A novel tunable Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺,Mn²⁺ phosphor with excellent thermal stability for white light emitting diodes

Abstract

In this paper, Ce³⁺ doped and Ce³⁺,Mn²⁺ co-doped Na₂Ba₆(Si₂O₇)(SiO₄)₂ phosphors were synthesized via a high temperature solid-state reaction. The Rietveld refinement analysis of the X-ray diffraction patterns confirmed the formation of the single phase of Na₂Ba₆(Si₂O₇)(SiO₄)₂. The PL spectrum of the Ce³⁺ single-doped phosphor shows a broad asymmetric band extending from 350 to 600 nm peaking at 420 nm under the excitation of UV light. Also, the low temperature (5 K) PL spectrum shows clearly three peaks at 375, 420, and 451 nm, which is in accordance with the Ce³⁺ ion site in the host. The Ce³⁺,Mn²⁺ co-doped phosphors show a blue emission band and an orange emission band, and the corresponding CIE coordinates intuitively indicate the tunable colors from the blue to green area. And the energy transfer mechanism from Ce³⁺ to Mn²⁺ in the host has been verified to be a dipole–quadrupole interaction. In addition, the emission intensity of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ phosphor slightly changed with the temperature increase from 300 to 450 K, revealing that the obtained phosphor possesses an excellent thermal stability, which makes it an attractive candidate phosphor for white lighting emitting diodes.

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1. Introduction

In the past decade, solid-state white light emission has confirmed its irreplaceable status for new solid-state light sources in the context of environmental and energy saving issues. As a result, phosphors, as an important part of white lighting emitting devices have aroused worldwide research interest.^1–3 Phosphors are always composed of two indispensable compositions as the matrix and luminescence center. The matrix is always developed from the various compounds. Always, silicate with diverse, rigid and very stable structures have been investigated widely for either their physicochemical phenomena and mechanisms or available application and practical usage in phosphor.⁴ Among various silicate compounds, Sr₃SiO₅ has been highlighted and chosen as an matrix for many years, because the Sr₂SiO₅ doped with Ce³⁺ ion phosphor shows excellent performance as a yellow component for WLEDs.^5,6 As to the luminescence center, we have to mention Ce³⁺ ion, which always acted as the luminescence centers in phosphor. The Ce³⁺ ion can be excited by either a UV chip or a blue chip. The emission band of Ce³⁺ ion is usually composed of a broad band and varies from ultraviolet to yellow light, depending on the host, which is caused by the Ce³⁺ ion 4f¹ ground state undergoing parity allowed f–d transition with 5d excited state.⁷ Therefore, matrices doped with Ce³⁺ ions show great potential as phosphor for white lighting emitting diodes, and indeed receive a lot of attention recently. As a result, many useful phosphors have been reported successively.^8,9 The famous Ce³⁺ doped yellow phosphor is Y₃Al₅O₁₂:Ce³⁺, however, the phosphor suffers a low R_a when pumped by a blue LED chip, which is caused by the lack of thermal stability at elevated temperatures during white LED operation. Therefore, most researchers are committed on developing novel phosphors with improved thermal stability now. In addition, the Ce³⁺ ion, apart from its own intense emission, can facilitate the energy transfer to various luminescent centers such as Tb³⁺ ion or Mn²⁺ ion, which can significantly solve the overheating problem associated with inappropriate excitation for WLEDs.^10,11

Here, we choose Na₂Ba₆(Si₂O₇)(SiO₄)₂ matrix as a research objectives. Notwithstanding the fact that the Na₂Ba₆(Si₂O₇)(SiO₄)₂ matrix has been first reported by Tamazyan et al.,¹² the most interesting feature of the co-existence structure of Si₂O₇ and SiO₄ in the Na₂Ba₆(Si₂O₇)(SiO₄)₂ matrix has not attracted much attention from materials scientists until recently. Also, to our best knowledge, neither investigations regarding on the luminescence properties of Na₂Ba₆(Si₂O₇)(SiO₄)₂ nor the energy transfer mechanism from Ce³⁺ to Mn²⁺ in the host has been reported. Therefore, we speculate whether the unique structure doped with Ce³⁺ or Mn²⁺ can produce some surprising phenomenon. Here, we focus on investigating the crystal structure, the thermal stability and the luminescent properties of Ce³⁺ or Ce³⁺,Mn²⁺ co-doped Na₂Ba₆(Si₂O₇)(SiO₄)₂ phosphors. The result indicates that this novel developed phosphor has excellent thermal stability and shows tunable blue to yellow emission excited by UV light.

2. Experimental section

2.1 Materials synthesis

Polycrystalline powder samples with the compositions of Na₂Ba₆(Si₂O₇)(SiO₄)₂:xCe³⁺,yMn²⁺ (NBSO:Ce³⁺,Mn²⁺) were synthesized via a conventional solid-state reaction with NaNO₃ (A.C), BaCO₃ (A.C), SiO₂ (A.C), CeO₂ (99.99%) and MnCO₃ (99.99%) as the raw materials. The stoichiometric amounts of the raw materials were weighed out and thoroughly mixed by grinding in an agate mortar. Afterward, the obtained product was then annealed at 1050 °C for 8 h with an intermediate regrinding under a 5% H₂–95% N₂ atmosphere. Finally, the temperature lowered to room temperature and some white polycrystalline powder was obtained.

2.2 Characterization

The crystalline phases of the samples were identified by X-ray diffraction (XRD), which were performed on a D8 Focus diffractometer (Bruker) operating at 40 kV and 40 mA with Cu.

Kα radiation. The X-ray diffraction data were recorded as follows: 2θ range: 15°–70°, scanning speed: 0.5 min, 0.02 step. The powder diffraction data were subjected to performa computer software General Structure Analysis System (GSAS) package.¹³ The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Thermoluminescence (TL) spectra were measured with a three-dimensional (3D)-TSL spectra instrument. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). All the measurements were performed at room temperature.

3. Results and discussion

3.1 Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺ phosphor

Fig. 1(a) shows the X-ray diffraction patterns of several representative Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ samples. It is obvious that all the diffraction peaks of the samples can be exactly indexed to the standard data of Na₂Ba₆(Si₂O₇)(SiO₄)₂ (JCPDS card no. 78-1728), which indicates that the prepared phosphors are of single phase and the doped Ce³⁺ and Mn²⁺ ions have been incorporated into the host successfully. To further investigate the structure of the Na₂Ba₆(Si₂O₇)(SiO₄)₂ host, Rietveld refinement of the X-ray diffraction patterns of the Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ sample has been done at room temperature as Fig. 1(b) shows.¹³ The initial structural model was constructed with crystallographic data previously reported for Na₂Ba₆(Si₂O₇)(SiO₄)₂ (JCPDS78-1728). The crystallographic cell parameters proceeded smoothly to convergence and do not show a significant change considering the standard deviations. All of the observed peaks satisfy the reflection condition, χ² = 7.85, R_p = 6.28% and R_wp = 8.819%. The refinement result indicates the Na₂Ba₆(Si₂O₇)(SiO₄)₂ has space group P2₁/a with unit cell parameters a = 11.52 Å, b = 9.508 Å, c = 7.856 Å, V = 820.64 ų, and Z = 2. Meanwhile, the refinement result further verifies that the structure of Na₂Ba₆(Si₂O₇)(SiO₄)₂ host is unchanged with the doping of luminescence ions.

Fig. 1 X-ray diffraction patterns of several representative Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ samples (y = 0, 0.01) (a) and results of structure refinement of X-ray diffraction patterns for Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ (b).

Fig. 2(a) and (b) represents a spatial view of the Na₂Ba₆(Si₂O₇)(SiO₄)₂ unit cell from different directions. One can see clearly the discrete [SiO₄] anions (pink polohedron) and isolated [Si₂O₇] anions (gray polohedron) in this unit cell. The discrete [SiO₄] anions and isolated [Si₂O₇] anions form unique layer in sandwich package way. The isolated [SiO₄] anions arrange in the inside of the cell, while the isolated [Si₂O₇] anions occupy the upper and lower plane. The entire above package manner to delimit right coordination cavities occupied by Ba²⁺ and Na⁺ cations, respectively. There are three crystallographic independent Ba²⁺ ions positions denoted as Ba(1), Ba(2), and Ba(3), in which Ba(1) and Ba(2) are in 9-coordination environment with tricapped trigonal prismatic geometry, while Ba(3) occupies 10-coordinated O polyhedra constructing bicapped square prism which builds distorted polyhedron with oxygens interconnected via common edges, respectively. Fig. 2(c) displays the different coordination environments of the three Ba²⁺ ions. Another cation in the host, the Na⁺ ion is all coordinated by six oxygen atoms in the form of an octahedra. On the basis of the above structure analysis, the coordination diversity of the Ba²⁺ sites in the host is benefit to the luminescence.¹²

Fig. 2 Crystal structure of Na₂Ba₆(Si₂O₇)(SiO₄)₂ in different directions (a) and (b); coordination of three different Ba²⁺ ions (c).

Fig. 3(a) shows the PLE and PL spectra of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺. The PLE spectrum is composed of a broad band range from 200 to 400 nm, a characteristic feature of the Ce³⁺ ions emission band from the 4f¹ ground state to the 5d level as described by Judd–Ofelt theory. The PL spectrum shows a broad asymmetric band extending from 350 to 600 nm peaking at 420 nm.^14,15 The asymmetry of PL band is always caused by the various luminescence sites in the host. Always, the value of Stokes shift is an important property of PL spectrum, which can measure the stiff of host lattice and the degree of the nonradiative relaxation after the luminescent ions excited. The Stokes shift can be roughly estimated as twice of the energy difference between the peak energy of the emission band and the zero-phonon line energy that was empirically determined as the intersection point of the excitation spectrum and emission spectrum.¹⁶ The Stokes shift of emission is roughly calculated to be 0.694 eV in energy here. In order to better understand the photoluminescence property of Ce³⁺ ion, we measure the PL spectrum of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ at 5 K (Fig. 3(b)). The emission curve has been well-fitted with a sum of three Gaussian functions in energy. One can see clearly a broad band with three peaks at 375, 420 and 451 nm, respectively. These components can be ascribed to the contributions of the transitions from the lowest 5d excited states to the ground states in three different Ce³⁺ luminescence centres, which is in accord with the number of Ba²⁺ sites in the host. It has been suggested the d-band edge (E) in energy of Ce³⁺ ion emission is sensitive to electron–electron repulsion, which always obey an empirical relation by Van Uitert as following:¹⁷

here Q* is the position in energy for the lower d-band edge for the free Ce³⁺ ion (Q* = 50 000 cm⁻¹), E is the position for the Ce³⁺ ion emission peak, V is the valence of the Ce³⁺ ion (V = 3), n is the number of anions in the immediate shell about the Ce³⁺ ion, E_a is the electron affinity of the atoms (eV), and r is the radius of the host cation replaced by Ce³⁺ ion (Å). E_a is a constant in the same host. Here, V = 3, Q* = 50 000 cm⁻¹, the value of E is directly proportional to the product of n and r. In our case, Ba(1) and Ba(2) sites are nine-coordinated by nine O atoms with Ba–O distance of 2.890 Å and 2.894 Å, respectively. The Ba(3) site is ten-coordinated with O atoms at an average Ba–O distance of 2.938 Å. Therefore, we can get a conclusion that the band centred at 375 nm is attributed to the 5d–4f emission of Ce³⁺ ion occupied the Ba(3) ion site with ten-coordinate, and the bands at 451 and 420 nm are due to the Ce³⁺ ion occupied Ba(1) and Ba(2) ion site with nine coordination.

Fig. 3 PLE spectrum observed at 420 nm (a); PL spectrum of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ at room temperature and 5 K under excited at 330 nm (b).

The thermal quenching property is an important technological parameter for phosphors used in practical solid-state lighting. Fig. 6 indicates the temperature-dependent relative emission intensities of the as-prepared Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ phosphor under the 340 nm excitation. With the temperature increasing from room temperature to 450 K, the integrated emission intensity of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ phosphor changes from 100% (300 K) to 93.7% (450 K), indicating that the phosphor possesses a excellent thermal stability. To verify the origin of temperature dependent emission intensity I_T, the activation energy ΔE (the electrons excited from 4f states to the lowest 5d states of Ce³⁺ ion) can be described as the following equation:^18,19

where I₀ and I_T represent the PL intensity of room temperature and testing temperature, respectively; k represents Boltzmann constant. The ΔE values for Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ were calculated to be 0.2446 eV as shown inset of Fig. 4. The larger ΔE (Na_xCa_1−xAl_2−xSi_2+xO₈:Eu²⁺ (x = 0.34), ΔE = 0.033 eV; NaBaScSi₂O₇, ΔE = 0.081 eV) is can be a theory evidence certifying the excellent thermal stability of this novel phosphor.^20,21 The excellent thermal stability of the Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce phosphor has aroused our great interest. The unusual features may originate from the bigger Ba²⁺ ions site and the unique crystal chemistry structure. Since the ionic radii of Ce³⁺ ions (CN = 9, R = 1.196; CN = 10, R = 1.25) is smaller than Ba²⁺ ions (CN = 9, R = 1.47; CN = 10, R = 1.52) with the same coordination in Na₂Ba₆(Si₂O₇)(SiO₄)₂ host lattice. With the increasing temperature, the interaction between the electrons is reduced, so that they spread out over wider orbitals which would result in the increasing covalence of Ce–O, as a result, the surrounding of the activator Ce³⁺ ions will be forced to expand, fortunately, the larger Ba²⁺ ion site makes Ce³⁺ ion match with the surrounding. On the other hand, another influence factor for the excellent thermal stability can be got from the Na₂Ba₆(Si₂O₇)(SiO₄)₂ structure. Compared with other silicate compounds as Na_xCa_1−xAl_2−xSi_2+xO₈:Eu²⁺ (x = 0.34) or NaBaScSi₂O₇, the crystal structure here, is composed by discrete [SiO₄] anions and isolated [Si₂O₇] anions, and these anions build in a unique stacking mode. The isolated [Si₂O₇] anions construct the main skeleton of Na₂Ba₆(Si₂O₇)(SiO₄)₂ host and the discrete [SiO₄] anions inserts into the inner space as Fig. 2(a) and (b) shows. Thus, we speculate that the coexistence of the two different anions and the unique stacking mode in geometry may improve the stability of compounds.²²

Fig. 4 PL intensity of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ from room temperature to 450 K (λ_ex = 340 nm). The inset shows the plot ln[I₀/I_T − 1] versus 1/kT.

3.2 Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺,Mn²⁺ phosphor

As shown in Fig. 5, the PL spectrum of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺ shows a broadband emission from 350 to 600 nm centered at 420 nm, which was attributed to the f–d transition. The PLE spectrum of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.03Mn²⁺ contains several bands centered at 344, 365, and 413 nm, corresponding to the transitions from the ⁶A₁(⁶S) ground state to the ⁴E(⁴D), ⁴T₂(⁴D), and [⁴A₁(⁴G), ⁴E(⁴G)] excited states, respectively. The significant spectral overlap between the PL spectrum of Ce³⁺ and PLE spectrum of Mn²⁺ indicates that the energy level of Ce³⁺ ions matches with well the energy level of Mn²⁺ ions. Thus, we can expect an effective resonance-type energy transfer from the Ce³⁺ to Mn²⁺ ions. Moreover, as shown in Fig. 6, another possible evidence, the PLE spectrum monitoring the emission (585 nm) of the Mn²⁺ and that (420 nm) of Ce³⁺ are similar. Under 340 nm excitation, the PL spectrum of the codoped sample Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,0.01Mn²⁺ in Fig. 6 shows both a blue band corresponding to the f–d transition of Ce³⁺ ions and a yellow band attributing to the ⁴T₁–⁶A₁ transition of Mn²⁺ ions. As a result, we can get various color tunes via adjusting the Mn²⁺ ions content.²³

Fig. 5 Overlap of Ce³⁺ emission spectrum to the Mn²⁺ excitation spectrum.

Fig. 6 The PLE and PL spectra of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,0.01Mn²⁺.

Fig. 7 depicts the PL spectra of several Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.11) phosphors. We can observe clearly that the PL luminescent intensity of Ce³⁺ ions decreases with the Mn²⁺ ions concentration up to 0.11. However, the emission intensity of Mn²⁺ ions shows an enhancement initially until 0.07, beyond which its intensity shows a drastic reduction. The phenomenon indeed indicate efficient energy transfer from Ce³⁺ to Mn²⁺ ions.

Fig. 7 PL spectra of Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.11) phosphors under 340 nm excitation.

The energy transfer efficiency (η_T) from Ce³⁺ to Mn²⁺ ions in Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ can be calculated by the formula:²⁴

where I_so is the intensity of the Ce³⁺ ions and I_s is the intensity of the Ce³⁺ ions in the presence of the Mn²⁺ ions. Thus, the energy transfer efficiency was calculated and presented in Fig. 8. The energy transfer efficiency was found to increase gradually with the increase of the Mn²⁺ concentration. As a result, a rough estimation of the critical distance (R_sa) for energy transfer from Ce³⁺ to Mn²⁺ ions can be calculated from the following relationship given by Blasse:
where V is the volume of the unit cell, x_c is the critical concentration of the activator ion, and N represents the number of sites that the Ce³⁺ ion can occupy in per unit cell. For the Na₂Ba₆(Si₂O₇)(SiO₄)₂ host, N = 12, x_c = 0.07, and V = 820.64 ų, Therefore, the R_sa value is calculated to be 12.3 Å. As we know, the resonant energy-transfer mechanism consists of two types: one is exchange interaction and another is multipolar interaction. It is known that if energy transfer takes the exchange interaction, the critical distance between the sensitizer and activator should be shorter than 3–4 Å. Here, the critical distance is longer than 3–4 Å, which indicates the more possibility of energy transfer via the multipolar interaction mechanism.

Fig. 8 Energy transfer efficiency from Ce³⁺ to Mn²⁺.

According to Dexter's energy transfer expressions of multipolar interaction, the following relation can be easily obtained:^25,26

where I_so and I_s are the luminescence intensities of the Ce³⁺ ions with and without the Mn²⁺ ions, and C is the sum of Ce³⁺ and Mn²⁺ concentration. α = 6, 8, and 10 are dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interaction, respectively. Fig. 9 has shown the plots of I_so/I_s of Ce³⁺ vs. α = 6, 8, 10. A line relation is well-fit at α = 8, indicating the energy transfer mechanism via a dipole–quadrupole interaction from the Ce³⁺ to Mn²⁺ ions. Considering the dipole–quadrupole mechanism, the critical distance of energy transfer from the Ce³⁺ to Mn²⁺ can be calculated by the spectral overlap method. The energy transfer probability P_sa (in s⁻¹) from the Ce³⁺ to Mn²⁺ can be written as:
where f_q is the oscillator strength of the involved absorption transition of the acceptor, λ_s (in Å) is the emission position of the sensitizer (in nm), R is the distance between the activator and acceptor, τ_S is the decay lifetime of the sensitizer (in seconds). E is the emission energy of the Ce³⁺ ion (in eV), and ∫F_s(E)F_a(E) dE/E⁴ represents the spectral overlap between the normalized shapes of the sensitizer emission and the acceptor excitation. The critical distance R_sa between the sensitizer and activator is defined as the distance at which the probability of energy transfer equals the probability of radiative emission of the Ce³⁺ ions. Namely, at the critical distance there has the equation P_saτ_s = 1. Then, R_sa can be obtained by the following formula:^27,28

Fig. 9 Dependence of I_so/I of (Ce + Mn) ion concentration (a) C^6/3, (b) C^8/3, and (c) C^10/3.

On the basis of the above equation combined with f_q = 10⁻¹⁰, the critical distance R_C was calculated to be 11.6 Å, which agrees approximately with that obtained by using the concentration-quenching method.

To determine the quantum efficiency of photoconversion for this phosphor. Herein we applied the integrated sphere method for the measurements of optical absorbance (A) and quantum efficiency (Φ) of the phosphors. The optical absorbance and quantum efficiency of Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺,Mn²⁺ phosphors were calculated by using the following two equations:

where L₀(λ) is the integrated excitation profile when the sample is diffusely illuminated by the integrated sphere's surface, L_i(λ) is the integrated excitation profile when the sample is directly excited by the incident beam, E_i(λ) is the integrated luminescence of the sample upon direct excitation, and E₀(λ) is the integrated luminescence of the sample excited by indirect illumination from the sphere. The term L_e(λ) is the integrated excitation profile obtained from the empty integrated sphere (without the sample present). On the basis of the above two equations, the measured values of quantum efficiency were 43.1%, 36.7%, 35.2%, 30.4%, 22.7%, 17.1% in Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ phosphor with y = 0, 0.03, 0.07, 0.09, 0.11 material at room temperature under the excitation wavelength of 340 nm. The higher quantum efficiency can be obtained by further improving the synthesis conditions to reduce the number of defects and impurities and to get a high crystallization of the phosphors.

Fig. 10 portrays the CIE chromaticity coordinates for the Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ (0.01, 0.03, 0.07, 0.09, 0.11) phosphors under 340 nm excitation. Generally, the coordinates shift from blue area (point A) to yellow area (point F) with the increase of Mn²⁺ ion concentration. It is found that the CIE chromatic coordinate is located at (0.348, 0.262) in the white-light area when the concentration of Mn²⁺ ions increases to y = 0.07. And the ideal CIE chromatic coordinate of WLEDs can be got by optimizing the amount of Ce³⁺ ions and Mn²⁺ addition simultaneously. The result indicates the novel Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺,Mn²⁺ with tunable color hue has a great potential phosphor fo UV-excited WLEDs.

Fig. 10 CIE chromaticity diagram Na₂Ba₆(Si₂O₇)(SiO₄)₂:0.01Ce³⁺,yMn²⁺ (0.01, 0.03, 0.07, 0.09, 0.11) phosphors under 340 nm excitation.

4. Conclusion

In summary, we have synthesized novel Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺ and Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺,Mn²⁺ phosphors by high temperature solid state method. The Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺ phosphor show a broad emission band under UV excitation and excellent thermal stability. The Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺,Mn²⁺ phosphor shows tunable color by controlling the composition of Mn²⁺ content. The energy transfer from Ce³⁺ ion to Mn²⁺ ion has been confirmed to be via a dipole–quadrupole mechanism on the basis of the Dexter's energy transfer theory. The involved critical distance of energy transfer has also been calculated by concentration quenching method and spectral overlap method. All the above investigations show that the novel Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺,Mn²⁺ phosphor with tunable emission property has great potential as novel phosphor for UV-WLEDs.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Grant no. 21271167) and the Fund for Creative Research Groups (Grant no. 21221061).

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