Bi³⁺-activated Sr₃SbAl₃Ge₂O₁₄: dual-site occupancy and broadband cyan emission for WLED spectral compensation

Abstract

Cyan-emitting phosphors are essential for near-ultraviolet LEDs and play a key role in achieving high-quality white light. In this work, a new series of Bi³⁺-doped Sr₃SbAl₃Ge₂O₁₄:xBi³⁺ (0.01 ≤ x ≤ 0.09) phosphors was prepared by a high-temperature solid-state method. The samples can be excited by ultraviolet to violet light and show a broad cyan emission centered at 518 nm (x = 0.03) with a full width at half maximum of 93 nm, indicating broadband luminescence. Analysis of excitation spectra, luminescence decay, and the Van Uitert model suggests that Bi³⁺ ions occupy both Al³⁺ and Sr²⁺ sites, giving rise to the wide cyan emission. The CIE chromaticity coordinates confirm that all compositions emit cyan light, and the optimal sample (x = 0.03) shows coordinates of (0.2271, 0.5026). Combining Sr₃SbAl₃Ge₂O₁₄:Bi³⁺ with commercial phosphors and near-UV LED chips enables the fabrication of white LEDs, demonstrating its potential as a cyan-emitting component for high-performance white light devices.

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1. Introduction

With the advancement of science and technology, rare-earth-doped luminescent materials have attracted increasing research interest.^1–5 Phosphors composed of a host matrix and an activator are key materials in optoelectronic applications, including solid-state lighting, bioimaging, and environmental monitoring.^6–10 Phosphor-converted white LEDs (pc-WLEDs) have rapidly developed in recent years,^11–13 and the demand for higher performance and better lighting quality continues to increase.^14–18 Current commercial phosphors show insufficient cyan emission, and the weak spectral output in the 475–525 nm range limits color rendering and color temperature control. Developing cyan phosphors excitable by near-ultraviolet light is therefore important. Studies on cyan phosphors doped with rare-earth ions such as Ca₂GdHf₂(AlO₄)₃:Ce³⁺ and Ca₃SiO₄(Cl, Br)₂:Eu²⁺ have been reported.^19,20

However, the extraction and refining processes of rare-earth elements are complex and may cause environmental pollution, such as acid leaching residues, electrolytic wastewater, and radioactive by-products, which can negatively impact sustainable development. Eu²⁺ and Ce³⁺ exhibit excitation in the visible region, leading to reabsorption and reduced luminous efficiency with color instability in LED devices.^21–23 To address these issues, Bi³⁺ was chosen as the activator due to its strong absorption in the near-ultraviolet region and minimal absorption in the visible range, which helps avoid reabsorption losses.^24,25 Its luminescence arises from the ¹S₀ → ³P₁/¹P₁ transitions, and the exposed 6s and 6p orbitals make it sensitive to the crystal field, enabling emission tunability from the ultraviolet to the near-infrared region.^26,27 Bi³⁺ is also low-cost, abundant, and environmentally friendly, and has been successfully applied in narrowband and broadband emitters, near-infrared phosphors, and persistent luminescent materials.²⁸ These characteristics make Bi³⁺ an ideal choice for designing NUV-excitable cyan phosphors.^29,30 In recent years, various Bi³⁺-doped cyan-emitting phosphors have been reported, which can generally be classified according to their host lattices into silicate, vanadate, germanate, and gallate systems.^31–33 In this work, we selected the germanate Sr₃SbAl₃Ge₂O₁₄ as the host matrix and successfully synthesized Bi³⁺-doped Sr₃SbAl₃Ge₂O₁₄ phosphors for the first time. This compound crystallizes in a trigonal system with high structural stability and multiple cation sites, providing favorable conditions for multi-site occupation. This characteristic allows the formation of multiple emission centers and a wide emission spectrum. The Sr₃SbAl₃Ge₂O₁₄:xBi³⁺ phosphors show strong excitation response in the near-ultraviolet region. X-ray diffraction, photoluminescence spectroscopy, and fluorescence lifetime measurements confirm that Bi³⁺ ions occupy both Al³⁺ and Sr²⁺ lattice sites, resulting in the observed broadband cyan emission.

2. Experimental

2.1. Sample preparation

A series of Sr₃SbAl₃Ge₂O₁₄:xBi³⁺ (SSAGO:xBi³⁺, 0 ≤ x ≤ 0.09) luminescent materials were synthesized using the high-temperature solid-state method. The raw materials included SrO (99.99%), Sb₂O₃ (99.99%), Al₂O₃ (99.99%), and Bi₂O₃ (99.99%), weighed according to stoichiometric ratios. These were mixed and ground in an agate mortar for 20 minutes. The well-mixed powders were then transferred to an alumina crucible and placed in a muffle furnace. Under an air atmosphere, the temperature was ramped at a rate of 5 °C min⁻¹ to 1500 °C and maintained for 6 hours. After natural cooling, the calcined products were ground into fine powders to obtain the SSAGO:xBi³⁺ phosphors.

2.2. Characterization

Phase identification was performed using a SmartLab X-ray diffractometer (Rigaku, Japan), scanning from 10° to 80° at a rate of 10° min⁻¹. The crystal structure was analyzed using VESTA software. Diffuse reflectance spectra were recorded using a Shimadzu UV-3900 Plus UV-Vis-NIR spectrophotometer. Steady-state and transient photoluminescence spectra were obtained using an Edinburgh FLS-1000 fluorescence spectrometer. A temperature-dependent fluorescence measurement was conducted using an Oxford Optistat DN2 system. Except for thermal emission measurements, all experiments were conducted at room temperature.

3. Results and discussion

3.1. Crystal structure analysis

Fig. 1 shows the crystal structure of Sr₃SbAl₃Ge₂O₁₄, which belongs to a hexagonal crystal system with space group P321. In this structure, Sr²⁺ ions coordinate with eight oxygen atoms to form [SrO₈] polyhedra, and Sb⁵⁺ ions coordinate with six oxygen atoms to form [SbO₆] octahedra. These two polyhedra are connected via edge-sharing to form Sr–Sb layers. Meanwhile, Al³⁺ and Ge⁴⁺ ions coordinate with four oxygen atoms each to form [AlO₄] tetrahedra and [GeO₄] tetrahedral structure. These units are connected by corner-sharing to form Al–Ge layers. The Sr–Sb and Al–Ge layers alternately stack along the c-axis, creating a stable three-dimensional framework.³⁴

Fig. 1 Crystal structure of Sr₃SbAl₃Ge₂O₁₄ and the coordination environments of the cations.

3.2. Phase analysis of SSAGO:Bi³⁺

Fig. 2(a) displays the X-ray diffraction (XRD) patterns of SSAGO:xBi³⁺(0 ≤ x ≤ 0.09). The diffraction peaks of the undoped host match well with the standard Sr₃SbAl₃Ge₂O₁₄ card (PDF#51-148). After Bi³⁺ doping, no new diffraction peaks appear, indicating that the samples retain high phase purity and that Bi³⁺ ions are successfully incorporated into the host lattice. The enlarged XRD patterns in Fig. 2(b) show that the diffraction peaks gradually shift to lower angles as the Bi³⁺ concentration increases, reflecting lattice expansion caused by ion substitution. Fig. 2c shows that the lattice parameters and cell volume increase with the rise of Bi³⁺ concentration. This expansion arises from the substitution of smaller lattice cations by the larger Bi³⁺ ions, thereby enlarging the crystal framework. Such behavior further confirms the successful incorporation of Bi³⁺ into the host matrix, in agreement with the XRD results.

Fig. 2 XRD patterns (a) and magnified XRD patterns (b) of SSAGO:xBi³⁺ (0 ≤ x ≤ 0.09); (c) crystal cell parameter variation diagram of SSAGO:xBi³⁺ (0 ≤ x ≤ 0.09).

3.3. Photoluminescence properties of SSAGO:Bi³⁺

Fig. 3(a) presents the diffuse reflectance spectra of the SSAGO host and the SSAGO:0.03Bi³⁺ sample with the corresponding calculated optical band gaps. The host shows almost no absorption from 300 to 380 nm, while the Bi³⁺-doped sample exhibits strong absorption in this region, attributed to the ¹S₀ → ³P₁ transition of Bi³⁺, and the absorption edge extends to about 400 nm.³⁵ The optical band gap (E_g) of SSAGO was determined using the following equations:³⁶

[F(R)hv]ⁿ = D(hv − E_g) (2)

D is a constant, hv represents the photon energy, and F(R) is the absorption function. The calculated optical band gap E_g is 5.55 eV. Fig. 3(b) shows the excitation spectra of SSAGO:xBi³⁺ (0.01 ≤ x ≤ 0.09), and Fig. 3(c) presents the corresponding emission spectra. The emission intensity reaches a maximum at x = 0.03 mol, with an excitation peak at 367 nm and an emission peak at 518 nm, and the emission band has a full width at half maximum of 93 nm. When the Bi³⁺ concentration continues to increase, the emission intensity decreases due to concentration quenching. Fig. 3(d) shows the relationship between log(I/x) and log(x), used to analyze the energy transfer mechanism responsible for the quenching behavior. The critical distance (R_c) is calculated using the following formula:³⁷V is the unit cell volume (299.72 ų), X_c is the critical concentration (0.03 mol), and N is the number of cations in one unit cell (N = 16). The calculated critical distance R_c is 1 nm, which is greater than 0.5 nm, indicating that concentration quenching is governed by multipolar interaction rather than exchange interaction.

Fig. 3 (a) Diffuse reflectance spectra of SSAGO:xBi³⁺ (x = 0, 0.03) and the inset show the x = 0 host band gap; excitation spectra (b) and emission spectra (c) of SSAGO:xBi³⁺ (0.01 ≤ x ≤ 0.09); (d) curve of lg(I/x) versus lg(x) for SSAGO:xBi³⁺.

According to Dexter's theory, the relationship between emission intensity (I) and concentration (x) follows:³⁸

In this expression, β and k are constants; θ is the electric multipole interaction factor. The types of electric multipole interactions are divided into three categories: electric dipole–electric dipole (d–d), electric dipole–electric quadrupole (d–q), and electric quadrupole–electric quadrupole (q–q) interactions. The corresponding θ exponents for these interaction types are 6, 8, and 10, respectively. Taking the logarithm of eqn (4) yields:³⁹

By fitting the experimental data, the slope of the linear fit gives −θ/3 = −1.99736, from which θ ≈ 6 is obtained. This result indicates that the dominant quenching mechanism is electric dipole–dipole (d–d) interaction.

We found that the emission spectrum is asymmetric, suggesting the presence of multiple emission centers. After converting the wavelength to energy for plotting, the curve remained asymmetric. We then performed Gaussian fitting, as shown in Fig. 4(a), from which two peaks can be observed at 2.28 and 2.47 eV, labeled as Bi1 and Bi2. These observations indicate that the phosphor has two emission centers. To confirm that Bi³⁺ occupies two lattice sites, spectral measurements were performed under different excitation and emission wavelengths, as shown in Fig. 4(b) and (c). When the excitation wavelength varied from 320 to 360 nm, the emission peaks of SSAGO:0.03Bi³⁺ shifted noticeably. Likewise, changing the emission wavelength from 518 to 560 nm resulted in different shapes of the excitation spectra, supporting the presence of two Bi³⁺ sites. Fluorescence lifetimes were also measured at emission wavelengths of 460 and 580 nm to further verify these results, as shown in Fig. 4(d). It can be seen that the two fluorescence lifetime curves are different. The average fluorescence lifetime (τ) was calculated using the following equation:^40,41

where A₁ and A₂ are the pre-exponential factors, and τ₁ and τ₂ are the fast and slow decay constants, respectively. When monitoring the emission wavelengths at 460 and 580 nm, the average lifetimes are 1.18 and 1.27 µs, respectively, indicating the presence of two luminescent centers. We employed the Van Uitert formula to determine the lattice sites occupied by Bi³⁺:⁴²

Fig. 4 (a) Gaussian fitting of the PL spectra of SSAGO:0.03Bi³⁺; (b) normalized spectra of SSAGO:0.03Bi³⁺ at various excitation wavelength; (c) normalized spectra of SSAGO:0.03Bi³⁺ at various emission wavelengths; (d) life decay curve of SSAGO:0.03Bi³⁺ samples.

In this formula, V is the valence of the activator ion (V = 3); n is the coordination number; E_a is the electron affinity of the anion, which is a constant in the same host lattice; and r is the ionic radius of the cation being substituted. Considering the ionic radii of potential substitution sites [Sr²⁺ (r_CN=8 = 1.26 Å), Al³⁺ (r_CN=4 = 0.39 Å), Ge⁴⁺ (r_CN=4 = 0.39 Å) and Sb⁵⁺ (r_CN=6 = 0.60 Å)], the calculations suggest that Bi³⁺ ions preferentially substitute for Al³⁺ and Sr²⁺ sites. Specifically, Bi2 is attributed to Bi³⁺ occupying the Sr²⁺ site, while Bi1 corresponds to Bi³⁺ occupying the Al³⁺ site. These results are in good agreement with the spectral deconvolution and lifetime analyses, collectively confirming the multi-site occupancy of Bi³⁺ in the SSAGO host lattice.^43,44

3.4. Thermal stability of the luminescence of SSAGO:Bi³⁺

Fig. 5(a) shows the temperature-dependent emission intensity of SSAGO:0.03Bi³⁺ in the range of 300 to 500 K. As the temperature increases, the emission intensity gradually decreases, indicating the occurrence of thermal quenching. Fig. 5(b) presents the relationship between temperature and both the FWHM and the integrated emission intensity. As the temperature rises, the gradual broadening of the FWHM and the decrease in the luminescence intensity of the phosphor with increasing temperature may be attributed to the enhanced interaction between electrons and phonons at elevated temperatures. As temperature increases, thermally excited electrons interact more with phonons and undergo non-radiative relaxation, gaining phonon energy that allows them to cross the barrier between the excited and ground states without emitting photons. This results in a reduction in radiative recombination and leads to thermal quenching of luminescence. Based on the analysis of Fig. 5(a) and (b), the luminescence thermal stability of this phosphor indicates that its emission intensity decreases by approximately 3% for every 1 K increase in temperature. To further understand the quenching mechanism, the thermal activation energy (E_a) was estimated using the Arrhenius equation:⁴⁵

Fig. 5 (a) 2D contour map of the emission spectra of SSAGO:0.03Bi³⁺ as a function of temperature; (b) the graph of half-peak width and integral intensity varying with temperature; (c) the functional relationship of ln(I₀/I_T − 1) versus 1/KT of SSAGO:0.03Bi³⁺.

Taking the natural logarithm on both sides:

where I₀ is the initial emission intensity at room temperature, I_T is the intensity at temperature T, C is a constant, k is the Boltzmann constant, and E_a is the thermal activation energy. As shown in Fig. 5(c), the calculated activation energy (E_a) for the SSAGO:0.03Bi³⁺ phosphor is 0.371 eV. We consider its moderate thermal stability to arise from the low thermal-quenching barrier, which enables excited Bi³⁺ electrons to relax non-radiatively back to the ground state when the sample is heated.⁴⁶

3.5. The chromaticity coordinates of the phosphor

Fig. 6(a) shows the chromaticity coordinates of the SSAGO:0.03Bi³⁺ phosphor, calculated from its emission spectrum. As seen from the figure, the coordinates are (0.2271, 0.5026), which fall within the cyan region of the CIE 1931 color space. The reported phosphor Bi³⁺, Ce³⁺, Eu²⁺ also exhibits CIE coordinates in the cyan region.^47–49 In comparison, the cyan-emitting phosphor synthesized in this work can similarly compensate for the cyan emission gap, demonstrating potential for applications in the development of WLEDs. Fig. 6(b) presents the emission spectra of a commercial WLED and the SSAGO:0.03Bi³⁺ phosphor. The commercial WLED exhibits two primary emission peaks at approximately 450 and 550 nm, with the strongest emission intensity observed at 450 nm. In contrast, the SSAGO:0.03Bi³⁺ phosphor exhibits a broad emission peak centered at 518 nm. Based on this spectral comparison, we speculate that this phosphor can be combined with commercial phosphors (Y₃Ga₅O₁₂:Ce³⁺) to compensate for the cyan gap. It exhibits promising potential in the field of WLEDs.

Fig. 6 (a) CIE of SAGO:0.03Bi³⁺ phosphor; (b) the emission spectrum of WLED and SSAGO:0.03Bi³⁺.

4. Conclusion

In this study, a Bi³⁺-doped cyan phosphor (Sr₃SbAl₃Ge₂O₁₄:xBi³⁺) was synthesized and its luminescence properties and lattice site occupancy were investigated. Under 380 nm excitation, a broad cyan emission band with a peak at 518 nm was observed. Gaussian fitting and spectral monitoring indicate that Bi³⁺ occupies two distinct sites. To further explore the site occupancy, calculations using the Van Uitert formula confirmed that the emission peaks at 503 and 544 nm originate from Bi³⁺ occupying the SrO₈ and AlO₄ sites, respectively. The CIE chromaticity coordinates of this phosphor fall within the cyan region, demonstrating its potential to fill the cyan spectral gap in conventional phosphors.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

All data supporting this study are available from the corresponding author upon reasonable request.

Acknowledgements

This work is sponsored by Natural Science Foundation of Xinjiang Uygur Autonomous Region (2024D01B19), the Basic Scientific Research Business Fund of Higher Education Institutions in the Autonomous Region (XJEDU2024P098), the Karamay City Innovation Environment Construction Plan (Innovation Talent) Project (2024hjcxrc0082), the 2024 School-level Research Fund Projects of Xinjiang Second Medical University (ZR202423).

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