Energy-efficient mechanochemical synthesis of MgWO₄ phase materials: (micro)-structural, thermal, and optical properties

Abstract

We prepared MgWO₄ phase materials using a single-step energy-efficient mechanochemical approach, which differs from existing studies on the phase formation, thermal, optical, and morphological features. The as-milled powder reveals a monoclinic structure with the lattice parameters, a = 0.4683(5) nm, b = 0.5686(6), c = 0.4956(6) nm, and β = 90.73°. The TGA curve shows a weight loss of 4.7% for the as-milled powder. The IR bands at 513 and 570 cm⁻¹ indicate the Mg–O vibration mode, whereas the IR bands seen at 794 and 825 cm⁻¹ are assigned to the W–O vibration mode for the MgWO₄ phase materials. The NIR reflectance of the MgWO₄ phase analysis achieved in the present work is novel in comparison to the existing studies, which reveal a maximum reflectance of ∼35% in the solar NIR reflectance region. The as-milled powder MgWO₄ has an E^direct_g of 3.97 eV. Agglomerated particles of MgWO₄ phases are observed in the SEM images.

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1. Introduction

The metal tungstates of the AWO₄ series with A²⁺ ions of IIA group elements (Mg, Ca, Sr, and Ba) and d-block elements (Mn, Fe, Co, Ni, Zn, and Cd) attract researchers owing to their wide range of applications in various sectors/areas.^1–10 The AWO₄ series has the tendency to form two crystal structures, such as wolframite and scheelite, that are decided by the ionic radii of the A²⁺ ions. The A²⁺ ions with smaller ionic radii, which are less than 0.077 nm, form the monoclinic wolframite structure, whereas the A²⁺ ions with larger ionic radii (greater than 0.099 nm) form the tetragonal scheelite structure.^1,2 For example, A²⁺ ions such as Mg, Zn, Co, Cd, Fe, Mn, and Ni stabilize the monoclinic wolframite structure. Conversely, A²⁺ ions such as Ca, Sr, Ba, Pb, and Eu form the tetragonal scheelite structure. Though the most common AWO₄ type phases of CaWO₄, SrWO₄, and BaWO₄ are well known for their suitability in numerous fields, the MgWO₄ phase materials have not been investigated to the extent of the former phases. Thus, our attention is focused on the latter one (MgWO₄) to explore new findings. In general, MgWO₄ has two phases: α-MgWO₄ and β-MgWO₄. The α-MgWO₄ phase stabilizes in a triclinic structure at high temperature, while the β-MgWO₄ phase crystallizes in a monoclinic structure below 1165 °C.³ Furthermore, Gancheva et al.¹¹ reported that MgWO₄ has the tendency to form a tetragonal structure in addition to a triclinic and monoclinic structure when the sample is prepared at lower temperatures which range from 400 to 850 °C. Several methods have been reported for the synthesis of MgWO₄ materials and some of them are solid state reaction methods, mechanochemical synthesis, single crystal growth, flux method, co-precipitation, hydrothermal synthesis, complex polymerization, chemical vapour deposition, electrospinning, and gamma-ray irradiation.^2,11–23 Some of the notable characteristics of MgWO₄ phases are listed in Fig. 1. The wide range of applications of MgWO₄ are catalysts, scintillators, photoluminescent materials, laser host materials, microwave applications, pigments, and solid-state lasers.^2,12,19

Fig. 1 Characteristics of monoclinic MgWO₄ phase materials screened from the cited studies.

Some of the research findings on MgWO₄ materials reported by different researchers are compared here. Sota et al.²⁴ prepared MgWO₄ materials after heat treatment of the reagents, MgO and WO₃ at 1000 °C for 12 h via the solid state route. By using a hydrothermal method, the MgWO₄ materials were prepared by Meng et al.¹⁵ and Hurley et al.²⁵ to study their photocatalytic behavior and photoluminescence and radioluminescence properties, respectively. Furthermore, the existing reports relevant to the synthesis of MgWO₄ phase materials based on a ball-milling process are briefed here.^1,11,20,22 Kim et al.²⁰ prepared MgWO₄ materials by a solid state reaction using the homogenized mixture (MgO and WO₃) by ball milling for 24 hours in ethanol solvent followed by heating at 900 °C/2 h. Gancheva et al.¹¹ aimed to synthesize the MgWO₄ materials through a mechanically activated (500 rpm/5–10 h) mixture of MgCO₃·3H₂O and WO₃ and the mixture of MgO and WO₃ in a planetary ball milling machine with the model of Fritsch-Premium line/Pulversette No. 7. The ball milling conditions employed for their work were as follows: a ball to powder mass ratio of 10 : 1. A ball milling duration of 15 minutes was succeeded by a 5 minute pause to prevent an excessive increase in temperature within the grinding chamber. Types of precursors and annealing temperature (600 and 850 °C) have influenced the phase formation and purity of the final products.¹¹ For a microwave dielectric ceramic study, Pullar et al.¹ obtained sintered MgWO₄ samples at 900–1200 °C/2 h using a ball milled powder made from MgO and WO₃ as the reaction mixture. According to them, the reaction mixture was initially ball milled for 1 day in zirconia milling media in the presence of water and then dried. The dried powder was then calcined at 900 °C/12 h and then milled at 300 rpm/3 h using water on a Fritsch Pulverisette ball milling machine. The sintered monoclinic MgWO₄ sample was nearly single-phase with a second phase at 2θ = 37.5°. In accordance with the report by Guo et al., the ball-milled powder (made from MgO and WO₃ as the reaction mixture) was obtained using ethanol as a solvent in a polyethylene jar using zirconia balls for 4 h and subsequent heat treatment at 1000 °C/3 h yielded the monoclinic MgWO₄ phase.²²

In this paper, we have synthesized monoclinic MgWO₄ phase materials by the ball-milling method (VBCC, Chennai), despite a few of the reports^1,11,22 dealing with ball milling followed by high-temperature treatment. The ball milling conditions employed by us in the present work are quite different as compared to the existing reports. The phase formation, thermal features, optical properties, and morphology were systematically studied, which distinguishes this work from previous mechanochemical studies. To our knowledge, there is hardly any report on the synthesis of MgWO₄ phase materials in a single step without the requirement of further heat treatment under the experimental conditions described below (see Section 2). Furthermore, the NIR reflectance study of the MgWO₄ phase materials conducted in the present work is novel as compared to the existing studies.

2. Experimental procedure

The starting materials used for the ball-milling experiment were MgO and WO₃. For this experiment, 0.45 (g) of MgO and 2.5885 (g) of WO₃ were ground for 20 min using an agate mortar and pestle. Then the reaction mixture was transferred into the tungsten carbide (WC) jar. In this, 30 WC balls (10 mm in diameter), 15 WC balls (8 mm in diameter), and 3 WC balls (20 mm in diameter) were used for ball milling the reaction mixture at 300 rpm for 10 h. After that, the as-milled powder was collected, and its phase purity was confirmed by X-ray diffraction (XRD) using a Bruker XRD diffractometer with Cu Kα radiation (λ = 0.154046 nm), scanned over a 2θ angle of 10°–80° with a step size of 0.02° and a counting duration of 0.5 s per step. After confirming the phase purity, different characterization techniques such as thermal gravimetric (TG) and differential thermal analyses (DTA), fourier-transform infrared (FTIR) spectroscopy, ultraviolet visible (UV-vis) spectroscopy, near-infrared (NIR) analysis, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis were performed in the present work. Thermal behavior was studied using TGA/DTA (TG/DTA6300, EXSTAR series, Seiko Instruments Inc., Japan) under a continuous air flow at a heating rate of 20 °C min⁻¹. Diffuse reflectance spectra were recorded between 200 and 2500 nm using a JASCO spectrophotometer with a D2/WI light source. FTIR spectra were obtained in KBr transmittance mode using a Thermo Fisher Scientific Nicolet iS50 spectrometer. The micro-structural images of MgWO₄ materials were collected using an FESEM with a resolution of 1.0 nm at 15 kV and 1.6 nm at 1 kV acceleration and a magnification level of up to 10 000 00× with a Smart EDX EDS analysis system (Carl Zeiss-Sigma 300, German).

3. Results and discussion

3.1. XRD

Fig. 2 represents the XRD patterns of the reaction mixture (left) and the as-milled product (right) of MgWO₄ nanomaterials. Interestingly, the as-milled product reveals monoclinic symmetry (P2/c) with the lattice parameters a = 0.4683(5) nm, b = 0.5686(6), c = 0.4956(6) nm, and β = 90.73°. These values are comparable with other existing reports.^12,22,24 According to the report by Guo et al.,²² the monoclinic MgWO₄ phase with lattice parameters of a = 0.46864 nm, b = 0.56755 nm, and c = 0.49284 nm has been noted for the ball-milled powder subjected to subsequent heat treatment at 1000 °C/3 h. The MgWO₄ powder obtained by a solid-state reaction carried out at 900 °C/4 h yielded the lattice parameters of a = 0.4642 nm, b = 0.5594 nm, c = 0.488 nm, and β = 90.56°.¹² Sota et al.²⁴ reported the significance of heating temperature between 850 and 1000 °C for the solid-state reaction between MgO and WO₃. At a higher temperature (1000 °C), they could obtain the phase pure MgWO₄ materials, while at a low temperature (850 °C), a couple of weak impurities of WO₃ were observed along with the MgWO₄ phase.²⁴ When compared with the cited ref. 12 by Bhuyan et al., an increase in lattice parameters was noted as a = 0.46906 nm, b = 0.56782 nm, and c = 0.49315 nm.²⁴ This may be due to the difference in synthesis methods and conditions. MgWO₄ single crystals grown using K₂W₂O₇ as a flux by the effective TSSG method¹⁸ revealed the lattice parameters of a = 0.4686(7) nm, b = 0.5674(1) nm, c = 0.4927(8) nm, and β = 90.710(7)°.

Fig. 2 XRD patterns: reaction mixture of MgWO₄ powder (left) and the as-milled monoclinic MgWO₄ phase powder (right). The diffraction peaks observed in the ball-milled sample are matched with the standard data (PDF card no. 01-080-0129).

The average crystallite size, D, was calculated to be 12.6 nm and 17.07 nm from Debye–Scherrer and Williamson–Hall (W–H) methods^26–28 for the present as-milled powder of MgWO₄ phase materials (see Fig. 3), whereas the electrospinning of (CH₃COO)₂Mg·4H₂O, (NH₄)₆W₇O₂₄·4H₂O, and PVA as precursors revealed a D of 69.8 nm upon calcining the as-obtained fibers at 700 °C/3 h.² This monoclinic phase had the structural values of a = 0.465 nm, b = 0.564 nm, and c = 0.49 nm.² However, D = 24.63 nm was obtained for the tetragonal MgWO₄ phase, which was obtained by a modified-combustion method using Mg(NO₃)₂·6H₂O, (NH₄)₆W₁₂O₃₉·2H₂O, and citric acid upon calcining the as-prepared product at 600 °C/1 h.²⁹ The formation of the tetragonal MgWO₄ phase is mainly based on the difference in the synthesis methods/conditions, precursors, and heat/radiation treatments. This tetragonal phase forms at relatively lower synthesis temperatures.^11,13,15,29

Fig. 3 W–H plot for the as-milled monoclinic MgWO₄ phase powder.

The sample having the oxides of MgO and WO₃ after ball milling for 10 h and subsequent heat treatment at 600 °C revealed the mixture of tetragonal and monoclinic MgWO₄ phases, MgO, and WO₃ as evidenced by the XRD patterns.¹¹ At 850 °C, the monoclinic MgWO₄ phase along with an impurity of the WO₃ phase has been formed.¹¹ Furthermore, the XRD patterns of the sample containing MgCO₃·3H₂O and WO₃ after 10 h milling time and thermal treatment at 600 °C yielded a pure tetragonal MgWO₄ phase, and at 850 °C a pure monoclinic MgWO₄ phase has been formed.¹¹ The radiation flux synthesis of tetragonal MgWO₄ materials¹³ involved the direct application of a high energy electron flux of 1.4 MeV with a flux power density of 15 and 18 kW cm⁻² directed at the powder mixture made from MgO and WO₃. Template-free hydrothermal synthesis of MgWO₄ nanoplates revealed the tetragonal phase of MgWO₄ with a = 0.563 nm and c = 1.081 nm.¹⁵

3.2. TG/DTA

Fig. 4 shows the TGA and DTA plots of the reaction mixture (left) and as-milled powder (right) of MgWO₄ materials, which resulted in weight losses of 2.6% and 4.7%, respectively. The slightly increased weight loss seen in the as-milled powder may be due to the formation of the desired MgWO₄ phase, as the planetary ball milling machine works on the combination of friction, shear, and impacts that are subsequently produced. The slight increase in weight loss seen in the as-milled MgWO₄ powder may be due to the surface-related effects induced by mechanical milling. Furthermore, this may be due to the presence of surface adsorption related to physisorbed water and surface OH⁻ species in the as-milled powder.³⁰ The review article by Amrute et al.³⁰ highlighted that low-temperature (<200 °C) weight loss is a signature of physisorbed water and surface OH⁻ species in the as-milled inorganic powder.³⁰ The saturation in weight loss of the as-milled powder started at 660 °C. The DTA curves of the reaction mixture and the as-milled MgWO₄ materials witnessed no endo- and exothermic events. The absence of endo/exothermic peaks in the DTA curve of the as-milled powder implies that the mechanochemical treatment of the reaction mixture at 300 rpm for 10 h had completed the formation of the MgWO₄ phase during milling. Subsequently, no additional thermal events were detected during successive heating in the thermal analysis. Mechanochemical milling develops interfacial contact between the precursors of MgO and WO₃, thus considerably enhancing the reactivity of the reaction mixture. Such mechanochemical activation can promote direct formation of the MgWO₄ phase. This finding supports the efficiency of mechanochemical milling in achieving phase formation without the need for additional thermal activation. The absence of endo/exothermic peaks in the DTA curve may also be ascribed to the nanocrystalline nature of the as-milled powder. Among the cited references, there are hardly any reports found on the TGA analysis of MgWO₄ phase materials. However, in order to find out the phase formation temperature, Gancheva et al.¹¹ conducted DTA analysis between room temperature and 1000 °C for the mixture of MgO and WO₃ following a milling duration of 10 hours. The DTA curve showed multiple exothermic peaks at temperatures of 530, 630, 920, and 960 °C. Among these, the peaks observed at 530 and 630 °C are associated with the formation of the MgWO₄ phase.

Fig. 4 TGA and DTA plots: reaction mixture of MgWO₄ powder (left) and the as-milled monoclinic MgWO₄ phase powder (right).

3.3. UV-vis-NIR spectral features

The UV-vis absorbance spectrum in Fig. 5 (left) exhibits a well-defined absorption maximum (λ^max) peak at 257 nm, and the estimation of the optical energy gap (E_g) of the MgWO₄ phase materials from the Kubelka–Munk (K–M) model is presented in Fig. 5 (right). The MgWO₄ phase materials obtained by Wang et al.¹⁹ and Meng et al.¹⁵ revealed a λ^max peak at ∼250 nm¹⁹ and at ∼260 nm,¹⁵ respectively. This main absorption edge/peak (λ^max) is related to the electronic transitions from the valence band (O 2p states) to the conduction band (W 5d orbitals), which are mainly related to the intra-group transitions within the [WO₄]²⁻ complex in the MgWO₄ structure.^12,16 The E_g of the MgWO₄ phase powder is estimated from the Tauc equation and the K–M model, which is derived from the absorbance and/or reflectance data obtained from DRS measurements for the direct transition in the band gap.^{12,15,16,19,31} The E^direct_g of 3.97 eV determined using the KM model for the present MgWO₄ phase materials is compared with the existing reports. The MgWO₄ phase has been found to show E_g values ranging from ∼3.8 to 4.33 eV with the direct assignments varying among different reports, synthesis methods and conditions, phase formation, and analytical methods.^{12,15,16,19,31} The absorption spectrum obtained from UV-visible analysis exhibited a E^direct_g of 4.17 eV.¹² The UV-vis DRS spectra of MgWO₄ samples synthesized through a single-step hydrothermal method, exhibiting various morphologies such as nanoplate shapes and nanoparticle structures, exhibited comparable optical absorption ranges or edges.¹⁵ The estimated E_g of the tetragonal MgWO₄ nanoparticles and nanoplates, as determined by the Tauc method, was found to be 4.15 and 4.10 eV, respectively.¹⁵ The UV-vis spectra and the E_g values of the MgWO₄ phases, which were obtained by a γ-ray irradiation method, showed variations in absorbance features and E_g values upon calcination of the as-prepared powder at various temperatures (400–900 °C).¹⁹ The reported E_g values were determined to be 4.07, 3.65, 2.82, 3.73, 4.03, and 3.54 eV for the calcination of the as-prepared powder at 400, 500, 600, 700, 800, and 900 °C, respectively.¹⁹ Dey et al.³¹ analyzed the optical properties and electronic structures of MgWO₄ materials obtained via a solid-state reaction method. In this compound, the E_g value is 4.06 eV.³¹ Gouveia et al.¹⁶ synthesized MgWO₄ powder by the complex polymerization method upon heat-treating the intermediate product at 900 °C/2 h. For this synthesized MgWO₄ powder, they obtained an E^direct_g of 4.33 eV using the KM model, while the band structure calculation showed an E^direct_g value of 4.49 eV. This difference is ascribed to the presence of localized electronic levels within the forbidden E_g.¹⁶ The intense absorption in the UV region and wide band gap indicate that the material could be a promising candidate for applications in photoluminescence, laser host materials, pigments, and scintillators.^31–33

Fig. 5 UV-vis absorbance spectrum (left) and E^direct_g estimation from the KM function (right) of the as-milled monoclinic MgWO₄ powder.

The scheelites and their derivative materials^34–43 have been found to exhibit significant reflectance in the near infrared (NIR) range between 750 and 2500 nm, which varies depending upon the phase compositions and the methods and conditions of synthesis (see Table 2). These NIR spectral features make them suitable for applications in solar reflective pigments and the NIR color pigmentation industry. However, there is a lack of reports regarding the NIR spectrum of MgWO₄ phase materials within these NIR regions. In the present work, the as-milled powder of monoclinic MgWO₄ phase materials exhibits a maximum NIR reflectance of ∼35% in the region between 750 and 1350 nm (Fig. 6). The observed relatively low reflectance of the as-milled powder could be attributed to the stress generated during the ball milling process of the reaction mixture. A decrease in NIR reflectance has been reported for various oxide-based materials produced through the ball-milling process.^44–46 It is quite interesting to compare this result with our previous work on the CaMoO₄ phase,⁴² which showed a relatively lower NIR reflectance of 37% (similar to the current MgWO₄ phase), whereas the co-precipitated (CaMoO₄) product showed a higher NIR reflectance of ∼85%. We believe that the reduced NIR reflectance of the ball-milled product is due to the stress induced by ball-milling of the reaction mixture.⁴² Furthermore, the reduced particle size due to ball-milling would increase the surface area, which enhances the scattering mechanism, thereby reducing the reflectance of the materials.

Fig. 6 NIR (reflectance) spectrum of the as-milled monoclinic MgWO₄ phase powder in the solar NIR region.

3.4. FTIR spectrum

The FTIR spectrum of MgWO₄ phase materials (Fig. 7) reveals IR bands at 3263, 1631, 825, 794, 570, and 513 cm⁻¹ in the range between 4000 and 400 cm⁻¹. The IR bands seen at 513 and 570 cm⁻¹ are associated with the Mg–O vibration mode. The IR bands of 794 and 825 cm⁻¹ are related to the W–O vibration mode. In addition to these main IR bands, the IR bands attributable to adsorbed water were also seen at 1631 cm⁻¹ (bending mode) and 3263 cm⁻¹ (stretching mode), validating the presence of adsorbed moisture and OH⁻ species, which is reflected in the TGA curve (Fig. 4(right)) of the as-milled MgWO₄ powder, showing weight loss in the lower temperature region (below 350 °C). These observed IR band assignments are corroborated by the reports on the MgWO₄ phase materials.^11,12,19,29 The transmittance curves of MgWO₄ materials shown in the report by Bhuyan et al.¹² illustrate intense IR bands in the wavenumber range between 400 and 900 cm⁻¹ and a weak band at 1022 cm⁻¹, which is due to the vibrational peak of CO₃²⁻ species. The intense IR peaks located at 435, 501, and 549 cm⁻¹ correspond to the Mg–O vibration mode, and the bands at 783 and 879 cm⁻¹ are attributed to the O–W–O vibration mode and W–O stretching mode, respectively. In another report,²⁹ the tetragonal MgWO₄ phase materials obtained via a modified combustion method show IR bands at 856, 794, and 743 cm⁻¹, which are attributed to the W–O asymmetric stretching mode, whereas IR bands located at 474, 398, and 384 cm⁻¹ correspond to the O–W–O asymmetric bending mode. The observed IR bands at 647 and 413 cm⁻¹ are due to Mg–O stretching vibrations.²⁹

Fig. 7 FTIR spectrum of the as-milled monoclinic MgWO₄ phase powder.

3.5. Morphology

The morphological features related to MgWO₄ phase materials are confirmed from Fig. 8. It reveals that the nanostructures of particles are agglomerates of smaller crystallites, and the EDX pattern confirms the presence of Mg, W, and O in the studied materials. Generally, the ball-milled powder has the tendency of showing agglomerated particles in a variety of oxide-based nanomaterials that exist in many reports in the literature. Monoclinic MgWO₄ particles with different sizes and shapes have been observed in the TEM images for the MgWO₄ materials obtained through a mechanically activated (500 rpm/5–10 h) mixture of MgCO₃·3H₂O and WO₃.¹¹ The obtained size of the MgWO₄ particles (∼120 nm) by TEM is in accordance with the XRD-derived average crystallite size of 100 nm.¹¹ According to the report by Pullar et al.,¹ the MgWO₄ samples obtained at 900–1200 °C/2 h through ball milling followed by sintering revealed well sintered grains with low density at 950 °C, while very large grains were grown at high temperature (1200 °C).¹ In another study, ball milling followed by heat treatment at 1000 °C/3 h showed dense microstructures of rod-shaped grains and blocky ones with the grain sizes of 3–10 µm.²² Furthermore, various microstructural features have been reported for the MgWO₄ samples obtained through different synthesis methods (Table 1). Densely arranged microcrystals revealing polyhedral shapes with typical dimensions of 2–5 µm are reported for the MgWO₄ phase materials crystallized in a tetragonal structure via the radiation flux method.¹³ Plate-like nanostructures that possess an irregular morphology with a thickness of ∼45 nm have been reported for the tetragonal MgWO₄ phase materials obtained by the hydrothermal method.¹⁵

Fig. 8 FESEM micro-images of the as-milled monoclinic MgWO₄ powder: images were taken at magnifications of 20 000× (a), 40 000× (b), and 65 000× (c), and the EDX spectrum (d).

Table 1 Micro-structural features of different MgWO₄ phase materials obtained through various methods from the cited studies

Synthesis method	Morphology	Particle size/shape	Ref.
Electrospinning	Fibers	—	2
Co-precipitation	Rod-like morphology	—	3
Solid-state reaction	Uneven grains (agglomerated)		12
Polymerization	Non-uniform agglomerated particles	—	16
Gamma-ray irradiation assisted polyacrylamide gel method	Irregular shape	150 and 220 nm	19
Hydrothermal	Flake shaped morphology	—	21
Hydrothermal	Spherical nanoparticles (small/large), wool balls, and stars	—	25

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Table 2 NIR spectral features of different scheelites and their derived compositions obtained through various methods from the cited studies

Scheelites and their derived compositions	Experimental method	Conditions employed	NIR spectral features	Ref.
a Different amounts (1.5, 3, 6, and 9.0 g) of reaction mixtures were ball-milled (300 rpm/10 h) using 3 and 30 balls of WC with 20 and 10 mm diameters, respectively.
BiVO4	Conventional ceramic	Calcined at 800 °C/9 h	46%	34
BiVO4	Conventional solid-state reaction route	Dried at 100 °C/1 h; calcined at 800 °C/6 h	∼50%	35
(BiV)0.2(CaMo)0.8O4	Conventional solid-state route	Calcined at 800 °C/6 h	91%	36
BiVO4	Ethylene glycol-assisted	Dried in an oven at 110 °C for 24 h	≥80%	37
BiVO4	Citrate-gel	Heated at 150 °C for 1 h; calcined at 500 °C for 3 h	75.94%	40
SrMoO4	Low temperature hydrothermal	150 °C for 16 h	70%	41
CaMoO4	Ball-milling	300 rpm/10 h	37%	42
	Co-precipitation	Dried at 100 °C/5 h	∼85%
CaWO4	Solid-state reaction	850 °C/4 h	90%	43
	Ball-milling	300 rpm/10 h under different conditions^a	40–50%

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4. Conclusion

We presented a simple, solvent-free, and energy-efficient mechanochemical approach for synthesizing phase-pure monoclinic MgWO₄ materials using a mixture of MgO and WO₃, which has a D value as 12.6 nm without post-annealing treatment. This approach is easy, cost-effective, and potentially scalable, producing nanoscale crystallites with structural stability and optical properties comparable to those of previously reported methods. The as-milled powder revealed a weight loss of 4.7% as evidenced by TGA. The absence of endo/exothermic peaks in the DTA curve indicates that the MgWO₄ phase is formed during mechanochemical milling, which is reminiscent of the findings reported for the ball-milled CaWO₄ phase materials.⁴³ The FTIR spectrum of MgWO₄ phase materials revealed IR bands at 513 and 570 cm⁻¹ related to the Mg–O vibration mode, and the IR bands at 794 and 825 cm⁻¹ were related to the W–O vibration mode. The UV-vis absorbance spectrum revealed a distinct λ^max peak at 257 nm. The KM model which is used in conjunction with the Tauc relation yielded an E^direct_g of 3.97 eV. FESEM analysis showed the nanostructure of agglomerated particles. The reported NIR reflectance (around 35%) is moderate, which can be significantly improved by tuning various synthesis methods and processing conditions. For cool roof or solar-reflective pigment applications, commercial cool pigments typically exhibit significantly higher NIR reflectance. Thus, extensive work on the MgWO₄ materials could provide insight into the NIR reflectance.

Author contributions

S.A. Ashika: data curation; formal analysis; investigation; methodology; resources; validation; writing – original draft preparation; writing – review and editing. S. Balamurugan: supervision; conceptualization; data curation; formal analysis; methodology; resources; project administration; validation; visualization; writing – original draft preparation; writing – review and editing.

Conflicts of interest

The authors state that there is no conflict of interest.

Data availability

All relevant data generated and analyzed during this study are included in this article.

Acknowledgements

The author, S. A. Ashika is indebted to the award of DST-INSPIRE Fellowship (Offer letter No. DST/INSPIRE Fellowship/[IF230137], GOVERNMENT OF INDIA, MINISTRY OF SCIENCE and TECHNOLOGY, Department of Science and Technology, Technology Bhawan, New Mehrauli Road, New Delhi – 110016) to conduct the research work under the guidance of Prof. Dr. S. Balamurugan, Department of Nanotechnology, Noorul Islam Centre for Higher Education. The authors thank the reviewers for providing their valuable comments/suggestions for significant improvements to the manuscript.

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