Sm³⁺ and Eu³⁺ codoped SrBi₂B₂O₇: a red-emitting phosphor with improved thermal stability

Abstract

The thermal stability of luminescence is very critical for white light-emitting diodes. However, it is a continuous challenge to improve the thermal stability of red phosphors. In this study, Sm³⁺ and Eu³⁺ codoped SrBi₂B₂O₇ was synthesized by a high temperature solid state reaction method. It was found that the thermal stability of the synthesized phosphors was improved as Sm³⁺ was used as the sensitizer for Eu³⁺ doped into SrBi₂B₂O₇. Combined with a local crystal environment study and the first-principles calculations, the origin of the improvement of this thermal stability was studied in detail. The doped Sm³⁺ and Eu³⁺ ions were inclined to occupy Bi(1) (6c) and Bi(2) (6c) sites simultaneously and the crystal structure of the SrBi₂B₂O₇:Sm³⁺, Eu³⁺ was more compact at high temperature than that at room temperature. Thus, the defect formation energy was very low in the Sm³⁺ and Eu³⁺ codoped SrBi₂B₂O₇ phosphor, which is the main reason to improve the thermal stability with Sm³⁺ and Eu³⁺ codoped into SrBi₂B₂O₇. This study provides a new idea for developing a new method to improve the thermal stability of red-emitting phosphors.

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1. Introduction

In high-power w-LEDs, when they are working the temperature of the layer deposited on the chip can rise to more than 150 °C because of the thermal effect from the p–n junction and the phosphor layer. Thus, the thermal stability of the phosphor has a great influence on the light output and color rendering index, and it is a vital factor for the application of w-LEDs. At about 150 °C, lots of the phosphors cannot show good thermal stability compared with those kept at room temperature. Up-to-now, some nitrides or oxynitrides have been proved to be good performance red phosphors because of their good thermal stability.^1,2 However, the synthesis of those phosphors requires a very high nitrogen pressure and high sintering temperature, which often results in much higher production costs. Therefore, it is highly desirable to develop new phosphors, especially red emitting phosphors, with excellent thermal stability and lower cost for warm white-light emission.

As a kind of rare earth ions, Sm³⁺ ions generate intense reddish orange emitting light because it can be excited to its ⁴F_7/2 energy level and then relaxed to the ⁴G_5/2 energy level through the non-radiative transition.^3–5 Eu³⁺ ions have been widely studied as an activator for red light emitting materials due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transition of Eu³⁺.^6–8 Presently most of used red phosphors are single doped by Sm³⁺ or Eu³⁺, such as Y₂O₂S:Eu³⁺,⁹ Y₂O₃:Eu³⁺,¹⁰ BaMoO₄:Sm³⁺,¹¹ SrCaMoO₆:Sm^3+ 12 et al. However, the majority of them show limited thermal stability.

As for the low-cost phosphors, rare-earth ions doped borates phosphors is a potential candidate because of their low synthesis temperature and high chemical and physical stability,¹³ such as Ba₂Tb(BO₃)₂Cl:Eu³⁺,¹⁴ and KSr₄(BO₃)₃:Sm³⁺.¹⁵ As Sm³⁺ and Eu³⁺ codoped into hosts, energy transfer from Sm³⁺ to Eu³⁺ can enhance the emission of the Eu³⁺ ions and extend the excitation spectrum. Thus energy transfer between Sm³⁺ and Eu³⁺ is important to adjust the photoluminescence properties. Up to now, this energy transfer has been investigated in some phosphors.^16–18 However, Sm³⁺ and Eu³⁺ codoped into suitable borates material, thus appearing energy transfer, especially improving thermal stability of the phosphor has not been reported to our knowledge.

In this study, SrBi₂B₂O₇ was considered as the host. SrBi₂B₂O₇, synthesized by J. Barbier's group in 2006,¹⁹ is crystallized in the hexagonal P6₃ space group. This is a non-centrosymmetric structure containing three crystallographic positions of cations: seven-fold coordinated Sr²⁺ (6c) sites, eight-fold coordinated Bi(1) (6c) sites, and eight-fold coordinated Bi(2) (6c) sites. Plenty of crystallographic sites are afforded for doped ions to occupy, thus SrBi₂B₂O₇ is a promising host to prepare phosphor with good luminescence properties. In this work, the new red-emitting borate phosphor SrBi₂B₂O₇:Sm³⁺,Eu³⁺, which has high thermal stability, is reported for the first time and the high thermal stability mechanism was discussed through defect formation energy calculations. The photoluminescence properties, the energy transfer from Sm³⁺ to Eu³⁺ and the preferred occupancy of the dopant were also investigated in detail.

2. Experimental

2.1 Materials and synthesis

The phosphor samples SrBi₂B₂O₇:Sm³⁺, SrBi₂B₂O₇:Eu³⁺, and SrBi₂B₂O₇:Sm³⁺, Eu³⁺ were prepared by high temperature solid-state reaction method. Analytical reagent SrCO₃, Bi₂O₃, H₃BO₃, and high purity (99.99%) Sm₂O₃ and Eu₂O₃ were used as raw materials. These powders were weighed out, well-mixed and ground thoroughly in an agate mortar. The mixtures were first heated at 500 °C for 24 h to decompose the carbonate and eliminate the water. When cooling down to the room temperature, the presintered samples were ground respectively and heated at 650 °C for 72 h, then cooling down to room temperature with the furnace cooling.

2.2 Characterization

X-ray diffraction (XRD) data for phase identification and structural refinement of as-prepared powders were collected by PANalytical powder X-ray diffractometer X'Pert Pro with Cu Kα radiation (40 kV, 40 mA) and the data were collected over a 2θ range from 10° to 140° at an interval of 0.017° with a counting time of 1 s per step. The temperature-dependent XRD patterns were measured on a Rigaku SmartLab X-ray Diffractometer with Cu Kα radiation (40 kV, 180 mA). Those samples to collect XRD data at high temperature are heated from RT to 150 °C with an intermittent heating rate of 3 °C min⁻¹ and then kept for 5 min at 150 °C before XRD data collection. High temperature XRD data were collected with a measurement time of 30 min. Photoluminescence (PL), photoluminescence excitation (PLE) spectra were recorded using a spectrofluorometer (Edinburgh Instruments, FLS920) equipped with a Xe light source and double excitation monochromators. Emitted fluorescence was detected by a photomultiplier (R928P) perpendicular to the excitation beam. A cutoff filter was used to eliminate the second-order emission of the source radiation. The luminescence decay was measured by a μF900 lamp (100 W) as a light source and a photomultiplier (R928P) was used as detector. The temperature-dependent luminescence properties were measured on the spectrofluorometer (Edinburgh Instruments, FLS920), combined with a self-made heating attachment (TAP-02). The internal quantum efficiency of optimized-composition phosphors SrBi₂B₂O₇:0.04Sm³⁺, SrBi₂B₂O₇:0.06Eu³⁺ and SrBi₂B₂O₇:0.04Sm³⁺, 0.06Eu³⁺ was determined on a standard Edinburgh Instruments FLS920 spectrometer equipped with an integrating sphere attachment.

2.3 Computational details

All the calculations were conducted by first-principles calculations, as implemented in the Vienna ab initio simulation package (VASP).^20,21 The projector augment wave (PAW) pseudopotential method was used to describe the interactions of elements. Electronic valence configurations of Sr, Bi, B, O, Sm and Eu were set as 5s², 6s²6p³, 2s²2p¹, 2s²2p⁴, 4f⁶5s²5p⁶6s², 4f⁷5s²5p⁶6s², respectively. The exchange–correlation potential used the Perdew–Burke–Ernzerhof (PBE) functional within the spin-polarized generalized gradient approximation (GGA).^22,23 A plane wave basis with cutoff energy of 400 eV was employed. The k-point sampling for Brillouin zone was generated using a 4 × 4 × 2Γ-centered grid. In structural optimized process, the energy change, maximum residual force acting on each ion was set as 1 × 10⁻⁴ eV per atom, 0.05 eV Å⁻¹, respectively. As for the formation of defects, they were all obtained through the process that Bi³⁺ ions are replaced equivalently by Sm³⁺, Eu³⁺ ions or both of them (Sm_Bi, Eu_Bi) in the conventional cell of SrBi₂B₂O₇ whose real stoichiometric formula is Sr₆Bi₁₂B₁₂O₄₂.²⁴ The formation energies of single defect such as Sm_Bi as well as Eu_Bi and the composite one Sm_Bi + Eu_Bi were calculated in our paper.

3. Results and discussion

3.1 Crystal structure

XRD patterns of phosphors SrBi₂B₂O₇:Sm³⁺, SrBi₂B₂O₇:Eu³⁺ and SrBi₂B₂O₇:Sm³⁺, Eu³⁺ are shown in Fig. 1. All XRD patterns are found to agree well with that reported in the Inorganic Crystal Structure Database (ICSD #245017), indicating that the doped Sm³⁺ and Eu³⁺ ions do not generate any impurity or induce significant changes in the host structure.

Fig. 1 XRD patterns of SrBi₂B₂O₇:Sm³⁺, SrBi₂B₂O₇:Eu³⁺, SrBi₂B₂O₇:Sm³⁺, Eu³⁺ and the ICSD (#245017) of SrBi₂B₂O₇.

3.2 Luminescence properties of SrBi₂B₂O₇:Sm³⁺,Eu³⁺

The PL and PLE spectra of SrBi₂B₂O₇:0.04Sm³⁺ are shown in Fig. 2(a). The strongest peak located at 403 nm in the excitation spectrum monitored at 598 nm is attributed to ⁶H_5/2 → ⁴F_7/2 transition. Under the 403 nm excitation, the emission spectra of Sm³⁺ show emission band at 561, 598, 644 and 704 nm due to the ⁴G_5/2 → ⁶H_J/2 (J = 5, 7, 9, 11) transitions (red curve in Fig. 2(a)). The quenching concentration of SrBi₂B₂O₇:Sm³⁺ is 4 mol%, as is shown in the inset of Fig. 2(a). Furthermore, the PL and PLE spectra of SrBi₂B₂O₇:0.06Eu³⁺ are shown in Fig. 2(b). Excited at 393 nm, two main emission bands at 611 nm and 701 nm are attributed to ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transition, and the three weak emission bands of the ⁵D₀ → ⁷F_J (J = 0, 1, 3) are located at around 578 nm, 587 nm and 653 nm. The excitation spectrum consists of the charge transfer band (CTB) and two excitation peaks at 393 and 464 nm, which are mainly caused by the strong f–f transition of ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂, respectively. The quenching concentration of SrBi₂B₂O₇:Eu³⁺ is 6 mol%, as shown in the inset of Fig. 2(b).

Fig. 2 PL and PLE spectra of SrBi₂B₂O₇:0.04Sm³⁺ (a) and SrBi₂B₂O₇:0.06Eu³⁺ (b). Inset: the dependence of the intensity on Sm³⁺ (a) and Eu³⁺ (b) concentration.

The PL and PLE spectra of Sm³⁺ and Eu³⁺-codoped SrBi₂B₂O₇ phosphor are shown in Fig. 3(a). The PLE spectrum monitored at 598 nm shows two strong excitation peaks at 393 nm and 403 nm, which correspond to the ⁷F₀ → ⁵L₆ transition of Eu³⁺ and ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺, respectively. In the PL spectrum excited at 403 nm, seven significant emission peaks located at 561, 598, 644, 578, 587, 611, 701 nm are observed, which are attributed to ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 transition of Sm³⁺ and ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transition of Eu³⁺, respectively. Fig. 3(b) shows the PL spectra of SrBi₂B₂O₇:0.04Sm³⁺,yEu³⁺ (y = 0.01–0.07) phosphors excited at 403 nm. All of the characteristic peaks of Sm³⁺ and Eu³⁺ can be observed in the PL spectra of SrBi₂B₂O₇:Sm³⁺,Eu³⁺.

Fig. 3 (a) PLE (left) and PL (right) spectra of SrBi₂B₂O₇:0.04Sm³⁺,0.06Eu³⁺ phosphor. (b) PL spectra of SrBi₂B₂O₇:0.04Sm³⁺, yEu³⁺ phosphor excited at 403 nm.

3.3 Thermal stability of SrBi₂B₂O₇:Sm³⁺,Eu³⁺

Thermal stability of the phosphors is very important for the application of w-LEDs, especially for the high-power one. Temperature-dependent PL spectra (excited with λ_ex = 403 nm) of SrBi₂B₂O₇:0.04Sm³⁺, 0.06Eu³⁺ at the temperature range of 25–300 °C are shown in Fig. 4(a). Compared to the emission intensity at room temperature, the integrated emission intensities of the characteristic peaks of Sm³⁺ and Eu³⁺ under the excitation of 403 nm at 150 °C were still remained about 87.9% and 83.4% of the initial value, which indicates that the thermal stability of SrBi₂B₂O₇:0.04 Sm³⁺, 0.06Eu³⁺ phosphor is good. The temperature dependent PL spectra of Sm³⁺ doped SrBi₂B₂O₇ and Eu³⁺ doped SrBi₂B₂O₇ were also measured. Fig. 4(b) is the normalized PL intensities of SrBi₂B₂O₇:Sm³⁺, Eu³⁺, SrBi₂B₂O₇:Sm³⁺, and SrBi₂B₂O₇:Eu³⁺. The PL intensities of the commercial red phosphor SrS:Eu²⁺ is also shown in Fig. 4(b). It is very interesting that the thermal stability of Sm³⁺ and Eu³⁺ codoped phosphor is better than those of Sm³⁺/Eu³⁺ single doped phosphors, which are all superior to that of SrS:Eu²⁺.

Fig. 4 (a) Temperature-dependent PL spectra of SrBi₂B₂O₇:0.04Sm³⁺, 0.06Eu³⁺ phosphor (λ_ex = 403 nm). (b) The comparison results of PL intensities of SrBi₂B₂O₇ (SrBiB):0.04Sm³⁺, 0.06Eu³⁺ (λ_ex = 403 nm), SrBi₂B₂O₇:0.04Sm³⁺ (λ_ex = 403 nm), SrBi₂B₂O₇:0.06Eu³⁺ (λ_ex = 393 nm) as a function of temperature. As a comparison, thermal quenching data of SrS:Eu²⁺ excited at 460 nm are also measured.

Why the thermal stability is enhanced as Sm³⁺ and Eu³⁺ codoped into the SrBi₂B₂O₇? Up to date, the mechanism of high thermal stability enhanced by rare ions codoping is rarely investigated.

To clarify this question, we first study the local crystal environment of Sm³⁺ and Eu³⁺ in SrBi₂B₂O₇. The host SrBi₂B₂O₇ crystallizes in the hexagonal space group P6₃ with a = 9.1404(4) Å, c = 13.0808(6) Å, V = 946.44(7) ų and Z = 6. In the non-centrosymmetric structure, the Bi atoms are weakly bonded to the O atoms with strongly asymmetric eight-fold coordination environment.

The BiO₈ coordination polyhedra and alternating BO₃ triangles are in one layer and linked by edge-sharings, while Sr atoms are in the adjacent layer, as is shown in Fig. 5. There are three crystallographic positions of cations in the unit cell: seven-fold coordinated Sr²⁺ (6c) sites, eight-fold coordinated Bi(1) (6c) sites, and eight-fold coordinated Bi(2) (6c) sites. As reported by Shannon,²⁵ the effective ionic radius (r) of Sm³⁺, Eu³⁺ and Bi³⁺ is 1.08 Å, 1.07 Å and 1.17 Å, respectively as the coordination number (CN) equals 8, whereas r_Sm³⁺ = 1.02 Å, r_Eu³⁺ = 1.01 Å and r_Sr²⁺ = 1.21 Å as CN = 7. Considering the r and CN, it seems that all the three sites can be occupied by Sm³⁺ and Eu³⁺.

Fig. 5 Crystal structure of SrBi₂B₂O₇ and site occupancy preference of Sm³⁺/Eu³⁺ ions to Bi1 and Bi2 in SrBi₂B₂O₇.

To clarify the structure and the local crystal environment of Sm³⁺ and Eu³⁺ in SrBi₂B₂O₇, the refinement on the XRD patterns of SrBi_1.96Sm_0.04B₂O₇, SrBi_1.94Eu_0.06B₂O₇, and SrBi_1.90Sm_0.04Eu_0.06B₂O₇ are performed by Rietveld method^26,27 within the Fullprof Program.²⁸ The final agreement factors converged to R_p = 8.50%, R_wp = 9.60%, and R_exp = 3.90% for SrBi_1.96Sm_0.04B₂O₇, R_p = 7.40%, R_wp = 9.79%, and R_exp = 4.03% for SrBi_1.94Eu_0.06B₂O₇, and R_p = 7.39%, R_wp = 9.64%, and R_exp = 3.97% for SrBi_1.90Sm_0.04Eu_0.06B₂O₇. Lattice parameters are refined to a = 9.1215(2) Å, c = 13.0550(9) Å, and V = 940.67(4) ų for SrBi_1.96Sm_0.04B₂O₇; a = 9.1128(3) Å, c = 13.0492(5) Å, and V = 938.47(6) ų for SrBi_1.94Eu_0.06B₂O₇; a = 9.1084(7) Å, c = 13.0452(6) Å, and V = 937.28(6) ų for SrBi_1.90Sm_0.04Eu_0.06B₂O₇.

Fig. 5 shows the crystal structure of SrBi₂B₂O₇. The final refinement patterns are given in Fig. 6. The crystallographic data, fractional atomic coordinates and occupancy are listed in Tables 1 and S1.†

Fig. 6 Final Rietveld refinement of the XRD profiles of SrBi_1.96Sm_0.04B₂O₇ (a), SrBi_1.94Eu_0.06B₂O₇ (b) and SrBi_1.90Sm_0.04Eu_0.06B₂O₇ (c) at room temperature. Small black circles and the red continuous lines represent the experimental and the calculated values respectively; vertical bars (|) indicate the position of Bragg peaks. The blue bottom trace depicts the corresponding residuals between the experimental and the calculated intensity values.

Table 1 Crystallographic data and Rietveld refinement data for SrBi_2−x−ySm_xEu_yB₂O₇ at room temperature

Chemical formula	x = 0.04, y = 0	x = 0, y = 0.06	x = 0.04, y = 0.06
Crystal system	Hexagonal	Hexagonal	Hexagonal
Space group	P63	P63	P63
a/Å	9.1215(2)	9.1128(3)	9.1084(7)
c/Å	13.0550(9)	13.0492(5)	13.0452(6)
V/Å^3	940.67(4)	938.47(6)	937.28(6)
Z	6	6	6
Rp (%)	8.50	7.40	7.39
Rwp (%)	9.60	9.79	9.64
Rexp (%)	3.90	4.03	3.97
RBragg (%)	8.61	9.52	9.86

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The final compositions determined by refinements are in good agreement with the nominal composition of the starting materials, as shown in Table S1.† It can be seen that the doped cations are preferred to occupy the Bi(1) (6c) sites and Bi(2) (6c) sites simultaneously for all of the Sm³⁺/Eu³⁺ single doped samples or codoped samples. This site occupancy is quite different from that of ZnBi₂B₂O₇:Eu³⁺,⁶ in which the Eu³⁺ ions are preferred to only occupy the Zn site instead of Bi sites. ZnBi₂B₂O₇ and SrBi₂B₂O₇ are nearly the same in the chemical formula. However, their structures are different. ZnBi₂B₂O₇ adopts an orthorhombic structure with space group Pba2 and Z = 4. The Bi³⁺ cations in ZnBi₂B₂O₇ occupy two distinct interlayer sites with asymmetric six-fold coordination environments. The bond valence sum for Bi(1) and Bi(2) in ZnBi₂B₂O₇ is 2.93 and 2.99, respectively,²⁹ which is close to the theoretical value of 3. However, the Bi³⁺ cations and O²⁻ anions are weakly bonded in SrBi₂B₂O₇. The weak bonding interactions can result in the fragility and easy cleavage. The bond valence sum for Bi(1) and Bi(2) in SrBi₂B₂O₇ is 2.39 and 2.29, respectively,¹⁹ which is much lower than those in ZnBi₂B₂O₇ and indicates a significant underbonding. Thus, the Bi³⁺ cations in SrBi₂B₂O₇ are much more unstable than that in ZnBi₂B₂O₇. Then the different occupancy preferences of the dopants are observed in ZnBi₂B₂O₇ and SrBi₂B₂O₇.

On this basis, the refinement on the XRD patterns of SrBi_1.96Sm_0.04B₂O₇, SrBi_1.94Eu_0.06B₂O₇ and SrBi_1.90Sm_0.04Eu_0.06B₂O₇ at 150 °C are further performed by Rietveld refinement within the Fullprof Program to clarify the change of the local crystal environment of Sm³⁺ and Eu³⁺ in SrBi₂B₂O₇. The final agreement factors converged to R_p = 7.91%, R_wp = 12.10% and R_exp = 3.96% for SrBi_1.96Sm_0.04B₂O₇, R_p = 8.90%, R_wp = 12.90% and R_exp = 4.03% for SrBi_1.94Eu_0.06B₂O₇, and R_p = 9.06%, R_wp = 13.10% and R_exp = 3.97% for SrBi_1.90Sm_0.04Eu_0.06B₂O₇. Lattice parameters are refined to a = 9.1194(3) Å, c = 13.0520(4) Å, and V = 940.02(5) ų for SrBi_1.96Sm_0.04B₂O₇, a = 9.1100(3) Å, c = 13.0440(5) Å, and V = 937.51(5) ų for SrBi_1.94Eu_0.06B₂O₇, and a = 9.1057(3) Å, c = 13.0406(5) Å, and V = 936.40(6) ų for SrBi_1.90Sm_0.04Eu_0.06B₂O₇. The final refinement patterns are given in Fig. 7. The crystallographic data, fractional atomic coordinates and occupancy are listed in Tables 2 and S2;† selected average bond lengths (Å) of Bi–O are reported in Table S3.†

Fig. 7 Final Rietveld refinement of the XRD profiles of SrBi_1.96Sm_0.04B₂O₇ (a), SrBi_1.94Eu_0.06B₂O₇ (b) and SrBi_1.90Sm_0.04Eu_0.06B₂O₇ (c) at 150 °C.

Table 2 Crystallographic data and Rietveld refinement data for SrBi_2−x−ySm_xEu_yB₂O₇ at 150 °C

Chemical formula	x = 0.04, y = 0	x = 0, y = 0.06	x = 0.04, y = 0.06
Crystal system	Hexagonal	Hexagonal	Hexagonal
Space group	P63	P63	P63
a/Å	9.1194(3)	9.1100(3)	9.1057(3)
c/Å	13.0520(4)	13.0440(5)	13.0406(5)
V/Å^3	940.02(5)	937.51(5)	936.40(6)
Z	6	6	6
Rp (%)	7.91	8.90	9.06
Rwp (%)	12.10	12.90	13.10
Rexp (%)	3.96	4.03	3.97
RBragg (%)	9.87	10.10	10.35

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As comparing the refined lattice parameters at 150 °C with those at room temperature, it is very interesting that the lattice parameters a, c, and V become smaller for both codoped and single doped samples. This unusual phenomenon is contrary to the rule of thermal expansion and contraction. Furthermore, as indicated in Table S3,† for the codoped and single doped phosphors, the average bond lengths between Bi (6c) and O (6c) sites at 150 °C is shorter than that at room temperature. Also, the average bond lengths of Bi–O in codoped phosphors at high temperature or RT is shorter than those in single doped phosphors at the same temperature. The above results indicates that the crystal structure of the codoped and single doped phosphors at high temperature is more compact than those of the phosphors at room temperature, which should contribute to the improved thermal stability as Sm³⁺ and Eu³⁺ codoped into SrBi₂B₂O₇.

In addition, to investigate whether the high thermal stability is related with the electronic band gap, the electronic band structures of Sm³⁺/Eu³⁺ singled doped SrBi₂B₂O₇, Sm³⁺ and Eu³⁺ codoped SrBi₂B₂O₇ and undoped SrBi₂B₂O₇ are computed with VASP and shown in Fig. 8. The electronic band gap of SrBi₂B₂O₇:Eu³⁺, SrBi₂B₂O₇:Sm³⁺, SrBi₂B₂O₇:Sm³⁺, Eu³⁺ and SrBi₂B₂O₇ are 2.57, 2.78, 2.60 and 2.81 eV, respectively. They are different somewhat. However, the difference among these electronic band gaps are small, which means that the electronic band structure should not the main reason to enhance the thermal stability as Sm³⁺ and Eu³⁺ codoped into SrBi₂B₂O₇.

Fig. 8 Electronic band structures of undoped and doped SrBi₂B₂O₇. The Fermi level is set to 0 eV.

As rare earth ions doped into the host, the corresponding defects will be formed. As studied on the site occupancy of the doped ions in this study, Sm³⁺ and Eu³⁺ ions are preferred to occupy Bi sites, thus the formed defects should mainly Sm_Bi and Eu_Bi. For the formed defects, the defect formation energy (E_f) will be different because of the different doped ions. In this study, to clarify the difference of E_f among Sm³⁺, Eu³⁺ single doped and codoped SrBi₂B₂O₇. E_f of SrBi_1.90Sm_0.04Eu_0.06B₂O₇, SrBi_1.94Sm_0.06B₂O₇ and SrBi_1.96Eu_0.04B₂O₇ defined as the following formula,^30,31 are calculated with VASP.

E_f = E(Sr₆Bi_12−x−yB₁₂O₄₂Sm_xEu_y) − E(Sr₆Bi₁₂B₁₂O₄₂) − xμ_Sm − yμ_Eu + (x + y)μ_Bi (1)

where E(Sr₆Bi_12−x−yB₁₂O₄₂Sm_xEu_y) and E(Sr₆Bi₁₂B₁₂O₄₂) are the total energy of the rare earth doped compound and pure compound, respectively. μ_Sm, μ_Eu and μ_Bi are the chemical potentials of bulk Sm, Eu and Bi, respectively. The total energy (E_tot) and defect formation energy (E_f) of the Sr(Bi,M)₂B₂O₇ (M = Sm, Eu) are listed in Table 3, in which case A, B, C represents SrBi_1.90Sm_0.04Eu_0.06B₂O₇, SrBi_1.94Sm_0.06B₂O₇ and SrBi_1.96Eu_0.04B₂O₇, respectively. It is indicated that the defect formation energy of the case A (i.e. Sm³⁺ and Eu³⁺ codoped sample) is −5.47 eV, which is the lowest value among case A, B and C (the defect formation energies of case B and case C are −5.20 eV and 0.30 eV, respectively). The defect formation energies of Sm³⁺ and Eu³⁺ co-doped and Sm³⁺ single doped phosphors are ∼5.77 eV and ∼5.50 eV lower than that of Eu³⁺ single doped phosphor, respectively. These big obvious energy differences indicate that it is much easier to form the double defects like Sm_Bi and Eu_Bi than single ones. For the single defect, Sm_Bi is much easier to form than Eu_Bi. From the total energy of the case A, B, and C, they also have the same trend as formation energy. Thus, as the phosphors were heated to high temperature, the double defects like Sm_Bi and Eu_Bi will be more stable than the single ones. So the improvement of thermal stability of Sm³⁺ and Eu³⁺ codoped into SrBi₂B₂O₇ should mainly because of the large defect formation energy of double defects.

Table 3 Total energy (E_tot) and formation energy (E_f) of the Sr(Bi,M)₂B₂O₇ (M = Sm, Eu)

Sample compositions	Etot (eV)	Ef (eV)
SrBi1.90Sm0.04Eu0.06B2O7	−530.84	−5.47
SrBi1.94Sm0.06B2O7	−517.67	−5.20
SrBi1.96Eu0.04B2O7	−510.15	0.30

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The active energy of Sm³⁺ and Eu³⁺ codoped SrBi₂B₂O₇ phosphor was also studied. It is well known that the decrease of the emission intensity at different temperature can be described by the Arrhenius equation:³²

I_T = I₀/[1 + exp(−E_a/kT)] (2)

where I₀ and I_T are the luminescence intensities of SrBi₂B₂O₇:0.04Sm³⁺,0.06Eu³⁺ at room temperature and the testing temperature, respectively. E_a is the activation energy and k is the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹). As is displayed in Fig. 9, E_a of Sm³⁺ and Eu³⁺ in SrBi₂B₂O₇:Sm³⁺, Eu³⁺ obtained to be 0.27 eV and 0.22 eV are bigger than the E_a of Sm³⁺ in SrBi₂B₂O₇:Sm³⁺ and Eu³⁺ in SrBi₂B₂O₇:Eu³⁺, which are calculated to be 0.15 eV and 0.20 eV, respectively. These results further indicate that the Sm³⁺ and Eu³⁺ codoped phosphor has good thermal stability.

Fig. 9 The ln(I₀/I_T − 1) vs. 1/kT activation energy graph for thermal quenching of the characteristic emission of Sm³⁺ (a) in SrBi₂B₂O₇:0.04Sm³⁺ phosphor, Eu³⁺ (b) in SrBi₂B₂O₇:0.06Eu³⁺ phosphor, Sm³⁺ (c) and Eu³⁺ (d) in SrBi₂B₂O₇:0.04Sm³⁺,0.06Eu³⁺ phosphor.

3.4 Quantum efficiency

Quantum efficiency is an important parameter for LED phosphor. To determine the absolute quantum efficiency of photo-conversion for the SrBi₂B₂O₇:Sm³⁺, Eu³⁺ phosphor, the optical absorbance (A) and internal quantum efficiency (η_int) was measured using the integrated sphere method. The absorbance can be calculated according to the equation:where L₀(λ) is the integrated excitation profile when the sample is diffusely illuminated by the integrated sphere's surface and L_i(λ) is the integrated excitation profile when the sample is directly excited by the incident beam. Furthermore, the internal quantum efficiency (QE) of the phosphors can be calculated by.where E_i(λ) is the integrated luminescence of the powder upon direct excitation, and E₀(λ) is the integrated luminescence of the powder 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). The internal quantum efficiency (QE) of the SrBi₂B₂O₇:0.04Sm³⁺ phosphor and SrBi₂B₂O₇:0.04Sm³⁺,0.06Eu³⁺ phosphor both under 403 nm excitation, SrBi₂B₂O₇:0.06Eu³⁺ phosphor excited at 393 nm are determined to be 33.2%, 42.6%, and 37.5% respectively, indicating the Sm³⁺ and Eu³⁺ codoped phosphor has better internal QE.

4. Conclusions

In conclusion, a red-emitting SrBi₂B₂O₇:Sm³⁺,Eu³⁺ phosphor with improved thermal stability was synthesized through solid state reactions. The origin of the reason why thermal stability is improved is studied. The structural study results indicate that the doped Sm³⁺ and Eu³⁺ ions are inclined to occupy Bi(1) (6c) and Bi(2) (6c) sites simultaneously and the crystal structure of the SrBi₂B₂O₇:Sm³⁺, Eu³⁺ was more compact at high temperature than that at room temperature. According to the first-principles calculation, the lowest defect formation energy −5.47 eV is obtained in the Sm³⁺ and Eu³⁺ codoped SrBi₂B₂O₇ phosphor, which discloses that the defect formation energy should be another intrinsically responsible for the thermal stable luminescence of red-emitting SrBi₂B₂O₇:Sm³⁺, Eu³⁺ phosphor.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (51372121, 61274053, U1460201, 51572132), Natural Science Foundation of Tianjin (14JCYBJC17800) and 111 Project (No. B07013) and YangFan Innovative & Entrepreneurial Research Team Project (2014YT02N037). We thank Mrs Xu of A02 group, Institute of Physics, Chinese Academy of Science for her great help in collecting the powder X-ray diffraction data.

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Footnote

† Electronic supplementary information (ESI) available: Sm³⁺ and Eu³⁺ codoped SrBi₂B₂O₇: a red-emitting phosphor with improved thermal stability. See DOI: 10.1039/c6ra26752a

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