Crystal structures of spinel-type Na2MoO4and Na2WO4revisited using neutron powder diffraction

Fortes, A. Dominic

doi:10.1107/s2056989015008774

Cited by 29 publications

(31 citation statements)

References 32 publications

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“…The unit-cell parameters for this compound are in excellent agreement with the literature [12]. However, the Na 2 WO 4 structure 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 the ones reported here or in many other simple tungstates [12]. Indeed, the ionic radii of four-coordinated W +6 obtained from our analysis of a large range of crystal structures are nearly identical, being 0.42 Å.…”

Section: Resultssupporting

confidence: 84%

“…The

W {O_{4}}^{- 2}

tetrahedra has perfect T d symmetry with W–O bond lengths of 1.7830 (2) Å. The unit-cell parameters for this compound are in excellent agreement with the literature [12]. However, the Na 2 WO 4 structure 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 the ones reported here or in many other simple tungstates [12].…”

Section: Resultssupporting

confidence: 83%

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Characterizations and morphology of sodium tungstate particles

et al. 2018

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A solid-state reaction technique was used to synthesize polycrystalline Na2WO4. Preliminary X-ray studies revealed that the compound has a cubic structure at room temperature. The formation of the compound has been confirmed by X-ray powder diffraction studies and Raman spectroscopy. Electrical and dielectric properties of the compound have been studied using complex impedance spectroscopy in the frequency range 209 Hz–1 MHz and temperature range 586–679 K. The impedance data were modellized by an equivalent circuit consisting of series of a combination of grains and grains boundary. We use complex electrical modulus M* at various temperatures to analyse dielectric data. The modulus plots are characterized by the presence of two relaxation peaks thermally activated. The morphologies and the average particle size of the resultant sodium tungstate sample were demonstrated by atomic force microscopy, scanning electron microscopy and transmission electron microscopy. The thicknesses and optical constants of the sample have been calculated using ellipsometric measurements in the range of 200–22 000 nm by means of new amorphous dispersion formula which is the objective of the present work. The results were obtained for Na2WO4 particles from experimental (EXP) and measured (FIT) data showed an excellent agreement. In addition, the energy gap of the Na2WO4 sample has been determined using ellipsometry and confirmed by spectrophotometry measurements.

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Section: Resultssupporting

confidence: 84%

“…The

W {O_{4}}^{- 2}

Section: Resultssupporting

confidence: 83%

Characterizations and morphology of sodium tungstate particles

et al. 2018

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“…Now, let us discuss the incorporation of Ni ions in the Na 2 WO 4 lattice: The most reliable Shannon radius [15] of the dopant Ni 2+ ion (0.069 nm) is close to that of hexa-valence tungsten ion (W 6+ ) (0.060 nm in sixfold coordination) but much smaller than that of Na 1+ (1.02 nm). Therefore, Ni 2+ ions prefer to locate in W 6+ sites without (2), c = 4.415 (8) causing high distortion in the molecular inversion centre. This conclusion could also be supported by observing that the electronegativity (en) of Ni (1.8) is close to that of W (1.7) but much higher than that of Na ion (0.9).…”

Section: Structural Characterisationmentioning

confidence: 98%

“…However, the stable form at room temperature (RT) is the β-Ag 2 MoO 4 cubic spinel structure type, space group Fd m, of lattice parameter 0.9133 nm [7]. The RT lattice constant of the cubic phase was also measured by neutron diffraction to be 0.910888 nm [8]. However, Na 2 WO 4 can also have at RT under atmospheric pressure an orthorhombic structure (Pmmm-47) of parameters a = 1.41420 nm, b = 1.58568 nm, c = 0.44117 nm, and V cell = 0.989310 nm 3 [9].…”

Section: Introductionmentioning

confidence: 99%

Creation of Ferromagnetic Properties in Ni-Doped Na2WO4—Effect of Hydrogenation Post-treatment

Dakhel

2016

J Supercond Nov Magn

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Pure and Ni-doped sodium tungstate (Na 2 WO 4 ) powders were synthesised by simultaneous crystallisation method. The effects of Ni doping on the structural, optical, and magnetic properties of the host Na 2 WO 4 powder were studied. The study of X-ray diffraction shows that the incorporated Ni ions occupy locations in interstitial positions and substitution for W ion in the Na 2 WO 3 lattice. A monophase cubic structure was obtained when the ascrystallised Na 2 WO 4 powder was doped with Ni ions or annealed in hydrogen gas atmosphere (hydrogenation). The optical properties were studied by diffuse reflectance spectroscopy (DRS) technique. It was established that the direct bandgap of Na 2 WO 4 exhibits red shift from 4.6 to 3.50 eV with Ni doping and blue shift to 5.13 eV with hydrogenation. The purpose of the present study is to study conditions necessary to prepare powders having room-temperature ferromagnetic (RT-FM) properties. Therefore, the Na 2 WO 4 nano-powder was doped with Ni ions. RT-FM properties were obtained with Ni-doped Na 2 WO 4 that was strongly enhanced by hydrogenation so that the energy product (EP) was increased by 213 %. This enhancement was attributed to the enhancement of the magnetic medium for the spin-spin (S-S) interaction inside the crystalline medium. In general, an experimental relationship was established between O vacancies, optical absorption, and magnetic properties of the studied crystal. Thus, it was proved, for the first A. A. Dakhel time, the possibility of producing Na 2 WO 4 having RT-FM, where magnetic characteristics can be tailored by doping and post-treatment under H 2 atmosphere, thus a new potential candidate to be used in magnetic applications of ferroelectric crystals.

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“…O 4 , A 6+ B 2 + O 4 and A 4+ B 2 2+ O 4 corresponding to compositions such as those of Mn 2 O 4 , MgCr 2 O 4 , MoNa 2 O 4 and SiMg 2 O 4 , respectively (Greedan et al, 1998;Lenaz et al, 2004;Kudoh et al, 2007;Fortes, 2015). The spinel structure is a cubic close-packed (c.c.p.)…”

Section: +mentioning

confidence: 99%

Chemical and structural variability in cubic spinel oxides

Bosi

2019

Acta Crystallogr Sect B

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The empirical relations between cubic spinel oxides of different compositions were investigated using data from 349 refined crystal structures. The results show that the spinel structure is able to tolerate many constituents (at least 36) by enlarging and decreasing the tetrahedra and octahedra. This is reflected in a large variation in tetrahedral and octahedral bond distances. The oxygen positional parameter (u) may be regarded as a measure of the distortion of the spinel structure from cubic close packing or of the angular distortion of the octahedron. The distortion can best be explained in terms of ionic potential (IP), which merges the size and charge properties of an ion. Sterically induced distortion depends on ion size, whereas electrostatically induced distortion is caused by cation-cation repulsion across faces of tetrahedra and shared edges of octahedra. The strong correlations between the u parameter and the IP at the T and M sites are consistent with the main role played by the both charge and size. Large distortions (u ) 0.27) result in oxygen-oxygen distances of the octahedron shorter than 2.50 Å , which would lead to structural instability because of increased non-bonded repulsion forces between the oxygen atoms.

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Crystal structures of spinel-type Na₂MoO₄and Na₂WO₄revisited using neutron powder diffraction

Abstract: 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.

Cited by 29 publications

References 32 publications

Characterizations and morphology of sodium tungstate particles

Characterizations and morphology of sodium tungstate particles

Creation of Ferromagnetic Properties in Ni-Doped Na2WO4—Effect of Hydrogenation Post-treatment

Chemical and structural variability in cubic spinel oxides

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