Site Occupancy Preference and Antithermal Quenching of the Bi2+

May 24, 2017 - In β-Ca2P2O7, the host lattice has four different calcium sites, at which .... in an agate mortar, the sample was sintered at 1100 °C...
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Site Occupancy Preference and Antithermal Quenching of the Bi2+ Deep Red Emission in β‑Ca2P2O7:Bi2+ Liyi Li,†,‡ Jiangkun Cao,†,‡ Bruno Viana,∥ Shanhui Xu,† and Mingying Peng*,† †

The State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, School of Materials Science and Technology, South China University of Technology, Guangzhou 510640, People’s Republic of China ∥ PSL Research University, Chimie ParisTech-CNRS, Institut de Recherche de Chimie Paris, UMR8247, 11 rue P. et M. Curie, 75005 Paris, France ABSTRACT: The resistance to thermal quenching is an essential factor in evaluating the performance of luminescent materials for application in white light emitting diodes (WLEDs). In this work, we studied the site occupancy preference and thermal quenching of luminescence in βCa2P2O7:Bi2+ red phosphor at low (10−300 K) and high temperatures (303−573 K). In β-Ca2P2O7, the host lattice has four different calcium sites, at which Bi2+dopant can be located. After comparing the change of bond energy when the Bi2+ ions are incorporated into the four calcium sites, we found out that Bi2+ would preferentially occupy the smaller energy variation sites Ci(2) and Ci(1) in this compound, which can be assigned to Bi(2) and Bi(1), respectively. Surprisingly, we noticed that the variation of emission intensity is different under different excitations when the temperature changes from 10 to 300 K. When exciting into the typical absorption of Bi(1) sites at 419 nm, the emission intensity at 300 K remains only 38% as compared to that at 10 K, while exciting into typical Bi(2) absorption at 460 nm, the emission intensity increases to 110%. When further increasing the temperature from 303 to 573 K, we observed a similar phenomenon, and the emission at 460 nm excitation starts to quench at 453 K. The emission intensity at 573 K still remains 86.1% of that at 303 K. This might be attributed to the Bi(2) → Bi(1) energy transfer. It is also evidenced by the time-resolved emission spectra and lifetime values. This work gives new insights into better understanding luminescent behaviors of Bi2+-doped materials with multiple cation sites. This should be helpful in the future when designing the bismuth doped phosphor for WLEDs with better resistance to thermal quenching.

1. INTRODUCTION White light emitting diodes (WLEDs) have recently attracted widespread attention due to their high luminous brightness, low energy consumption, extreme long lifetime, high durability, and environmental friendliness, over traditional light sources.1−20 The typical conventional method to obtain white light is to combine blue InGaN chips (λem = 450−470 nm) with yellow light-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphors.6,21,22 However, this approach suffers from certain disadvantages, especially the low color rendering index (CRI) and high correlated color temperature due to a lack of the red component of the spectra. To solve this problem, additional red phosphors should be incorporated, especially for the WLED applications in excellent rendering of light and full color display. An important aspect of WLED red phosphors is the resistance to thermal quenching. Thus, the design, fabrication, and improvement of red phosphors with a good color rendering index and excellent resistance to thermal quenching have been sparking much interest in the academia and industry fields. Generally, when the bismuth ions are stabilized in their divalent state, the bismuth doped phosphors exhibit yellow to red light with the absorption of near-ultraviolet (NUV) or blue light.23−25 So, the Bi2+-doped phosphor can be a candidate for © 2017 American Chemical Society

red phosphor to improve the low CRI of WLED. An important aspect of WLED phosphors is the resistance to thermal quenching. Although certain Bi2+-doped alkaline earth borates,26,27 sulfates,28 phosphates,29,30 and borophosphates31 have been reported as promising candidates for red phosphor under NUV or blue excitation, not all the compounds show a good resistance to the thermal quenching, and some variations are observed. For instance, comparison of the emission intensities between room temperature and 20 K indicates that with Bi2+-doped strontium borate, the emission at 300 K remains at about 76% of the intensity observed at 20 K in SrB4O7:Bi2+. This emission is only 43% in the case of SrB6O10:Bi2+.26 Then thermal quenching remains a big challenge for the Bi2+-doped red phosphors. Recently, we have evidenced that Bi2+-heavily-doped Sr2P2O7 phosphors exhibit better resistance of thermal quenching, which may due to the energy transfer between the luminescent centers.32 In the present paper, we will report photoluminescence (PL) and thermal quenching in the Ca2P2O7:Bi2+ phosphor to better understand the thermal quenching behavior and energy transfer Received: March 3, 2017 Published: May 24, 2017 6499

DOI: 10.1021/acs.inorgchem.7b00564 Inorg. Chem. 2017, 56, 6499−6506

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Inorganic Chemistry

that the radius is slightly larger than r(Ca2+) = 1.12 Å as r(Bi3+) = 1.17 Å and r(Bi+) = 1.774 Å with a coordination number (CN) of 8.35,36 As shown in Figure 2, the crystal structure of β-

between luminescent centers. We will report crystallographic investigations and Rietveld analyzing and focus on the static and dynamic excitation, emission spectra, and decay curves at various temperatures (10 to 300 K, 303 to 573 K) to investigate the energy transfer mechanism between luminescent centers.32−34 Our work may help to design and fabricate the bismuth doped red phosphors for WLEDs with better resistance to thermal quenching.

2. EXPERIMENTAL PROCEDURE 2.1. Sample Synthesis. β-Ca2P2O7:Bi2+ samples also reported as β-Ca1.999Bi0.001P2O7 (or Ca1.999P2O7:0.05%Bi) were prepared by conventional solid state reaction. For the synthesis, the starting raw materials were analytical-grade reagents CaCO3, NH4H2PO4, and Bi2O3. In order to slowly decompose the ammonia phosphates, the mixture was put into a corundum crucible and first preheated at 500 °C for 6 h in the air. After grinding homogeneously in an agate mortar, the sample was sintered at 1100 °C for 24 h in the air. In order to fully promote the reduction of bismuth from the trivalent to divalent state, the sample was sintered at 1100 °C for 1 h in a reducing CO atmosphere. 2.2. Characterization. The structure and crystallinity of the sample was confirmed by X-ray diffraction (XRD, Rigaku D/max-IIIA, Cu Kα radiation, λ = 1.5405 Å, 40 kV, 40 mA, 1.2° min−1) at room temperature. Excitation and emission spectra as well as decay profiles were obtained by a high-resolution spectrofluorometer (Edinburgh FLS 920) equipped with a red-sensitive photomultiplier (Hamamatsu R928P) at 10 to 300 K (in a closed cycle helium cryostat) and 303 to 573 K (in a high-temperature cell). Excitation curves were corrected over the lamp intensity with a silicon photodiode, and all emission spectra were corrected from the PMT spectral response.

Figure 2. (a) Crystal structure of Ca2P2O7:Bi2+ and (b) Ca2+ lattice sites coordinated by different oxygen atoms.

Ca2P2O7 consists of four independent calcium atoms. Two calcium atoms (Ca(3) and Ca(4)) are coordinated by seven oxygen atoms, while the other two calcium atoms are coordinated by eight (Ca(1)) and nine (Ca(2)) oxygen atoms, respectively.37,38 The bond length d and calculated chemical bond covalency fc in the four types of calcium sites are summarized in Table 1. The chemical bond covalency fc is calculated by using the Born−Haber thermochemical cycle improved by Zhang et al.39−42 3.2. Site Occupancy and PL Properties of βCa2P2O7:Bi2+ at Room Temperature. In the previous reports, we investigated the photoluminescence (PL) properties of Bi2+ ions in the host lattice containing two types of cation sites, such as Sr2P2O7:Bi2+ and Ba2P2O7:Bi2+.29,30,43 In Sr2P2O7:Bi2+, the Bi2+ ions are incorporated into the two different Sr sites (Sr(1) and Sr(2)). Incorporation preferentially occurs on Sr(2) sites at a low doping concentration due to the good agreement in size. In β-Ca2P2O7:Bi2+, there are four different calcium sites where the Bi2+ dopant might be located. In order to determine which calcium sites are preferentially occupied by Bi ions, we have studied the local structure associated with Bi2+ ions by using the bond energy method.44 The bond energy method provides a strategy for us to better understand the relationship between its crystal properties and its composition and structure.44,45 In our case, we have crystallographic data of β-Ca2P2O7 from the ICSD database, and we obtained the crystallographic data of β-Ca2P2O7:Bi2+ by Rietveld refinement. So, we can calculate the bond energy of Ca−O (ECa−O) and Bi−O (EBi−O) in the four sites and compare the bond energy variation after Bi ions substitute Ca ions. The bond energy calculation is evaluated by the bond valence model (BVM).44,46−50 As we know, the crystal formula can be divided into the subformula expression (the bond−valence formula CamOn or BimOn). The bond energy ECa/Bi−O is the average value of all the bond energies of Ca−O and Bi−O in each site. The equation is shown as follows:44

3. RESULTS AND DISCUSSION 3.1. Phase Identification of β-Ca2P2O7:Bi2+. Figure 1 displays the XRD pattern of β-Ca2P2O7:Bi2+ and results of the

Figure 1. XRD pattern of Ca1.999P2O7:0.05%Bi and Rietveld refining results (·), Bragg reflections (|), and profile difference between experimental and calculated values ().

Rietveld refinement implemented with the crystallographic information file (ICSD #73712) from the ICSD database (Findit version 1.4.6). The sample phase purity can be ensured by the low Bragg R-factor (Rb = 4.961%), RF-factor (Rf = 3.579%), and the goodness of fit parameter χ2 = 1.86. And the composition is confirmed by EPMA measurement. This compound exhibits a tetragonal space group P41(76) with eight formula units per unit cell. The refinement also indicates the lattice parameters values a = b = 6.6878 Å and c = 24.1513 Å, which are slightly larger than the lattice constants in the crystallographic information file (a = b = 6.6858 Å, c = 24.1470 Å). This may indicate that the bismuth cations are successfully incorporated into the Ca2P2O7 lattice. Although an accurate Bi2+ ionic radius has not been accurately reported, we can infer 6500

DOI: 10.1021/acs.inorgchem.7b00564 Inorg. Chem. 2017, 56, 6499−6506

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Table 1. Bond Lengths d and Calculated Chemical Bonding Covalency fc for the Ca(1), Ca(2), Ca(3), and Ca(4) Sites in Ca2P2O7 M−O bonds for Ca(1) site

d (Å)

fc

M−O bonds for Ca(2) site

d (Å)

fc

Ca(1)−O(10) × 1 Ca(1)−O(9) × 1 Ca(1)−O(6) × 1 Ca(1)−O(1) × 1 Ca(1)−O(2) × 1 Ca(1)−O(5) × 1 Ca(1)−O(7) × 1 Ca(1)−O(11) × 1 mean value

2.3682 2.3741 2.3989 2.4429 2.4692 2.5213 2.7990 2.8759 2.5312

0.268 0.2677 0.2665 0.1388 0.1381 0.1368 0.1312 0.2502 0.1997

2.3406 2.4201 2.4245 2.5056 2.5640 2.6378 2.6853 2.7216 2.8385 2.5709 d (Å)

0.5154 0.2256 0.2254 0.1106 0.1093 0.1079 0.2157 0.1064 0.1046 0.1912 fc

2.3013 2.3421 2.3426 2.3632 2.4449 2.5166 2.7037 2.4306

0.3253 0.3231 0.3231 0.1804 0.1777 0.1757 0.1714 0.2395

M−O bonds for Ca(3) site

d (Å)

fc

Ca(2)−O(3) × 1 Ca(2)−O(9) × 1 Ca(2)−O(8) × 1 Ca(2)−O(1) × 1 Ca(2)−O(14) × 1 Ca(2)−O(7) × 1 Ca(2)−O(12) × 1 Ca(2)−O(13) × 1 Ca(2)−O(2) × 1 mean value M−O bond for Ca(4) site

Ca(3)−O(8) × 1 Ca(3)−O(13) × 1 Ca(3)−O(2) × 1 Ca(3)−O(5) × 1 Ca(3)−O(7) × 1 Ca(3)−O(14) × 1 Ca(3)−O(4) × 1 mean value

2.3581 2.3758 2.3984 2.4002 2.4158 2.4508 2.7961 2.4565

0.3223 0.1799 0.1792 0.1791 0.1786 0.1776 0.3061 0.2175

Ca(4)−O(10) × 1 Ca(4)−O(12) × 1 Ca(4)−O(6) × 1 Ca(4)−O(1) × 1 Ca(4)−O(13) × 1 Ca(4)−O(5) × 1 Ca(4)−O(14) × 1 mean value

⎛ d − dM − O ⎞⎛ VCa ⎞ ⎟⎜ EM − O = J exp⎜ 0 ⎟ ⎝ 0.37 ⎠⎝ VM ⎠

energy formation argument, the priority order for the Bi2+ incorporation of the luminescent centers is Bi(2) [CN = 9], Bi(1) [CN = 8], Bi(3) [CN = 7], and Bi(4) [CN = 7]. In the following, we will try to corroborate the optical features with this crystallographic approach. Figure 3 illustrates the excitation and emission spectra of βCa2P2O7:Bi2+ at room temperature. For the emission spectra,

(1)

where coefficient J is constant, it is equal to the standard atomization energy divided by the number of cations in the formal molecule and by cation valence.50 The bond-valence parameter d0 is a empirically determined parameter by processing all available crystallographic data and is a constant for a given atom pair.48 VCa and VM present the valence state of Ca and M, respectively. In this case, if M is dopant Bi2+, VCa/VBi is equal to 1. Bond length dM−O (dCa/Bi−O) can be obtained in crystallographic data. So the value of ECa/Bi−O can be evaluated. Therefore, we can study the preferential occupation of Bi2+ dopant by comparing the variation of bond energy (ΔECa Bi−O) in the independent calcium sites by the following equation:44 |ΔE BiCa− O| = |E Bi − O − ECa − O|

(2)

where EBi−O is the mean bond energy of Bi−O in the Ca2P2O7:Bi2+, ECa−O represents the mean bond energy of Ca− O in different calcium sites of the pure Ca2P2O7. All the values are presented in Table 2. Afterward, we can obtain the values of bond energy variation |ΔECa Bi−O| in the four types of calcium sites, which are also listed in Table 2. The result indicates that the value of bond energy variation increases in the order Ca(2) < Ca(1) < Ca(3) < Ca(4). According to the bond energy method, Bi2+ ions should preferentially occupy the sites with smaller alterations of bond energy values |ΔECa Bi−O|. Within this

Figure 3. Normalized emission and excitation spectra of Ca2P2O7:Bi2+ at room temperature. In red, Bi(2): λex = 460 nm and broad emission between 600 and 700 nm, which can be adjusted in two bands at 647 and 680 nm; in green, Bi(1): λex = 419 nm and λem = 680 nm). fwhm of the individual emission peaks ranges between 40 and 50 nm.

the shape is indeed very different according to the excitation wavelength. When monitoring the emission peak at 680 nm, the excitation spectrum consists of bands at 254 (276), 419, and 628 nm, while monitoring the emission peak at 647 nm, the excitation spectrum consists of bands at 254 (276), 419 (460), and 609 nm. These excitation bands can be assigned respectively to the transitions 2P1/2 → 2S1/2, 2P1/2 → 2P3/2(2), and 2P1/2 → 2P3/2(1), which are typical transitions for divalent bismuth cations.26,27 Their corresponding emission peaks at 680 and 647 nm can be assigned to the 2P3/2(1) → 2P1/2 transition of Bi2+, exhibiting a full width at half-maximum (fwhm) of 40 and 60 nm, respectively. The broad emission band can be well decomposed into two emission bands,

Table 2. Mean Bond Energy (EBi−O) of Bi−O in the Ca2P2O7:Bi2+, Mean Bond Energy (ECa−O) of Ca−O in Ca2P2O7, and Variation of Bond Energy (|ΔEBi Ca−O|) When the Bi Ion Locates at Different Calcium Sites

Ca(1)−O Ca(2)−O Ca(3)−O Ca(4)−O

EBi−O (kcal/mol)

ECa−O (kcal/mol)

|ΔEBi Ca−O| (kcal/mol)

29.586 26.526 32.356 34.787

30.552 26.905 35.713 38.176

0.966 0.379 3.357 3.389 6501

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Figure 3 under the same excitation wavelength. The overlap of Bi(2) and Bi(1) emission leads to the formation of this broad band. Within a few microseconds, a gradual change can be observed in the shape of the emission. The emission peak at 647 nm (Bi(2) emission) decays more rapidly, and the emission peak at about 680 nm (Bi(1) emission) becomes dominant. This might result from the energy transfer from Bi(2) to Bi(1). 3.3. Photoluminescence Properties of β-Ca2P2O7:Bi2+ at Low Temperature. Figure 5 illustrates the emission spectra

centered at 647 and 679 nm (dotted lines shown in Figure 3). The emission varies according to the excitation wavelength at 254, 419, or 628 nm (excitation peaks for 680 nm emission) or 276, 460, and 609 nm (excitation peaks for 647 nm emission). This indicates that Bi2+ ions are incorporated into two lattice calcium sites. As we all known, bismuth can exist in different valences. But in our sample, we used Bi2O3 as the raw material, so we do not believe that there is a higher valence of Bi ions, such as Bi5+. This is because we prepared the sample in a reducing atmosphere of CO, which promotes reduction of bismuth from a trivalent to divalent state. Bi5+ is very unstable and such a high oxidizing species cannot coexist with Bi2+ in the same compound.51 The existence of Bi2+ has been confirmed by its spectroscopic data. We also did not observe the typical lifetimes or emissions from Bi3+ and bismuth ions with a valence lower than +2. According to the previous report, the emission from the former Bi3+ usually lies in a wide range from ultraviolet (UV) to red light with a lifetime of ∼500 ns at room temperature,1,17 while the emission from the latter usually lies in 1000−1700 nm with a lifetime of hundreds of microseconds.52,53 In this paper, we only discussed Bi2+. In βCa2P2O7:Bi2+, the site occupancy is first expected in the Ca(1) and Ca(2) sites due to the smaller bond energy variation as discussed above. Within the previous calculation, one could propose the formation of the two luminescent centers Bi(1) and Bi(2). In addition, we can assign the excitation and emission spectra to the luminescent centers according to the covalency fc variation of the chemical bonds in Bi−O.30 We can get a hint from the covalency fc of the chemical bonds in Ca−O as shown in Table 1. Generally, higher covalency fc leads to an increasing red shift in the emission spectra. 54 In βCa2P2O7:Bi2+, the covalency in Ca(1)−O ( fc = 0.1997) is higher than that in Ca(2)−O ( fc = 0.1912). Therefore, the deep red 680 nm emission and its excitation peaks can be attributed to Bi2+ on Ca(1) sites (labeled as Bi(1)), while the 647 nm emission and its excitation peaks can be attributed to Bi2+ on Ca(2) sites (labeled as Bi(2)). We can also point out in Figure 3 that under direct pumping into the excitation level of Bi(2) (with the excitation of 460 nm), besides of the Bi(2) emission, one can still observe the Bi(1) emission merged. So this reveals that energy transfer between Bi(1) and Bi(2) could occur. Figure 4 displays the time-resolved emission spectra of Ca1.999P2O7:0.05%Bi with excitation of 460 nm. At the initial stage, the emission exhibits a broad band ranging from 600 to 800 nm, which is similar to the emission band reported in

Figure 5. Normalized emission spectra of β-Ca2P2O7:Bi2+ at different temperatures excited by (a) 419 nm and (b) 460 nm in the temperature range of 10−300 K. (c) Temperature dependence of normalized integrated emission intensity on β-Ca2P2O7:Bi2+ under excitation of 460 and 419 nm at various temperatures.

of β-Ca2P2O7:Bi2+ under 419 and 460 nm excitation in the temperature range of 10−300 K. Under 419 nm excitation, thermal quenching occurs, which means that the emission intensity strongly decreases with a temperature increase. Herein, Bi(2) emission is hardly observed, and the intensity of Bi(1) emission dramatically decreases, I300K/I10K = 38.3% (see Figure 5c). In parallel, the emission exhibits a slight red shift from 679 to 681 nm. This might be due to the strengthened electron−phonon interaction with increasing temperature. However, under excitation of 460 nm, one can observe a quite opposite thermal quenching behavior. The integrated intensity of the overall emission slightly increases (I300K/I10K = 110.3%; see Figure 5c). This broad emission band as previously presented can be divided into two Gaussian peaks centered at 647 and 680 nm from Bi(2) and Bi(1), respectively. The intensity dependence of both emissions on temperature is plotted in Figure 6. As temperature increases, the Bi(2) emission decreases, but the Bi(1) emission increases. This could indicate the energy transfer from Bi(2) to Bi(1).

Figure 6. Temperature dependence of normalized integrated emission intensity of Bi(2) and Bi(1) on β-Ca2P2O7:Bi2+ at temperatures ranging from 10 to 300 K.

Figure 4. Time-resolved emission spectra of Ca1.999P2O7:0.05%Bi (excited with 460 nm) at room temperature. 6502

DOI: 10.1021/acs.inorgchem.7b00564 Inorg. Chem. 2017, 56, 6499−6506

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Inorganic Chemistry Therefore, thanks to the energy transfer, β-Ca2P2O7:Bi2+ shows limited thermal quenching under 460 nm excitation. Generally, broadenings of excitation and emission bands are observed at higher temperatures. And more details will be revealed when we inspect the spectra at low temperatures. Figures 7 and 8 show the excitation spectra of the Bi(2) and

(see Figure 8 inset). As the temperature increases, the two excitations (Bi(2) and Bi(1)) decrease, and Bi(2) excitation is hardly observed at 300 K. This implies that the energy transfer from Bi(1) to Bi(2) is not effective. The energy splitting for Bi(1) between 2P3/2 (1) and 2P3/2 (2) levels increases from 7479 to 7614 cm−1, which leads to the decrease in 2P3/2(1) energy level and results in a red shift of the emission (2P3/2(1) → 2P1/2). Finally, we observed the energy transfer between Bi(1) and Bi(2), but the Bi(2) → Bi(1) energy transfer is more effective than Bi(1) → Bi(2) with an increase in temperature. In addition, increased splitting energy could enhance interaction between luminescent centers and vary the local crystal field. Under excitation at 419 and 460 nm, the dependence of emission lifetime on temperature is presented in Figure 9a.

Figure 7. Normalized excitation spectra of β-Ca2P2O7:Bi2+ by monitoring the emission at 647 nm (Bi(2) emission) in the 10−300 K temperature range. Figure 9. (a) Temperature dependence (10−300 K) of luminescence lifetimes for β-Ca2P2O7:Bi2+ upon excitation at 419 and 460 nm. Luminescence decay curves for β-Ca2P2O7:Bi2+ for (b) λex = 460 nm and λem = 647 nm and (c) λex = 419 nm and λem = 680 nm.

Their corresponding luminescence decay curves are shown Figure 9b and c. Under excitation at 419 nm, all the decay curves follow single exponential decay. The emission lifetime increases from 7.8 to 9.3 μs with the elevation of temperature, while the emission lifetime is independent of the temperature under the excitation of 460 nm. When decay profiles are not exponential, one uses a mean lifetime calculated through the equation τm = (A1τ12 + A2τ22)/(A1τ1 + A2τ1). Thus, the mean emission lifetimes vary in the range 6.5−6.8 μs, which is shorter than the lifetime values obtained with 419 nm excitation. This again results from Bi(2) → Bi(1) energy transfer. 3.4. Photoluminescence Properties of β-Ca2P2O7:Bi2+ at High Temperature. For high-power WLED applications, the increasing power consumption leads to more generated heat at the p−n junction, resulting in the high temperature of the phosphor layer up to 450 K.55 Thus, the resistance of a phosphor to thermal quenching at such a high temperature is essential for application in high power WLEDs. Figure 10 illustrates emission spectra of β-Ca2P2O7:Bi2+ excited by different excitations at 419 and 460 nm in the temperature range of 303−573 K. Under excitation of 419 nm, the emission decreases by 45% with the increasing temperature from 303− 573 K. The emission shows a similar monotonic decrease as observed at low temperatures from 10 to 300 K. On the other hand, under excitation of 419 nm, the emission intensity first slightly increased, reached a maximum at 453 K (I453K/I303K = 106.8%), and then decreased (I573/I303 = 86.1%). This might be due to the balance between the emission quenching and Bi(2) → Bi(1) energy transfer. Normally, the emission intensity decreases as the temperature increases because of classic thermal quenching. But in our case, the energy transfer from Bi(2) to Bi(1) can be a supplement to the emission excited at 460 nm, leading to the favorable thermal resistance of

Figure 8. Normalized excitation spectra of β-Ca2P2O7:Bi2+ by monitoring the emission at 680 nm (Bi(1) emission) in the 10−300 K temperature range.

Bi(1) emissions, at different temperatures in the range 10 to 300 K. At 10 K, for the emission at 647 nm corresponding to Bi(1), the excitation peaks of Bi(1) and Bi(2) can be clearly observed in the UV range. Similarly for the emission at 680 nm corresponding to Bi(2), excitation peaks of Bi(1) and Bi(2) with strong intensity differences are also observed. This indicates that energy transfer occurs between the two luminescent centers. As the temperature increases from 10 to 300 K, the excitation intensity between 400 and 500 nm attributed to 2P1/2 → 2P3/2(2) transition changes, as shown in the insets of Figures 7 and 8. First, by monitoring the Bi(2) emission at 647 nm, the intensity of Bi(2) excitation decreases, while the Bi(1) excitation increases (see Figure 7 inset). This means that energy transfer from Bi(2) to Bi(1) occurs. Additionally, the intensity variation is accompanied by a slight red shift in Bi(1) excitation and a slight blue shift in Bi(2) excitation. Then, for Bi(2), the splitting energy between 2P3/2 (1) and 2P3/2 (2) increases from 4506 to 5158 cm−1. Second, by monitoring the emission at 680 nm for Bi(1) species, the Bi(1) excitation between 400 and 500 nm for the 2P1/2 → 2P3/2(2) transition shows weak satellite spectra at higher wavelengths, 6503

DOI: 10.1021/acs.inorgchem.7b00564 Inorg. Chem. 2017, 56, 6499−6506

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we heated the sample. For the emission lifetime at 419 nm excitation, it decreases rapidly with increasing the temperature, and the value is shortened from 8.1 to 6.5 μs. Therefore, this compound shows a better resistance to thermal quenching under excitation of 460 nm.

4. CONCLUSIONS In this paper, site occupation preference and a good resistance to thermal quenching were observed under excitation of 460 nm in the divalent bismuth doped β-Ca2P2O7. The structure of this compound consists of four independent calcium sites, at which dopant Bi2+ ions could be located. We used the bondenergy method to study the site occupancy preference. Bi2+ dopant cations are more likely preferentially incorporated into the cation sites with a smaller variation of bond energy. In this case, Bi2+ cations will substitute the calcium sites in the order of Bi(2), Bi(1), Bi(3), and Bi(4). However, we can only observe two types of site for the optical spectroscopy of Bi2+ in this compound. This indicates that Bi2+ ions will be preferentially incorporated into Bi(1) and Bi(2). Energy transfer occurs between these two luminescent centers. With an increase in temperature, Bi(2) → Bi(1) energy transfer is more efficient, leading to the increase of Bi(1) emission. Indeed, the emission shows better resistance to thermal quenching under excitation of 460 nm. Thermal quenching can even be suppressed by the energy transfer from one luminescent center to the other one, and this could be indeed favorable for further use as a new deep red phosphor.

Figure 10. Normalized emission spectra of β-Ca2P2O7:Bi2+ at different temperatures excited by (a) 419 nm and (b) 460 nm in the temperature range of 303 to 573 K. (c) Temperature dependence of normalized integrated emission intensity on β-Ca2P2O7:Bi2+ under excitation of 460 and 419 nm at various temperatures.

Ca2P2O7:Bi2+ red phosphor. As we can see in Figures 6 and 11, the Bi(1) emission intensity increases, while Bi(2) emission

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Figure 11. Temperature dependence of normalized integrated emission intensity of Bi(2) and Bi(1) on β-Ca2P2O7:Bi2+ at temperatures ranging from 303 to 573 K.

ASSOCIATED CONTENT

Accession Codes

CCDC 1545947 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

decreases before 513 K. This indicates that the energy transfer is predominant at lower temperatures. But at higher temperatures, the thermal quenching appears to dominate, and this is accompanied by a rapid decrease in emission intensity. The dependence of emission lifetime on temperature from 303 to 573 K under excitation at 419 and 460 nm is presented in Figure 12a. The corresponding luminescence decay curves are shown Figure 12b and c. The emission lifetime at 460 nm excitation is shorter than that at 419 nm excitation. This is in agreement with the observations in the temperature range of 10−300 K. However, under excitation at 460 nm, the emission lifetime is no longer maintained at 6.5 μs at higher temperatures. It slowly becomes shorter (6.5 → 5.6 μs) when

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 22236910. Fax: +86 20 87114204. E-mail: [email protected]. ORCID

Mingying Peng: 0000-0002-0053-2774 Author Contributions ‡

These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Key Program of Guangzhou Scientific Research Special Project (Grant No. 201607020009), the National Natural Science Foundation of China (Grant No. 51672085), the Department of Education of Guangdong Province (Grant No. 2013gjhz0001), Fundamental Research Funds for the Central Universities, and the Hundred, Thousand and Ten Thousand Leading Talent Project in Guangdong Program for Special Support of Eminent Professionals.

Figure 12. (a) Temperature dependence (303−573) of luminescence lifetimes for β-Ca2P2O7:Bi2+ upon excitation at 419 and 460 nm. Luminescence decay curves for β-Ca2P2O7:Bi2+ for (b) λex = 460 nm and λem = 647 nm and (c) λex = 419 nm and λem = 680 nm. 6504

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