	1.7716 (3)	Na^iv—O^vii	2.3986 (2)
Mo—Oⁱ	1.7716 (3)	Na^iv—O^viii	2.3986 (2)
Mo—Oⁱⁱ	1.7716 (3)	Na^iv—O^ix	2.3986 (2)
Mo—Oⁱⁱⁱ	1.7716 (3)	Na—Na^iv	3.2205 (1)
Na^iv—O	2.3986 (2)	Mo—Na^iv	3.7763 (1)
Na^iv—O^v	2.3986 (2)	O—O^vi	3.2288 (2)
Na^iv—O^vi	2.3986 (2)	O—O^v	3.5479 (4)

O—Mo—Oⁱ	109.4712 (1)	O—Na^iv—O^v	95.393 (6)
O—Mo—Oⁱⁱ	109.4712 (1)	O—Na^iv—O^ix	180.000 (1)
O—Mo—Oⁱⁱⁱ	109.4712 (1)	O—Na^iv—O^viii	84.607 (6)
Oⁱ—Mo—Oⁱⁱ	109.4712 (1)	O—Na^iv—O^vii	95.393 (6)
Oⁱ—Mo—Oⁱⁱⁱ	109.4712 (1)	O^v—Na^iv—O^vi	180.000 (1)
Oⁱⁱ—Mo—Oⁱⁱⁱ	109.4712 (1)	Mo—O—Na^iv	129.178 (5)
O—Na^iv—O^vi	84.607 (6)

Symmetry codes: (i) −x+3/4, y, −z+3/4; (ii) −x+3/4, −y+3/4, z; (iii) x, −y+3/4, −z+3/4; (iv) −x+1/4, −y+1/4, z; (v) x, −y+1/4, −z+1/4; (vi) −y+1/2, x+1/4, z−1/4; (vii) −x+1/4, y, −z+1/4; (viii) y+1/4, −x+1/2, z−1/4; (ix) −y+1/2, −x+1/2, −z.

(Na2WO4) Disodium tungsten(VI) oxide top

Crystal data top

Na₂WO₄	Melting point: 969 K
M_r = 293.83	Neutron radiation
Cubic, Fd3m	µ = 0.01+ 0.0097 * λ mm⁻¹
Hall symbol: -F 4vw 2vw 3	T = 298 K
a = 9.12974 (4) Å	white
V = 760.98 (1) Å³	cylinder, 27 × 11 mm
Z = 8	Specimen preparation: Prepared at 673 K and 100 kPa
D_x = 5.129 Mg m⁻³

Data collection top

HRPD, High resolution neutron powder diffractometer	Absorption correction: analytical Data were corrected for self shielding using σ_scatt = 28.088 barns and σ_ab(λ) = 19.361 barns at 1.798 Å during the normalisation procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.014 + 0.0097 * λ [mm^-1]
Radiation source: ISIS Facility, Neutron spallation source	T_min = 1.000, T_max = 1.000
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): 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.
R_p = 0.037	Profile function: TOF profile function #3 (21 terms). Profile coefficients for exp pseudovoigt convolution [Von Dreele, 1990 (unpublished)] (α) = 0.1603, (β₀) = 0.026115, (β₁) = 0.004558, (σ₀) = 0, (σ₁) = 237.2, (σ₂) = 45.0, (γ₀) = 0, (γ₁) = 14.21, (γ₂) = 0, (γ_2s) = 0, (γ_1e) = 0, (γ_2e) = 0, (ε_i) = 0, (ε_a) = 0, (ε_A) = 0, (γ₁₁) = 0, (γ₂₂) = 0, (γ₃₃) = 0, (γ₁₂) = 0, (γ₁₃) = 0, (γ₂₃) = 0. Peak tails ignored where intensity <0.0005x peak. Aniso. broadening axis 0.0 0.0 1.0
R_wp = 0.044	24 parameters
R_exp = 0.024	0 restraints
R(F²) = 0.06245	0 constraints
χ² = 3.423	(Δ/σ)_max = 0.01
7716 data points	Background function: GSAS Background function # 1 (10 terms). Shifted Chebyshev function of 1st kind 1: 0.884779, 2: 4.212470x10^-2, 3: 4.210950x10^-2, 4: -4.489520x10^-2, 5: -2.683690x10^-2, 6: -1.892450x10^-2, 7: -2.248710x10^-2, 8: -2.821970x10^-3, 9: 6.467340x10^-3, 10: 6.167050x10^-3

Crystal data top

Na₂WO₄	Z = 8
M_r = 293.83	Neutron radiation
Cubic, Fd3m	µ = 0.01+ 0.0097 * λ mm⁻¹
a = 9.12974 (4) Å	T = 298 K
V = 760.98 (1) Å³	cylinder, 27 × 11 mm

Data collection top

HRPD, High resolution neutron powder diffractometer	T_min = 1.000, T_max = 1.000
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 = 28.088 barns and σ_ab(λ) = 19.361 barns at 1.798 Å during the normalisation procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.014 + 0.0097 * λ [mm^-1]

Refinement top

R_p = 0.037	χ² = 3.423
R_wp = 0.044	7716 data points
R_exp = 0.024	24 parameters
R(F²) = 0.06245	0 restraints

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

	x	y	z	U_iso*/U_eq
O	0.262246 (15)	0.262246 (15)	0.262246 (15)	0.01312
W	0.375	0.375	0.375	0.00903
Na	0.0	0.0	0.0	0.01538

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O	0.01312 (6)	0.01312 (6)	0.01312 (6)	−0.00161 (5)	−0.00161 (5)	−0.00161 (5)
W	0.00903 (11)	0.00903 (11)	0.00903 (11)	0.0	0.0	0.0
Na	0.01538 (11)	0.01538 (11)	0.01538 (11)	−0.00045 (12)	−0.00045 (12)	−0.00045 (12)

Geometric parameters (Å, º) top

W—O	1.7830 (2)	Na^iv—O^vii	2.3995 (1)
W—Oⁱ	1.7830 (2)	Na^iv—O^viii	2.3995 (1)
W—Oⁱⁱ	1.7830 (2)	Na^iv—O^ix	2.3995 (1)
W—Oⁱⁱⁱ	1.7830 (2)	Na—Na^iv	3.2279 (1)
Na^iv—O	2.3995 (1)	W—Na^iv	3.7850 (1)
Na^iv—O^v	2.3995 (1)	O—O^vi	3.2356 (2)
Na^iv—O^vi	2.3995 (1)	O—O^v	3.5441 (4)

O—W—Oⁱ	109.4712 (3)	O—Na^iv—O^v	95.211 (6)
O—W—Oⁱⁱ	109.4712 (3)	O—Na^iv—O^ix	180.000 (1)
O—W—Oⁱⁱⁱ	109.4712 (3)	O—Na^iv—O^viii	84.789 (6)
Oⁱ—W—Oⁱⁱ	109.4712 (3)	O—Na^iv—O^vii	95.211 (6)
Oⁱ—W—Oⁱⁱⁱ	109.4712 (3)	O^v—Na^iv—O^vi	180.000 (1)
Oⁱⁱ—W—Oⁱⁱⁱ	109.4712 (3)	W—O—Na^iv	129.043 (4)
O—Na^iv—O^vi	84.789 (6)

Experimental details

	(Na2MoO4)	(Na2WO4)
Crystal data
Chemical formula	Na₂MoO₄	Na₂WO₄
M_r	205.92	293.83
Crystal system, space group	Cubic, Fd3m	Cubic, Fd3m
Temperature (K)	298	298
a (Å)	9.10888 (3)	9.12974 (4)
V (Å³)	755.78 (1)	760.98 (1)
Z	8	8
Radiation type	Neutron	Neutron
µ (mm⁻¹)	0.01+ 0.0018 * λ	0.01+ 0.0097 * λ
Specimen shape, size (mm)	Cylinder, 25 × 11	Cylinder, 27 × 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 = 29.198 barns and σ_ab(λ) = 3.541 barns at 1.798 Å during the normalization procedure. The linear absorption coefficient is wavelength dependent and is calculated as: µ = 0.014 + 0.0018 * λ (mm^-1).	–
T_min, T_max	1.000, 1.000	–
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 = 0.037, R_wp = 0.043, R_exp = 0.022, R(F²) = 0.06364, χ² = 3.842	R_p = 0.037, R_wp = 0.044, R_exp = 0.024, R(F²) = 0.06245, χ² = 3.423
No. of data points	7716	7716
No. of parameters	24	24

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

Acknowledgements

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

References

Arnold, O., & 27 co-authors (2014). Nucl. Instrum. Methods Phys. Res. A, 764, 156–166. Google Scholar
Atovmyan, L. O. & D'yachenko, O. A. (1969). J. Struct. Chem. 10, 416–418. CrossRef Google Scholar
Becka, L. N. & Poljak, R. J. (1958). Anales Asoc. Quim. Arg. 46, 204–209. CAS Google Scholar
Berg, A. J. van den & Juffermans, C. A. H. (1982). J. Appl. Cryst. 15, 114–116. CrossRef Web of Science IUCr Journals Google Scholar
Bramnik, K. G. & Ehrenberg, H. (2004). Z. Anorg. Allg. Chem. 630, 1336–1341. Web of Science CrossRef CAS Google Scholar
Breitinger, D. K., Emmert, L. & Kress, W. (1981). Ber. Bunsenges. Phys. Chem. 85, 504–505. CrossRef CAS Google Scholar
Busey, R. H. & Keller, O. L. Jr (1964). J. Chem. Phys. 41, 215–225. CrossRef CAS Web of Science Google Scholar
Cadbury, W. E. Jr (1955). J. Phys. Chem. 59, 257–260. CrossRef CAS Web of Science Google Scholar
Farrugia, L. J. (2007). Acta Cryst. E63, i142. Web of Science CrossRef IUCr Journals Google Scholar
Fleck, M., Schwendtner, K. & Hensler, A. (2006). Acta Cryst. C62, m122–m125. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Funk, R. (1900). Ber. Dtsch. Chem. Ges. 33, 3696–3703. CrossRef CAS Google Scholar
Gatehouse, B. M. & Leverett, P. (1969). J. Chem. Soc. A, pp. 849. Google Scholar
Gürmen, E. (1971). J. Chem. Phys. 55, 1093–1097. Google Scholar
Horiuchi, H., Morimoto, N. & Yamaoka, S. (1979). J. Solid State Chem. 30, 129–135. CrossRef CAS Web of Science Google Scholar
Ibberson, R. M. (2009). Nucl. Instrum. Methods Phys. Res. A, 600, 47–49. Web of Science CrossRef CAS Google Scholar
Koster, A. S., Kools, F. X. N. M. & Rieck, G. D. (1969). Acta Cryst. B25, 1704–1708. CrossRef CAS IUCr Journals Web of Science Google Scholar
Larsen, A. C. & Von Dreele, R. B. (2000). General Structure Analysis System (GSAS). Los Alamos National Laboratory Report LAUR 86-748, Los Alamos, New Mexico, USA. http://www.ncnr.NIST.gov/Xtal/software/GSAS.html . Google Scholar
Liebertz, J. & Rooymans, C. J. M. (1967). Solid State Commun. 5, 405–409. CrossRef CAS Web of Science Google Scholar
Lindqvist, I. (1950). Acta Chem. Scand. 4, 1066–1074. CrossRef CAS Web of Science Google Scholar
Luz Lima, C., Saraiva, G. D., Freire, P. T. C., Maczka, M., Paraguassu, W., de Sousa, F. F. & Mendes Filho, J. (2011). J. Raman Spectrosc. 42, 799–802. Google Scholar
Luz Lima, C., Saraiva, G. D., Souza Filho, A. G., Paraguassu, W., Freire, P. T. C. & Mendes Filho, J. (2010). J. Raman Spectrosc. 41, 576–581. CAS Google Scholar
Lynch, G. F. & Segel, S. L. (1972). Can. J. Phys. 50, 567–572. CrossRef CAS Google Scholar
Mantid (2013). Manipulation and Analysis Toolkit for Instrument Data; Mantid Project. http://dx.doi.org/10.5286/SOFTWARE/MANTID . Google Scholar
Okada, K., Morikawa, H., Marumo, F. & Iwai, S. (1974). Acta Cryst. B30, 1872–1873. CrossRef IUCr Journals Web of Science Google Scholar
Pistorius, C. W. F. T. (1966). J. Chem. Phys. 44, 4532–4537. CrossRef CAS Web of Science Google Scholar
Pistorius, C. W. F. T. (1975). J. Solid State Chem. 13, 325–329. CrossRef CAS Web of Science Google Scholar
Preudhomme, J. & Tarte, P. (1972). Spectrochim. Acta Part A, 28, 69–79. CrossRef CAS Web of Science Google Scholar
Putz, H. & Brandenburg, K. (2006). DIAMOND. Crystal Impact GbR, Bonn, Germany. http://www.crystalimpact.com/diamond . Google Scholar
Sears, V. F. (2006). Neutron News, 3, 26–37. CrossRef Google Scholar
Shannon, R. D. (1976). Acta Cryst. A32, 751–767. CrossRef CAS IUCr Journals Web of Science Google Scholar
Singh Mudher, K. D., Keskar, M., Krishnan, K. & Venugopal, V. (2005). J. Alloys Compd. 396, 275–279. Web of Science CrossRef Google Scholar
Swanson, H. E., Gilfrich, N. T. & Cook, M. I. (1957). Natl. Bur. Stand. (US) Circ. 539, Vol. 7, p. 45. Google Scholar
Swanson, H. E., Morris, M. C., Stinchfield, R. P. & Evans, E. H. (1962). NBS Monograph No. 25, sect. 1, pp. 46–47. Google Scholar
Toby, B. H. (2001). J. Appl. Cryst. 34, 210–213. Web of Science CrossRef CAS IUCr Journals Google Scholar
Wandahl, G. & Christensen, A. N. (1987). Acta Chem. Scand. Ser. A, 41, 358–360. CrossRef Web of Science Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
Wyckoff, R. W. G. (1922). J. Am. Chem. Soc. 44, 1994–1998. CrossRef CAS Google Scholar
Zachariasen, W. H. & Plettinger, H. A. (1961). Acta Cryst. 14, 229–230. CrossRef IUCr Journals Web of Science Google Scholar
Zhilova, S. B., Karov, Z. G. & El'mesova, R. M. (2008). Russ. J. Inorg. Chem. 53, 628–635. Web of Science CrossRef Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 71| Part 6| June 2015| Pages 592-596

doi:10.1107/S2056989015008774

Open

access

Format		BIBTeX
		EndNote
		RefMan
		Refer
		Medline
		CIF
		SGML
		Plain Text

Format		BIBTeX
		EndNote
		RefMan
		Refer
		Medline
		CIF
		SGML
		Plain Text

Search IUCr Journals		doi		Advanced search
Author		volume	page

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