Structural, physical, and luminescence properties of sodium–aluminum–zinc borophosphate glass embedded with Nd³⁺ ions for optical applications

2.1 Samples preparation

2.1.2 Preparation of Nd³⁺-doped SAZBP glass

For Nd³⁺-doped SAZBP glass, Nd₂O₃ (99.99%) was introduced into the batch composition (typically 0–6 wt%) alongside the undoped precursors. The raw materials were homogenized and melted under identical conditions (1,300, 30 min). To ensure uniform Nd³⁺ distribution and avoid phase separation, the melt was stirred intermittently with a platinum rod. The quenching, annealing, and cooling steps followed the same protocol as for the undoped glass. The incorporation of Nd³⁺ was confirmed through optical absorption spectroscopy, with no observable precipitation or opacity in the final glass. As illustrated in Figure 1, the preparation workflow (raw material mixing → melting → quenching → annealing → final glass) applies to both undoped and Nd³⁺-doped variants, with the doping step explicitly integrated into the mixing stage.

Figure 1

Schematic illustration of the melt-quenching synthesis process for (60−x)P₂O₅–10B₂O₃–10Na₂O–5Al₂O₃–15ZnO–xNd₂O₃ glasses (x = 0–6 wt%).

The prepared glass samples were polished into disks with a 10 mm diameter and a thickness of approximately 2 mm for further optical property analysis. The melt-quenching technique is optimal for this study compared with other methods, because it guarantees high-purity, homogeneous, and amorphous Nd³⁺-doped glasses with tunable properties, directly supporting the investigation of structure-property relationships for photonic applications.

The melt-quenching technique is a reliable method for producing homogeneous, high-quality glass materials with tailored compositions for optical applications.

2.2 Characterization techniques

The Nd³⁺-doped SAZBP glasses were characterized using the following advanced techniques and instruments:

2.3 Theoretical modeling of structural and optical parameters

3.1 Analysis of density and molar volume

3.1.1 Physical properties

The incorporation of varying concentrations of Nd₂O₃ into the glass network (10% Na₂O–5% Al₂O₃–15% ZnO–10% B₂O₃–60% P₂O₅) leads to notable changes in both density and molar volume, as shown in Figure 2. Specifically, the density of the glass increases with rising Nd₂O₃ content. This suggests that by replacing P₂O₅ with the denser Nd₂O₃, the molecular weight of the oxide ions in the glass increases, resulting in the observed density increase [44]. There are some key recent studies supporting the replacement of P₂O₅ with Nd₂O₃ in glass/ceramic systems [45,46,47,48].

Figure 2

Density and molar volume of (60−x)P₂O₅–10B₂O₃–10Na₂O–5Al₂O₃–15ZnO–xNd₂O₃ (x = 0–6 wt%) glasses as a function of Nd₂O₃ concentration.

Similarly, the molar volume of the glass system also rises with increasing Nd₂O₃ content. This behavior, shown in Figure 2, can be attributed to the creation of additional non-bridging oxygen (NBO) atoms, which expand the glass network. Furthermore, the addition of Nd₂O₃ is believed to result in an extension of the glass network, thereby influencing its structural properties [49,50].

The observed simultaneous increase in density (ρ) and molar volume (Vₘ) with higher Nd³⁺ concentrations (Figure 2) arise from two competing effects.

3.2 XRD analysis

The obtained XRD patterns, as illustrated in Figure 3, display broad, diffuse humps centered around 2θ ≈ 30°, characteristic of the amorphous nature of the glass samples. Notably, the intensity of these humps increases with the addition of Nd₂O₃ concentrations, indicating a greater interaction of the dopant ions with the glass matrix. The absence of sharp crystalline peaks or secondary crystalline phases confirms the successful incorporation of Nd³⁺ ions into the glass network while maintaining structural homogeneity.

Figure 3

XRD patterns of (60−x)P₂O₅–10B₂O₃–10Na₂O–5Al₂O₃–15ZnO–xNd₂O₃ glasses with varying Nd₂O₃ concentrations (x = 0–6 wt%).

The structural homogeneity is critical for optical applications, as crystalline phases or secondary precipitates would introduce light scattering centers and degrade performance [65]. The lack of Nd-rich crystalline phases (e.g., NdPO₄ or Nd₂O₃) suggests that Nd³⁺ ions primarily occupy modifier sites within the borophosphate network, consistent with reports for similar RE-doped systems [66] This uniform dispersion is further evidenced by the unchanged full width at half maximum of the amorphous halo across all compositions, indicating negligible nanoscale ordering [67]. The observed amorphous stability aligns with the SAZBP system’s glass-forming ability, where the combined effects of B₂O₃ (glass former) and ZnO/Al₂O₃ (intermediates) suppress crystallization during quenching, a behavior well-documented in multi-component phosphate glasses [68]. This makes Nd³⁺-doped SAZBP glasses promising for photonic devices requiring structural uniformity, such as fiber lasers or optical amplifiers

3.3 Vibrational spectroscopy and structural analysis of glasses

Vibrational spectroscopy is a widely used technique for analyzing molecular structures and bonding in materials. Its utility extends to understanding the structural framework of glasses, which is crucial for tailoring their physical and chemical properties. IR transmitting glasses offer significant functional potential in various advanced technological applications.

Figure 4 presents the IR transmission spectra of the vitreous glass system with the composition (10% Na₂O-5% Al₂O₃-15% ZnO-10% B₂O₃-60% P₂O₅: xNd₂O₃ wt%) recorded over the spectral range of 4,000–400 cm⁻¹. The positions of the absorption bands identified in the spectra are detailed in Table 2, along with their corresponding vibrational modes. The presence of broad, strong, and weak absorption bands across the range highlights the amorphous nature of these glasses.

Figure 4

FTIR spectra of (60−x)P₂O₅–10B₂O₃–10Na₂O–5Al₂O₃–15ZnO–xNd₂O₃ glasses (x = 0–6 wt%) recorded in the 400–4,000 cm⁻¹ range.

In the pure SAZBP glass, the primary absorption bands are located at the following wavenumbers:

• 519 cm ⁻¹ : P–O–P harmonic bending

• 760 cm ⁻¹ : P–O–P anti-symmetric stretching

• 960 cm ⁻¹ : P–O–P symmetric stretching

• 1,080 cm ⁻¹ : PO 4 3 − (υ₃-normal tetrahedral ion stretching)

• 1,285 cm ⁻¹ : P═O symmetric stretching.

Additionally, absorption bands at 1,625, 2,360, 2,945, and 3,452 cm⁻¹ are attributed to water absorbed by the glass samples during pellet preparation [69,70].

Upon doping with varying concentrations of Nd₂O₃, significant changes are observed in the IR spectra. The mid-band positions of the absorption bands shift to higher wave numbers with increasing Nd₂O₃ content, indicating structural modifications within the glass network. Furthermore, the intensity of bands associated with P═O, PO 4 3 − , P–O–P, and O═P–O vibrations decrease as Nd₂O₃ concentration increases. This phenomenon is attributed to the replacement of phosphorus ions by Nd³⁺ ions, confirming the entry of Nd³⁺ into the glass network [44]. A new absorption peak emerges at 465 cm⁻¹, which is associated with Nd–O vibrations. This observation supports the hypothesis that Nd³⁺ ions act as network modifiers, occupying interstitial spaces within the glass. As network modifiers, Nd³⁺ ions contribute to the depolymerization of the phosphate glass structure. This occurs through the breaking of P–O–P linkages and the formation of ionic cross-bonds between phosphate chains, resulting in structural reorganization.

The structural changes induced by Nd₂O₃ doping have significant implications for the physical and optical properties of the glass. These alterations can be exploited to design materials with tailored properties for applications in optical devices, lasers, and other photonic technologies. This study demonstrates how vibrational spectroscopy serves as a powerful tool for elucidating the intricate relationships between composition, structure, and functionality in glass materials.

3.4 Analysis of optical absorption

The absorption spectra of Nd³⁺-doped glass systems with the composition (10% Na₂O-5% Al₂O₃-15% ZnO-10% B₂O₃- 0% P₂O₅: xNd₂O₃ wt%) for various Nd₂O₃ concentrations are presented in Figure 5. The absorption peaks observed in the spectrum correspond to transitions originating from the ⁴I_9/2 ground state to various excited states, specifically ⁴D_1/2 (357 nm), ²P_1/2 (436 nm), ⁴G_11/2 (465 nm), ²G_9/2 (477), ⁴G_9/2 (517 nm), ⁴G_7/2 (530 nm), ⁴G_5/2 (586 nm), ⁴F_9/2 (687 nm), ⁴F_7/2 (753 nm), ⁴F_5/2 (808 nm), and ⁴F_3/2 (879 nm) [75]. These spectral features are consistent with those reported for other Nd³⁺-doped glasses [76,77,78,79].

Figure 5

Room-temperature UV-Vis-NIR absorption spectra of (60−x)P₂O₅–10B₂O₃–10Na₂O–5Al₂O₃–15ZnO–xNd₂O₃ glasses (x = 0–6 wt%) recorded in the 250–1,000 nm range.

Among these, the absorption band at 586 nm, attributed to the ⁴I_9/2 → ⁴G_5/2 transition of Nd³⁺, shows the highest absorption coefficient among all observed transitions.

Consequently, this wavelength was selected for investigating the NIR emission spectra of the glass samples, as illustrated in Figure 5. Furthermore, the intensities of the energy levels associated with these transitions are observed to vary with Nd³⁺ concentration, with the absorption intensity of the bands increasing as the Nd₂O₃ concentration rises. Due to the amorphous nature of the glass samples, the optical absorption edges are not sharply defined, consistent with previous studies [80]. The transition energy levels in the absorption spectra vary with Nd₂O₃ concentration. This variation is influenced by the covalent character and asymmetry of the Nd–O local structure within the host matrices [81]. An increase in Nd₂O₃ concentration results in a noticeable enhancement in the absorption intensity of the observed bands. The spectra exhibit multiple distinct absorption peaks, each corresponding to transitions from the ground state ⁴I_9/2 to various excited states within the 4f³ electronic configuration of the Nd³⁺ ions. These transitions include notable peaks arising from intra-⁴f transitions, which are characteristic of rare-earth-doped materials. Furthermore, the observed increase in absorption intensity with higher Nd₂O₃ content indicates an elevated population of Nd³⁺ ions in the glass matrix. This enhancement can improve the material’s suitability for optical applications such as lasers and amplifiers, as the increased rare-earth ion concentration can enhance the efficiency of light absorption and subsequent emission processes. These findings highlight the impact of Nd₂O₃ doping on the optical properties of the glass system, suggesting its potential for applications in photonic and laser technologies. Detailed data on the absorption transitions and intensities provide a valuable foundation for further exploration of these materials.

The distinction between direct and indirect optical band gaps in Nd³⁺-doped SAZBP glasses is critical for understanding their electronic and photonic behavior. In direct band gap transitions, electrons absorb a photon and transition between the valence and conduction bands without a change in momentum (Δk = 0), typical of allowed transitions in Nd³⁺ ions (e.g., ⁴I₉/₂ → ⁴F₅/₂). These transitions dominate the sharp absorption peaks observed in the UV-Vis-NIR spectra and are highly efficient for luminescence. In contrast, indirect band gap transitions involve phonon-assisted processes (Δk ≠ 0), where momentum conservation requires lattice vibrations. Such transitions are common in the amorphous SAZBP matrix due to its disordered structure, manifesting as broad absorption edges in the band gap analysis. The interplay between these mechanisms’ direct transitions (Nd³⁺-related) and indirect transitions (host glass-related) explains the concentration-dependent shifts in band gap energy.

The band structure surrounding the energy gap is visible at the fundamental absorption edge. The red shift of the absorption edge for undoped glass (SAZBP-1) at 280–295 nm (SAZBP-8) occurs as the concentration of Nd₂O₃ ions gradually increases which attributed to (i) NBO formation: Higher Nd³⁺ concentrations create more NBOs. (ii) Polaronic effects: Nd³⁺–O²⁻ charge transfer transitions require lower energy at higher doping levels. (iii) Ligand field modification: Enhanced Nd³⁺-ligand interactions alter the electronic density of states near band edges.

Band gaps in optical energy, both direct and indirect E _g of the glasses were calculated by projecting the linear region to the x-axis where (αhν)² or (αhν)^1/2 = 0 as illustrated in Figure 6(a) and (b) from the Tauc plots generated between (hv) with (αhv)^1/2 and (αhν)². As the concentration of Nd₂O₃ increased, the direct and indirect optical band gaps were observed to gradually diminish. The direct and indirect optical band gap were observed to be high for SAZBP-1 glass (3.82 eV), (3.76 eV) and low for SAZBP-8 glass (3.72 eV), (3.65 eV) respectively. The optical band gap decreasing with increasing Nd₂O₃ concentration, attributed to an increase in NBOs and bonding defects [53].

Figure 6

(a) Direct and (b) indirect optical band gap analysis with error bars of (60−x)P₂O₅–10B₂O₃–10Na₂O–5Al₂O₃–15ZnO–xNd₂O₃ (x = 0–6 wt%) glasses using Tauc plots.

The standard deviations (σ) of the direct ( E g dir ) and indirect ( E g indir ) bandgap values derived from Tauc plots, calculated across three independent measurements for each sample. The results in (Table 3) confirm that the observed changes in E _g with Nd₂O₃ concentration are statistically significant and not attributable to measurement uncertainty.

The systematic reduction in both E g dir and E g indir with increasing Nd₂O₃ (e.g., Δ E g dir = 0.10 eV from 0.0 to 6 wt%) exceeds the experimental errors (σ ≤ 0.04 eV), confirming that the trend arises from Nd³⁺ incorporation. This aligns with studies Kunawat et al. [82], where rare-earth doping modifies glass network topology, and by Nain et al. [83], which correlates bandgap shifts with ligand field effects.

The optical bandgap (E _g) of the SAZBP glass for direct allowed transitions was most appropriate for amorphous materials like glasses where the lack of long-range order typically favors direct transitions. This approach aligns with prior studies on oxide glasses [80] and is justified by the dominant electronic transitions observed in our UV-Vis-NIR spectra.

The differences in the E _g values are responsible for the structural modifications that arise according to stoichiometry [53,84]. The E _g values observed in glass that contained Nd₂O₃ were in line with findings from prior investigations [74,53,85]. A prior study found that glasses with high refractive indices and small energy gaps are more suited for producing strong optical fields. The direct and indirect optical energy gaps of prepared samples decrease from 3.77–3.65 eV to 3.83–3.71 eV, respectively, when Nd₂O₃ concentrations in the glass network rise.

3.5 PL

Figure 7 displays the NIR emission spectra of Nd³⁺-doped SAZBP glasses recorded under 586 nm excitation. Three prominent emission bands are observed at 903, 1,062, and 1,331 nm, corresponding to the transitions ⁴F₃/₂ → ⁴I₉/₂, ⁴F₃/₂ → ⁴I₁₁/₂, and ⁴F₃/₂ → ⁴I₁₃/₂, respectively. These assignments align with previously reported data for Nd³⁺ in similar glass matrices [75,76]. In bismuth zinc borate glasses, Nd³⁺ doping shows emission peaks at 919, 1,063, and 1,337 nm, with optimal concentration at 1 mol% [86]. Similarly, Zn–Al–Ba borate glasses exhibit the strongest emission at 1,056 nm with 1.0 mol% Nd₂O₃ [22]. Zinc-borophosphate glasses demonstrate a strong emission peak at 1,060 nm with a 30 nm width and 95 μs decay time [87]. Across various borate glass systems, the hypersensitive transition for Nd³⁺ occurs at 580–585 nm [88,89,90]. All studies utilized JO theory to analyze radiative properties. The emission intensity generally increases with Nd³⁺ concentration up to an optimal point, after which concentration quenching occurs. These glasses show potential for NIR laser applications, with slight variations in emission wavelengths and optimal doping concentrations depending on the host matrix.

Figure 7

NIR PL spectra of (60−x)P₂O₅−10B₂O₃−10Na₂O−5Al₂O₃− 15ZnO −xNd₂O₃) glasses (x = 0–6 wt%) under 586 nm excitation (room temperature). Peaks at 903, 1,062, and 1,331 nm correspond to Nd³⁺ transitions from ⁴F₃/₂ to ⁴I₉/₂, ⁴I₁₁/₂, and ⁴I₁₃/₂, respectively.

The 1,062 nm transition (⁴F₃/₂ → ⁴I₁₁/₂) exhibits the highest intensity and sharpness, making it the most promising for laser applications.

The fluorescence intensity increases with Nd₂O₃ concentration up to 5 wt%, beyond which it declines due to concentration quenching (Figure 7). This quenching is attributed to enhanced non-radiative energy transfer between neighboring Nd³⁺ ions at higher dopant levels [91,92].

The line shape of the emission bands remains consistent across all concentrations, suggesting uniform local environments for Nd³⁺ ions. However, the bandwidth of the 1,062 nm peak broadens notably at 5 wt% Nd₂O₃, a desirable trait for low-threshold lasing and high-gain amplification.

The invariance in band positions indicates minimal changes in the crystal field symmetry around Nd³⁺, while the intensity variations reflect changes in ion–ion interactions.

The 5 wt% Nd₂O₃ sample demonstrates the highest emission intensity, identifying it as the optimal composition for fluorescence efficiency. Beyond this threshold, spectral diffusion (from Nd³⁺–Nd³⁺ coupling) and reabsorption processes dominate, reducing overall efficiency [79,93,94].

The broad emission bandwidth and high intensity at 1,062 nm make this glass system suitable for: solid-state lasers, optical amplifiers, photonic devices,

The monotonic increase in 1,062 nm emission intensity (Figure 6) under 586 nm excitation reflects both the rising density of Nd³⁺ luminescent centers and the SAZBP matrix’s exceptional ability to suppress concentration quenching up to 6 wt% Nd₂O₃. This contrasts with conventional phosphate glasses [79] and is enabled by (i) B₂O₃-Al₂O₃ hybridization isolating Nd³⁺ ions [80], and (ii) optimized ⁴G₅/₂ excitation matching Nd³⁺’s absorption peak [81]. The absence of emission saturation suggests potential for higher doping in this host.

This study highlights the critical role of Nd³⁺ concentration in balancing fluorescence efficiency and quenching effects, providing a roadmap for optimizing rare-earth-doped glasses. Further investigation of the ⁴F₃/₂ → ⁴I₁₁/₂ transition dynamics could refine these materials for targeted applications.

3.6 JO analysis

A critical tool for understanding the spectroscopic behavior of rare-earth-doped glasses is the JO theoretical analysis. This well-established approach provides a quantitative description of the radiative properties of rare-earth ions in different host matrices by determining the intensity parameters Ω _λ (λ = 2, 4, 6). These parameters are derived from absorption spectra as shown in Figure 3 and are key to predicting important radiative parameters, including transition probabilities (A _R), branching ratios (β _R), and stimulated emission cross-sections (σ(λ _p)). The JO analysis not only helps to elucidate the interaction between the rare-earth ions and the host matrix but also serves as a reliable method for identifying transitions with high lasing potential.

The SAZBP-7 (Nd = 5 wt%) glass was analyzed to explore its laser potential using JO analysis. The ⁴I_9/2 → ⁴G_5/2 transition exhibited the highest experimental (f _exp) and calculated (f _cal) at 586 nm in the visible region, as illustrated in Table 3.

This transition, known as the hypersensitive transition, adheres to the selection rules | ΔS | = 0, |ΔL| ≤ 2, and |ΔJ|≤ 2 [80,68,83]. The hypersensitive ⁴I_9/2→⁴G_5/2 transition is highly sensitive to the surrounding environment of the Nd³⁺ ions, making it more intense compared to other transitions. Furthermore, the small root mean square deviation (σ _rms) value of 1.07 × 10⁻⁶ indicates excellent agreement between the experimental results and the theoretical JO calculations.

Note: The root mean square deviation (σ _rms = 0.12 × 10⁻⁶) between f _exp and f _cal values validates the quality of the JO analysis.

Table 4 summarizes the JO parameters, their trends, and the spectroscopic quality factor for SAZBP-7 glass. The Ω ₂ parameter primarily indicates the site symmetry and the covalent nature of the Nd–O bonds, while Ω ₄ and Ω ₆ are linked to bulk properties, such as the viscosity and stiffness of the host glass [83,95]. The JO parameters for SAZBP-7 glass follow the trend Ω ₆ > Ω ₂ > Ω ₄, suggesting a stronger viscosity interaction between the Nd³⁺ ions and the surrounding ligand anions. The spectroscopic quality factor (χ), defined as the ratio Ω ₄/Ω ₆ [83], plays a crucial role in determining the luminescence efficiency. An increase in the χ value typically suggests enhanced luminescence efficiency for the ⁴F_3/2→⁴I_11/2 transition. For SAZBP-7 glass, the χ value is calculated as 0.66, indicating potential for efficient laser action at 1,062 nm.

The emission peak position (λₚ), radiative transition probability (A _R), stimulated emission cross-section (σ(λₚ)), and branching ratio (β _R), which are radiative parameters, were derived from the emission spectrum of SAZBP-7 (Nd = 5 wt%) glass using JO analysis, as presented in Table 5. The 1,062 nm emission spectrum, exhibiting the highest values of stimulated emission cross-section (1.07 × 10⁻²² cm²) and branching ratio (0.66), is identified as a highly promising lasing transition (Table 6).

The SAZBP glass demonstrates a balanced combination of emission cross-section (σₑ) and branching ratio (ꞵ), positioning it as a competitive alternative to conventional Nd³⁺ host glasses. While fluorophosphate (σₑ = 1.8 × 10⁻²² cm²) and phosphate (σₑ = 1.9 × 10⁻²² cm²) glasses exhibit higher σₑ, they often suffer from drawbacks such as poor chemical durability (phosphate) or complex synthesis (fluorophosphate). In contrast, SAZBP glass achieves a favorable σₑ (1.07 × 10⁻²² cm²) alongside a high branching ratio (0.66), comparable to tellurite and silicate systems but with potential advantages in stability and ease of fabrication making it a promising candidate for tunable laser applications.

3.7 Strengths, weaknesses, and future directions of the current study

This study demonstrates successful structural and optical tuning of Nd³⁺-doped SAZBP glasses through comprehensive characterization of physical, structural, and optical properties. The research identifies 5 wt% Nd₂O₃ as the optimal doping concentration, showing maximum 1,062 nm emission intensity with minimal quenching, supported by JO analysis. The glass system exhibits excellent potential for photonic applications, particularly solid-state lasers and optical amplifiers, due to its broad emission bandwidth, high stimulated emission cross-section (1.07 × 10⁻²² cm²), and stable amorphous nature.

The study faces the following limitations:

1. mitigating concentration quenching above 5 wt% Nd₂O₃ due to unresolved Nd³⁺ clustering issues, requiring advanced techniques like EXAFS for better ion distribution analysis,

2. lacking thermal property data (e.g., glass transition temperature) crucial for practical applications,

3. exhibiting spectral resolution challenges in FTIR/XRD that could mask important structural details, suggesting the need for higher-resolution methods like Raman mapping. These limitations highlight areas needing improvement for more comprehensive material characterization and application potential.

Future research should explore:

1. co-doping with Yb³⁺/Ce³⁺ to reduce quenching,

2. developing glass-ceramic nanocomposites for improved luminescence,

3. prototyping photonic devices for performance testing,

4. advanced characterization methods to analyze thermal and decay properties. These approaches will build on the current foundation to optimize Nd³⁺-doped glasses for practical photonic applications.