Iovka Koseva

IR and Raman spectra of pure and chromium doped glasses and glass–ceramics from the system Li₂O-CaO-GeO₂-B₂O₃ are investigated and analyzed. The IR spectra of the pure and chromium doped glass samples contain bands typical of germanate and borate glasses. The amorphous network in the undoped glass is built by deformed GeO₄ tetrahedra, BO₃ groups and LiO₄ tetrahedra. Doping with 0.5 at% Cr₂O₃ leads to the appearance of BO₄ groups and an increase in the symmetry of the GeO₄ groups. The bands associated with chromium are not observed. The comparison between glass and glass–ceramic IR spectra shows the more intensive bands in the latter. Raman spectra of pure glass–ceramic samples show an increase in the number of Raman bands compared with glass samples. The shoulder at 850 cm⁻¹ is associated with internal stretching vibrations of the CrO₄ tetrahedron, so Cr⁴⁺ ions occupy the Ge⁴⁺ tetrahedron position in the Li₂CaGeO₄ crystal phase in glass–ceramic samples. A change in the symmetry of the GeO₄ units after doping with Cr₂O₃ in the glass–ceramic is also observed. Results from IR and Raman studies of non-doped and Cr-doped glass, and glass–ceramics containing the crystal phase of Li₂CaGeO₄, show that the IR spectra clearly show the change in the local environment of Li, Ge, and B group, but the detection of CrO₄ is questionable. In contrast, Raman spectroscopy is a more suitable method for the detection of CrO₄ units in the glass–ceramic samples. Raman analysis of the glass–ceramic confirmed the formation of CrO₄ units.

The pure and xDy³⁺-doped SrMoO₄ series (x = 0.5, 1.0, 1.5 and 2.0 at.%) were synthesized using a direct mechanochemical route. We found that a milling speed of 850 rpm and a milling time of 30 min result in a complete chemical reaction at different concentrations of dopant ions. The phase formation, structural units, and optical properties of the obtained samples were investigated by XRD, IR, UV-Vis and PL analyses. It has been established that Dy₂O₃ mainly influences the lattice parameters, unit cell volumes, crystallite sizes, and microstrains. The symmetry of MoO₄ groups was investigated using IR spectroscopy, and it showed that pure and Dy³⁺-doped SrMoO₄ samples are built up of deformed structural units. The calculated optical band gap of the obtained crystal phases decreases with increasing concentrations of Dy³⁺ ions. The host SrMoO₄ matrix shows broad blue emission centered at 430 nm under an excitation wavelength of 230 nm. All doped samples display a strong yellow emission at 570 nm, belonging to the ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ ions. The highest luminescence intensity was observed when the concentration of the Dy³⁺ ion was 0.5 at.%. The mechanism of concentration quenching was mainly caused by the electric dipole–dipole interaction. The calculated CIE chromaticity coordinates of the doped samples fall in the yellow range. This study demonstrates that mechanochemical treatment is an appropriate route for the fast preparation of yellow phosphors.

Nanocrystalline CoWO₄ sampled were synthesized using a simple mechanochemical approach and a solid-state reaction, respectively. The formation of nanocrystalline CoWO₄ was characterized by X-ray diffraction (XRD) and infrared spectroscopy (IR). The optical properties of the obtained samples were explored by diffuse reflectance UV–visible (DRS) and photoluminescence (PL) techniques. A milling speed of 850 rpm led to the direct synthesis of monoclinic CoWO₄ with a short reaction time (1 h). The complete reaction did not occur in the solid-state synthesis. The obtained samples had monoclinic crystal systems with different lattice parameters. The average crystallite sizes of CoWO₄ were in the range of 20 to 180 nm. The TEM investigation showed that the morphology of the CoWO₄ particles differed depending on the preparation conditions. The values of the determined optical bandgap of CoWO₄ were the range of 1.89 to 2.18 eV, according to diffusion reflectance spectroscopy in the ultraviolet-to-visible range. Broader blue–green emission spectra with peaks at 430 nm were observed for samples prepared via both routes. The CIE color coordinates of the CoWO₄ samples lay in the blue and purple regions. The quantum yields of the CoWO₄ samples synthesized after 1 h and 5 h milling times at 850 rom were 0.34 and 0.67%, respectively. This study proposes an affordable mechanochemical approach for blue–green phosphors that could possibly be used in various light-emitting diodes (LEDs).

This work investigated the optical characteristics of BaWO₄ nanoparticles that were produced through direct mechanochemical synthesis at varying speeds and times. This research expands upon our previous study. We demonstrated that the mechanochemical activation of the precursor of BaCO₃ and WO₃, at elevated milling speeds (850 rpm), facilitates the formation of tetragonal BaWO₄ in a reduced reaction time. The final products were characterized by scanning electron microscopy (SEM), as well as Raman, infrared (IR), UV-Vis diffuse reflectance, and photoluminescence spectroscopies. The crystallite sizes and particles shapes were determined by X-ray diffraction and SEM analysis. Round particles with a size below 50 nm formed under different milling conditions. The Raman spectra of the synthesized samples confirmed the presence of a scheelite-type structure with the typical six distinct vibrational peaks. The symmetry of the structural WO₄ groups was determined by IR spectroscopy. The absorption spectra of both samples exhibited intensive peaks at 210 nm, and the calculated optical band gaps of BaWO₄ were 5.10 eV (3 h/500 rpm) and 5.24 eV (1 h/850 rpm). A strong (400 nm) and weak (465 nm) emission were observed for the BaWO₄ that was obtained at a higher milling speed, while wider emission at 410 nm was visible for the BaWO₄ that was prepared at a lower milling speed. The CIE coordinates of the mechanochemically synthesized BaWO₄ were located within the blue area, exhibiting various positions.

Glasses from the system Na₂O‐Al₂O₃‐SiO₂‐B₂O₃‐Dy₂O₃ are prepared with three different concentrations of the Dy³⁺ ion—0.5, 1, and 2 at%. Glass‐ceramics are obtained after thermal treatment of the parent glass at 580°С and 630°С during 2, 5, and 24 h. The crystallizing phases, the crystallinity degree, and the nano crystalline size are determined. The crystallizing phases after thermal treatment are NaAlSiO₄ in two different modifications and Na₃B₃O₆. XRD analyses show the presence of third unknown phase in some samples. Emission and excitation spectra of the glass‐ceramics show the characteristic peaks of Dy³⁺ (⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 at 483, 575, and 668 nm, respectively). The yellow peak is dominating in all samples. The spectra show different intensity depending on the Dy³⁺ concentration, temperature, and time of the thermal treatment. This result suggests that the emission color can be controlled by these three factors.

BaMoO₄ was obtained via facile mechanochemical synthesis at room temperature and a solid-state reaction. An evaluation of the phase composition and structural and optical properties of BaMoO₄ was conducted. The influence of different milling speeds on the preparation of BaMoO₄ was explored. A shorter reaction time for the phase formation of BaMoO₄ was achieved using a milling speed of 850 rpm. A milling speed of 500 rpm led to partial amorphization of the initial reagents and to prolongation of the synthesis time of up to 3 h of milling time. Solid-state synthesis was performed via heat treatment at 900 °C for 15 h. X-ray diffraction analysis (XRD), infrared (IR) and UV diffuse reflectance (UV-Vis) and photoluminescence (PL) spectroscopy were carried out to characterize the samples. Independently of the method of preparation, the obtained samples had tetragonal symmetry. The average crystallite sizes of all samples, calculated using Scherrer’s formula, were in the range of 240 to 1540 Å. IR spectroscopy showed that more distorted structural MoO₄ units were formed when the compound was synthesized via a solid-state reaction. The optical band gap energy of the obtained materials was found to decrease from 4.50 to 4.30 eV with increasing crystallite sizes. Green- and blue-light emissions were observed for BaMoO₄ phases under excitation wavelengths of 330 and 488 nm. It was established that the intensity of the PL peaks depends on two factors: the symmetry of MoO₄ units and the crystallite sizes.

Powder samples of Li₂CaGeO₄, Ca₂GeO₄, and Ca₅Ge₃O₁₁ doped by 0.5, 1, 2, 3, 4 and 5 at% Eu³⁺ relative to the Ca²⁺, were prepared using a conventional solid‐state synthesis technique. X‐ray diffraction (XRD) analyses confirmed obtaining the pure phases at all dopant concentrations. In parallel, single crystals of the three compounds with the experimentally found optimal Eu³⁺ concentration were grown using a flux method. Structural investigation on the single crystals were done with a special attention to the form of the Ca–O polyhedron, the mean Ca–O distance, the Ca–Ca distance in the structure, the distortion degree of the polyhedron, as well as the Eu–Ca substitution site. The main spectral characteristics were analyzed and several relationships between the structural and spectra features were found. The optimal dopant concentration was 3 at% for Ca₂GeO₄ and 4at% for Ca₅Ge₃O₁₁ and Li₂CaGeO₄. Commission Internationale de l'éclairage coordinates of the samples showed emission colours in the red region close to the standard red coordinates and slightly influenced by the active ion concentration. The obtained results showed that europium‐doped Li₂CaGeO₄, Ca₂GeO₄, and Ca₅Ge₃O₁₁ could be used as red phosphors.