The Influence of High-Energy Milling on the Phase Formation, Structural, and Photoluminescent Properties of CaWO4 Nanoparticles

CaWO4 nanoparticles were obtained by facile mechanochemical synthesis at room temperature, applying two different milling speeds. Additionally, a solid-state reaction was employed to assess the phase composition, structural, and optical characteristics of CaWO4. The samples were analyzed by X-ray diffraction (XRD), transition electron microscopy (TEM), and Raman, infrared (IR), ultraviolet–visible (UV–Vis) reflectance, and photoluminescence (PL) spectroscopies. The phase formation of CaWO4 was achieved after 1 and 5 h of applied milling speeds of 850 and 500 rpm, respectively. CaWO4 was also obtained after heat treatment at 900 °C for 12 h. TEM and X-ray analyses were used to calculate the average crystallite and grain size. The Raman and infrared spectroscopies revealed the main vibrations of the WO4 groups and indicated that more distorted structural units were formed when the compound was synthesized by the solid-state method. The calculated value of the optical band gap of CaWO4 significantly increased from 2.67 eV to 4.53 eV at lower and higher milling speeds, respectively. The determined optical band gap of CaWO4, prepared by a solid-state reaction, was 5.36 eV. Blue emission at 425 (422) nm was observed for all samples under an excitation wavelength of 230 nm. CaWO4 synthesized by the solid-state method had the highest emission intensity. It was established that the intensity of the PL peak depended on two factors: the morphology of the particles and the crystallite sizes. The calculated color coordinates of the CaWO4 samples were located in the blue region of the CIE diagram. This work demonstrates that materials with optical properties can be obtained simply and affordably using the mechanochemical method.


Direct Mechanochemical Synthesis
The reagents used in the mechanochemical treatment were CaCO 3 (Merck KGaA, Amsterdam, The Netherlands, 99.9% purity) and WO 3 (Merck KGaA, Amsterdam, The Netherlands, 99.9% purity).The stoichiometric ratio of the initial materials was 1:1, which corresponds to the crystal CaWO 4 phase.The high-energy ball milling of the initial mixture was carried out in a planetary ball mill (Fritsch premium line, Pulverisette No 7, FRITSCH GmbH, Idar-Oberstein, Germany), where two different milling speeds were applied: 500 and 850 rpm.The activation was performed in air atmosphere, and the ball-to-powder weight ratio was 10:1.To minimize the temperature during milling, the process was carried out in periods of 15 min, with rest periods of 5 min according to our previous studies [15,[39][40][41].The labels of the milled samples were as follows: CaWO 4 -I for the milling speed of 500 rpm; CaWO 4 -II for the milling speed of 850 rpm.

Solid-State Reaction
The initial reagents for the solid-state reaction were identical to those used in the mechanochemical activation i.e., CaCO 3 (Merck KGaA, Amsterdam, The Netherlands, 99.99%) and WO 3 (Merck KGaA, Amsterdam, The Netherlands, 99.99%).The stoichiometric ratio was the same (1:1).The initial mixture was homogenized in an agate mortar.Subsequently, the mixture was transferred to an aluminum crucible and heated at 900 • C for 12 h in an electrical furnace.The heat treatment was carried out according to the data in reference [13].The prepared sample was labeled CaWO 4 -III.

Characterization
The XRD powder patterns were collected using the Bruker D8 Advance X-ray powder diffractometer, Karlsruhe, Germany, equipped with a CuKa radiation source (1.542 A) and the LynxEye PSD detector.The measurement range was from 5.5 • to 120.0 • 2θ, with a step of 0.02 • 2θ and a counting time of 1.0 s/strip (for a total of 175.0 s/step, according to the PSD detector).The qualitative phase analysis was performed using the DIFFRAC.EVA v.4 software program [45] and the ICDD PDF-2 (2021) reference database.The unit cell parameters and crystallites size were calculated using the whole profile of the powder pattern and Topas v.4.2 software [46].TEM observation was performed by a JEOL JEM-2100 microscope (Akishima, Japan) at an accelerating voltage of 200 kV.The preparation procedure of the specimens consisted of dispersing them in ethanol by ultrasonic treatment and then dripping them onto standard Cu grids.The Raman spectra were recorded using the Via Qontor Raman Confocal Microscope (Renishaw plc, Wotton-under-Edge, UK) with a laser wavelength of 532 nm (Nd:YAG-Laser).The laser power on the sample was maintained at 1% of the nominal power, so no heating effects on the powder sample could be observed.The excitation light was focused and collected using a ×50 LWD objective lens.Infrared spectra were registered in the range of 1200-400 cm −1 on a Nicolet-320 FTIR spectrometer (Madison, WI, USA) using the KBr pellet technique with a spectral resolution of 2 nm.The diffuse-reflectance spectra were recorded with a Thermo Evolution 300 UV-Vis Spec-trophotometer equipped with a Praying Mantis device (Madison, WI, USA) with Spectralon reflectance standard.For recording the background, Spectralon was used.The PL emission spectra were measured on a Horiba Fluorolog 3-22 TCS spectrophotometer (Longjumeau, France) equipped with a 450 W Xenon Lamp as the excitation source.The automated modular system, with the highest sensitivity among those available on the market, was used, allowing for the measurement of light emission of practically any type of sample.Double-grating monochromators were used for emissions in the range of 200-950 nm.All spectra were measured at room temperature.

X-ray Powder Diffraction Analysis
A comparative analysis of the phase formation, morphology, and symmetry of the structural units and optical properties of the mechanochemically and solid-state-obtained CaWO 4 powders was carried out.The influence of the milling speed on the reaction time, crystallite size, and defects in the CaWO 4 was studied by X-ray diffraction analysis (Figure 1A,B).The XRD pattern of the initial mixture shows the principal peaks of the monoclinic WO 3 (PDF# 01-072-0677) and the orthorhombic CaCO 3 (PDF# 00-041-1475).The milling speed of 500 rpm led to a decrease in the intensity and the broadening of all diffraction lines after 1 h of milling time (Figure 1A).This was a result of a decrease in the particle size, destruction of the long-range order, and the partial amorphization of both reagents.Increasing the milling time to 3 h caused the appearance of reflections characteristic for tetragonal CaWO 4 (PDF#00-041-1431).This is an indication of the beginning of the chemical reaction between the activated reagents.The reaction finished after 5 h of milling time (Figure 1A).The possibility of phase formation after a short time was checked using a higher milling speed of 850 rpm.In this case, the principal peaks of CaWO 4 were observed after 15 min of milling time.But the diffraction lines typical of the initial WO 3 at 2θ = 23.20 and 23.95 • were detected (Figure 1B).The pure CaWO 4 was synthesized after 30 min of milling time, which is shorter compared to the synthesis at the 500 rpm milling speed.The increase in the milling time to 60 min did not affect the phase composition.In both cases, the diffraction lines were broad, which was attributed to the smaller size of the mechanochemically synthesized CaWO 4 particles.The TEM images of these samples also show the formation of particles with lower dimensions (Figure 2A,B).Compared to the research reported in Ref. [20], we found that the amount of 2.5 g of the reaction mixture, higher milling speeds of 500 and 850 rpm, and dry media were more suitable conditions for the rapid preparation of CaWO 4 .Figure 1C exhibits the XRD pattern of CaWO 4 after heat treatment at 900 • C for 12 h.A remarkable narrowing of the diffraction lines was observed due to the higher crystallinity of the CaWO 4 in comparison to the mechanochemically obtained CaWO 4 powders.No additional diffraction lines were found, meaning that the obtained sample was pure single phase.The lattice parameters (a, c, unit cell), lattice strain, and crystallite size of the CaWO 4 obtained from the XRD refinement are shown in Table 1.

X-ray Powder Diffraction Analysis
A comparative analysis of the phase formation, morphology, and symmetry of the structural units and optical properties of the mechanochemically and solid-state-obtained CaWO4 powders was carried out.The influence of the milling speed on the reaction time, crystallite size, and defects in the CaWO4 was studied by X-ray diffraction analysis (Figure 1A,B).The XRD pattern of the initial mixture shows the principal peaks of the monoclinic WO3 (PDF# 01-072-0677) and the orthorhombic CaCO3 (PDF# 00-041-1475).The milling speed of 500 rpm led to a decrease in the intensity and the broadening of all diffraction lines after 1 h of milling time (Figure 1A).This was a result of a decrease in the particle size, destruction of the long-range order, and the partial amorphization of both reagents.Increasing the milling time to 3 h caused the appearance of reflections characteristic for tetragonal CaWO4 (PDF#00-041-1431).This is an indication of the beginning of the chemical reaction between the activated reagents.The reaction finished after 5 h of milling time (Figure 1A).The possibility of phase formation after a short time was checked using a higher milling speed of 850 rpm.In this case, the principal peaks of CaWO4 were observed after 15 min of milling time.But the diffraction lines typical of the initial WO3 at 2θ = 23.20 and 23.95° were detected (Figure 1B).The pure CaWO4 was synthesized after 30 min of milling time, which is shorter compared to the synthesis at the 500 rpm milling speed.The increase in the milling time to 60 min did not affect the phase composition.In both cases, the diffraction lines were broad, which was attributed to the smaller size of the mechanochemically synthesized CaWO4 particles.The TEM images of these samples also show the formation of particles with lower dimensions (Figure 2A,B).Compared to the research reported in Ref. [20], we found that the amount of 2.5 g of the reaction mixture, higher milling speeds of 500 and 850 rpm, and dry media were more suitable conditions for the rapid preparation of CaWO4. Figure 1C exhibits the XRD pattern of CaWO4 after heat treatment at 900 °C for 12 h.A remarkable narrowing of the diffraction lines was observed due to the higher crystallinity of the CaWO4 in comparison to the mechanochemically obtained CaWO4 powders.No additional diffraction lines were found, meaning that the obtained sample was pure single phase.The lattice parameters (a, c, unit cell), lattice strain, and crystallite size of the CaWO4 obtained from the XRD refinement are shown in Table 1.Table 1.Unit cell parameters, crystallite size, and lattice microstrain values of the obtained samples.
Table 1 presents the calculated unit cell parameters, crystallite sizes, and isotropic microstrains of the three synthesized samples.These were compared with data published in the literature [18].It is clear from the values that there was no significant change in the parameters or volumes of the unit cells of the CaWO4 regardless of the conditions and method of preparation.Quite expectedly, as the mechanochemical activation energy increased, the crystallite size of the treated sample decreased.This was particularly clearly observed in the obtained values for the crystal lattice microstrains, which almost doubled when the milling speeds increased from 500 rpm to 850 rpm.The lattice strain decreased with an increasing crystallite size due to the prolongation of the heat treatment at a high temperature (CaWO4-III).

TEM Analysis
The size and morphology of the particles are important factors for the optical application of CaWO4. Figure 2A-C presents the TEM images, particle size distribution, and EDS mapping of the investigated samples.The particles of CaWO4-I and II obtained by mechanochemical treatment (milling speeds of 500 and 850 rpm) are well separated from each other, with a quasihexagonal form (Figure 2A,B).The size histograms illustrate that both CaWO4 nanoparticles exhibited good dispersion and a narrower size distribution.The particle size distribution of the CaWO4-I obtained at the lower milling speed of 500 rpm was between 5 and 30 nm, with most particles in the range between 10 and 15 nm (Figure 2A).At the higher milling speed, the particles had an average size of 20 nm (Figure 2B).Agglomeration of the particles was not observed in either of the mechanochemically synthesized samples.As might be expected, the particles obtained by solid-state synthesis were much larger, with an oval shape and average particle size of 1200 nm (Figure 2C).Table 1 presents the calculated unit cell parameters, crystallite sizes, and isotropic microstrains of the three synthesized samples.These were compared with data published in the literature [18].It is clear from the values that there was no significant change in the parameters or volumes of the unit cells of the CaWO 4 regardless of the conditions and method of preparation.Quite expectedly, as the mechanochemical activation energy increased, the crystallite size of the treated sample decreased.This was particularly clearly observed in the obtained values for the crystal lattice microstrains, which almost doubled when the milling speeds increased from 500 rpm to 850 rpm.The lattice strain decreased with an increasing crystallite size due to the prolongation of the heat treatment at a high temperature (CaWO 4 -III).

TEM Analysis
The size and morphology of the particles are important factors for the optical application of CaWO 4 .Figure 2A-C presents the TEM images, particle size distribution, and EDS mapping of the investigated samples.The particles of CaWO 4 -I and II obtained by mechanochemical treatment (milling speeds of 500 and 850 rpm) are well separated from each other, with a quasihexagonal form (Figure 2A,B).The size histograms illustrate that both CaWO 4 nanoparticles exhibited good dispersion and a narrower size distribution.The particle size distribution of the CaWO 4 -I obtained at the lower milling speed of 500 rpm was between 5 and 30 nm, with most particles in the range between 10 and 15 nm (Figure 2A).At the higher milling speed, the particles had an average size of 20 nm (Figure 2B).Agglomeration of the particles was not observed in either of the mechanochemically synthesized samples.As might be expected, the particles obtained by solid-state synthesis were much larger, with an oval shape and average particle size of 1200 nm (Figure 2C).The particle distribution was between 500 and 2500 nm, with most particles tending to be between 1000 and 1500 nm in size.In this case, the agglomeration of the particles was visible.We can infer from the TEM results that significantly smaller particles were produced by high-energy ball milling.The EDS mapping demonstrates that the elements Ca, W, and O were disseminated on the entire surface of the samples prepared by mechanochemical activation and solid-state reaction.

Raman and Infrared Spectroscopy
Both types of spectroscopic spectra of the obtained products evidence the formation of CaWO 4 with a scheelite type structure with different degrees of WO 4 destruction.Figure 3A shows the Raman spectra of the CaWO 4 phase excited at 532 nm (Nd:YAG-Laser).The Raman bands can be divided into two groups-internal and external modes-due to the weak coupling between the WO 4 and CaO 8 groups and keeping in mind the literature data [5,18,23,47].The internal modes are attributed to the oscillations inside the [WO 4 ] 2− units with an immovable mass center.The external or lattice phonons correspond to the motion of the Ca 2+ cations and WO 4 units [47,48].The spectrum of CaWO 4 -III produced by the solid-state reaction was used to assign the Raman peaks: the strong band at 911 cm −1 was due to the symmetric stretching ν 1 of W-O bond in the WO 4 tetrahedra [5,18,23,47,48].The appearance of several absorption bands in the range of 838-718 cm −1 can be attributed to the elimination of the degeneracy of the ν 3 vibration of different crystallographically nonequivalent WO 4 tetrahedra with different local symmetry (T d , C 3 , C 2v ) [49].The observed band at 332 cm −1 and the weak band at 400 cm −1 are the result of the ν 2 vibrations of the W-O bond [47,48].The low frequency bands at 273 and 210 cm −1 could be assigned to the translational mode of the ν(Ca-O) and the bending vibration of the [WO 4 ] 2− group in the CaWO 4 , respectively [23,50].The absence of the peaks in the range from 840 to 400 cm −1 in the CaWO 4 -II sample after 1 h of milling time at 850 rpm indicates the generated WO 4 units were more symmetric compared to the other samples (CaWO 4 -I and CaWO 4 -III).These results were confirmed by the infrared (IR) spectroscopy (Figure 3B).The IR spectra of the CaWO 4 -I and CaWO 4 -II obtained by mechanochemical synthesis display one absorption band around 810 (820) cm −1 due to the ν 3 vibration of the WO 4 structural unit building the crystalline structure of the CaWO4 [15,17,25].The low-intensity band at 440 cm −1 is attributed to the ν 4 modes of the same structural groups [51].In contrast, the IR spectrum of the CaWO 4 -III prepared by solid-state reaction exhibited more absorption bands between 900 and 700 cm −1 .In this case, the appearance of the bands at 830, 800, and 780 cm −1 are assigned to the elimination of the ν 3 vibration degeneracy of the WO 4 tetrahedral with a lower local symmetry [52].Similar results were established for BaMoO 4 obtained by solid-state reaction [41].The results above show that more symmetrical WO4 structural units were formed due to the high-energy ball milling (Figure 3B).

UV-Vis Absorbance Spectroscopy
The optical behavior of the CaWO 4 powders obtained by both techniques was investigated using UV-Vis (Figure 4A,B) and photoluminescence spectroscopies (Figures 5 and 6).The UV-visible absorption spectra was transformed in the Kubelka-Minck function using the following formula [F(R)]: F(R∞) = (1 − R∞) 2 /(2R∞) = K/S.The R is the diffuse reflectance of the sample, K is the absorption coefficient, and S is the scattering coefficient.The reflectance of the sample depends on the ratio of K to S but not on the absolute values of K and S [53,54].A strong absorption in the range of 210-250 nm was observed for all obtained samples (Figure 4A).The position of the main peak depends on the processing conditions i.e., the milling speeds (CaWO 4 -I and CaWO 4 -II) and the solid-state method (CaWO 4 -III).In the UV-Vis spectrum, the CaWO 4 -I sample exhibited a wide range from 210 to 370 nm with a maximum peak at 250 nm.The absorption band shifted up to 220 nm when the milling speed was increased to 850 rpm.CaWO 4 -III, obtained by solid-state reaction, also showed a tendency toward a blue shift, with a notable absorption peak recorded at 210 nm (Figure 4A).The narrowest absorption line was detected for this sample.This fact can be attributed to the higher crystallinity nature of the solid-state reaction-obtained powder with the highest crystalline size (D = 370 nm).The bands between 210 and 250 nm were attributed to the charge-transfer transitions within the WO 4 2− complex in the scheelite type inorganic phases [5,7,17,24,25,30].In addition, a secondary absorption band between 250 and 320 nm was also visible.Keeping in mind the reported data, this band corresponds to the creation of the excitonic state in A 2+ ions (A 2+ = Ba, Sr, Ca) [33,55].A similar UV-Vis spectra was reported for CaWO 4 obtained by different methods, including the sonochemical method [5], solvothermal method [9,23], solid-state reaction [12,13], and hydrothermal route [16].
The calculated Eg values of the CaWO 4 powders obtained by mechanochemical activation at 500 and 850 rpm were 2.88 and 4.59 eV, respectively.The optical band gap value of CaWO 4 -I (5 h, 500 rpm) is similar to those of CaWO 4 obtained by 12 h of milling time and heat-treated at 1100 • C [19].The calculated optical band gap of CaWO 4 -III produced by the solid-state technique is 5.27 eV (Figure 4B).This increase in the band gap is a result of the structural improvements, according to the XRD analysis.The value is close to that of the CaWO 4 thin film obtained by the chemical solution method [21] and the CaWO 4 powders obtained by solid-state reaction [13], the polymeric precursor method, and the microwave-assisted hydrothermal method [56].The changes in the crystallite size and microstructure of the CaWO 4 obtained by both preparation methods influenced the electron structure, and this is reason for the difference in value of the optical band gaps and their optical features [57].
obtained powder with the highest crystalline size (D = 370 nm).The bands between 210 and 250 nm were attributed to the charge-transfer transitions within the WO4 2− complex in the scheelite type inorganic phases [5,7,17,24,25,30].In addition, a secondary absorption band between 250 and 320 nm was also visible.Keeping in mind the reported data, this band corresponds to the creation of the excitonic state in A 2+ ions (A 2+ = Ba, Sr, Ca) [33,55].A similar UV-Vis spectra was reported for CaWO4 obtained by different methods, including the sonochemical method [5], solvothermal method [9,23], solid-state reaction [12,13], and hydrothermal route [16].The calculated Eg values of the CaWO4 powders obtained by mechanochemical activation at 500 and 850 rpm were 2.88 and 4.59 eV, respectively.The optical band gap value of CaWO4-I (5 h, 500 rpm) is similar to those of CaWO4 obtained by 12 h of milling time and heat-treated at 1100 °C [19].The calculated optical band gap of CaWO4-III produced by the solid-state technique is 5.27 eV (Figure 4B).This increase in the band gap is a result of the structural improvements, according to the XRD analysis.The value is close to that of the CaWO4 thin film obtained by the chemical solution method [21] and the CaWO4 powders obtained by solid-state reaction [13], the polymeric precursor method, and the microwave-assisted hydrothermal method [56].The changes in the crystallite size and microstructure of the CaWO4 obtained by both preparation methods influenced the electron structure, and this is reason for the difference in value of the optical band gaps and their optical features [57].Figure 5 compares the emission spectra of the CaWO4 synthesized by both procedures: mechanochemical activation (500 and 850 rpm) and solid-state reaction (900 °C for 12 h).All samples show a broad blue band with a maximum at around 422-425 nm upon excitation at 230 nm (absorption of the WO4 groups).The emission lines show no variation in shape, but there is a noticeable difference in the emission intensity.The emission peaks of all prepared CaWO4 are in good agreement with the literature data for blue emission [58,59].In our case, although the obtained crystalline phases possessed different particle size and morphologies, the emission spectrum profiles were the same.The blue emission at 425 nm, with a full width at the half maximum of 98 nm, was registered for CaWO4-I obtained using the lower milling speed (500 rpm), while the blue emission was changed at 422 nm, and a lower full width at the half maximum for CaWO4-II was obtained after the short milling time (1 h) at 850 rpm.The same blue maximum emission (422 nm) was observed with a full width at the half maximum of 100 nm for the solid-state-synthesized sample (CaWO4-III).This demonstrates that the as-prepared broader blue CaWO4 materials are promising candidates for WLEDs.
The PL intensities at 425-422 nm gradually increased with an increasing crystallite size.The value of the emission intensity was: ~3.02 × 10 8 for the CaWO4-I prepared after 5 h of milling time and a milling speed of 500 rpm; ~8.5 × 10 8 was recorded for the CaWO4-II obtained after a 1 h milling time and a milling speed of 850 rpm; ~9.3 × 10 9 was recorded for the CaWO4-III obtained by the solid-state technique.The highest blue emission of CaWO4-III was probably due to its good crystallinity, oval-shaped particles, and highly asymmetric WO4 structural units.Similar results regarding the emission intensity were discussed by F. Lei et al. [60].According to the literature data, the PL intensity and emission behavior of MWO4 with the scheelite and wolframite type structure depends on the surface chemistry, morphology, and particle size [61].The lower crystallite size that results from mechanochemical treatment may be the cause of the lowest blue emissions for CaWO4-I and CaWO4-II [62].The Commission International de I Eclairage (CIE)ʹs chromaticity diagram of the CaWO4 samples excited at 230 nm is shown in Figure 6.As the main peak in the luminescence spectra is in the range between 400 and 600 nm and peaks at 425 (422) nm, it is expected that the emitted light will be blue.The calculated chromaticity coordinates are very close values and fall into the blue range on the CIE diagram (Figure 6 and Table 2).Blue light is in high demand because it can be mixed with yellow or participate in the tricolor (red, green, blue) to produce white light.

Conclusions
The variation in the milling speed promoted the rapid synthesis of the CaWO4 from 5 h at 500 rpm to 1 h at 850 rpm without additional heat treatment.We demonstrated that the mechanochemical treatment led to the formation of particles with a quasihexagonal form and a narrower size distribution.CaWO4 was also obtained by solid-state reaction at 900 °C for 12 h.Agglomeration of the oval particles was obtained using the solid-state technique.It was established that the method of preparation influences the crystallinity nature, macrostrains, and the symmetry of the WO4 entity and its optical properties.It was established that the optical band gap of CaWO4 strongly depends on the milling conditions, i.e., an Eg increase from 2.85 eV to 4.59 eV with an increase in the milling speed.The CaWO4-III obtained by the solid-state reaction possessed higher crystallinity, asymmetric WO4 units, and oval particles, which resulted in a higher blue emission intensity compared to the mechanochemically synthesized samples.

Luminescent Properties
Figure 5 compares the emission spectra of the CaWO 4 synthesized by both procedures: mechanochemical activation (500 and 850 rpm) and solid-state reaction (900 • C for 12 h).All samples show a broad blue band with a maximum at around 422-425 nm upon excitation at 230 nm (absorption of the WO 4 groups).The emission lines show no variation in shape, but there is a noticeable difference in the emission intensity.The emission peaks of all prepared CaWO 4 are in good agreement with the literature data for blue emission [58,59].In our case, although the obtained crystalline phases possessed different particle size and morphologies, the emission spectrum profiles were the same.The blue emission at 425 nm, with a full width at the half maximum of 98 nm, was registered for CaWO 4 -I obtained using the lower milling speed (500 rpm), while the blue emission was changed at 422 nm, and a lower full width at the half maximum for CaWO 4 -II was obtained after the short milling time (1 h) at 850 rpm.The same blue maximum emission (422 nm) was observed with a full width at the half maximum of 100 nm for the solid-state-synthesized sample (CaWO 4 -III).This demonstrates that the as-prepared broader blue CaWO 4 materials are promising candidates for WLEDs.
The PL intensities at 425-422 nm gradually increased with an increasing crystallite size.The value of the emission intensity was: ~3.02 × 10 8 for the CaWO 4 -I prepared after 5 h of milling time and a milling speed of 500 rpm; ~8.5 × 10 8 was recorded for the CaWO 4 -II obtained after a 1 h milling time and a milling speed of 850 rpm; ~9.3 × 10 9 was recorded for the CaWO 4 -III obtained by the solid-state technique.The highest blue emission of CaWO 4 -III was probably due to its good crystallinity, oval-shaped particles, and highly asymmetric WO 4 structural units.Similar results regarding the emission intensity were discussed by F. Lei et al. [60].According to the literature data, the PL intensity and emission behavior of MWO 4 with the scheelite and wolframite type structure depends on the surface chemistry, morphology, and particle size [61].The lower crystallite size that results from mechanochemical treatment may be the cause of the lowest blue emissions for CaWO 4 -I and CaWO 4 -II [62].The Commission International de I'Eclairage (CIE)'s chromaticity diagram of the CaWO 4 samples excited at 230 nm is shown in Figure 6.As the main peak in the luminescence spectra is in the range between 400 and 600 nm and peaks at 425 (422) nm, it is expected that the emitted light will be blue.The calculated chromaticity coordinates are very close values and fall into the blue range on the CIE diagram (Figure 6 and Table 2).Blue light is in high demand because it can be mixed with yellow or participate in the tricolor (red, green, blue) to produce white light.

Conclusions
The variation in the milling speed promoted the rapid synthesis of the CaWO 4 from 5 h at 500 rpm to 1 h at 850 rpm without additional heat treatment.We demonstrated that the mechanochemical treatment led to the formation of particles with a quasihexagonal form and a narrower size distribution.CaWO 4 was also obtained by solid-state reaction at 900 • C for 12 h.Agglomeration of the oval particles was obtained using the solid-state technique.It was established that the method of preparation influences the crystallinity nature, macrostrains, and the symmetry of the WO 4 entity and its optical properties.It was established that the optical band gap of CaWO 4 strongly depends on the milling conditions, i.e., an Eg increase from 2.85 eV to 4.59 eV with an increase in the milling speed.The CaWO 4 -III obtained by the solid-state reaction possessed higher crystallinity, asymmetric WO 4 units, and oval particles, which resulted in a higher blue emission intensity compared to the mechanochemically synthesized samples.

Figure 2 .
Figure 2. (A) TEM image, particle size distribution, and EDS mapping of CaWO4-I obtained after 5 h of milling time at 500 rpm.(B) TEM image, particle size distribution, and EDS mapping of CaWO4-II obtained after 1 h of milling time at 850 rpm.(C) TEM image, particle size distribution, and EDS mapping of CaWO4-III obtained by solid-state reaction.

Figure 2 .
Figure 2. (A) TEM image, particle size distribution, and EDS mapping of CaWO 4 -I obtained after 5 h of milling time at 500 rpm.(B) TEM image, particle size distribution, and EDS mapping of CaWO 4 -II obtained after 1 h of milling time at 850 rpm.(C) TEM image, particle size distribution, and EDS mapping of CaWO 4 -III obtained by solid-state reaction.

Table 1 .
Unit cell parameters, crystallite size, and lattice microstrain values of the obtained samples.

Table 2 .
Emission peaks, FWHM, and CIE coordinates (x, y) of the synthesized samples.

Table 2 .
Emission peaks, FWHM, and CIE coordinates (x, y) of the synthesized samples.