MECHANOCHEMICAL-ASSISTED SYNTHESIS AND CHARACTERIZATION OF PRISTINE AND GD3+ DOPED COWO4 COMPOUNDS

European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 95 MECHANOCHEMICAL-ASSISTED SYNTHESIS AND CHARACTERIZATION OF PRISTINE AND GD3+ DOPED COWO4 COMPOUNDS V. Anbalagan*, K. Dinakaran** & P. Prabukanthan*** * Materials Chemistry Lab, Department of Chemistry, Muthurangam Government Arts College, Vellore, Tamil Nadu, India ** Department of Chemistry, Thiruvalluvar University, Vellore, Tamil Nadu, India *** Department of Chemistry, Government Arts and Science College, K.V. Kuppam, Vellore, Tamil Nadu, India Cite This Article: V. Anbalagan, K. Dinakaran, P. Prabukanthan. (November 2025). MechanochemicalAssisted Synthesis and Characterization of Pristine and GD3+ Doped COWO4 Compounds. In Proceedings of the European Summit on Interdisciplinary Research and Development (pp. 95-102). Perambalur, Tamil Nadu, India: Crystal Pen Publication. ISBN: 978-93-49435-80-3 Publisher Website: www.crystalpen.in Copy Right: © 2025 Crystal Pen Publication (CPP). All rights reserved. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. DOI: Abstract: Pristine and Gd3+ doped CoWO4 compounds were synthesized by mechanochemical assisted. The XRD, EDAX, FT-IR spectroscopy study indicates that pure phase formation of the worframite-type with monoclinic system. Scanning electron microscopy study suggests that smooth surface morphological with uniform distribution of particles. Optical absorption, photoluminescence, EPR and EDAX analysed revel that Gd3+ is substituted in the Co2+ into CoWO4 lattice. When the Gd3+ was incorporated in the CoWO4 compound the absorption peak position significantly changed, whereas luminescence property was killed. There is a deviation in the composition of Co, its indicate that when the concentration of Gd increases in CoWO4. Key Words: CoWO4; XRD; optical properties; EPR; and TG/DTA studies. 1. Introduction: Metal tungstates are attractive materials and have received considerable research interest due to their intriguing luminescence and structural properties. As a p-type semiconductor, CoWO₄ has been the most widely studied material for various technological applications such as catalysis, sensors, displays, and optoelectronics. It is well established that the additional doping will induce the structural and magnetic changes in the host systems [1-2].A. Sen, P. Pramanik [3] (2001) reported the photoluminescent properties of hydrothermally synthesized Eu³⁺-doped ZnWO₄. Jani et al [5] (1995) study the growth and spectral analysis of Ho³⁺-doped ZnWO₄.. Recently, Nikl et al [6] (2000) reported the luminescent features of the CoWO₄:Ce³⁺ nanostructure. Scott et al (2002) [7] reported the Ln³⁺-doped YPO₄ nano/microstructures with tunable luminescent colors. The effect of Nd impurities on the optical, dielectric, and electrical properties of PbWO₄ single crystals was reported by Dai et al (2007) [8]. The bright white upconversion luminescence from Er³⁺, Tm³⁺, and Yb³⁺ doped CaSnO₃ powders has been reported by Thongtem et al. (2009) [9]. Further, it is reported that the lanthanide ions possessing special 4f intra shells are recognized as excellent candidates for luminescence centers of the doped inorganic phosphor systems due to their many optical advantages. Recently, Dy, as one element belonging to the lanthanide family, doped phosphors have been the focus of numerous investigations because of their unique optical properties [8] and promising applications in optoelectronics [9]. Many metal tungstates exhibit such properties as will make them suitable for potential applications in different areas of technology. The CoWO₄ is a material that has been considered for extensive studies of photoluminescence (PL), optoelectronic devices, scintillating material, microwave dielectrics, and catalysis [6] where the properties of these materials can be achieved by a well-intended synthetic technique [7-8]. The tungstates are found to be highly attractive and suitable for different methodological applications such as solid-state lasers [5], scintillators [6], optoelectronic devices, besides being promising candidates for super-hard materials [7], which are useful and promising candidates. To date, several chemical synthesis processes, such as sonochemical, hydrothermal, co-precipitation, etc., have been used for the preparation of metal tungstate, especially for the wolframite nanocrystals. Synthesis of metal tungstate nanocrystallites was done with low-temperature ethylene glycol as a solvent. Similarly, Dai et al (2007) [8] proposed the room temperature synthesis and properties of multifunctional doped tungstate nanorods. Among these different methods, the solution-based chemical synthetic methods play a crucial role in the design and production of fine ceramics and have been successful in overcoming many of the limitations of the traditional solid-state, high-temperature methods. The use of solution chemistry can eliminate major problems such as long diffusion paths, impurities, and agglomeration, which will result in a product with European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 96 improved homogeneity. However, wet methods have disadvantages such as complicated synthetic steps, use of expensive equipment, high synthetic temperature, and long sintering time. On the other hand, due to excessive energy consumption, complex apparatus, and techniques, the solid-state reaction becomes gradually unpopular and unsatisfactory. However, solid-state material synthesis by the metathetic route is emerging as a viable alternative approach to synthesizing high-quality novel inorganic materials in a short amount of time. Here we report the synthesis of pristine and Gd-doped CoWO₄ by a novel solid-state mechanochemical approach method, and the effects of the dopant on the structures and surface morphology with composition and optical properties were reported. 2. Experimental - Synthesis of pristine and Gd3+ ions doped CoWO4 Compounds: Na₂WO₄.2H₂O, Gd(CH₃COO)₃, and CoCl₂.6H₂O obtained from Alfa Aesar, USA, were used as precursors for the synthesis of the cobalt tungstate (CoWO₄) and Gd³⁺-doped CoWO₄ compounds. Synthesis of CoWO₄ was carried out by reacting a well-ground mixture of CoCl₂ and Na₂WO₄ in a molar ratio of 1:1 in the mechanochemical approach method. The attain product were washed with deionised water to remove the sodium chloride reaction by product and dried at 90°C. Synthesis of various mole concentrations (1, 3, and 5 mole %) of Gd-doped CoWO₄ compounds was carried out in a similar manner by reacting to a well-ground mixture of Na₂WO₄, Gd(CH₃COO)₃, and CoCl₂, as mentioned above. Synthesis of cobalt tungstate (CoWO₄) and Gd-doped CoWO₄ compounds was according to the following chemical reactions: Na2WO4 + CoCl2  CoWO4 + NaCl Na2WO4 + CoCl2 + (1, 3 and 5mole %) Gd(CH3COO)3  CoWO4:Gd + NaCl 3. Result and Discussion: 3.1 XRD Analysis: Figure 1 shows the XRD patterns for the synthesized pristine and Gd-doped CoWO₄ tungstates. The pristine and Gd-doped tungstates follow a wolframite-like monoclinic crystal structure with the space group of P2/c. Their lattice parameters are as follows: a = 4.947 Å, b = 5.682 Å, c = 4.66 Å, and b = 90.0° for pristine and Gd-doped CoWO₄.. The lattice parameters of the CoWO₄ compound under consideration are consistent with the values reported in [10] and tabulated by the Joint Committee on Powder Diffraction Standards (JCPDS); file No. (72-0479) for cobalt tungstate. The pattern of pristine and Gd-doped CoWO₄ indicates the main diffraction peaks of the monoclinic CoWO₄ phase at 2q = 31.6°, 39.1°, 45.2°, 56.2°, 66.14°, and 75.3°, which correspond to the (020), (012), (211), (231), and (232) crystallographic planes, respectively. As Figure 1 reveals, no characteristic peaks from non-reacting starting materials are detected on the XRD patterns of the synthesized CoWO₄ tungstates, indicating that the products obtained are single-phase materials. With the doping of Gd, no new peak was found due to Gd(CH₃COO)₃, which is ascribed to the incorporation of Gd into the Co lattice site. With respect to the Gd doping concentration of 1, 3, and 5 mole %, there is no peak shift observed, but the intensity increased gradually. This may be associated with the lattice distortion induced by Gd ions into the CoWO₄ crystal structure, which leads to a larger ionic radius of dopant ions, and also Gd³⁺ ions in the Co² site enlarge the crystalline size of the CoWO₄.. Table 1 lists the full width at half maximum (FWHM) values of the (020) reflection peak of pristine and Gd-doped CoWO₄ compounds. The FWHM of the (020) peak of Gd-doped CoWO₄ is higher than that of pristine CoWO₄ due to crystal lattice strain occurring in Gd-doped CoWO₄.. In conformity, the average crystalline size of the pristine and Gd-doped CoWO₄ assessed using Scherrer’s equation [D = 0.9 l/b cos q, where b is the FWHM of the diffraction line in radians and l is the X-ray wavelength] is based on using the (020) reflection at 31.6°.The average crystalline size of the pristine and CoWO₄ compound could be determined by employing Scherrer’s formula, and it was found to be 973-1059 nm. Table 1: FWHM, crystalline size and atomic concentration of pristineand Gd doped CoWO4 compounds Compounds FWHM of GAXRD peak (020) (deg.) Crystalline size (nm) Composition (Atomic Percentage) Co W O Gd PristineCoWO4 0.02847 973 16.92 16.38 66.7 0.00 1 mole % Gd doped CoWO4 0.02893 1012 16.08 16.32 66.62 0.98 3 mole % Gd doped CoWO4 0.02921 1034 14.32 16.31 66.58 2.79 5 mole % Gd doped CoWO4 0.03023 1059 12.88 16.29 66.51 4.32 European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 97 Figure 1: Powder XRD spectra of pristineand Gd-doped CoWO4 3.2 UV-Visible Spectral Studies: Optical absorption measurement is a widely used technique for confirming dopant substitution into the host lattice. At the same time, the change in electronic band structure also can be identified using UV-Vis absorption spectra. The UV-Vis absorption spectra of synthesized pristine and Gd-doped CoWO₄ compounds are shown in Figure 2. The strong absorption maximum of 231-385 nm was observed for all the samples. Due to the high crystallinity, the samples have shown more prominent UV absorption maxima, which imply that there is direct charge transfer between the metal and ligand within the (WO₄²⁻) complex. There was no additional absorption due to the Gd seen in the UV-Vis spectra, demonstrating the substitution of Gd into the Co lattice [11]. By the addition of Gd, only the small shift in the absorption band was observed towards lower wavelengths for the samples. This may be due to the active interaction of trivalent cations with divalent ions. The absorption peak at 231-385 nm does change significantly, whereas peak broadening decreases in magnitude, possibly due to the formation of a wide distribution of Gd ions in the wolframite structure of the CoWO₄ compound. The strong d-d transition band was observed in the visible and far-visible regions (490-656 nm and 1057-1800 nm) and corresponds to the d-d transition of Co²⁺ ions in the CoWO₄ system, which also shifted slightly towards lower wavelengths with respect to Gd doping concentration. Figure 2: UV-Vis absorption spectra of pristine and Gd-doped CoWO4 European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 98 3.3 Scanning electron microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDAX) Analysis: Figure 3 (a-d) represents the SEM micrographs for pristine and Gd-doped CoWO₄ compounds. From the SEM images, more information on the grains and nature of the samples can be seen. Captured SEM images clearly show the formation of self-aggregated particles, highly homogeneous and cubical in shape. It is clearly seen that the particles are highly aggregated and emerge in a well-crystalline nature. The addition of Gd into CoWO₄ made no changes in the surface morphology except the crystal diameter, which is in the range of 9001100 nm. This result indicates that the addition of Gd retains their microstructure without altering the crystalline structure and morphology. With an increase in the doping concentration, the particle size was increased significantly, and the uniform particle distribution was also observed. This result further confirms the complete incorporation of Gd onto the Co lattice site. The elemental chemical analysis has carrying out using EDAX directly attached to the SEM instrument. The experimental EDAX spectra of pristine and 5 mole % Gd-doped CoWO₄ were exposed in Figures 3 (e-f). The atomic concentrations of the primary elements present in the pristine and Gd-doped CoWO₄ compounds are given in Table 1.EDAX spectra obviously designate the presence of functional elements of Co, Gd, W, and O with appropriate concentrations without any impurities, which confirms the stoichiometric concentration of the elements. From Table 1, it can be observed that there is deviation in the compositions of Co when the concentration of Gd increases in CoWO₄ compounds. The results indicate that Gd might act as a substitute at the Co site in the CoWO₄ compound system. European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 99 Figure 3 (a-f): SEM micrographs (a) pristine CoWO4 , 1 mole % Gd3+ ions doped CoWO4 (b), 3 mole % Gd3+ ions doped CoWO4 (c), 5 mole % Gd3+ ions doped CoWO4 (d), EDAX spectra of pristine CoWO4 (e) and 5 mole % Gd3+ ions doped CoWO4 (f) 3.4 FT-IR Spectra Studies: To determine the chemical structure of the pristine and Gd³⁺-doped CoWO₄ compounds, the Fouriertransform infrared (FT-IR) spectra were observed over the frequency range of 4000-400 cm⁻¹ as shown in Figure 4. The intense band that appeared in the low-frequency region of 400-1000 cm⁻¹ belonged to the characteristic deformation modes of Co-O, W-O, and W-O-W bridges. The IR spectra below 565 cm⁻¹ could be ascribed to the deformation modes of W-O bonds in WO₆ octahedra or the deformation of W-O-W bridges [12]. The weak bands appearing at 849 and 726 cm⁻¹ could be associated with the asymmetrical stretching of the OW-O vibration mode and the W-O bond stretching in the (W₂O₄)ₙ chain, respectively. When compared to the pristine CoWO₄, the Gd³⁺-doped sample has shown the broad vibrational band at around 430 cm⁻¹, which corresponds to the Co-O and ascribes the incorporation of Gd into the Co lattice. While increasing the Gd concentration, there is no visible band due to the Gd-O-Co or Gd-O being seen, this confirms the homogenous dispersion of Gd within the CoWO₄ matrix. And also, when Gd concentrations were increased in the CoWO₄ lattice, does not change the band position. The absorption of CO₂ from the atmosphere at the CoWO₄ surface was identified from the sharp band positioned at about 2353 cm⁻¹ in the FTIR spectra. From the figure it can be seen that the intensity of the O-C-O stretching band increases with increasing the concentration of Gd³⁺.This may be due to the presence of Gd³⁺ ions in the CO₂ site that absorbs more CO₂ from the air atmosphere. The OH stretching vibration of surface-absorbed or internally bonded water molecules accounted for the strong intense band appearing at 3759 and 3556 cm⁻¹.. The bending mode of H-O-H was located at about 1634 cm⁻¹.This mainly arises from internally bonded water molecules or physically adsorbed water molecules. Figure 4: FT-IR spectra of pristine and Gd3+ doped CoWO4 compounds European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 100 3.5 Photoluminescence (PL) Studies: Figure 5 shows typical room temperature photoluminescence (PL) spectra of pristine and Gd³⁺-doped CoWO₄ compounds. From the figure it can be seen that the three characteristic emission bands at around 364, 467, and 634 nm for the pristine CoWO₄ compound ascribe the intrinsic optical behavior of the host system. The luminescent efficiency seems to be closely related to the process of synthesis for obtaining particles that are not agglomerate but well-formed crystals. In comparison to the pristine CoWO₄ compound, the 1, 3, and 5 mole % Gd-doped CoWO₄ exhibit a red shift to a higher wavelength region in the emission band, with a decrease in the intensity. The luminescent band in higher wavelength should be associated to the temperament of semiconducting band gap. It is noticeable in the PL spectra that the sudden drop in the intensity at doping Gd (1, 3, and 5 mole %) concentration demonstrates that the small to large amount of Gd acts as the luminescent killer in the CoWO₄ matrix [13-16]. Accordingly stern luminescent reduce was experiential in different concentrations of Gd-doped CoWO₄ compounds. While be capable of seen starting the figure, there is no other band related to the Gd in CoWO₄ that has been pragmatic, and yet high concentrations supplementary confirm the whole incorporation of Gd in Co lattices. The deficiency of deep-level visible luminance in PL spectra confirms the defect-free crystalline nature of the pristine CoWO₄ compound. Figure 5 : Photoluminescence spectra of pristine and Gd-doped CoWO4 3.6 EPR Spectral Studies: An analysis of Co2+ EPR spectra is not straightforward because the spectra can be recorded usually at low temperatures because of a very fast electron spin-lattice relaxation, which produces strong EPR line broadening. Moreover, Co2+ ions (3d7 configuration with S = 3/2, I = 7/2) can appear both at high-spin (S = 3/2) and at low-spin (S = 1/2) configurations and have different characteristics in tetrahedral and octahedral coordination. Figure 6 shows the EPR spectra of pristine and Gd3+ doped CoWO4 in solid state recorded at 77 K. In this figure two groups of cobalt hyperfine lines are shown. The broad EPR feature from 123 and 210 gauss unambiguously indicates the presence of the high spin Co2+ {S =3/2, d7} in which the EPR transition occurs within the lower Kramers doublet {ms =  ½} at 77 K for the pristine CoWO4 compound. But 1 mole % Gd3+ doped CoWO4 compound has this region well-resolved hyperfine structure appear. However Gd ions concentration has increased this region of unresolved spectra [17-18]. Unresolved hyperfine structure at gyy  gzz {g} may be attributed to the 59Co nuclei {I = 7/2}. The g value is 5.7  0.2. In second region the narrow intense signal with g11 = 2.004 this attributed to an electron which transferred from WO2to Co2+, when Gd3+ ions concentration this high, the intensity peaks were decreased but not change the peaks position. This phenomenon is attributed to significant contributions from excited orbital states and spin-orbit coupling. Whereas g>g11 when the A2g state has the lowest energy. European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 101 Figure 6: EPR spectra of pristine and Gd3+ doped CoWO4 compounds 3.7 Thermogravimetry/Differential Thermal Analysis (TG/DTA): Typical temperature versus mass change of the pristine and 5 mole % Gd³⁺-doped CoWO₄ are shown in (TGA) Figure 7a.The stepwise crystallization with respect to temperature is as follows: the initial weight loss in the TGA curve at around 200°C is associated with the decomposition of metal salt and impurity evaporation. In continuous heating, the major weight loss observed above 200°C is mainly attributed to the crystallization process taking place. Further, very small and gradual weight loss followed by the plateau region in the TGA curve beyond 400°C.By adding the Gd into the Co site, the crystallization temperature reduces significantly, as shown in Figure 7a. In relation to thermal decomposition (different thermal analysis) of the pure and 5 mole % Gd³⁺-doped CoWO₄, Figure 7b is shown. In the pristine CoWO₄ compound, four thermal decomposition steps were observed {81, 302, 648, and 755°C}.Their peak indicates an exothermic nature. The first one had an exothermic peak assigned to the elimination of water and gases absorbed on the powder surface. Other peaks were exothermic, being assigned to a combustion reaction that led to the formation of an intermediate meta-stable compound with the respective metal oxide. Figure 7 (a-b): (a) TGA spectra of pristine and 5 mole % Gd3+ doped CoWO4 (b) DTA spectra of pristine and 5 mole % Gd3+ doped CoWO4 4. Conclusion: A novel mechanochemical solid-state metathesis approach has been employed to synthesize technologically important pristine and Gd-doped CoWO₄ materials. This method has distinct advantages in terms of simplicity and easy scale-up and is relatively inexpensive with high yield. The X-ray diffraction analysis clearly shows that the Gd-doped CoWO₄ compounds are single-phase in the monoclinic (wolframite) structure. Optical absorption edge energies for the pristine and Gd-doped CoWO₄ synthesized in this study have been determined. Optical studies of pristine and Gd-doped CoWO₄ exhibits the intense absorption position European Summit on Interdisciplinary Research and Development - An International Research Conference Published By Crystal Pen Publication, Perambalur, Tamil Nadu, India - www.crystalpen.in ESIRD - 2025 Proceedings, Date: November 30, 2025, ISBN Number: 978-93-49435-80-3 102 between 231-385 nm, which confirms the excellent optical behavior of the monoclinic (wolframite) structure. The small wavelength shift was towards the lower wavelength region for the doped samples, addressing the interaction of the Gd³⁺ ion with the CoWO₄ structure. SEM showed well-defined morphology for pristine and Gd-doped CoWO₄ prepared by this method. By Gd doping there is no finding of any other possible secondary phases in luminescence, and also the absence of deep-level visible emission in the PL spectra confirms the defect-free crystalline nature. 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