Preparation of Glass-Ceramic Materials by Controlled Crystallization of Eu₂O₃ Doped WO₃-B₂O₃-La₂O₃ Glasses and Their Luminescent Properties

1. Introduction

In recent years, rare earth-based tungstates have gathered increased attention as host materials due to their distinct crystal structure, remarkable emission properties, strong near-ultraviolet (NUV) absorption, thermal and chemical stability, low hygroscopicity, environmental friendliness, and self-activating luminescence properties. In addition, tungstates exhibit a broad charge transfer band (CTB) in the range of 200–350 nm. The CTB can transfer energy to rare earths, making them ideal for UV excitations [1]. In fact, rare earth tungstates are a big family including various compounds of RE₂W₃O₁₂, RE₂WO₆, RE₆WO₁₂, RE₂W₂O₉, RE₁₀W₂₂O₈₁, and so forth, according to W/RE stoichiometry, and can be viewed as pseudo-binary compositions between RE₂O₃ and WO₃ [2,3]. Among the rare earth tungstates, the compounds of La and tungsten are of great potential for luminescent host materials. For example, La₂W₂O₉:Eu³⁺ shows promising photoluminescence and has quantum yield as high as 77% [3,4,5,6]. La₂(WO₄)₃:Tb³⁺ phosphorsk were successfully synthesized via optimized microwave assisted co–precipitation technique [7]. The luminescence characteristics and structural analyses of Sm³⁺ activated La₁₀W₂₂O₈₁ microphosphors were also reported [8]. Eu³⁺: La₁₀W₂₂O₈₁ (LWO) microphosphor rods with intense red emission were synthesized using a hydrothermal assisted solid-state method [9]. To the best of our knowledge, there is no data for the preparation of lanthanum tungstates glass-ceramics by controlled glass crystallization. In recent years, a great deal of attention has been paid to rare earth-doped glass-ceramic materials, which play a crucial role in many optical applications such as up-conversion fibers, solid-state lasers, medical sensors, optical electronic chips, luminescence labels, optical amplifiers, 3D displays, etc. [10,11] Glass-ceramics possess excellent characteristics from both glasses and crystals and have no disadvantages of these two materials. Similar to glasses, glass-ceramics have a large capacity for accommodating an active rare earth dopant, are isotropic and have evenly distributed activators within their bodies. Similar to single crystals, glass-ceramics contain rare earth ions within strictly ordered ligand surroundings. As a result, the presence of a crystalline environment around a rare earth ion allows high absorption and emission cross-section reduction in the non-radiative relaxation process because of the lower phonon cut-off energy and tailoring of the ion–ion interaction by control of the rare earth ion partition [10,12]. The glass-ceramic materials are usually obtained by subsequent thermal treatment of a glass, first melted and annealed as usual. This conventional method relies on thermally induced phase separation and in situ crystallization processes, which are however very complex to experimentally control [12]. The choice of an appropriate glass composition is very important for luminescent glass-ceramics elaboration. The search for more efficient glass compositions and guiding structures for rare earth-doped glass-ceramics continues. In our earlier paper we have determined the glass formation region in the ternary La₂O₃-B₂O₃-WO₃ system. It was established that transparent colorless glasses are obtain in the central part of the system between 20 and 35 mol% La₂O₃ [13]. We have also studied their structure and crystallization behavior. The main crystallization mechanism in the glasses is the surface crystallization and the glass in mol % of 50WO₃:25La₂O₃:25B₂O₃ crystallized as a single phase LaBWO₆ [14]. The luminescence properties of of Eu³⁺doped glass, glass-crystalline product and polycrystalline sample with the same composition of 50WO₃:25La₂O₃:25B₂O₃ have been verified [13]. It was found that the intensity of emission increases drastically in the glass-crystalline sample consisting of LaBWO₆:Eu³⁺ nanoscrystals compared with the glass and polycrystalline samples. These results provoked our interest to continue the investigation on La₂O₃-B₂O₃-WO₃ system. The purpose of the present study is to investigate crystallization behavior of one selected glass composition from the isothermal section LaBWO₆-La₂W₂O₉ of the phase diagram [15] in order to check the possibility to obtain lanthanum tungstate glass-ceramics doped with Eu³⁺ ions and to study their photoluminescence properties. The crystallization behavior of glass-ceramics was characterized by X-ray diffraction and transmission electron microscopy. Emission spectra were measured, and color coordinates of the materials were determined.

2. Results and Discussion

2.1. XRD Data

Figure 1 shows the powder diffraction patterns after crystallization of glass with composition 52WO₃:22B₂O₃:26La₂O₃:0.5Eu₂О₃ at a temperature of 680 °C and a crystallization time of 3, 6, 9, 12, 15 18 and 21 hours, respectively. The glass-ceramics obtained after heat treatment of the parent glass for different times are designated as GC-3 h, GC-6 h, GC-9 h, GC-12 h, GC-15 h, GC-18 h and GC-21 h, respectively. From the phase analysis performed, the best match was found to be β-La₂W_1.7Mo_0.3O₉ (PDF # 01-073-9174), which is isostructural to the high-temperature polymorphic modification of La₂Mo₂O₉ and crystallizes in cubic space group P2₁3 with parameter a = 7.1327 Å [16]. After 21 hours of glass crystallization, the appearance of an additional crystalline phase of Orthorhombic La₂WO₆ was observed. To the best of our knowledge, no La₂W₂O₉ with a cubic structure at room temperature has been reported so far.

Generally, the stabilization at room temperature of the high-temperature polymorphic modification of La₂Mo₂O₉ and La₂W₂O₉ is carried out with suitable cationic substitutions, and only β-La₂W_1.7Mo_0.3O₉ is reported as single phase at room temperature. Based on this, we suggest that in our study, the isomorphous substitution of La³⁺ by Eu³⁺ has an influence on the stabilization of the cubic symmetry phase of La₂W₂O₉. Moreover, in the chemical composition of the resulting glass there are no other possibilities for isomorphic substitutions in the structure of β-La₂W₂O₉.

Table 1 presents the calculated unit cell parameters and crystallite size for all stages of the crystallization of cubic β-La₂W₂O₉.

No significant change in unit cell parameters as a function of crystallization time is observed from the data. The small variations observed are most likely due to the conditions during powder pattern collection. The study was conducted on a bulk sample of initial glass, which inevitably has some surface irregularities and leads to systematic errors during X-ray diffraction. The calculated crystallite size also remains within narrow limits, between 27.8(6) nm and 22.1(4) nm regardless of crystallization time. The reasons for this are most likely kinetic, the limiting factor in this case being the relatively low crystallization temperature of 680 °C.

2.2. Luminescent Properties

To investigate the effect of leat treatment (level of crystallinity) on luminescent properties, we compared the excitation and emission spectra of 0.5% Eu³⁺ doped 52WO₃:22B₂O₃:26La₂O₃ glass with those of the corresponding glass-ceramic (GC) samples obtained at different times of heat treatment at 680°C. The excitation spectrum (Figure 2) monitored by 612 nm wavelength (⁵D₀ → ⁷F₂ transition of Eu³⁺ ions) contains a continuous excitation band below 350 nm and several sharp peaks in the range from 350 nm to 600 nm at 361 nm, 374 nm 381 nm, 392 nm, 413 nm, 462 nm, 523 nm, 530 nm and 576 nm, which corresponds to characteristic 4f - 4f transitions of Eu³⁺ ion from ⁷F₀ → ⁵D₄, ⁷F₀ →⁵G₂, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, ⁷F₀ → ⁵D₂, ⁷F₀ → ⁵D₁, ⁷F₁ → ⁵D₁, ⁷F₀ → ⁵D₀, respectively [17].

Among the above transitions, the ⁷F₀ → ⁵L₆ one, centered at 392 nm, stands out as the most prominent. Therefore, subsequent emission spectrum measurements were conducted under excitation at 392 nm. The wide band peaking at about 290 nm is assigned to the ligand - to metal charge transfer - states (LMCT) appeared due to the combined effect of the electronic transition from 2p orbital of O²⁻ ions to vacant 4f energy level of central Eu³⁺ ions in Eu - O polyhedral and O^2- → W⁶⁺ in WO_n groups (WO_n=WO₄ and WO₆) [18,19,20,21,22].

The presence of WO_n groups’ excitation band, recorded at the characteristic Eu³⁺ emission wavelength of 612 nm is suggesting that non-radiative energy transfer from the glass matrix to the active rare-earth ion is occurring. [22,23]. This mechanism is called host - sensitized luminescence.

As can be seen from Figure 2, the excitation gradually increases with the heat treatment time and reaches a maximum at 18 hours, then decreases at 21 hours. The same trend is observed in the emission spectra (Figure 3). This enhancement is most likely attributed to the progressive increase in the amount of the crystalline phase in the samples, which alters the crystal field environment inside the matrix and hence more efficient excitation of Eu³⁺ can be expected in the glass-ceramic samples.

Additionally, when comparing the intensity of LMCT and the f-f transitions, it is evident that the intensity is stronger for narrow Eu³⁺ bands. This is helpful for attaining proper excitation by near - UV and blue LED chips, as the intensity of these Eu³⁺ transitions is generally weak because they are forbidden by Laporte’s selection rule [24]

The emission spectra at Figure 3 show the conventional emission lines of Eu³⁺, which corresponds to the transitions from ⁵D₀ excited energy level towards the ⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃, and ⁷F₄ ground states at about 578, 592, 612, 651, and 700 nm, respectively.

The glass sample possesses the lowest emission intensity, and after obtaining glass-ceramic samples by heat treatment at 680°C and crystallization of La₂W₂O₉ phase, the intensity increases up to 18h of thermal treatment. This enhancement is most likely attributed to the increase in the crystallinity of the sample and due to the incorporation of Eu³⁺ in the obtained crystal phase. It is well known that the distorting of the local environment around Eu³⁺ ions in the host lattice can increase the emission. It is obvious that the site symmetry around the active ion modifies with the glass → crystal transition, leading to the enhancement of the intensity. Additionally, when holding heat treatment time from 6 hours to 9 hours, the emission intensity increased by approximately twofold, which is a result of the significant increase in the amount of crystalline phase on the surface (see Figure 1). Further maintaining the heating temperature for up to 21 hours leads to a drop in the emission intensity due to the quenching effect of europium ions, probably as a result of the formation of new crystalline phase - La₂WO₆. A redistribution of Eu³⁺ ions in the different crystalline compounds is most likely taking place, which is not favorable for the emission intensity. The X-ray structural data and TEM analysis support these observations.

It is established that the ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transitions may provide structural hints about the distortion of crystal field environment around the rare earth ions. Тhe ⁵D₀→⁷F₁ is a magnetic dipole (MD) transition among Eu³⁺ emission spectrum, and is permitted by Jude-Offelt theory. Its intensity is not influenced by the environment near Eu³⁺ ions. In contrast, the ⁵D₀→⁷F₂ is an electric dipole transition (ED) and its intensity is strongly dependent of the ligand environment. The dominance of the ⁵D₀ → ⁷F₂ transition over the ⁵D₀ → ⁷F₁ transition in the emission spectra (Figure 3) is an indication that the environment surrounding Eu³⁺ in the samples is non-centrosymmetric. In particular, the site symmetry of Eu³⁺ can be evaluated by using the integrated emission intensity ratio (R) of these two transitions ⁵D₀→⁷F₂/⁵D₀→⁷F₁. Higher values of R indicate more site asymmetry of the rare-earth ion, high covalency between Eu³⁺ and O^2- ions, increased emission intensity and hence greater potential for optical applications [17,18,25].

The calculated intensity ratios R (from 4.88 to 6.12) of the Eu³⁺ doped 52WO₃:22B₂O₃:26La₂O₃:0.5Eu₂O₃ glass and glass-ceramics (Table 2) are mostly higher than other reported Eu³⁺ oxide compositions [26,27,28,29,30,31,32,33]. It is also evident from Table 2 that the asymmetry ratio of the glass-ceramics is higher than that of glass and progressively rises with increasing heat treatment time of the samples up to 18h (GC-18h), which demonstrates that the degree of symmetry of Eu³⁺ ions decreases, and a strong Eu-O covalence is formed at a higher level of crystallinity. Since the larger the asymmetry, the greater the luminous intensity, this fact is directly related to the higher emission intensity of glass-ceramics. This result is consistent with the intensity trend in PL spectra, where the intensity increases with the increasing of the crystallinity level. At 21 hours of heat treatment (CG-21), a decrease in R value is observed, which agrees with the decrease of the emission intensity of this specimen.

Additionally, the appearance of the sensitive to the crystal field and forbidden based on the standard Judd–Ofelt theory [34] ⁵D₀→⁷F₀ transition, indicates that the Eu³⁺ ion occupies non-centrosymmetric sites with C_2ν, C_n or C_s symmetry [35].

The Commission International de I’Eclairage (CIE) 1931 chromatic diagram is a standardized method for characterizing colors of emission. The chromaticity coordinates of Eu³⁺ doped 52WO₃:22B₂O₃:26La₂O₃:0.5Eu₂O₃ glass and glass-ceramics at Table 3 are calculated from the emission spectra by using color calculator software SpectraChroma (CIE coordinate calculator) [36] and are illustrated at Figure 4. It can be observed that all values are located in the red region of the diagram. The estimated coordinates of glass-ceramic samples are represented at diagram as one point, as the values are almost identical, and are very similar to the CIE coordinate of the commercial red phosphor Y₂O₂S:Eu³⁺ (0.658; 0.340) [37]. The obtained results show that the investigated glass and glass-ceramics have the potential to be used as red-emitting materials.

2.3. TEM Investigations

A glass-ceramic sample obtained after an 18 hour thermal treatment of the parent glass, which exhibited the most intensive luminescent emission, was analysed using TEM and HRTEM techniques. Morphological studies (Figure 5, Figure 6 and Figure 7) revealed that the sample contains spherical and rectangular nanosized particles. The inset in Figure 7 illustrates the particle size distribution. The nanosized particles range between 10 and 80 nanometres, with the majority falling within the 20–40 nm range, although some larger particles were also observed. The average particle size of the glass-ceramic sample was determined to be 22 nm.

The HRTEM results align well with those obtained from X-ray phase analysis. HRTEM analysis (Figure 5 and Figure 6) identified the presence of cubic LaW₂O₉ and orthorhombic La₂WO₆ phases in the sample. The cell parameters were measured as a = 7.33 Å for the cubic phase and a = 8.886 Å, b = 16.555 Å, c = 5.521 Å for the orthorhombic phase. The corresponding interplanar distances were determined to be d = 3.2 Å and d = 4.4 Å, respectively

3. Materials and Methods

Glass with nominal compositions 52WO₃:22B₂O₃:26La₂O₃:0.5Eu₂O₃ was obtained by applying the conventional melt-quenching method, using commercial powders of reagent grade WO₃, H₃BO₃, La₂O₃ and Eu₂O₃ as starting materials. The homogeneous batch (10 g) have been melted for 20 min in air in platinum crucible at 1240 °C. The glass was obtained by press quenching between two coper plated (coling rate 10²K/s). To prepare the glass-ceramics (GC), the precursor glass 52WO₃:22B₂O₃:26La₂O₃:0.5Eu₂O₃ was subjected to heat treatment at 680 °C for 3 h, 6 h, 9 h, 15 h, 18 h and 21 h. X-ray powder patterns of crystallized La₂W₂O₉ glass ceramics were collected on a Bruker D8 Advance diffractometer equipped with CuKa X-ray source. Monochromatization of the diffracted beam was achieved using nickel filter in front of a Bruker LynxEye silicon-strip detector. During data collection, spinner with rotation speed of 15 rpm was used to average the measurement over the entire volume of the sample. The powder patterns were measured in the angular range of 5.5 - 80 degrees 2theta with a step of 0.04 degrees 2theta and acquisition time of 0.4 sec/step per one detector channel. The phase composition of the samples was determined using Match! v.3.16 program [38] and ICDD PDF-2 database (2021) as a referent files. The program Topas v.4.2 [39] and Pawley fit procedure were used to determine the crystallite size and unit cell parameters of collected patterns. The TEM observations were carried out using a transmission electron microscope JEM 2100 (JEOL, Tokio, Japan) with GATAN Orius 832 SC1000 CCD camera (AMETEK, Berwin, Pennsylvania, USA) at an accelerating voltage of 200 kV. The specimen for TEM investigation was prepared by grinding the sample in an agate mortar and then disintegrating it in the form of ethanol suspension by ultrasonic treatment for 6 min. A droplet of the suspension was coated on standard carbon film on a Cu grid. The size distribution of the particles was performed with the image-processing program ImageJ, and the measurements of the interplanar distances were performed with the specialized software Digital Micrograph (Version 2.31.734). Photoluminescence (PL) excitation and emission spectra at room temperature for all studied samples were measured with a FluoroLog3-22 spectrofluorometer, 2014 (Horiba Jobin-Yvon, Longjumeau, France).

4. Conclusions

Glass-ceramic materials with enhanced photoluminescence emission were obtained by controlled crystallization of the 52WO₃:22B₂O₃:26La₂O₃:0.5Eu₂O₃ glass sample. Structural characterizations, i.e., TEM and XRD, verify the crystalline nature of the resulting glass-ceramics. X-ray studies show that during the heat treatment of the glass the main crystalline phases are La₂W₂O₉ cubic phase and orthorhombic La₂WO₆. Morphological studies revealed that the sample contains spherical and rectangular nanosized particles.. The majority of the particles are in the range of 20–40 nm. Photoluminescence spectra revealed that the obtained glass-ceramics can emit red light under excitation of 392 nm originating from the dominant dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ ions. Based on the results from the emission spectra, it was established that the formation of La₂W₂O₉:Eu³⁺ nanocrystals and the appropriate degree of crystallinity are decisive factors for improving the luminescence properties of the samples. Formation of second crystalline phase - La₂WO₆ leads to a drop in the emission intensity, because of the redistribution of Eu³⁺ ions in different crystalline phases. The optical properties confirm that the obtained glass-ceramics are suitable hosts for the incorporation of Eu³⁺ ions and have potential applications in the field of red light-emitting diodes.