The effect of Eu 3+ ion on photoluminescence and scintillator properties in QS-P 2 O 5 -CaO-BaO-Gd 2 O 3 glasses

1 Introduction

Scintillators are functional materials capable of converting high-energy radiation, such as X-rays or g-rays, into detectable visible or near-visible photons. These materials play a critical role in medical imaging, radiation monitoring, and high-energy physics instrumentation (Liu et al., 2023; Zhang C. et al., 2025). The performance of scintillators is primarily determined by several key parameters, including high light yield, fast decay time, good thermal stability, and chemical durability (Ronda, 2024; Wang et al., 2023; Wibowo et al., 2023). Among the various classes of scintillator materials, glass-based systems have attracted increasing attention due to their advantages in large-scale fabrication, compositional flexibility, optical homogeneity, and ease of incorporating luminescent activators (Han et al., 2022; Kohli et al., 2022; Lu et al., 2024; Mader et al., 2021). In particular, rare-earth-ion-doped glasses exhibit excellent optical properties owing to their well-defined intra-4f electronic transitions, which are only weakly influenced by the host matrix (Galleani et al., 2024; Li et al., 2021; Payungkulanan et al., 2024).

Europium (Eu³⁺) ions are widely recognized as efficient luminescent centers due to their intense red emission originating from the ⁵D₀ →⁷F₂ transition, long radiative lifetime, and high quantum efficiency (Wantana et al., 2020). Eu³⁺-doped phosphate glasses have been extensively studied for applications in solid-state lasers, light-emitting diodes (LEDs), and radiation detection systems, owing to their high transparency, low phonon energy, and excellent rare-earth solubility (Kaur et al., 2024; Rajagukguk et al., 2022; Wang et al., 2019). However, pure phosphate glasses tend to exhibit hygroscopic behavior and relatively low chemical durability, which can limit their practical applications (Tomáš and Aleš, 2025). To overcome these limitations, suitable network modifiers and intermediate oxides are incorporated into the glass matrix (Raju et al., 2021). In this study, P₂O₅ acts as the primary glass former, providing a highly transparent and rare-earth-friendly network structure (Khrongchaiyaphum et al., 2023; Meejitpaisan et al., 2025; Rajaramakrishna et al., 2025). The incorporation of CaO serves to improve the chemical durability and thermal stability of the glass while also modifying the phosphate network to enhance energy transfer processes. BaO, as a heavy metal oxide, contributes to increased density and effective atomic number, thereby improving radiation absorption capability, which is crucial for scintillation applications (Wantana et al., 2018). Furthermore, Gd₂O₃ plays a dual role as both a structural modifier and a sensitizer. Gd³⁺ ions possess a high neutron and X-ray absorption cross-section and exhibit strong UV emission (notably around 311 nm), making them highly effective in transferring excitation energy to Eu³⁺ ions. This Gd³⁺→Eu³⁺ energy transfer mechanism is essential for enhancing luminescence efficiency under high-energy excitation conditions (Jiménez and Thomas, 2024; Khrongchaiyaphum et al., 2022; Sadovnikova et al., 2022; Zhang et al., 2022). An important novelty of this work lies in the utilization of locally sourced quartz sand (QS) from Huta Ginjang, Indonesia, as a substitute for conventional high-purity SiO₂. This quartz sand exhibits a high silica content (∼99.08%) after appropriate processing, making it a viable and sustainable raw material for glass fabrication (Rajagukguk et al., 2025). The use of QS not only reduces production costs but also promotes the valorization of abundant local natural resources. In addition, the incorporation of QS contributes to the formation of a stable silicate–phosphate hybrid network, which can improve mechanical strength and chemical resistance while maintaining desirable optical properties.

Based on these considerations, this study aims to systematically investigate the effect of Eu³⁺ concentration (0–2.0 mol%) on the photoluminescence and scintillation properties of QS-P₂O₅-CaO-BaO-Gd₂O₃ glasses (POCaGE system). The glasses were synthesized using the melt-quenching technique to ensure homogeneous amorphous structures with uniform dopant distribution. Particular emphasis is placed on understanding absorption characteristics, excitation-emission behavior, Gd³⁺→Eu³⁺ energy transfer mechanisms, radiative properties, chromaticity performance, and X-ray induced luminescence.

This work provides both fundamental insights into rare-earth-doped phosphate glass systems and a practical pathway toward the development of cost-effective, high-performance scintillator materials based on locally available resources.

2 Methods

The Huta Ginjang quartz sand (QS) was first immersed in water for 24 h and cleaned with detergent to minimize impurities, followed by drying in direct sunlight for 2 days. The quartz sand was ground in a ball mill for 4 h. The compound content of quartz sand was analyzed using XRF MiniPal 4, PANalytical and it was confirmed that the silica oxide content was >98.65% and several other elements were confirmed like K₂O (0.83%), CaO (0.47%), Cr₂O₃ (0.02%), Fe₂O₃(0.03%). Glass composition based on quartz sand (QS) is determined following chemical formula (15-x) QS-50P₂O₅-15CaO-10BaO-10Gd₂O₃-xEu₂O₃ (with x = 0; 0.5; 1.0; 1.5 and 2.0 mol%). Reagent-grade NH₄H₂PO₄, Gd₂O₃, CaCO₃ and BaCO₃ (99.9% purity) were used as raw materials. Using the melt quenching technique, 20 g were mixed homogeneously in an alumina crucible, then stored in a container containing silica gel to reduce the water content. It was then placed in a furnace at 1,200 °C for 3 h, then cast into preheated graphite molds. The resulting glasses were subsequently annealed at 500 °C for 3 h to relieve thermal stresses, followed by slow cooling to room temperature. The glass was then cut into dimensions of 1 cm × 1.5 cm × 0.3 cm. The melt quenching technique was used for glass sample synthesis, which is a process of melting a homogeneous mixture of chemicals at a certain temperature (Meejitpaisan et al., 2024; Sarumaha et al., 2022). The glass density was measured using four-digit microbalance (AND, HR 200) with weight in air and distilled water. Refractive indices of glass were measured by Abbe refractometer (Atago, DR-M2 model) at 589 nm). Optical absorption spectra were obtained with a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer in the 240–2500 nm range with a step size of 1 nm. Photoluminescence (PL) studies, including excitation, emission, and lifetime measurements, were performed using an Agilent Cary Eclipse spectrofluorometer equipped with a xenon lamp. PL quantum yield (PLQY) values were determined with a Horiba FluoroMax-4+ spectrofluorometer coupled to an integrating sphere (integration time: 0.3 s; excitation/emission slit: 1 nm; emission range: 400–800 nm). For scintillation studies, X-ray induced luminescence spectra were recorded using a Cu-target X-ray generator (50 kV, 30 mA), with the emission collected via an optical fiber coupled to a spectrometer.

3 Results and discussion

3.1 Prepared glass sample

The physical appearance of the prepared glass samples is a critical indicator of their structural quality and suitability for optical applications. The POCaGE glass series was successfully fabricated with varying Eu₂O₃ concentrations (x = 0.0, 0.5, 1.0, 1.5, and 2.0 mol%), denoted as POCaGE_0.0, POCaGE_0.5, POCaGE_1.0, POCaGE_1.5, and POCaGE_2.0 respectively. As presented in Figure 1, all samples exhibit excellent transparency and smooth surfaces without any visible cracks, bubbles, or phase separation, indicating the formation of a homogeneous amorphous structure. The high optical clarity of the glasses confirms the effectiveness of the melt-quenching process in producing uniform glass matrices with well-dispersed dopant ions. Furthermore, a slight coloration difference is observed when the Eu³⁺ concentration increases to 2.0 mol%. The same conditions occurs for the glasses samples before and after cutting and polishing as shown in Figures 1a,b, which can be attributed to the characteristic absorption of Eu³⁺ ions in the visible region. This gradual change in appearance suggests successful incorporation of Eu³⁺ ions into the glass network without compromising transparency.

3.2 Absorption spectra and optical bandgap

The absorption spectra of POCaGE glasses doped with Eu³⁺ ions are presented in Figure 2. Several characteristic absorption bands corresponding to electronic transitions from the Eu³⁺ ground state were observed. Six distinct absorption peaks appear at approximately 394 nm, 413 nm, 465 nm, 534 nm, 2092 nm, and 2214 nm, which can be attributed to the transitions ⁷F₀ →⁵L_6, ⁷F₀ →⁵D_3, ⁷F₀ →⁵D₂, ⁷F₁ → ⁵D₁, ⁷F₀→⁷F₆, ⁷F₁ →⁷F₆ respectively. Among these transitions, the band centered at 394 nm (⁷F₀→⁵L₆) exhibits the most intense and sharp absorption peak in the ultraviolet region. This transition is partially allowed according to the ΔJ selection rule and is commonly employed as an efficient excitation wavelength for recording the emission spectra of Eu³⁺-activated materials (Rajagukguk et al., 2016; Sreedhar et al., 2019). The absorption band associated with the ⁷F₀ →⁵D₂ transition at 465 nm is stronger than the band at 534 nm (⁷F₁ → ⁵D₁) because the former corresponds to an induced electric dipole transition with hypersensitive character, whereas the latter is mainly attributed to a magnetic dipole transition, which typically exhibits lower intensity. In contrast, the transitions belonging to the to ⁷F₀→⁵D_J group are relatively weaker due to their spin-forbidden nature; however, weak intensity is still observed as a result of J-mixing induced by the crystal field environment in the glass matrix (Sarumaha et al., 2023a; Starzyk et al., 2022). Additionally, strong absorption bands are observed in the near-infrared region at 2092 nm (⁷F₀→⁷F₆) and 2214 nm (⁷F₁ →⁷F₆). It is also evident that the overall absorbance of the POCaGE glass system increases with increasing Eu³⁺ concentration, indicating the progressive incorporation of Eu³⁺ ions into the glass network and the enhancement of optical transition probability (Jiménez and Sendova, 2023; Luewarasirikul et al., 2023a).

The absorption bands corresponding to the ⁷F₀→⁵L₆ (394 nm) and ⁷F₀ →⁵D₃ (413 nm) transitions are relatively weak and partially overlapped with the intense absorption edge in the ultraviolet region; therefore, they are not clearly distinguishable in the main spectrum. To highlight these transitions, an inset figure is included in Figure 2, which enlarges the spectral region around 380–420 nm. The inset clearly reveals the two absorption features at 394 nm and 413 nm, corresponding to the ⁷F₀→⁵L₆ and ⁷F₀ →⁵D₃ transitions, respectively.

The integrated absorption intensities of the observed bands were calculated in order to determine the experimental oscillator strengths (f _exp) of Eu³⁺ ions in the POCaGE glass system. The oscillator strengths were obtained from the integrated area under the absorption bands using the standard expressions reported in Judd-Ofelt (J-O) theory (Sarumaha et al., 2023a). These values were subsequently compared with the calculated oscillator strengths (f _cal) derived from theoretical predictions based on the Judd-Ofelt intensity parameters. Table 1 summarizes the experimental and calculated oscillator strengths corresponding to the observed transitions of Eu³⁺ ions in the (15-x)QS-50P₂O₅-15CaO-10BaO-10Gd₂O₃-xEu₂O₃ glass system (POCaGE_x, where x = 0.5–2.0 mol%). The absorption transitions observed in the spectra correspond to the characteristic Eu³⁺ transitions from the ground multiplet levels ⁷F₀ and ⁷F₁ to higher excited states such as ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁, and ⁷F₆.

TABLE 1. Energy level, experimental (f _exp x 10^–6) and calculated (f _cal x 10^–6) oscillator strength of POCaGE glasses.

Among these transitions, the ⁷F₀ → ⁷F₆ transition (2092 nm) exhibits the largest oscillator strength, indicating a relatively strong absorption probability for this transition in the POCaGE glass host. The higher oscillator strength observed for this band suggests that the transition probability is significantly influenced by the local environment surrounding the Eu³⁺ ions within the glass network (Rajagukguk et al., 2022; Wantana et al., 2019). A comparison between f _exp and f _cal shows reasonable agreement for all investigated samples, indicating that the experimental data are well described by the Judd–Ofelt theoretical model. The reliability of the fitting procedure can be evaluated from the root mean square deviation (δ_rms) values listed at the bottom of Table 1. The obtained δ_rms values (0.406, 0.249, 0.191, and 0.256 for POCaGE_0.5, POCaGE_1.0, POCaGE_1.5, and POCaGE_2.0 respectively) are relatively small, confirming the good correlation between experimental and theoretical oscillator strengths. The lowest deviation observed for the POCaGE_1.5 sample indicates the best fitting accuracy among the studied compositions.

The Judd–Ofelt intensity parameters (Ω₂, Ω₄, and Ω₆) were determined using a least-squares fitting method applied to the experimental and calculated oscillator strengths obtained from the absorption spectra. These parameters are fundamental spectroscopic quantities that describe the electric dipole transition probabilities of rare-earth ions in different host materials (Lalla et al., 2020). The obtained J-O parameters for the POCaGE glasses are listed in Table 2, together with data reported for similar Eu³⁺-doped glass systems from the literature for comparison. The Ω₂ parameter is generally associated with the local symmetry and covalency of the rare-earth ion environment, while Ω₄ and Ω₆ are more sensitive to long-range effects such as the rigidity, viscosity, and structural characteristics of the glass network (Herrmann et al., 2023). For all investigated compositions, the Ω₂ parameter is larger than Ω₄ and Ω₆, indicating that the Eu³⁺ ions in the POCaGE glass matrix occupy highly asymmetric coordination sites (Jha and Jayasimhadri, 2017). A relatively large Ω₂ value also suggests a significant degree of covalent interaction between Eu³⁺ ions and surrounding oxygen ligands (Eu³⁺-O^2- bonds) within the glass structure. Such asymmetry is typical for rare-earth ions embedded in amorphous glass networks (Thanyaphirak et al., 2025). The variation of Judd–Ofelt parameters with Eu³⁺ concentration reveals two distinct trends. For the POCaGE_1.0 and POCaGE_2.0 samples, the relationship between the parameters follows the order Ω₂>Ω₄>Ω₆ whereas for POCaGE_0.5, and POCaGE_1.5 the trend become Ω₂> Ω₆ > Ω₄. These variations may be attributed to structural modifications in the glass network induced by different Eu³⁺ concentrations, which can influence the local ligand field and coordination environment around the Eu³⁺ ions. The dominance of Ω₂ over the other parameters further supports the conclusion that asymmetry and covalent bonding characteristics play a significant role in determining the optical properties of the POCaGE glasses (Sarumaha et al., 2023b).

TABLE 2. Comparison of the J-O parameters Ω_λ (λ = 2, 4 and 6) x 10^–20 of POCaGE glasses.

As shown in Table 2, the Judd-Ofelt parameters obtained for the POCaGE glass system are comparable to those reported for other Eu³⁺-doped glass matrices such as WGB glass (Wantana et al., 2020),CaGdSiB glass (Wantana et al., 2018), LANaKP1.0 glass (Sarumaha et al., 2023a), MGBEu glass (Rajaramakrishna et al., 2020) and ZFPEu glass (Vijaya and Jayasankar, 2013). Although the absolute values vary due to differences in glass composition and structural environments, the general trend Ω₂ > Ω₄ > Ω₆ observed in many Eu³⁺-activated glass systems is also consistent with the present results. This agreement suggests that the phosphate-germanate-silicate glass network provides a suitable host environment for Eu³⁺ ions, enabling efficient electric dipole transitions and favorable spectroscopic properties for potential photonic and luminescent applications (Rajaramakrishna et al., 2025).

3.3 Optical bandgap

The optical bandgap was calculated to assess the level of disorder in the absorption coefficient. For glass materials, it is more appropriate to calculate the indirect bandgap, as electron transitions in amorphous materials typically do not happen directly due to structural defects or disorders (Al-Hadeethi et al., 2019). In addition, the calculation of the optical band gap in this study has been described in previous literature (Judd, 1962; Deopa et al., 2018). The optical absorption coefficient α (ν) was determined from the measure absorbance (A) using the equation:

α ν = 1 d ln I 0 I

Where I _*0* and I represent the intensities of the incident and transmitted light, respectively, and d is the thickness of the glass sample. The absorption coefficient obtained from this expression provides important information about the interaction between photons and the electronic structure of the glass material. The optical band gap energy was estimated using the Tauc relation, which describes the dependence of the absorption coefficient on the photon energy:

E o p t = h v − h v B 1 / 2

This parameter is crucial for understanding the photon absorption mechanism, electronic transitions, and the influence of structural defects within the glass matrix. Furthermore, the optical band gap provides insight into charge carrier transport properties, electronic band structure, and the role of dopant ions in modifying the glass network (Allaham et al., 2024). Figure 3 illustrates the indirect optical band gap and summarizes the results in Table 3 for the POCaGE sample with different Eu³⁺ concentrations.

TABLE 3. Indirect optical band gaps energies of POCaGE glasses with varying Eu³⁺ concentrations.

The results reveal that the indirect optical band gap decreases progressively with increasing Eu³⁺ concentration, ranging from approximately 3.75 eV–2.67 eV for the investigated compositions. The reduction in band gap energy with increasing Eu³⁺ content can be attributed to structural modifications induced by the incorporation of rare-earth ions into the glass network (Rezgui and Soltani, 2023). The presence of Eu³⁺ ions may introduce additional localized energy states within the band structure, which facilitates electronic transitions and consequently narrows the effective band gap. Moreover, the substitution of Eu³⁺ ions into the glass matrix can alter the bonding configuration and structural connectivity of the phosphate-based glass network. The incorporation of rare-earth ions typically leads to the formation of non-bridging oxygen (NBO) sites, which modify the electronic structure by increasing the density of localized states near the band edges. As the concentration of Eu³⁺ increases, these defect-related states become more prominent, thereby reducing the energy required for electronic excitation from the valence band to the conduction band (Padalia et al., 2024). Additionally, the presence of heavy rare-earth ions such as Eu³⁺ may enhance the polarizability of the glass network, which also contributes to the narrowing of the band gap. This behavior is commonly observed in rare-earth-doped oxide glasses and reflects the strong interaction between dopant ions and the host matrix (Mariselvam et al., 2019).

3.4 Luminescence properties

Figure 4 presents the excitation spectra of POCaGE monitored at the characteristic Eu³⁺ emission wavelength of 613 nm, over the spectral range of 190–550 nm. The spectra exhibit several well resolved excitation bands arising from both Eu³⁺ and Gd³⁺ ions, together with a broad high-intensity band in the deep-ultraviolet region. The broad excitation band centered at approximately 225 nm is assigned to the charge transfer (CT) band associated with the O^2- → Eu³⁺ transition (Khan et al., 2024; Xie et al., 2024). This band originates from electron transfer between oxygen ligands and Eu³⁺ ions and is commonly observed in Eu³⁺-activated oxide glasses. Its strong intensity indicates efficient host-to-activator excitation in the ultraviolet region (Pier and Jüstel, 2025; Xiong et al., 2023).

In addition to the CT band, a series of narrow excitation lines characteristic of 4f–4f transitions of Eu³⁺ are observed at 287, 298, 320, 362, 382, 394, 414, 464, 525, and 533 nm, corresponding to the transitions ⁷F₀→⁵(I,⁵H)₆, ⁷F₀→⁷F₄, ⁷F₀→⁵H₄, ⁷F₀→⁵D₄, ⁷F₀→⁵L₇+⁵G₂, ⁷F₀→⁵L₆, ⁷F₁→⁵D₃, ⁷F₀→⁵D₂, ⁷F₀→⁵D₁, ⁷F₁→⁵D₀, respectively. Among these transitions, the excitation band at 394 nm, assigned to the ⁷F₀→⁵L₆ transition of Eu³⁺, is the most intense sharp line in the near UV region. This result confirms that 394 nm is the most efficient wavelength for direct excitation of Eu³⁺ ions in the present glass system, which is highly favorable for near-UV LED pumping applications. The strong intensity of this band is also consistent with the absorption results, where the ⁷F₀→⁵L₆ transition was identified as one of the dominant excitation channels.

Two excitation bands associated with Gd³⁺ are also observed at 275 nm and 311 nm, corresponding to the ⁸S_7/2→⁶I_9/2 and ⁸S_7/2→⁶P_7/2 transitions, respectively. The simultaneous presence of Gd³⁺ and Eu³⁺ excitation features suggests that both ions actively participate in the excitation process. In particular, the existence of intense Gd³⁺ excitation bands together with Eu³⁺ emission monitored at 613 nm indicates the possibility of energy transfer from Gd³⁺ to Eu³⁺ (Monika et al., 2023).

The inset of Figure 4 further compares the evolution of the excitation intensities at 275 nm and 394 nm as a function of Eu³⁺ concentration. The intensity of the 394 nm Eu³⁺ excitation band increases progressively with increasing Eu³⁺ content, reflecting the higher number of optically active Eu³⁺ luminescence centers in the glass matrix. In contrast, the 275 nm Gd³⁺-related excitation band exhibits a non-monotonic variation, indicating that the Gd³⁺-assisted excitation pathway is influenced by the relative Gd³⁺/Eu³⁺ population and by the efficiency of interionic energy transfer (Sarumaha et al., 2023b). Overall, the excitation spectra demonstrate that both direct Eu³⁺ excitation and indirect Gd³⁺mediated sensitization contribute to the luminescence behavior of the POCaGE glasses.

The emission spectra of POCaGE glasses excited at 275 nm are shown in Figure 5. Under this excitation wavelength, both Gd³⁺ and Eu³⁺ emissions are detected, confirming that the excitation energy can be absorbed by Gd³⁺ ions and subsequently transferred, at least in part, to Eu³⁺ ions. A strong emission band appears at 311 nm, which is assigned to the ⁶P_7/2 → ⁸S_7/2 transition of Gd³⁺. This ultraviolet emission is particularly intense in the Eu-free or low-Eu samples and decreases significantly with increasing Eu³⁺ concentration. Such behavior strongly suggests that Gd³⁺ acts as a sensitizer, transferring a portion of its absorbed energy to neighboring Eu³⁺ ions (Khan et al., 2019). In addition, a weak band near 623 nm is attributed to the ⁶P_7/2 → ⁸P_7/2 transition of Gd³⁺.

In the visible region, several characteristic emission bands of Eu³⁺ are observed at 578, 592, 613, 652, and 699 nm, corresponding to the ⁵D₀ →⁷F_0, ⁵D₀ →⁷F_1, ⁵D₀ →⁷F_2, ⁵D₀ →⁷F₃, and ⁵D₀ →⁷F₄ transitions, respectively. Among the Eu³⁺ emissions, the ⁵D₀ →⁷F₂ transition at 613 nm is the dominant band, producing the characteristic red emission of Eu³⁺. The predominance of this electric-dipole transition over the ⁵D₀ →⁷F₁ magnetic-dipole transition indicates that Eu³⁺ ions occupy non-centrosymmetric sites in the glass network, in good agreement with the Judd–Ofelt analysis, particularly the relatively large Ω₂ values obtained for the POCaGE glasses (Vu et al., 2025). The progressive decrease in the Gd³⁺ emission intensity at 311 nm with increasing Eu³⁺ concentration, together with the appearance and enhancement of Eu³⁺ red emission, provides strong evidence for Gd³⁺ → Eu³⁺ energy transfer. Upon 275 nm excitation, Gd³⁺ ions are initially excited and then transfer energy non-radiatively to Eu³⁺ ions, which subsequently relax radiatively through the ⁵D₀ →⁷F_J transitions (Priya et al., 2023; Priyanka et al., 2020). This sensitization mechanism is beneficial because it broadens the accessible excitation window and enhances Eu³⁺ luminescence under deep-UV pumping (Priya et al., 2021).

However, the Eu³⁺ emission intensity under 275 nm excitation does not increase indefinitely with Eu³⁺ concentration. After reaching an optimum at low or intermediate Eu³⁺ content, the visible emission tends to decrease at higher concentrations, which may be attributed to concentration quenching arising from increased Eu³⁺–Eu³⁺ interactions and non-radiative energy migration. Therefore, under indirect excitation via Gd³⁺, the luminescence performance reflects a balance between sensitization efficiency and concentration-dependent quenching processes.

Figure 6 shows the emission spectra of POCaGE glasses under direct excitation at 394 nm, corresponding to the ⁷F₀→⁵L₆ excitation transition of Eu³⁺. In contrast to the 275 nm excitation case, the spectra obtained under 394 nm excitation exhibit only the characteristic Eu³⁺ emission bands in the 550–725 nm region, confirming that the luminescence is dominated by direct excitation of Eu³⁺ ions. The observed emission bands at 578, 592, 613, 652, and 699 nm are assigned to the transitions ⁵D₀ →⁷F_0, ⁵D₀ →⁷F_1, ⁵D₀ →⁷F_2, ⁵D₀ →⁷F₃, and ⁵D₀ →⁷F₄, respectively. Among these bands, the ⁵D₀ →⁷F₂ transition at 613 nm is the most intense for all compositions, producing a strong red emission characteristic of Eu³⁺-activated materials. Since the ⁵D₀ →⁷F₂ transition is an electric-dipole transition hypersensitive to the local symmetry, its dominance indicates that Eu³⁺ ions are located in highly asymmetric coordination environments in the POCaGE glass matrix (Sarumaha et al., 2023a). By contrast, the ⁵D₀ →⁷F₁ band at 592 nm, which is mainly magnetic-dipole in nature and less sensitive to local symmetry, remains weaker than the 613 nm emission. This intensity relationship further supports the conclusion that the local environment around Eu³⁺ ions lacks inversion symmetry (Li et al., 2025). A significant feature of Figure 6 is that the Eu³⁺ emission intensity increases systematically with increasing Eu³⁺ concentration, and the POCaGE_2.0 sample exhibits the strongest red emission, which are similar with previous reported by Rajagukguk et al. (2016). This trend indicates that direct excitation at 394 nm effectively activates a greater number of Eu³⁺ luminescence centers as the Eu³⁺ content increases. Within the investigated concentration range, no severe concentration quenching is observed under direct Eu³⁺ excitation, suggesting that the POCaGE host can accommodate Eu³⁺ ions efficiently while maintaining strong radiative emission (Kaewkhao et al., 2015; Zaman et al., 2018).

3.5 PLQY and lifetime analysis

Figure 7 shows the luminescence decay curves of the POCaGE glasses recorded at l_exc = 275 nm and l_em = 311 nm corresponding to the Gd³⁺ emission channel. The decay profiles can be satisfactorily described by a single-exponential function:

I t = I 0 exp t τ R

Where I 0 is the initial intensity at t = 0 dan τ is the decay lifetime. The single-exponential nature of the decay indicates that the emitting Gd³⁺ centers experience a relatively uniform deactivation pathway, without strong evidence of multiple independent decay components in the analyzed time window (Anni, 2023). The Photoluminescence Quantum Yield (PLQY) values of the Eu³⁺-doped POCaGE glasses measured under 275 nm and 394 nm excitations are listed in Table 4. The measured lifetimes decrease systematically with increasing Eu³⁺ concentration, from 4.154 m for POCaGE_0.0–2.394 m for POCaGE_0.5, 1.617 m for POCaGE_1.0, 1.126 m for POCaGE_1.5, and 0.845 m for POCaGE_2.0. This progressive shortening of the Gd³⁺ lifetime clearly indicates the introduction of an additional non-radiative depopulation pathway after Eu³⁺ incorporation. Since the monitored emission corresponds to Gd³⁺, the decrease in lifetime is strong evidence that the excitation energy initially absorbed by Gd³⁺ ions is increasingly transferred to Eu³⁺ ions as the Eu³⁺ concentration rises. Thus, the lifetime results confirm the occurrence of sensitizer-to-activator energy transfer, namely, Gd³⁺ → Eu³⁺, in the POCaGE glass system (Bisht et al., 2022; Rao et al., 2020).

TABLE 4. PLQY and Energy transfer efficiency of POCAGE glasses.

The lifetime values of Eu³⁺ emission monitored at 613 nm under 394 nm excitation remain nearly constant, varying only slightly from 2.269 to 2.342 m across the doped samples. This weak variation suggests that the radiative decay of the Eu^{3+ 5}D₀ level is only marginally affected by Eu³⁺ concentration in the investigated range (Manasa and Jayasankar, 2016). In other words, severe Eu³⁺-Eu³⁺ concentration quenching is not dominant under direct Eu³⁺ excitation, which is consistent with the relatively stable PLQY values. Therefore, while increasing Eu³⁺ concentration strongly enhances Gd³⁺→Eu³⁺ energy transfer, it does not significantly shorten the ⁵D₀ lifetime of Eu³⁺ under direct excitation, indicating that the Eu³⁺ emission centers remain reasonably efficient within the studied composition range (Rachniyom et al., 2025; Xu et al., 2026).

Taken together, the PLQY, decay, and transfer-efficiency results demonstrate that the POCaGE glasses possess a favorable luminescence mechanism in which Gd³⁺ ions serve as sensitizers and Eu³⁺ ions act as activators. The progressive decrease in Gd³⁺ lifetime and the corresponding increase in transfer efficiency provide clear evidence of efficient Gd³⁺→Eu³⁺ energy transfer, while the relatively high PLQY values under 394 nm excitation confirm that the Eu³⁺ centers can emit efficiently once populated (Zhang Y. et al., 2025).

3.6 Radiative properties

The radiative properties of Eu³⁺ ions in the POCaGE glasses were evaluated using the Judd–Ofelt formalism and the parameters are presented in Table 5. The calculation of important spectroscopic parameters such as radiative transition probability (A_R), branching ratio (β_R), stimulated emission cross section (σ_e), and radiative lifetime. These parameters provide quantitative insight into the radiative behavior of the ⁵D₀ → ⁷F_J transitions of Eu³⁺ ions, which determine the emission efficiency and potential photonic applications of the material (Sarumaha et al., 2022). The peak emission wavelengths listed in Table 5 correspond to the characteristic Eu³⁺ emission transitions originating from the ⁵D₀ excited level. The observed emission peaks at 578, 592, 613, 652, and 700 nm are assigned to the transitions ⁵D₀ →⁷F_0, ⁵D₀ →⁷F_1, ⁵D₀ →⁷F_2, ⁵D₀ →⁷F₃, and ⁵D₀ →⁷F₄, respectively. Among these transitions, the ⁵D₀ → ⁷F₂ transition at 613 nm consistently exhibits the largest radiative parameters across all investigated compositions. This transition shows the highest radiative transition probability (A_R), larger branching ratio (β_R). For example, the A_R value for POCaGE_0.5 reaches 164.57 s^-1, which is significantly higher than the radiative probabilities associated with the other transitions. Correspondingly, the experimental branching ratios b_exp for the ⁵D₀ → ⁷F₂ transition exceed 0.70, indicating that more than 70% of the total radiative decay from the ⁵D₀ level occurs through this transition. This behavior is characteristic of Eu³⁺ luminescence in oxide glasses, where the electric-dipole hypersensitive transition ⁵D₀ → ⁷F₂ dominates the emission spectrum when Eu³⁺ ions occupy non-centrosymmetric coordination sites (Zaman et al., 2018).

TABLE 5. Radiative properties such as peak emission wavelength (λp), effective bandwidth (Δλ_eff), stimulated emission cross section (σ_e (λp) x 10^–22), branching ratio (β_R) and radiative transition probability (A_R) of POCaGE glass.

The bold values in Table are meanings to destination of spin-orbit levels.

The effective emission bandwidth (Δλ_eff) provides information about the spectral broadening of each emission band. As shown in Table 5, the (Δλ_eff) values for the ⁵D₀ → ⁷F₂ transition lie in the range of 11.55–12.63 nm, which is slightly broader than the bandwidths observed for several other Eu³⁺ transitions. Such moderate bandwidth values are typical for Eu³⁺ emission in glass hosts due to the structural disorder inherent to amorphous materials. The stimulated emission cross section (σ_e) is another important parameter that characterizes the potential of a material for optical amplification and laser applications. According to Table 5, the σ_e values corresponding to the ⁵D₀ → ⁷F₂ transition are 11.1 x 10⁻²², 2.36 x 10⁻²², 2.02 x 10⁻²², and 2.24 x 10⁻²² cm² for the POCaGE_0.5, POCaGE_1.0, POCaGE_1.5, and POCaGE_2.0 samples, respectively. Among these compositions, POCaGE_0.5 exhibits the highest emission cross-section value. This relatively large σ_e value suggests that the emission probability of the red transition is strongest at this Eu³⁺ concentration, which is consistent with the relatively high radiative transition probability observed for the same composition (Mariselvam et al., 2019).

A comparison of the radiative lifetimes (τ_R) calculated from the J-O analysis and the experimental lifetimes is also presented in Table 5. The calculated radiative lifetimes range from 4.501 to 11.273 m, whereas the experimentally measured lifetimes are approximately 2.26–2.34 m. The shorter experimental lifetimes compared with the theoretical radiative lifetimes indicate that non-radiative relaxation processes are present in the glass host, which is expected in oxide glasses due to multiphonon relaxation and structural disorder (Selvaraj et al., 2026). Nevertheless, the experimental lifetimes remain relatively consistent across the different Eu³⁺ concentrations, suggesting that severe concentration quenching of Eu³⁺ emission does not occur within the investigated compositional range.

A focused comparison of the radiative parameters for the ⁵D₀ → ⁷F₂ transition is presented in Table 6. The radiative transition probabilities A_R and emission cross-sections σ_e obtained for the present POCaGE glasses are compared with values reported for other Eu³⁺-doped glass systems. The obtained values fall within the range reported for several oxide glass hosts in the literature, confirming that the radiative characteristics of Eu³⁺ ions in the present glass matrix are consistent with established spectroscopic behavior. Differences in absolute values can arise from variations in glass composition, local ligand field strength, and the covalency of Eu–O bonding, which influence the electric-dipole transition probabilities. Overall, the radiative property analysis demonstrates that the ⁵D₀ → ⁷F₂ transition is the dominant radiative channel in the POCaGE glass system, exhibiting the highest radiative transition probability, branching ratio, and stimulated emission cross section. These results are consistent with the excitation and emission spectra presented earlier and confirm that the POCaGE glasses possess favorable spectroscopic characteristics for red-emitting photonic materials.

TABLE 6. Radiative transition probabilities A_R (s⁻¹), stimulated emission cross-sections σ_e (λp x 10^–22) and quantum efficiency η (%) of POCaGE glasses for ⁵D₀→⁷F₂ transition.

3.7 The chromaticity of CIE (commission internationale de l'Éclairage)

The chromaticity characteristics of the POCaGE glasses were evaluated using the CIE 1931 color space, and the corresponding coordinates are presented in Figure 8 and Table 7. The chromaticity coordinates (x; y) were calculated from the emission spectra by integrating the spectral power distribution with the standard CIE color matching functions, which provides an accurate representation of the perceived emission color (Shaw and Fairchild, 2002) As shown in Table 7, the chromaticity coordinates for all Eu³⁺ doped POCaGE glass are located at approximately x ≈ 0.649–0.650; y ≈ 0.348. These values place the emission firmly in the red region of the CIE diagram, as illustrated in Figure 8. The minimal variation in chromaticity coordinates with increasing Eu³⁺ concentration indicates that the spectral shape of the emission remains essentially unchanged, and only the emission intensity varies (Zhao et al., 2024). This behavior is consistent with the dominance of the ⁵D₀ → ⁷F₂ transition at 613 nm, which governs the overall emission profile. The color purity values, also listed in Table 7, are remarkably high, ranging from 94.01% to 94.28%. Such high color purity indicates that the emission is highly saturated and concentrated within a narrow spectral region. This observation is consistent with the relatively sharp emission band of Eu³⁺ ions and confirms that the POCaGE glasses produce highly pure red emission.

TABLE 7. CIE, CCT and color purity values of POCaGE glasses.

The correlated color temperature (CCT) values lie in the range of 2594–2618 K, which corresponds to warm light emission. According to standard colorimetric classification, CCT values below 4000 K indicate warm (reddish-yellow) emission, whereas higher values correspond to cooler (bluish) light (Wantana et al., 2020). The relatively low CCT values obtained in this study are consistent with the dominant red emission and further confirm the suitability of the material for warm-light applications such as display devices and solid-state lighting.

3.8 X-ray induced luminescence spectra

The X-ray induced luminescence (XIL) spectra of POCaGE glasses, along with the reference Bi₄Ge₃O₁₂ (BGO) crystal, are presented in Figure 9. Upon X-ray irradiation, high-energy photons are absorbed by the host matrix, generating electron–hole pairs that subsequently transfer their energy to luminescent centers, resulting in photon emission in the UV–visible region (Yodkantee et al., 2024). The XIL spectra exhibit emission bands associated with both Gd³⁺ and Eu³⁺ ions. A strong ultraviolet emission peak centered at 311 nm is attributed to the ⁶P_7/2 → ⁸S_7/2 transition of Gd³⁺, indicating that Gd³⁺ ions play a significant role in the initial energy absorption and relaxation processes under X-ray excitation (Khan et al., 2024). The coexistence of Gd³⁺ and Eu³⁺ emissions under X-ray excitation suggests that the scintillation process involves energy absorption by the host matrix followed by energy transfer to both rare-earth ions. The presence of a strong Gd³⁺ emission at 311 nm indicates that Gd³⁺ ions act as primary energy absorbers, while Eu³⁺ ions function as secondary emission centers (Luewarasirikul et al., 2023b). This behavior is consistent with the previously observed Gd³⁺ → Eu³⁺ energy transfer mechanism under optical excitation. In the visible region, characteristic Eu³⁺ emission bands are observed at 578, 592, 613, 652, and 699 nm, corresponding to the ⁵D₀ →⁷F_0, ⁵D₀ →⁷F_1, ⁵D₀ →⁷F_2, ⁵D₀ →⁷F₃, and ⁵D₀ →⁷F₄ transitions, respectively. The inset of Figure 9, which enlarges the spectral region from 560 to 720 nm, provides a clearer visualization of the Eu³⁺ emission bands and their relative intensities.

The inset further reveals that the relative spectral shape of the Eu³⁺ emission remains nearly unchanged with increasing Eu³⁺ concentration, while the emission intensity increases. This indicates that the local environment of Eu³⁺ ions remain spectroscopically similar across compositions, and the increase in intensity is primarily due to the higher number of luminescent centers rather than changes in the emission mechanism (Albert and Kennedy, 2025).

The dominance of the ⁵D₀ →⁷F₂ electric-dipole transition over the ⁵D₀ →⁷F₁ magnetic-dipole transition in the inset spectrum confirms that Eu³⁺ ions occupy non-centrosymmetric sites in the glass matrix. This observation is consistent with the Judd-Ofelt analysis (high Ω₂ values) and the photoluminescence results discussed earlier, providing strong internal consistency across different characterization techniques (Gökçe et al., 2017).

When compared with the BGO crystal, which exhibits a broad intrinsic emission band, the POCaGE glasses display narrow and well-defined emission lines characteristic of rare-earth ions. Although the overall emission intensity of the glasses is lower than that of BGO, the presence of sharp Eu³⁺ emission peaks, particularly the strong red emission at 613 nm, indicates that the POCaGE glasses are capable of converting X-ray energy into visible light with good spectral selectivity (Rajaramakrishna et al., 2023).

4 Conclusion

Eu³⁺ -doped QS-P₂O₅-CaO-BaO-Gd₂O₃ (POCaGE) glasses were successfully synthesized using the melt-quenching method with locally sourced quartz sand. The study systematically investigated the effect of Eu³⁺ concentration on the photoluminescence and scintillation-related properties of the glass system. Absorption spectra confirmed characteristic Eu³⁺ transitions, with dominant bands at 394 nm (⁷F₀ → ⁵L₆) and 2092 nm (⁷F₀ → ⁷F₆). Judd-Ofelt analysis yielded Ω₂ > Ω₄, Ω₆, indicating that Eu³⁺ ions occupy asymmetric and covalent environments. The ⁵D₀ → ⁷F₂ transition at 613 nm was identified as the dominant emission channel, exhibiting the highest radiative transition probability, branching ratio, and emission cross-section. Photoluminescence results revealed strong red emission at 613 nm, with 394 nm as the most efficient excitation wavelength. Under 275 nm excitation, Gd³⁺ acts as a sensitizer, enabling Gd³⁺ → Eu³⁺ energy transfer, supported by a significant decrease in Gd³⁺ lifetime and transfer efficiency reaching 80%. In contrast, Eu³⁺ lifetimes remained nearly constant, indicating minimal concentration quenching under direct excitation. The optical band gap decreased with increasing Eu³⁺ content, reflecting structural modification of the glass network. PLQY values peaked at intermediate Eu³⁺ concentrations, indicating an optimal balance between radiative emission and non-radiative losses. CIE analysis showed stable red emission with coordinates (x ≈ 0.65; y ≈ 0.348), high color purity (∼94%), and low CCT (∼2600 K). X-ray induced luminescence confirmed emissions from both Gd³⁺ (311 nm) and Eu³⁺ (613 nm), with Eu³⁺ red emission dominating. Eu³⁺ doping significantly enhances both photoluminescence and scintillation properties, making POCaGE glasses promising candidates for red-emitting photonic devices and radiation detection applications.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Footnotes

Edited by: Carlos DS Brites, University of Aveiro, Portugal

Reviewed by: Om Prakash Pandey, Thapar Institute of Engineering & Technology, India

M.Z.H. Mayzan, Universiti Tun Hussein Onn Malaysia, Malaysia