Spectroscopic analysis of Sr₆Ca₄(PO₄)₆F₂:Dy³⁺/Eu³⁺ phosphors for color-tunable LED applications

Abstract

Currently, the development of luminescent materials is a major focus of research. In particular, inorganic phosphor-based compounds have been widely utilized for different types of applications, including in cathode ray tubes (CRTs), lamps, field emission displays (FEDs), radiation dosimetry, and white light-emitting diodes (WLEDs). White light emission was achieved in this study by employing a wet chemical method to successfully synthesize single-host Sr₆Ca₄(PO₄)₆F₂:Dy³⁺/Eu³⁺. The Sr₆Ca₄(PO₄)₆F₂ phase purity was validated by XRD analysis. In photoluminescence (PL) tests, Sr₆Ca₄(PO₄)₆F₂:Dy³⁺ doped phosphors demonstrated effective excitation at 349 nm, exhibiting noticeable emission bands at 487 nm and 576 nm. The color characteristics of the produced luminous samples were calculated using the Commission internationale de l'éclairage (CIE) coordinates. These findings suggest that Sr₆Ca₄(PO₄)₆F₂ phosphors co-doped with Dy³⁺/Eu³⁺ could be suitable for making white LEDs that can be excited by UV-LEDs.

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1 Introduction

Initially, light-emitting diodes (LEDs) were made to emit monochromatic light. Afterwards, in the early 1990s, LEDs were available in almost all colors, and this provided a breakthrough for the generation of white light-emitting diodes (wLEDs). The very first wLED came into the picture in the year 1996.¹ The gigantic capabilities of this transforming innovation to counter the issues of natural contamination and energy preservation were promptly noted. However, the research community has continuously focused on designing effective and improved versions of wLEDs. It is considered that this field has not been explored to its full depth, and hence much more is to come to benefit researchers and humanity. wLEDs possess lots of benefits, such as high luminous efficiency, extended lifetimes, low energy requirements, good economy, increased reliability and a non-hazardous nature;^2–4 all of these factors make them a great alternative lighting source. Several factors determine whether phosphors exhibit strong luminescence, including good crystallinity, broad emission bands, and efficient energy absorption. To improve the luminescence, various synthesis techniques have been employed to obtain phosphors, and also the use of sensitizers has been proposed. At present, the foremost solid-state lighting devices are fabricated using wLEDs, replacing conventional fluorescent lamps.⁵ In addition, this kind of diode is used in several devices, such as lasers, biosensors, amplifiers, display panels, solar cells and field emission displays.^6,7

To produce multicolored light, including white light, from single-phase phosphors, the first alternative could be to dope them with rare-earth ions.^8–11 The trivalent dysprosium ion (Dy³⁺), one of the rare-earth ions, is a significant activator of luminous materials. Owing to its distinctive f–f transitions, it produces white-light radiation, having two peaks with blue and yellow color.^12–14 The development of effective Dy³⁺-including phosphors offering better thermal stability is still appealing for wLED applications, despite the fact that several luminescence-based studies of these materials have been published. Moreover, Dy³⁺ ions offer profuse energy levels, and their high degree of colour purity makes them an ideal candidate to act as an activator for white light emission.^15,16

Few researchers have explored the color-tuning capabilities of Dy³⁺ in Eu³⁺-activated phosphors in recent years.^17,18 Tikale et al.¹⁹ reported employing urea as a fuel to produce a Dy³⁺/Eu³⁺-doped/co-doped KMg₃Si₃AlO₁₀F₂ phosphor by combustion. When excited at wavelengths of 260 nm and 395 nm, KMg₃Si₃AlO₁₀F₂:Eu³⁺ showed two strong peaks at 595 nm and 614 nm in its photoluminescence (PL) emission spectrum, indicating orange-red and strong-red emission. When stimulated at 349, 364, and 387 nm, KMg₃Si₃AlO₁₀F₂:Dy³⁺ exhibits two emission peaks at 484 nm and 576 nm.¹⁹ Dy³⁺ to Eu³⁺ ion energy transfer was effective, and the emission of tunable warm white light was observed. Deshmukh et al.²⁰ reported K₂Ba(PO₄)F doped/co-doped with a series of Dy³⁺/Eu³⁺ ions produced by a simple low-temperature combustion process. K₂Ba(PO₄)F:Dy³⁺,Eu³⁺ phosphors can be efficiently stimulated at 365 nm, according to photoluminescence (PL) measurements. They likewise possess main emission bands centered at 489 nm, 570 nm and 613 nm. K₂Ba(PO₄)F:0.7 mol% Dy³⁺,x mol% Eu³⁺ phosphors have CIE chromaticity coordinates computed in the blue-white part of the visible spectrum. Kadam et al.²¹ described a new single-phase Eu³⁺, Dy³⁺-doped KAlF₄ down-conversion phosphor that was prepared and characterized using a sol–gel method. Under NUV wavelength excitation at a 0.7 mol% concentration of the RE activator ions, the computed CIE chromaticity coordinates show that KAlF₄:0.7 mol% Dy³⁺, x mol% Eu³⁺ phosphors occupy the white area of the visible spectrum. Because of the high concentration of rare-earth ions utilized in the synthesis procedures, all of these phosphors employed solid-state reaction methods. Furthermore, Co-activation of a Sr₆Ca₄(PO₄)₆F₂ phosphor has been reported to date,^22–24 but activation with Dy³⁺ and Eu³⁺ ions has not yet been documented.

The novelty of this research lies in the preparation and optical characterization of Dy³⁺/Eu³⁺-co-doped Sr₆Ca₄(PO₄)₆F₂ phosphors synthesized via a simple wet chemical route, which has not yet been reported for this host lattice. In contrast to earlier studies that primarily employed solid-state synthesis, the adopted method provides enhanced homogeneity and phase purity at comparatively lower temperatures. The co-incorporation of Dy³⁺ (sensitizer) and Eu³⁺ (activator) ions within a single Sr₆Ca₄(PO₄)₆F₂ matrix enables efficient Dy → Eu energy transfer, producing tunable emission ranging from blue-yellow to red under near-UV excitation. This controllable color tunability, together with good structural stability and improved color rendering, demonstrates the potential of the developed phosphor for advanced white-light and display applications.

2. Material preparation and characterization

By mixing an appropriate quantity of Dy₂O₃ and Eu₂O₃ powder with diluted HNO₃, RE(NO₃)₃ (RE = Dy and Eu) solutions with a range of concentrations were obtained. Any surplus HNO₃ was then removed, and the solutions were allowed to cool to room temperature. Lastly, the volumetric flask was filled with distilled water.

Sr₆Ca₄(PO₄)₆F₂:Dy³⁺,Eu³⁺ phosphors were prepared by a wet chemical method.^25–27 All samples, i.e. Ca(NO₃)₂, Zn(NO₃)₂, and Al(NO₃)₃, were taken in a stochiometric ratio and dissolved in distilled water. A solution of rare-earth ions dissolved in distilled water was then added to the solution of raw materials under continuous stirring by a magnetic stirrer. The solution was kept under magnetic stirring at a temperature of 60 °C for better homogeneity. The solution was then removed from the magnetic stirrer, and the obtained wet precipitate was kept in a hot air oven at 90 °C for 12 h to remove excess water and impurities. The sample was collected from the beaker after 12 h and crushed to make a fine powder. The sample was then used for further characterization.

Using Cu Kα radiation (λ = 0.154059 nm) and patterns recorded from 10–90° in small steps, a Rigaku Miniflex 600 X-ray diffractometer operating at 40 kV and 15 mA was used to analyze the phosphor's crystalline structure and phase purity using X-ray diffraction (XRD). Fullprof Suite software has been used for Rietveld refinement for a detailed structural study. The crystal structure of the proposed phosphor has been derived from the output of Rietveld refinement. Vesta software was used here to draw the final crystal structure of the phosphor. Fourier-transform infrared (FTIR) spectroscopy was used to measure the sample's vibrational characteristics. Excitation and emission studies of photoluminescence (PL) spectra were conducted using a SHIMADZU RF-5301pc spectrofluorophotometer. The quality of the prepared material was also assessed using Commission internationale de l'éclairage (CIE) criteria.

3. Results and discussion

3.1. XRD and crystal structure analysis

XRD patterns from all co-doped samples from the series are shown in Fig. 1. Employing the standard ICSD database card no. 01-072-2399, all samples were compared. All of the samples match the pattern from the ICSD database. Dopants increase peak intensities when compared to the pure host lattice, as shown in Fig. 1. This intensity shift could be the result of modifications to the host lattice's local crystal structure. The XRD pattern seen in Fig. 2 was used to perform Rietveld refinement analysis and determine the crystal structure of the pure Sr₆Ca₄(PO₄)₆F₂ phosphor. Fullprof used the pseudo-Voigt peak profile function for Rietveld refining.^28,29 The R_wp (13.62), R_p (7.72), R_exp (8.23), and χ² (1.95) reliability factors grouped together, revealing the reflection conditions. In accordance with the refinement study, the Sr₆Ca₄(PO₄)₆F₂ phosphor crystallizes into a hexagonal structure with the space group P63 and lattice parameters a = b = 9.6300 Å, c = 7.2200 Å, α = β = 90.00°, and γ = 120.00°. The unit cell's volume is 579.85750 ų. The XRD patterns of the Dy³⁺/Eu³⁺-doped samples closely match the pure Sr₆Ca₄(PO₄)₆F₂ host phase without any additional impurity peaks, confirming the successful incorporation of rare-earth ions into the lattice. As the ionic radii of Dy³⁺ (0.912 Å) and Eu³⁺ (0.947 Å) are comparable to those of Ca²⁺ and Sr²⁺, only a negligible structural distortion is expected, which is below the resolution limit of the present diffraction setup. Hence, a qualitative comparison of diffraction intensities and peak consistency sufficiently validates the structural integrity and dopant accommodation in the host matrix.

Fig. 1 The XRD patterns of Sr₆Ca₄(PO₄)₆F₂ samples co-doped with Dy³⁺ and Eu³⁺ ions.

Fig. 2 Rietveld refinement of the Sr₆Ca₄(PO₄)₆F₂ phosphor.

Fig. 3 gives the crystal structure of Sr₆Ca₄(PO₄)₆F₂ phosphor determined using Rietveld refinement. Sr₆Ca₄(PO₄)₆F₂ crystallizes in the hexagonal P63 space group. Sr–Al is joined to nine O–Al atoms in nine-coordinate geometry. The bond distances between Sr and O vary from 2.53–2.80 Å. Two Ca–Al sites are non-equivalent. Nine O–Al atoms are joined to Ca–Al in a nine-coordinate geometry at the initial Ca–Al site. The range of Ca–O bond distances is 2.42–2.91 Å.

Fig. 3 The crystal structure of the Sr₆Ca₄(PO₄)₆F₂ phosphor.

3.2. FTIR analysis

The FTIR spectrum of the Sr₆Ca₄(PO₄)₆F₂ phosphor in the range of 500–4000 cm⁻¹ is shown in Fig. 4. The asymmetric bending vibration v₄(F₂) of PO₄³⁻ for the sample is represented by the peaks at 500 and 556 cm⁻¹.³⁰ The peaks around 950 and 1200 cm⁻¹ correspond to the asymmetric mode of the P–O bonds in PO₄³⁻ groups.^31,32 All these peaks are caused by vibrations of the phosphate group in Sr₆Ca₄(PO₄)₆F₂. Furthermore, an extra peak was identified at 740 cm⁻¹, which is possibly due to the F–OH–F arrangement.³³

Fig. 4 FTIR analysis of the Sr₆Ca₄(PO₄)₆F₂ phosphor.

3.3. TGA-DTA

Fig. S1 shows the TGA-DTA investigation of the Sr₆Ca₄(PO₄)₆F₂ phosphor. Thermal lattice vibrations of Sr₆Ca₄(PO₄)₆F₂ particles generate an exothermic peak in the DTA curve between 300 and 400 °C.³⁴ The TGA curve depicts the sample's weight loss as a function of temperature. The material is not stable across the temperature range after the first step, indicating weight loss due to melting, and then the second step occurs, indicating weight loss due to crystallization. The last phase occurs when the sample decomposes.

3.4. Photoluminescence investigation

The photoluminescence (PL) excitation and emission spectra of Dy³⁺-doped Sr₆Ca₄(PO₄)₆F₂ phosphors are shown in Fig. 5. The excitation spectra were recorded by monitoring an emission wavelength of 576 nm. The spectra show several sharp peaks at wavelengths of 349, 364 and 387 nm, corresponding to transitions from the ground state ⁶H_15/2 to the excited states ⁶P_7/2, ⁶P_5/2, and ⁴F_7/2, respectively^35–37 (Fig. 5a). As illustrated in Fig. 5, these phosphors can be excited by ultraviolet (UV) light and near-ultraviolet (NUV) light. The most prominent peak is observed at an excitation wavelength of 349 nm, highlighting the material's potential for NUV light and white LED applications.³⁸ The emission spectra of Dy³⁺-doped phosphors measured at an excitation wavelength of 349 nm show two distinct peaks at 487 nm and 576 nm (Fig. 5b). These peaks correspond to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively.^39–41 The strong blue emission around 487 nm is attributed to the magnetic-dipole transition ⁴F_9/2 → ⁶H_15/2, while the emission at 576 nm is related to the hypersensitive (forced electric dipole) transition ⁴F_9/2 → ⁶H_13/2. Typically, when yellow emission dominates the spectrum, it indicates that Dy³⁺ is located at a site with low symmetry. Conversely, when blue emission is more intense, it suggests that Dy³⁺ is situated at a site with high symmetry (with an inversion symmetry center). The stronger blue emission indicates that Dy³⁺ occupies a high-symmetry site. PL emission spectra at excitation wavelengths of 364 and 387 nm are given in Fig. 5c and d, respectively. A series of Sr₆Ca₄(PO₄)₆F₂:xDy³⁺ phosphors was synthesized to study the impact of different Dy³⁺ doping concentrations. The emission spectra at all concentrations exhibit a similar pattern. However, the peak intensity rises as the Dy³⁺ concentration increases from 0.2 to 0.8 mol% and then declines with further increase in the Dy³⁺ concentration. The optimum concentration, yielding the maximum PL peak intensity, is approximately 0.8 mol%. Above this critical concentration, the PL intensity diminishes due to concentration quenching. This occurs because of the movement of excitation energy between emitting ions or the dissipation of this energy through non-radiative transitions.

Fig. 5 (a) PL excitation spectra of Dy³⁺-activated Sr₆Ca₄(PO₄)₆F₂ phosphors at an emission wavelength of 576 nm. (b) PL emission spectra of Dy³⁺-activated Sr₆Ca₄(PO₄)₆F₂ phosphors at an excitation wavelength of 349 nm. (c) PL emission spectra of Dy³⁺-activated Sr₆Ca₄(PO₄)₆F₂ phosphors at an excitation wavelength of 364 nm. (d) PL emission spectra of Dy³⁺-activated Sr₆Ca₄(PO₄)₆F₂ phosphors at an excitation wavelength of 387 nm.

The phenomenon of concentration quenching is related to the critical distance, represented as R_C. When the activator concentration reaches the ideal level, X_C, the close-ion activator satisfies the close-distance criterion. There are a number of reasons why an activator and host exchange energy. Either exchange contact or electric multipole interactions can accomplish this energy transfer. Blasse's formula is used to approximate the critical distance (R_C):^42,43

where, in this context, V represents the volume of the unit cell, X_C denotes the optimum concentration of the activator ion, and N is the number of ions within each unit cell. Specifically, for this scenario, X_C is 0.8, N equals 2 and V is 579.8563 ų. Consequently, the calculated R_C value between Dy³⁺ ions is 8.8472 Å. As the R_C value exceeds 5 Å for rare-earth ions, this suggests that electric multipole interactions are predominant, resulting in the concentration quenching of Dy³⁺ in the phosphors. Dexter's theory discusses the type of interaction involved in the energy transfer (ET) process by expressing the emission intensity (I) per activator ion (X) through the following equation:^44,45where x is the concentration of the rare-earth ions, β is a constant that quantifies the overlap of the wavefunctions of the donor and acceptor molecules, and k and θ are constants under similar material matrix and excitation conditions. Fig. 6 illustrates the correlation between log(x) and log(I/x). Only four points are displayed in the plot because using more points may introduce complex interactions that lead to concentration quenching of luminescence. The slope of the graph, as determined by Van Uitert's theory, is −0.4456, and θ = 1.3368, close to 3, verifying that dipole–dipole interactions are dominant.

Fig. 6 Linear fitting of log(X) versus log(I/X) for Sr₆Ca₄(PO₄)₆F₂:Dy³⁺ phosphors.

The ideal excitation wavelength for the Sr₆Ca₄(PO₄)₆F₀:Eu³⁺ phosphors was found by analysing their observed photoluminescence excitation spectra (Fig. 7). In addition to the transitions from ⁵L₆ → ⁵D₂, a couple of distinctive absorption peaks from the Eu³⁺ ion (4f orbital) were observed, centered at 395 and 465 nm. The ⁷F₀ → ⁵L₆ transition was responsible for the main peak at 395 nm, which suggested that this wavelength was the most appropriate for excitation in this investigation.⁴⁶Fig. 8 and 9 show the photoluminescence (PL) emission spectra of the Sr₆Ca₄(PO₄)₆F₂:xEu³⁺ phosphors, which were measured at excitation wavelengths of 395 and 465 nm, respectively. The distinctive emission peaks of the Eu³⁺ ion, which match the transitions from ⁵D₀ → ⁷F₁ and ⁷F₂, and are situated at roughly 593 and 614 nm, respectively, were seen in these PL emission spectra.^47,48 The most noticeable peak in the PL spectra was the ⁵D₀ → ⁷F₂ peak at 614 nm, indicating that the Sr₆Ca₄(PO₄)₆F₂:Eu³⁺ phosphors emit in the orange-red area when excited at 395 nm. Additionally, at a Eu³⁺ concentration of 0.8 mol%, the PL emission intensity peaked, and it then declined as the concentration rose further, suggesting concentration quenching at x = 0.8 mol%. Therefore, it was found that the ideal doping concentration for the Sr₆Ca₄(PO₄)₆F₂:Eu³⁺ phosphors was around 0.8 mol%. Further research is necessary to understand the underlying mechanism of this concentration quenching phenomenon, which is widespread in luminous materials. Fig. 10 illustrates the relationship between log(X) and log (I/X), exhibiting a slope of 0.2079. From this, θ is calculated to be 0.6237, so we can conclude that exchange interactions are the cause of the concentration quenching (CQ) of Sr₆Ca₄(PO₄)₆F₂:Eu³⁺ phosphors.

Fig. 7 PL excitation spectra of Sr₆Ca₄(PO₄)₆F₂:Eu³⁺ phosphors at an emission wavelength of 614 nm.

Fig. 8 PL emission spectra of Sr₆Ca₄(PO₄)₆F₂:Eu³⁺ phosphors at an excitation wavelength of 395 nm.

Fig. 9 PL emission spectra of Sr₆Ca₄(PO₄)₆F₂:Eu³⁺ phosphors at an excitation wavelength of 465 nm.

Fig. 10 Linear fitting of log(X) versus log(I/X) for Sr₆Ca₄(PO₄)₆F₂:Eu³⁺ phosphors.

In order to determine the energy transfer requirements for rare-earth elements, spectral overlap analysis is essential. Emission–emission overlap, excitation–excitation overlap, and excitation–emission overlap are the three main techniques used to evaluate energy transfer.^49,50 This study confirmed that singly doped rare-earth minerals fit these requirements upon analysing photoluminescence (PL) emission and excitation.⁴⁵ In particular, the overlap of the excitation spectra of Eu³⁺ and Dy³⁺ satisfies the excitation–excitation requirement. Energy transfer takes place during the co-doping of Dy³⁺ with Eu³⁺, indicating that the suggested dopants satisfy the requirements. The considerable overlap between the excitation spectra of Eu³⁺ and Dy³⁺ in Sr₆Ca₄(PO₄)₆F₂ is shown in Fig. 11. This implies the possibility of effective energy transfer between Eu³⁺ activators and Dy³⁺ sensitizers.

Fig. 11 Spectral overlap of the emission of Dy³⁺ and excitation of Eu³⁺ ions in the Sr₆Ca₄(PO₄)₆F₂ phosphor.

In Fig. 12, the emission spectra of Sr₆Ca₄(PO₄)₆F₂:0.8 mol% Dy³⁺,yEu³⁺ (y = 0.2, 0.4, 0.6, 0.8 and 1.0 mol%) phosphors at an excitation wavelength of 395 nm are depicted. In the Sr₆Ca₄(PO₄)₆F₂:0.8 mol% Dy³⁺,xEu³⁺ samples, the spectra show distinctive peaks at 487, 576, and 614 nm. Remarkably, in addition to the peaks in the blue and yellow regions, new red emission appears at 614 nm in these phosphors, which is ascribed to the Eu³⁺ ion's ⁵D₀ → ⁷F₂ transition.⁵¹ It is evident from Fig. 12 that the emission intensity at 487 and 576 nm from Dy³⁺ significantly drops as a result of increased energy transfer to Eu³⁺, but the intensity at 614 nm from Eu³⁺ significantly increases as the Eu³⁺ concentration rises.^52–54 As a result, all of these comprehensive descriptions demonstrate that energy is being effectively transferred from Dy³⁺ to Eu³⁺ ions.¹⁹ A representation of the energy transfer procedure in the Dy³⁺/Eu³⁺-co-doped Sr₆Ca₄(PO₄)₆F₂ phosphor is shown in Fig. 13.

Fig. 12 PL emission spectra of Sr₆Ca₄(PO₄)₆F₂ phosphors co-doped with Dy³⁺ and Eu³⁺ at an excitation wavelength of 395 nm.

Fig. 13 A schematic representation of the energy transfer process in the Dy³⁺/Eu³⁺-co-doped Sr₆Ca₄(PO₄)₆F₂ phosphor.

3.5. Photochromic investigation

The photochromic properties of the prepared Dy³⁺ and Eu³⁺ co-doped Sr₆Ca₄(PO₄)₆F₂ phosphors are described using the Commission internationale de l’éclairage (CIE) coordinates. Fig. 14 illustrates the CIE coordinates of Dy³⁺ and Eu³⁺ co-doped Sr₆Ca₄(PO₄)₆F₂ phosphors, demonstrating that the emission in the red area upon an increase in the Eu³⁺ ion concentration can be used in WLEDs to reduce the red color deficiency. The color purity of the phosphors at different concentrations is calculated using the given formula:^55–57where (X, Y) and (X_i, Y_i) denote the color coordinates of the sample point and the CIE equal-energy illuminant, respectively. The coordinates (X_d, Y_d) correspond to the chromaticity of the light source's dominant wavelength. The color correlated temperature (CCT) is calculated using the following equation:^58–60

CCT = −499n³ + 3525n² − 6823.3n + 5520.33 (4)

where the epicenter (X_C, Y_C) is located at (0.332, 0.186), and n is defined as (X − X_C)/(Y − Y_C). This method was used to calculate the CCT values, which are presented in Table 1. The observed values range from 4755 K to 8133 K for all the Dy³⁺ and Eu³⁺ co-doped phosphors. Furthermore, the correlated color temperature (CCT) indicates that the phosphor produces cool white-light emission.⁵¹

Fig. 14 CIE chromaticity coordinates of Sr₆Ca₄(PO₄)₆F₂:0.8 mol% Dy³⁺,y mol% Eu³⁺ phosphors. The letters correspond to the entries in Table 1.

Table 1 CIE color coordinates, CCT and color purity values of Dy³⁺ and Eu³⁺ co-doped Sr₆Ca₄(PO₄)₆F₂ phosphors

No.	Sample	X	Y	X d	Y d	Color purity (%)	CCT (K)
A	Sr6Ca4(PO4)6F2:0.8 mol% Dy^3+,0.5 mol% Eu^3+	0.2702	0.3726	0.6821	0.3178	18.42	8133
B	Sr6Ca4(PO4)6F2:0.8 mol% Dy^3+,0.7 mol% Eu^3+	0.2840	0.3724	0.6821	0.3178	14.47	7518
C	Sr6Ca4(PO4)6F2:0.8 mol% Dy^3+,1.0 mol% Eu^3+	0.3367	0.3711	0.6821	0.3178	10.47	5349
D	Sr6Ca4(PO4)6F2:0.8 mol% Dy^3+,1.5 mol% Eu^3+	0.3670	0.3700	0.6821	0.3178	10.12	4346
E	Sr6Ca4(PO4)6F2:0.8 mol% Dy^3+,2.0 mol% Eu^3+	0.3540	0.3704	0.6821	0.3178	6.41	4755

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4. Conclusions

In summary, we synthesized Sr₆Ca₄(PO₄)₆F₂:0.8 mol% Dy³⁺,xEu³⁺ (x = 0.2, 0.4, 0.6, 0.8, or 1.0 mol%) phosphors with tunable colors using a wet chemical method. XRD analysis validated that the phosphors crystallized in a phase-pure manner. When excited at 349 nm or below, the phosphors doped with Dy³⁺ showed emission peaks at 487 nm and 576 nm, which correspond to blue and yellow emission, respectively. Upon excitation at 395 nm, Sr₆Ca₄(PO₄)₆F₂ phosphors with 0.8 mol% Dy³⁺ and x mol% Eu³⁺ exhibited four prominent emission peaks at 487 nm, 576 nm, 593 nm, and 614 nm, resulting from the transitions of Dy³⁺ ions (⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2) and the transitions of Eu³⁺ ions (⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂). The PL and PLE spectra demonstrated energy transfer from Dy³⁺ to Eu³⁺ in the Sr₆Ca₄(PO₄)₆F₂ phosphors with 0.8 mol% Dy³⁺ and x mol% Eu³⁺, with calculations indicating that the transfer mechanism involved exchange interactions. The sample color was shown to shift from blue to red based on the CIE chromaticity coordinates. Consequently, these results indicate that the Sr₆Ca₄(PO₄)₆F₂:Dy³⁺/Eu³⁺ phosphors show potential as candidates for use in white LEDs.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Due to privacy/ethical/commercial restrictions, some data are not publicly available.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5nj03034j.

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