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Article Cite This: Chem. Mater. 2017, 29, 8792-8803

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Color-Tunable Luminescence Properties of Bi3+ in Ca5(BO3)3F via Changing Site Occupation and Energy Transfer Xue Li, Panlai Li,* Zhijun Wang,* Simin Liu, Qi Bao, Xiangyu Meng, Keliang Qiu, Yuebin Li, Zhiqiang Li, and Zhiping Yang College of Physics Science & Technology, Hebei Key Lab of Optic-Electronic Information and Materials, Hebei University, Baoding 071002, China ABSTRACT: Generally, 6s electron and 6p electron of Bi3+ ions are located in its outermost layer; thus, luminescence properties of Bi3+ are strongly associated with the coordination environment around Bi3+. Bi3+ ions occupy different cationic positions in hosts, which may cause the movement of the emission spectrum. In order to investigate the luminescent property of Bi3+, a series of Bi3+ doped Ca5(BO3)3F species are synthesized. There are three types of Ca2+ sites in the host, which could be substituted by Bi3+. Upon the 322 nm excitation of Bi3+, a broad emission band can be observed, which is ascribed to the 1s0 → 3p1 transition of Bi3+. Meanwhile, there is the emission shift of Ca5(BO3)3F:xBi3+, and its emission color can be altered from blue to cyan. It may result from Bi3+ occupying different positions of Ca2+ in the host, which can give rise to different degrees of a nephelauxetic effect and crystal field splitting. In order to explore the relationship between the luminescence properties of Bi3+ and the nepelauxetic effect, the value of the centroid shift (∈c) is calculated. Centroid shift (∈c) is related to the covalence and average bond length of an octahedron in which the influence of covalence is primary. The relationship between the luminescence properties of Bi3+ and the crystal field splitting is discussed. The crystal field splitting is related to the interaction between the Bi3+ species, the crystal field splitting energy (Δ), and the distortion of the crystal. Emission spectra are asymmetric; meanwhile, the emission spectra have remarkable changes at various excitation wavelengths. This proves that the broadband emission band consists of at least two emission centers. In order to assess this hypothesis, the decay curves are measured. This confirms that there are three luminescence centers in a host. On one hand, considering the effect of the centroid shift (∈c) and crystal field splitting (∈cfs), the sources of three luminescence centers are confirmed by calculating the total shift (D(A)) of the 6s6p level of Bi3+ in a host. On the other hand, the source of three luminescence centers is determined by the changing trend of the average bond length of the octahedron. In addition, the luminescence properties of Ca5(BO3)3F:Bi3+, Eu3+, are investigated as well. There is efficient energy transfer from the Bi3+ to the Eu3+ ion, and the color-tunable phosphor can be achieved by the combination of the appropriate proportion of Bi3+ and Eu3+ ions. The emission color can gradually change from cyan to red. Bi3+, with increasing Bi3+ concentration, the tunable emission may be obtained by varying the coordination environment around Bi3+.10,11 The emission spectrum of Bi3+ shows a broad band due to the electronic transfer of Bi3+ from the ground state to the excited state. The ground state of Bi3+ is 1s0, which originates from the 6s2 configuration. Bi3+ ions have four excited states, which are 3P0, 3p1, 3p2, 1p1, respectively. They are originating from 6s6p configuration. The electron transitions from 1s0 to 3P0 and 3p2 are spin forbidden; the electron transitions from 1s0 to 3p1 and 1p1 are allowed. It is commonly the 1s0 → 3p1 transition due to the excited state 1p1 which has higher energy state and bad stability compared with the 3p1 state. On the basis of the strong sensitivity to the coordination environment, the emission of Bi3+ can be tunable from the

1. INTRODUCTION Phosphors have attracted much attention, having wide and continuously tunable excitation and emission wavelengths, because of their potential applications in many fields, including displays, solid-state lighting, and other photonic devices.1−3 On the basis of the energy transfer between codoped rare-earth ions, which is the usual method to achieve tunable excitation and emission by adjusting the ratio of different rare-earth doped ions,4−6 rare-earth ion doped phosphors can obtain tunable emission by crystal chemical substitution, such as Ca3Si2O7:Eu2+ and Lu3−xYxAl5O12:Mn4+.7,8 Bismuth is a non-rare-earth metal ion, and Bi3+ doped materials have attracted more and more attention in recent years owing to their optical properties. Among these optical properties, the tunable emission is the most attractive.9 This is due to the sensitivity of Bi3+ naked 6s electrons and 6p electrons to the crystal field particularly surrounding the Bi3+ ions. Therefore, Bi3+ is sensitive to its coordination environment. According to the characteristic of © 2017 American Chemical Society

Received: July 25, 2017 Revised: September 15, 2017 Published: September 15, 2017 8792

DOI: 10.1021/acs.chemmater.7b03151 Chem. Mater. 2017, 29, 8792−8803

Article

Chemistry of Materials

2.2. Material Characterization. Crystal structure and phase purity of samples were identified by X-ray diffractometer (XRD) using a D8-A25 Focus diffractometer at 40 kV and 40 mA. In order to determine the change in crystal structure, the Rietveld structure refinements were performed by General Structure Analysis System (GSAS) software. The spectral property was detected by a Hitachi F-4600 fluorescence spectrophotometer. A diffuse reflectance spectrum of a sample was measured on a Hitachi U4100 machine, in a scanning wavelength range 300−700 nm. The decay curve was collected by a 320 nm pulse laser radiation (nano-LED) and 450 W Xe lamp as the excitation resource at room temperature. Commission International de I’Eclairage (CIE) chromaticity coordinates of the sample were recorded on a PMS-80 UV−vis−NIR spectral analysis system.

ultraviolet (UV) to the yellow region due to the different covalence, bond length, distortion, and so on.1−3,12,13 Moreover, Bi3+ is not only a good activator, but also a good sensitizer through strengthening and broadening UV excitation bands.3,14−16 Thus, codoping Bi3+ with rare-earth ions, such as Eu3+,17,18 has been adopted to achieve color-tunable phosphors by energy transfer. It well-known that host compounds also play an important role in the luminescence process. Borate contains many different structures and allows for the replacement of a larger number of doped ions. Thus, the borate crystal structure, especially the multiborate crystal structure, is important for investigating the optical properties.2,19 Ca5(BO3)3F, described early in 1989 by Lei et al, is a new promising nonlinear optical crystal of borate host due to its possessing large secondharmonic-generation coefficients, a wide transparency range, and good chemical stability.20−22 However, the optical properties of Ca5(BO3)3F are not studied in detail. According to the above reason, here, the Ca5(BO3)3F compound with various cationic sites is selected as a host compound. There are three kinds of octahedra, which are called Ca(1)O5F, Ca(2)O6, and Ca(3)O4F2, respectively. They have different covalence, average bond length, and distortion. A series of Ca5(BO3)3F:xBi3+ species were synthesized by a solid-state method. When Bi3+ substituted for the different Ca2+ sites, the luminescence properties of Ca5(BO3)3F:xBi3+ are investigated by analyzing the occupation of Bi3+, and by calculating the values of centroid shift (∈c) and crystal field splitting (∈cfs). The purpose is that Bi3+ doped phosphors with tunable emission can be obtained; the emission color can turn from blue to cyan. In additional, the energy transition mechanism from Bi3+ to Eu3+ is studied in Ca5(BO3)3F:Bi3+, yEu3+; the tunable full color emitting phosphors are achieved, and the emission color can change from cyan to red.

3. RESULTS AND DISCUSSION 3.1. Phase Characterization and Crystal Analysis. Figures 1a and 2b depict the XRD patterns of Ca5(BO3)3F:xBi3+

Figure 2. Crystal structure of Ca5(BO3)3F and the coordination environment of Ca1, Ca2, and Ca3.

2. EXPERIMENTAL SECTION 2.1. Material and Synthesis. A series of Ca5(BO3)3F:xBi3+, yEu3+ were synthesized by the high temperature solid-state method. H3BO3 (Analytical Reagent, A. R.), CaF2 (A.R.), CaCO3 (A.R.), Bi2O3 (99.99%), and Eu2O3 (99.99%) were used as raw materials. Stoichiometric amounts of raw materials were weighed by an electronic scale with 0.0001 g accuracy and ground thoroughly in an agate mortar for 30 min. Then, mixed powder samples were transferred into an alumina crucible and sintered at 1150 °C for 4 h. Finally, all obtained powder samples were cooled to room temperature in the furnace and ground into powder for further measurement.

and Ca5(BO3)3F:0.15 Bi3+, yEu3+. The diffraction peaks of all single or codoped samples are well matched with the standard cards of Ca5(BO3)3F (ICSD-65763). This means that the samples are purity phase and doping Bi3+/Eu3+ ions do not cause significant changes to the crystal structure. Figure 2 illustrates the crystal structure of Ca5(BO3)3F; it belongs to a monoclinic crystal and locates in space group C1m1 which possesses three kinds of Ca2+ ion coordination environments named Ca1, Ca2, and Ca3. Ca1 coordinates with

Figure 1. XRD patterns and the standard of Ca5(BO3)3F (ICSD-65763): (a) Ca5(BO3)3F:xBi3+ and (b) Ca5(BO3)3F:0.15 Bi3+, yEu3+. 8793


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Chemistry of Materials five oxygen atoms and one fluorine atom; Ca2 coordinates with six oxygen atoms, and Ca3 coordinates with four oxygen atoms and two fluorine atoms. Ca2 connects with Ca1 by two common oxygen atoms between them; Ca2 connects with Ca3 through one common oxygen atom between them as well. The numbers of Ca1, Ca2, and Ca3 atoms are 4, 4, 2 in a unit cell, respectively. To get further knowledge regarding crystal structure information and sites of Ca5(BO3)3F:xBi3+, the XRD Rietveld refinements of Ca5(BO3)3F:xBi3+ (x = 0, 0.03, 0.15, 0.25) were performed by the GSAS program with the single crystallographic data of Ca5(BO3)3F as the initial model. The results of refinement are presented in Figure 3 and Table 1. It is seen that red lines and black lines stand for experimental and calculated

patterns, respectively, which matched well. The result of refinement strongly proves that Ca5(BO3)3F:xBi3+ phosphors are single phase without any impurity phase as well. It is wellknown that the unit cell volume will commonly expand when Ca2+ ions (r = 1 Å, CN = 6 where CN means coordination number) were substituted by Bi3+ ions with larger size (r = 1.03 Å, CN = 6). However, the refinement results indicate that cell parameters (a, b, c) and unit cell volume gradually decrease with increasing Bi3+ concentration; then, they obviously increase when the concentration of Bi3+ ions is over 0.15. It is known that the generation of cationic vacancies should contribute to the shrinkage of the crystal cell volume. The cationic vacancies should be produced when the Ca2+ ions positions were occupied by Bi3+ ions because of the integral charge balance. It can be expressed by the following formula:23 . 2Bi 3 + → 2Bi Ca + V″Ca

In addition, from Table 1, it is found that the summation of Bi3+ is lower than the doped concentration from the result of refinement. This result confirms that Bi3+ ions do not completely occupy the position of the lattice through refining the occupation of Bi3+ ions. It is possible that part of the Bi3+ ions enter into the crystal interstitial, which will induce the expansion of the crystal. Hence, the expansion of crystal volume may be caused by a mass of Bi3+ entering into the crystal interstitial in Ca5(BO3)3F:xBi3+ when Bi3+ concentration exceeds 0.15.24 It is known that the electronegativity of ions is stronger; the covalence of ions is weaker. For the three kinds of octahedra, the covalences are Ca(2)O6 > Ca(1)O5F > Ca(3)O4F2 in Ca5(BO3)3F, because the electronegativity of F− ions is stronger than that of O2− ions. According to the covalence, Bi3+ ions take priority for replacing Ca3 sites first, and then, Ca1 and Ca2 sites should be gradually substituted with increasing Bi3+ concentration, which is well-matched with the result of the refinement for the occupation of Ca2+ ions and Bi3+ ions. 3.2. Luminescence Property of Ca5(BO3)3F:xBi3+. Figure 4a shows the excitation and emission spectra of Ca5(BO3)3F:xBi3+. The excitation spectrum of Ca5(BO3)3F:Bi3+ consists of a single broadband absorption ranging from 250 to 400 nm with the strongest absorption peak at 322 nm, which could be attributed to the electronic transitions from the ground state (1S0) to the excited state (3P1) of Bi3+ ions. Upon the 322 nm excitation, Ca5(BO3)3F:xBi3+ exhibited a broad emission band in the range 350−650 nm, which originates from the 3P1 → 1S0 electronic transitions of Bi3+ ions. With increasing Bi3+ concentration, it can be seen that the emission gradually exhibits a red-shift and the position of emission peak shifts from 418 to 462 nm; meanwhile, the emission intensity displays an increase first when the Bi3+ concentration is lower than 0.01, and then the emission intensity suffers a continuous decrease when the Bi3+ concentration is higher than 0.01. However, the emission spectrum shows a slight blue shift when the Bi3+ concentration exceeds 0.15. The range of shift becomes 462−454 nm. At the same time, the emission intensity shows a little increase, as shown in Figure 4b. Because of the 6s electronic structure and 6p electronic structure of Bi3+ located in the outermost layers of ions, Bi3+ ions are sensitive to surround the crystal field environment and have obviously a nephelauxetic effect. The energy transition significantly reduces with an enhancement of the nephalauxetic effect and a deepening of the crystal field splitting. Thus,

Figure 3. Rietveld refinement results of Ca5(BO3)3F:xBi3+ with (a) x = 0.00, (b) x = 0.03, (c) x = 0.15, (d) x = 0.25. (e) Stand for relative shifts in the cell parameters (a, b, c) and changes in the unit cell volume of Ca5(BO3)3F:xBi3+ compared with Ca5(BO3)3F.

Table 1. Results of Occupying Refinement for Ca2+ and Bi3+ in Ca5(BO3)3F:xBi3+ Occupancy & Content

x=0

x = 0.03

x = 0.15

x = 0.25

Ca1 Bi1 Ca2 Bi2 Ca3 Bi3 summation of Bi3+

1 0 1 0 1 0 0

0.9968 0.0032 0.9995 0.0005 0.9876 0.0124 0.0161

0.9517 0.0483 0.9600 0.0400 0.9616 0.0384 0.1267

0.9180 0.0820 0.9529 0.0471 0.9421 0.0579 0.1870

(1)

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Figure 4. (a) Excitation spectra of Ca5(BO3)3F:0.15Bi3+ and emission spectra of Ca5(BO3)3F:xBi3+. (b) Variation of intensity and shift of wavelength for Ca5(BO3)3F:xBi3+. (c) Normalized emission spectra of Ca5(BO3)3F:xBi3+ (x = 0, 0.003, 0.007, 0.1, 0.3, 0.7, 0.15) under 322 nm excitation. (d) Normalized emission spectra of Ca5(BO3)3F:xBi3+ (x = 0.15, 0.17, 0.2, 0.22, 0.25, 0.27, 0.3) under 322 nm excitation.

Figure 5. Schematic diagrams of the red shift. (a) Schematic diagram of nephauxetic effect; inset shows average bond length of three kinds of octahedra (Ca(1)O5F, Ca(2)O6, and Ca(3)O4F2). (b) Schematic diagram of crystal field splitting; inset shows distortion of three kinds of octahedra (Ca(1)O5F, Ca(2)O6, and Ca(3)O4F2) and the whole crystal.

length. For Ca5(BO3)3F, it does not have to consider the influence of coordination number, because the coordination numbers of Ca2+ ions are all six-coordination. As shown in the inset of Figure 5a, it is seen that the average bond lengths all gradually get longer for the Ca(1)O5F octahedron and the Ca(3)O4F2 octahedron with increasing concentration of Bi3+ ions. However, the average bond length gradually gets shorter for the Ca(2)O6 octahedron when the Bi3+ ion contents are more than 0.03. Hence, the average bond length gets longer for the whole crystal lattice, which leads to a reduction of the nephelauxetic effect. With consideration of various factors, the nephelauxetic effect is reinforced owing to mainly being influenced by the covalence of the ligand atoms. Finally, the emission spectrum red-shifts. The strength of the nephelauxetic effect gives rise to a centroid shift. Due to a stronger crystal field interaction, the average position of the 6s6p level (the position of centroid) in the host is lower relative to the position for the free ion of Bi3+, and the position of the centroid shifts down. Therefore, the emission spectrum shifts to the red.

with respect to the reason for the red shift for the emission spectrum, it can be explained by the following two aspects: nephalauxetic effect and crystal field splitting. Figure 5a describes the schematic diagram of the nephauxetic effect. In theory, the movement of the emission spectrum is attributed to the nephelauxetic effect which is mainly influenced by the electronegativity of the ligands. The covalence of the ligands gradually increases with the electronegativity of the ligands decreasing which leads to the nephelauxetic effect getting stonger. As mentioned above, the covalences are Ca(2)O6 > Ca(1)O5F > Ca(3)O4F2 in Ca5(BO3)3F; Bi3+ ions should preferentially substitute for the Ca3 site, and then the Ca1 site and Ca2 site should be substituted. Thus, with an increase in the Bi3+ concentration, the nephauxetic effect gets stronger, leading to the emission spectra producing a red shift. The nephelauxetic effect is related to the coordination numbers of the polyhedron and the average bond length of the polyhedron as well. This gets stronger with the reduction in coordination number and shortening of the average bond 8795


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in the emission spectrum shifting to long wavelengths and generating a red shift.29,30 Metal ion charge can directly affect the size of crystal field splitting energy (Δ), which is the energy difference between the lowest level of 6s6p and the highest level of 6s6p after the 6s6p level generating splitting. The charge of metal ions changes from +2 into +3 resulting in the corresponding increase in crystal field splitting energy (Δ) when Ca2+ ions are substituted by Bi3+ ions. This is due to the fact that ligands get closer to metal ions with increasing metal ion charge, which has a great effect on the 6s6p level of Bi3+ ions. The crystal field splitting energy (Δ) is also related to the ligands of the center atom; the value of Δ is halogen < oxygen < nitrogen < carbon. From the analysis of occupation and covalence, ligands of the center atom change from O2− and F− to all O2−; thus, the value of Δ gradually increases. Eventually, the crystal field splitting occurs, and the emission spectrum generates a red shift. If the bond length of the polyhedron changes, the distortion of the polyhedron will change as well; the distortion can be calculated by the following formula as for bond length.31−33

The distance of the centroid shift can be calculated by the following equation:25−28 ∈c =

A=

e2 ( r2 4π ∈0

e2 ( r2 4π ∈0

N 6s6p

6s6p

− r2

− r2

6s

6s2

2

)∑ i=1

αsp 6

(R i − 12 ΔR)

)

(2)

(3)

For O: αspo = 0.33 + 4.8/χav2

(4)

2 For F: αspF = 0.15 + 0.96/χaV

(5)

Here, ⟨r⟩ stands for the radial radius of an electron in 6s6p configuration or 6s2 configuration, e is the elementary charge, and ∈0 is the permittivity of vacuum. Thus, the value of e2 ( r2 − 4π ∈ 0 6s6p −20 2

r2

6s2

) is constant, which is defined as A (unit

m ). Ri stands for the distance between the center of 10 atom (Ca2+/Bi3+) and the anion in the lattice. The summation 1 is over all N anions that coordinate Bi3+. 2 ΔR represents a correction for lattice relaxation around Bi3+, and ΔR is the difference in ionic radius of the Bi3+ and the Ca2+. αisp (in units of 10−30 m3) is the spectroscopic polarizability of the anion, which is related to the average polarizability of the N nearest neighbor anions around Bi3+ and the average covalence between Bi3+ and ligands. χav is the electronegativity of the cation. In this case, χav = (10χCa2+ + 6χB2+)/16 = 1.39 (χCa2+ = 1, χB2+ = 2.04) in Ca5(BO3)3F. The calculating values of αosp and αFsp are 2.82 × 10−30 m−3 and 0.65 × 10−30 m−3, respectively. ΔR = 0.03. There are three kinds of Ca2+ sites which are substituted by Bi3+. For Ca5(BO3)3F:0.15 Bi3+, the values of ∈c for Bi3+(1), Bi3+(2), and Bi3+(3) are calculated to 1.17A, 1.23A, and 1.1A eV, respectively. The value of Ri is listed in Table 2

6

D(TO) = (∑ |TOi − TOm |)/6TOm i=1

Here, TOi stands for the individual distances from the cation to ligand anions, TOm stands for the average bond length, and the distortion D(TO) stands for the average deviation of bond length. The inset of Figure 5b illustrates the distortion of Ca(1)O5F, Ca(2)O6, Ca(3)O4F2, and the whole unit cell, respectively. With increasing Bi3+ concentration, it is seen that the distortion of Ca(3)O4F2 gradually increases when the concentration of Bi3+ ions is lower than 0.15, but the distortion of Ca(3)O4F2 slightly decreases when the Bi3+ ion content is over 0.15. The distortion of Ca(1)O5F gets larger when the Bi3+ ion content is less than 0.03, which becomes smaller when the Bi3+ ion content exceeds 0.03. The distortion of Ca(2)O6 becomes bigger when the Bi3+ ion content is lower than 0.15, and then, the distortion of Bi3+ gets smaller when the Bi3+ ion content exceeds 0.15. The change of the distortion for them can be explained as follows. The average bond length gets longer when the minor radius of the Ca2+ ions is substituted by the large radius Bi3+ ions; the variation in bond length gives rise to the change in the symmetry of the crystal. According to covalence and site analysis from above the result of refinement, it is known that Bi3+ ions should give priority to Ca(3)O4F2, and then Ca(1)O5F, Ca(2)O6 should be substituted. When the doping content is low, Bi3+ ions mainly occupy the position of Ca3 and Ca1, which gives rise to a lowering of the symmetry of the Ca(3)O4F2 octahedron and the Ca(1)O5F octahedron. The reason for the reduction of symmetry is as follows. As shown in Figure 6a, by comparing Ca5(BO3)3F and Ca5(BO3)3F:0.03 Bi3+, on the left side of the Ca(1)O5F octahedron, it is seen that the bond lengths (Ca1−O4, Ca1−O1, and Ca1−O1) all get shorter. However, on the right side of the Ca(1)O5F octahedron, the bond lengths (Ca1−O2, Ca1−O2, and Ca1−F1) get longer. Thus, the symmetry of the Ca(1)O5F octahedron gets lower leading to the distortion of Ca(1)O5F becoming larger. Meanwhile, as shown in Figure 6b, when the concentration of Bi3+ changes from 0% to 3%, for the Ca(3)O4F2 octahedron, the bond lengths (Ca3−O3, Ca3−O3, Ca3−O5, Ca3−O5, and Ca3−F1) all become shorter in the blue region. However, the bond length of Ca3−F1 gets longer in the pink region which turns from 2.398 to 2.534 Å.

Table 2. Value of Ri for Ca1/Bi1, Ca2/Bi2, and Ca3/Bi3 (Ca5(BO3)3F:0.15 Bi3+) Ca1/Bi1 O4 O2 O2 O1 O1 F1 av bond

Ca2/Bi2 2.283 2.360 2.281 2.386 2.411 2.322 2.341

O1 O5 O5 O4 O4 O2 av bond

Ca3/Bi3 2.483 2.531 2.250 2.335 2.311 2.397 2.385

O5 O3 O3 O5 F1 F1 av bond

(6)

2.427 2.499 2.031 2.427 2.488 2.339 2.369

for Ca2+(1)/Bi3+(1), Ca2+(2)/Bi3+(2), and Ca2+(3)/Bi3+(3), respectively. The crystal field splitting of Bi3+ makes emission shift to a long wavelength as well. The reason for crystal field splitting can be explained by the following three aspects: the interaction between Bi3+ and Bi3+ ions, the enlargement of the crystal field splitting energy (Δ), and the distortion of the crystal. The schematic diagram of crystal field splitting is discribed in Figure 5b. With increasing Bi3+ concentration, the Bi3+−Bi3+ distance becomes shorter, leading to the interaction between the Bi3+ ions strengthening. Hence, the ligand field strength surrounding the Bi3+ is reinforced, which leads to the crystal field splitting of Bi3+ ion generation. The distance from the excited state to the ground state of Bi3+ ions becomes shorter resulting 8796


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crystal interstitial position of Ca(1)O5F and Ca(3)O4F2 octahedra leading to the expansion in volume. Ca(1)O5F and Ca(3)O4F2 octahedra form a relatively symmetrical environment again resulting in the distortion getting smaller. On the contrary, owing to the Ca(2)O6 octahedron gradually getting pressed by Ca(1)O5F and Ca(3)O4F2 octahedra, the volume of the Ca(2)O6 octahedron gradually shrinks. Thus, the Ca(2)O6 octahedron also forms a relatively symmetrical environment again leading to the distortion getting smaller. For the effect of the distortion on the crystal’s bond length, it can be quantitatively calculated by the following equation. Dw = [D(TOCa1) + D(TOCa2 ) + D(TOCa3)]/3

(7)

Here, Dw is the distortion of the crystal; D(TOCa1), D(TOCa2), and D(TOCa3) stand for the distortion of Ca(1)O5F, Ca(2)O6, and Ca(3)O4F2 octahedra, respectively. As shown in the inset of Figure 5b, it is seen that the distortion of the crystal increases when the concentration of Bi3+ ions is lower than 0.15; however, there is a small decrease when the concentration of the Bi3+ ions is higher than 0.15. Thus, the distortion of the crystal gets larger, resulting in a deepening in the crystal splitting and emission spectrum shifting to the red when the concentration of Bi3+ is less than 0.15. The crystal field splitting is related to the shape of polyhedron and the bond length between cations and ligands. The crystal field splitting of the 6s6p level can be defined by the following equation:25,26

∈cfs = βploy R−2

(8)

βploy is a constant that depends on the type of the coordination polyhedron; the βploy value is 1.35 × 109 pm2 cm−1, 0.89βocta, 0.79βocta, 0.42βocta, and 0.42βocta for octahedral (octa), cubal, dodecahedral (ddh), tricapped trigonal prism (sctp), and cuboctahedral (cubo), respectively. R is the average distance between the cation and ligands. In this case, for Ca5(BO3)3F:0.15 Bi3+, βploy = 1.35 × 109 pm2 cm−1; the value of R is shown in Table 2. Thus, the values of ∈cfs are calculated to be 3.05, 2.94, and 2.98 eV for Bi1, Bi2, and Bi3, respectively. According to the emission spectra of Ca5(BO3)3F:xBi3+ in Figure 4a,c,d, it is found that the emission spectra are asymmetric due to the Bi3+ ions occupying different lattice sites in Ca5(BO3)3F. Therefore, they formed different kinds of luminescence centers; the numbers are different leading to difference emission intensity. Ultimately, the emission spectra show an asymmetric single peak by the superposition of different emission bands. Figure 7a shows the emission spectra of Ca5(BO3)3F:0.15 Bi3+ under 290−340 nm different excitation. There is a remarkable change in emission spectra with changing excitation wavelength. The variation of the emission peak is caused by site-selective excitation; it also strongly proves that the broad emission band consists of at least two emission centers. There are three Ca3+ sites with different coordination environments in Ca5(BO3)3F, and the Bi3+ ions are sensitive to the surrounding coordination environment. Thus, the asymmetric emission peak may be caused by substitution for different Ca2+ sites. In order to determine the source of the asymmetric emission peak, Gaussian fitting is performed. Taking into account the three kinds of Ca2+ ion sites in Ca5(BO3)3F, thus, the emission peak could be fitted to three subemission peaks by Gaussian functions of Ca5(BO3)3F:0.15Bi3+. The positional fits for the three peaks are 446, 490, and 538 nm, respectively, as shown in Figure 7b. The fluorescent decay curves

Figure 6. (a) Change of the bond length for Ca(1)O5F in Ca5(BO3)3F:xBi3+ (x = 0%, 3%). (b) Change of the bond length for Ca(3)O4F2 in Ca5(BO3)3F:xBi3+ (x = 0%, 3%). (c) Change of the bond length for Ca(2)O6 in Ca5(BO3)3F:xBi3+ (x = 3%, 15%, 25%). (d) Change of the volume for Ca(1)O5F, Ca(2)O6, and Ca(3)O4F2 in Ca5(BO3)3F:xBi3+ (x = 3%, 15%, 25%).

Thus, the symmetry of the Ca(1)O5F octahedron also gets lower leading to the distortion of the Ca(3)O4F2 octahedron becoming larger as well. Although a few of the Ca2 sites are replaced by Bi3+, the Ca(2)O6 octahedron not only connects with the Ca(1)O5F octahedron through two conjunct oxygen atoms between them, but also linked with the Ca(3)O4F2 octahedron through a common connection of the oxygen atom. The change of the Ca(3)O4F2 octahedron and the Ca(1)O5F octahedron should have an effect on the Ca(2)O6 octahedron. Therefore, the distortion of the Ca(2)O6 octahedron gets larger as well. As shown in Figure 6c, when the concentration of Bi3+ changes from 15% to 25%, it can be seen that the bond lengths of Ca2−O4 and Ca2−O1 all get shorter; they connect with the Ca(1)O5F octahedron. The bond length of Ca2−O5 also gets shorter which links with the Ca(3)O4F2 octahedron. Meanwhile, as shown in Figure 6d, the volumes of the Ca(1)O5F octahedron and the Ca(3)O4F2 octahedron gradually get bigger, respectively; on the contrary, the volume of the Ca(2)O6 octahedron gradually gets smaller. The reason for the distortion of the octahedron (Ca(1)O5F, Ca(2)O6, Ca(3)O4F2) getting lower can be explained as follows. The numbers of substitution increase for a Ca2 site; a mass of Bi3+ ions enters into the 8797


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Figure 7. (a) Emission spectra of Ca5(BO3)3F:0.15 Bi3+ under different excitation wavelengths. (b) Gaussian fitting of the emission band of Ca5(BO3)3F:0.15 Bi3+. (c) Average lifetime monitored at 446, 490, and 538 nm, respectively. (d) Schematic diagram of spectral red shift (D(A)).

Ca1 site, and Ca2 site, respectively. The schematic diagram of the red shift is shown in Figure 7d. On the other hand, the change in the average bond length of octahedron also affects the nephelauxetic effect and the crystal field splitting. As shown in the inset of Figure 5a, a comparison with the Ca(1)O5F and Ca(3)O4F2 octahedra, it is found that the average bond length gradually gets shorter for the Ca(2)O6 octahedron. Therefore, a comparison with Ca(1)O5F and Ca(3)O4F2 octahedra, the nephelauxetic effect and crystal field splitting gradually get deeper for the Ca(2)O6 octahedron. This implies that the 538 nm emission peak belongs to the Ca2 site. The average bond length gradually gets longer for Ca(1)O5F and Ca(3)O4F2 octahedra, respectively. Thus, the nephelauxetic effect and crystal field splitting all gradually get weaker for Ca(1)O5F and Ca(3)O4F2 octahedra. However, the average bond length for the Ca(3)O4F2 octahedron is longer than that for the average bond length for the Ca(1)O5F octahedron; the nephelauxetic effect and crystal field splitting for the Ca(3)O4F2 octahedron are weaker than those of the Ca(1)O5F octahedron. Hence, the 446 and 490 nm emissions belong to the Ca3 site and the Ca1 site, respectively. By analyzing the changing trend of average bond length of three octahedra (Ca(1)O5F, Ca(2)O6, Ca(3)O4F2) and calculating the value of D(A), it can determine that the 446, 490, and 538 nm peaks belong to the Ca3 site, Ca1 site, and Ca2 site, respectively. There is the concentration quenching when the concentration of Bi3+ is 0.01, which may occur to activator saturation effect. As the concentration of Bi3+ ions increases to a certain degree, the Bi3+−Bi3+ distance becomes shorter resulting in generating the interaction between the Bi3+ species in the excited state. It can induce energy consumption leading to a reduction in the emission intensity. As for the concentration quenching, there are two major mechanisms for concentration quenching: exchange interaction and multipolar interaction.

of three peaks are measured under 320 nm pulse laser radiation (nano-LED) excitation, as shown in Figure 7c; all the decay curves can be fitted successfully by the second-order exponential. The average lifetimes are calculated to 0.93, 1.07, and 1.29 μs at the monitoring 446, 490, and 538 nm, respectively. It is found that the values of lifetime are same as the lifetime of Bi3+;17,34,35 thus, it proves that the three emission peaks (446, 490, and 538 nm) are derived from Bi3+, which occupies the position of three kinds of Ca2+ sites in the host. The source of three emission peaks can be confirmed by the changing trend of the average bond length of three octahedra and the total shift (D(A)) of the 6s6p level of Bi3+ arising from the nephelauxetic effect and crystal field splitting. The total shift can be defined by the following equation:25,26,28,36 D(A) = εc(A) +

εcfs(A) −B r(A)

(9)

where ∈c(A) is the centroid shift, which is the difference value between the centroid position of free Bi3+ ions and the centroid position of Bi3+ in the host. ∈cfs(A) is the crystal field splitting, which is defined as the energy difference between the lowest and highest 6s6p level. The value of a fraction 1/r(A) depends on the type of polyhedron, which leads to the red shift; the value is 2.4 for octahedral coordination. B is a constant, which is the energy difference between the centroid position and the lowest 6s6p level (3p0) of the free Bi3+ ion. According to the average lifetime of three emission peaks, this indicates that the emission band has three emission centers. On one hand, the different crystal lattices have different luminescence properties in the same host, which are influenced by the covalence and crystal field. Covalence is the primary factor that affects it. Thus, the value of D(A) is D(Ca2) > D(Ca1) > D(Ca3) for Ca5(BO3)3F:0.15 Bi3+; the result can infer that the 446 nm peak, 490 nm peak, and 538 nm peak belong to the Ca3 site, 8798


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Figure 8. (a) Value of parameter Δβ for Ca5(BO3)3F:xBi3+ (x = 0, 0.03, 0.15, 0.25). (b) Gaussian fitting emission band of Ca5(BO3)3F:xBi3+ (x = 0.15, 0.2, 0.25). (c) Intensity of fitting peak. (d) Lifetime of the fitting peak for Ca5(BO3)3F:xBi3+ (x = 0.15, 0.2, 0.25).

increasing Bi3+ concentration. Figure 8a shows the difference value between the monoclinic crystal and the cubic crystal on β. The difference value gradually reduces between them. Furthermore, these significantly reduce when the concentration of Bi3+ ions is over 0.15. This result indicates that the distortion of the Ca5(BO3)3F crystal is reducing bond angles. As the analysis above shows, the distortion of the Ca5(BO3)3F crystal is also reducing the bond length when the concentration of Bi3+ ions is higher than 0.15; hence, the distortion of the total crystal is reduced. Although the interaction between the Bi3+ species still gets stronger, the crystal field splitting energy (Δ) still increases, which deepens the crystal field splitting. However, the distortion of the total crystal shows an obvious decrease for a large concentration of doping, which gives rise to the crystal field splitting getting weakened slightly and generating a blue shift to a small extent. The emission intensity increases when the Bi3+ concentration is over 0.15. The possible reason is that energy transfer exists among Ca1, Ca2, and Ca3.37−39 The fitting three peaks are defined as peak1, peak2, and peak3, belonging to Ca3, Ca1, and Ca2, respectively, as shown in Figure 8b. From Figure 8c, it is seen that the emission intensities of three peaks all increase, and the growing rates of the Ca1 site and Ca3 site are larger; however, the growing rate of the Ca2 site is small with the occupation of the Ca2 site increasing. It may be that the Ca2

The distance Rc between the sensitizer and activator is an important parameter. It is reasonable that the critical distance Rc between activators should be shorter than 5 Å for exchange interaction. The critical distance Rc between Bi3+ ions can be determined by the following formula8,36 ⎡ 3V ⎤1/3 R c = 2⎢ ⎥ ⎣ 4πXCN ⎦

(10)

where V is the volume of the unit cell, XC is the critical concentration of Bi3+ ions, and N is the number of host cationic in the unit cell. In this case, V = 454.784 Å3, N = 10, and the critical concentration is 0.01 for Ca5(BO3)3F:xBi3+. Thus, the critical distance of Ca5(BO3)3F:Bi3+ is estimated to be 20.556 Å. The result means that the concentration quenching of Ca5(BO3)3F:Bi3+ belongs to multipolar interaction. It should be noted that not only does the bond length change leading to the octahedron distortion, but also the bond angles change with increasing Bi3+ content, which has an effect on the distortion of crystal as well.32 Ca5(BO3)3F is a monoclinic crystal; the values of cell parameters are α = γ = 90°, β ≠ 90°. The crystal has certain distortion. Cell parameters of cubic crystal are α = β = γ = 90°. The distortion degree of the crystal is zero. The cubic crystal is to be a reference; the result of refinement could be found that the value of β declines with 8799


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Here, A is the absorption constant, Eg is the optical band gap, hv represents the photon energy, F(R∞) is the absorption, and R is the reflectance (%) coefficient, respectively. The inset of Figure 8 describes the plots of [F(R∞)hv]1/2 versus hv. A tangent line is drawn in the near edge region of the obtained curve. The optical band gap of Ca5(BO3)3F:Bi3+ can be defined as the intersection of a tangent line with a photon energy axis. It is seen that the values for the optical band gap of Ca5(BO3)3F:xBi3+ have evidently decreased when the concentration of Bi3+ ions is lower than 0.15 and then slightly increased when the concentration of the Bi3+ ion is higher than 0.15. This result also demonstrates that the nephelauxetic effect gets larger and the crystal field splitting gets deeper with increasing Bi3+ concentration; however, the crystal field splitting has a little decrease resulting in the band gap getting larger when the Bi3+ concentration is higher than 0.15. 3.3. Luminescence Property of Ca5(BO3)3F:0.15 Bi3+, yEu3+. Ca5(BO3)3F:0.15 Bi3+, yEu3+ phosphors are synthesized aiming to study the energy transfer mechanism of Bi3+−Eu3+. Figure 10a shows the emission spectra of Ca5(BO3)3F:0.15Bi3+, yEu3+ (y = 0−0.1) upon 322 nm excitation; the emission spectra display not only a broad emission band of Bi3+ corresponding to the electronic transitions of 3P1 → 1S0, but also several sharp lines belonging to the characteristic excitation of Eu3+ ions (5D0 →7FJ; J = 0, 1, 2, 3, 4) transitions. It is known that the 580 nm emission peak corresponds to the forbidden transition 5D0 → 7F0, the 590 nm emission peak stands for the magnetic dipole transition 5D0 → 7F1, and the 617 nm emission peak is ascribed to the hypersensitive electric dipole transition 5 D0 → 7F2, which is the strongest emission peak. The 633 and 650 nm emission peaks originate from 5D0 → 7F3; the 694 and 714 nm emission peaks belong to 5D0 → 7F4. When the 617 nm emission was moinitored, there are several sharp lines consisting of 322, 464, and 538 nm, which correspond to

site transfers energy to the Ca1 site and the Ca3 site, leading to the intensity of these rapidly increasing, which contributes to enhancing the emission intensity and the blue shift of the emission spectrum. Thus, the lifetimes of peak1, peak2, and peak3 are measured, as shown in Figure 8d. The lifetimes of peak1 and peak2 gradually become longer; however, the lifetime of peak3 is quickly getting shorter. This result strongly proves that Ca2 transfers energy to Ca1 and Ca3, which contributes to a blue shift of the emission spectrum to some extent. The diffuse reflectance spectra of Ca5(BO3)3F:xBi3+ are illustrated in Figure 9; there are obvious absorption bands in

Figure 9. Diffuse reflectance spectra of Ca5(BO3)3F:xBi3+; inset shows the relationship of [F(R∞)hv]1/2 vs hv.

the region 320−350 nm. According to the theory given by Mott and Davis, the optical band gap (Eg) of Ca5(BO3)3F:Bi3+ can be determined by the following equation:40 [F(R ∞)hv]1/2 = A(hv − Eg )

(11)

F(R ∞) = (1 − R )2 /2R

(12)

Figure 10. (a) Emission spectra of Ca5(BO3)3F:0.15 Bi3+, yEu3+ (y = 0−0.1) under the 322 nm excitation. Inset of part a shows the intensity of Bi3+ and Eu3+ in Ca5(BO3)3F:0.15 Bi3+, yEu3+ (y = 0−0.1). (b) Emission spectra of Bi3+ in Ca5(BO3)3F:0.15 Bi3+ under 322 nm excitation and the excitation spectra of Eu3+ in Ca5(BO3)3F:0.15 Bi3+, Eu3+ monitored at 617 nm. (c) Lifetime of Bi3+ and Eu3+ in Ca5(BO3)3F:0.15 Bi3+, 0.03Eu3+. (d) Energy transfer efficiency ηT from Bi3+ to Eu3+. 8800


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Chemistry of Materials F0 → 6H5, 7F0 → 5D2, and 7F1 → 5D1 of Eu3+, respectively.41 As shown in Figure 10b, it is clearly seen that there is an obvious spectral overlap between the excitation spectrum of Eu3+ ions and the emission spectrum of Bi3+ ions, which indicates that the efficient energy transfer is expected to occur from the Bi3+ to Eu3+ ions. With increasing Eu3+ content, the emission intensity of Bi3+ ions gradually decreases; meanwhile, the emission intensity of Eu3+ ions obviously increases, as shown in the inset of Figure 10a. This result also proves that the efficient energy transfer surely occurs from Bi3+ to Eu3+ ions. The decay curves of Bi3+ ions under 320 nm nano-LED excitation (λex = 320 nm, λem = 457 nm) and Eu3+ ions under 322 nm Xe-lamp excitation (λex = 322 nm, λem = 617 nm) were recorded for Ca5(BO3)3F:0.15 Bi3+, yEu3+, as shown in Figure 10c, and the values of the lifetimes are listed in Table 3. 7

Figure 11. Schematic diagram of energy transfer from Bi3+ to Eu3+ and the chromaticity coordinates of Ca5(BO3)3F:0.15 Bi3+, yEu3+(y = 0−0.1).

Table 3. Lifetime of Bi3+ and Eu3+ of Ca5(BO3)3F:0.15 Bi3+, yEu3+ (y = 0−0.1) Bi3+ (μs) y=0 y = 0.01 y = 0.02 y = 0.03 y = 0.05 y = 0.07 y = 0.1

compared with those of the 3p1 state. Thus, Bi3+ at higher energy 1p1 is relaxed to the lower energy 3p1 by nonradioactive relaxation. Bi3+ ions generate a cyan emission from the 3p1 to 1 s0 electron transition; part of cyan emission will be reabsorbed by Eu3+ due to its existence in the overlap between the emission spectrum of Bi3+ and the excitation spectrum of Eu3+. The electronic transition will generate from the ground state 7 FJ (J = 0, 1) to the excited state 6H5, 5D2, and 5D1 for Eu3+ due to the reabsorbed energy of Eu3+. Then, the higher energy of 6 H5, 5D2, and 5D1 relaxes to the lower energy of 5D0 via nonradioactive relaxation and the electronic transition of Eu3+ from 5D0 to 7FJ (J = 0, 1, 2, 3, 4) occurs. The color-tunable phosphors can be achieved by changing the proportions of Bi3+ and Eu3+, and the color of phosphors gradually changes from cyan to red. The chromaticity coordinates are measured for Ca5(BO3)3F:0.15 Bi3+, yEu3+, and the values of chromaticity coordinates are described in Figure 11. It could be found that the chromaticity coordinates gradually vary from cyan (X = 0.21, Y = 0.30) to red (X = 0.54, Y = 0.34) under 254 nm excitation.3,17

Eu3+ (ms) τ = 1.05 τ = 0.92 τ = 0.88 τ = 0.84 τ = 0.77 τ = 0.72 τ = 0.66

y=0 y = 0.01 y = 0.02 y = 0.03 y = 0.05 y = 0.07 y = 0.1

τ=0 τ = 1.46 τ = 1.71 τ = 1.80 τ = 1.83 τ = 1.87 τ = 1.90

The decay curves fit well to a second-order exponential decay model based on the following formula:19,42,43 ⎛ t ⎞ ⎛ t⎞ I(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

(13)

I(t) is the luminescence intensity at times t. A1 and A2 are fitting constants; τ1 and τ2 are exponential component of the decay time, and t is the time, respectively. The average decay time (τ*) can be described by the following formula:19,43 τ * = (A1τ12 + A 2 τ22)/(A1τ1 + A 2 τ2)

4. CONCLUSIONS In order to explore the relationship between the luminescence properties of Bi3+ and its the site occupation, a series of Ca5(BO3)3F:xBi3+ materials are synthesized via a solid-state method. There are three types of Ca2+ sites in the host, which could be substituted by Bi3+. The emission spectra occur with some movement when Bi3+ occupies different positions for Ca2+ in the host. The emission color can alter from blue to cyan, which is induced by the coordination environment of Bi3+. With increasing Bi3+ content, the coordination environment around Bi3+ gradually varies. It gives rise to different degrees of nephelauxetic effect and crystal field splitting. In order to explore the relationship between the luminescence properities of Bi3+ and the nepelauxetic effect, the value of the centroid shift (∈c) is calculated. Centroid shift (∈c) is related to the covalence and average bond length of the octahedron in which the influence of covalence is primary. The relationship is discussed between the luminescence properities of Bi3+ and the crystal field splitting as well. The crystal field splitting is related to the interaction between the Bi3+ species, the crystal field splitting energy (Δ), and the distortion of the crystal. The emission spectra are asymmetric, and the emission spectra have remarkably changed at various excitation wavelengths. This proves that the broad band emission consists of at least two emission centers. In order to confirm this assumption, the

(14)

As the Eu3+ ion content increases, apparently the luminescent lifetimes of Bi3+ ions decrease while the luminescent lifetimes of Eu3+ ions increase, which is consistent with the luminescent behavior of Bi3+ ions and Eu3+ ions, respectively. This strongly proves that there is the energy transfer from Bi3+ to Eu3+ ions as well. On this basis, the energy transfer efficiency ηT can be calculated by the equation19,42,43

ηT = 1 − τS/τS0

(15)

where τS and τS0 are the lifetimes of Bi ions in the presence and absence of Eu3+, respectively. These values are plotted as Figure 10d; it is clearly found that the energy transfer efficiency ηT increases with increasing Eu3+ content, which also confirms energy transfer from Bi3+ to Eu3+ ions. Thus, the tunable emission is obtained by the combination of an appropriate proportion of Bi3+ ions. In order to get further knowledge about the energy transfer mechanism of Bi3+−Eu3+, the relationships of the energy levels between Bi3+ and Eu3+ are investigated in detail. Figure 11 illustrates the schematic of energy transfer between Bi3+ and Eu3+. The electron transition from 1s0 to 3p1 and 1p1 is allowed for Bi3+ ions. It is commonly a 1s0 → 3p1 transition due to the excited state 1p1 having a higher energy state and bad stability 3+

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(10) Kang, F.; Peng, M.; Xu, S.; Ma, Z.; Dong, G.; Qiu, J. Broadly tunable emission from CaMoO4:Bi phosphor based on locally modifying the microenvironment around Bi3+ ions. Eur. J. Inorg. Chem. 2014, 2014, 1373−1380. (11) Kang, F.; Peng, M.; Yang, X.; Dong, G.; Nie, G.; Liang, W.; Xu, S.; Qiu, J. Broadly tuning Bi 3+ emission via crystal field modulation in solid solution compounds (Y, Lu, Sc)VO4:Bi for ultraviolet converted white LEDs. J. Mater. Chem. C 2014, 2, 6068−6076. (12) Zheng, Y.; Hu, J.; Deng, S.; Gao, J.; Wang, Q. Variable Emission Changes in Bi3+/Ln3+, (Ln = Eu, Sm, Dy) Co-doped Lutetium Vanadates (LuVO4). J. Electron. Mater. 2016, 45, 2974−2980. (13) Awater, R. H. P.; Dorenbos, P. The Bi3+, 6s and 6p electron binding energies in relation to the chemical environment of inorganic compounds. J. Lumin. 2017, 184, 221−231. (14) Li, P.; Xu, Y.; Qin, Z.; Wang, Y.; Li, H.; et al. An intense broadband sensitized near-infrared luminescence from Yb3+, and Bi3+, co-doped zeolite L crystals. Microporous Mesoporous Mater. 2017, 239, 96−100. (15) Ju, G.; Hu, Y.; Chen, L.; Wang, X.; Mu, Z.; Wu, H.; Kang, F. Luminescence properties of Y2O3:Bi3+, Ln3+, (Ln = Sm, Eu, Dy, Er, Ho) and the sensitization of Ln 3+, by Bi 3+. J. Lumin. 2012, 132, 1853−1859. (16) Zhang, L.; Han, P.; Wang, K.; Lu, Z.; Wang, L.; et al. Enhanced luminescence of Sr2SiO4:Dy3+, by sensitization (Ce3+/Bi3+) and its composition-induced phase transition. J. Alloys Compd. 2012, 541, 54− 59. (17) Li, K.; Fan, J.; Shang, M.; Lian, H.; Lin, J. Sr2Y8(SiO4)6O2:Bi3+/ Eu3+: a single-component white-emitting phosphor via energy transfer for UV w-LEDs. J. Mater. Chem. C 2015, 3, 9989−9998. (18) Zhou, H.; Wang, Q.; Jin, Y. Temperature dependence of energy transfer in tunable white light-emitting phosphor BaY2Si3O10:Bi3+, Eu3+ for near UV LEDs. J. Mater. Chem. C 2015, 3, 11151−11162. (19) Chen, P.; Mo, F.; Guan, A.; Wang, R.; Wang, G.; Xia, S.; Zhou, L. Luminescence and energy transfer of the color-tunable phosphor Li6Gd(BO3)3:Tb3+/Bi3+, Eu3+. Appl. Radiat. Isot. 2016, 108, 148−153. (20) Lei, S.; Huang, Q.; Zheng, Y.; Jiang, A.; Chen, C. Structure of calcium fluoroborate, Ca5(BO3)3F. Acta Crystallogr. 1989, C45, 1861− 1863. (21) Hu, C. L.; Xu, X.; Sun, C. F.; Mao, J. G. Electronic structures and optical properties of Ca5(BO3)3F: a systematical first-principles study. J. Phys.: Condens. Matter. 2011, 23, 395501−395508. (22) Xu, K.; Loiseau, P.; Aka, G.; Lejay, J. A New Promising Nonlinear Optical Crystal for Ultraviolet Light Generation: Ca5(BO3)3F. Cryst. Growth Des. 2009, 9, 2235−2239. (23) Wang, Y.; Ding, J.; Wang, Y. Preparation and photoluminescence properties with the site-selected excitations of Bi3+‑activated Ba3Sc4O9 phosphors. J. Am. Ceram. Soc. 2017, 100, 2612−2620. (24) He, L.; Zou, X.; Wang, T.; Zheng, Q.; Jiang, N.; Xu, C.; et al. Red/Blue-Shift Dual-Directional Regulation in Blue-Emitting Ca0.8 Ba1.2SiO4:Eu2+, Phosphor on Incorporation of Eu2+/Mg2+ Ions. J. Electron. Mater. 2017, 46, 1777. (25) Dorenbos, P. 5d-level energies of Ce3+ and the crystalline environment. III. Oxides containing ionic complexes. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 125117. (26) Wang, F.; Wang, W.; Zhang, L.; Zheng, J.; Jin, Y.; Zhang, J. Luminescence properties and its red shift of blue-emitting phosphor Na3YSi3O9:Ce3+ for UV LED. RSC Adv. 2017, 7, 27422−27430. (27) Dorenbos, P. Calculation of the energy of the 5d barycenter of La3F3[Si3O9]:Ce3+. J. Lumin. 2003, 105, 117−119. (28) Dorenbos, P. 5d-level energies of Ce3+, and the crystalline environment. I. Fluoride compounds. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 15640−15649. (29) Liu, X. Y.; Guo, H.; Liu, Y.; Ye, S.; Peng, M. Y.; Zhang, Q. Y. Thermal quenching and energy transfer in novel Bi3+/Mn2+ co-doped white-emitting borosilicate glasses for UV LEDs. J. Mater. Chem. C 2016, 4, 2506−2512.

decay curves are measured; this confirms that there are three luminescence centers in the host. Considering the effect of the centroid shift (∈c) and the crystal field splitting (∈cfs), the sources of the three emission centers are confirmed by calculating the total shift (D(A)) of the 6s6p level of Bi3+ and the changing trend of the average bond length of the octahedron. In addition, the energy transfer mechanism of Bi3+−Eu3+ is also investigated in Ca5(BO3)3F:Bi3+, Eu3+. A color-tunable emission phosphor can be observed by efficient energy transfer from Bi3+ to Eu3+ ions, and the corresponding chromaticity coordinates gradually can vary from cyan (X = 0.21, Y = 0.30) to red (X = 0.54, Y = 0.34) under 254 nm excitation. This indicates that color-tunable phosphors could achieve a change in site occupancy or energy transfer.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 312 5977068. *E-mail: [email protected]. Phone: +86 312 5977068. ORCID

Panlai Li: 0000-0003-0972-9343 Zhijun Wang: 0000-0002-3574-3985 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (51672066, 50902042), the Funds for Distinguished Young Scientists of Hebei Province, China (A2015201129), and the personnel training project of Hebei Province, China (A2016002013).

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REFERENCES

(1) Ju, H. D.; Liu, J.; Wang, B. L.; Tao, X. T.; et al. Bi3+-doped Sr3Al2O6: An unusual color-tunable phosphor for solid state lighting. Ceram. Int. 2013, 39, 857−860. (2) Yao, S.; Chen, L.; Huang, Y.; Li, W. Enhanced luminescence of CaSb2O6:Bi3+, blue phosphors by efficient charge compensation. Mater. Sci. Semicond. Process. 2016, 41, 265−269. (3) Jiang, Z. Q.; Yu, X. C.; Gou, J.; Duan, L.; Su, X. H.; et al. Design, luminescence and energy transfer of single-phased color-tunable YNbO4:Bi3+, Eu3+, phosphor for UV pumped white light-emitting diodes. J. Mater. Sci.: Mater. Electron. 2017, 28, 3630−3636. (4) Yang, W. J.; Luo, L. Y.; Chen, T. M.; Wang, N. S. Luminescence and Energy Transfer of Eu- and Mn-Coactivated CaAl2Si2O8 as a Potential Phosphor for White-Light UVLED. Chem. Mater. 2005, 17, 3883−3888. (5) Yang, W. J.; Chen, T. M. White-light generation and energy transfer in SrZn2(PO4)2:Eu, Mn phosphor for ultraviolet light-emitting diodes. Appl. Phys. Lett. 2006, 88, 101903−101903−3. (6) Huang, C. H.; Chen, T. M. Ca9La(PO4)7:Eu2+, Mn2+: an emission-tunable phosphor through efficient energy transfer for white light-emitting diodes. Opt. Express 2010, 18, 5089−5099. (7) Long, J.; Wang, Y.; Ma, R.; Ma, C.; Yuan, X.; Wen, Z.; et al. Enhanced Luminescence Performances of Tunable Lu3‑xYxAl5O12:Mn4+ Red Phosphor by Ions of Rn (Li+, Na+, Ca2+, Mg2+, Sr2+, Sc3+). Inorg. Chem. 2017, 56, 3269−3275. (8) Kasturi, S.; Sivakumar, V. Eu2+ luminescence in Ca3Si2O7 and spectral widening and tuning of Eu2+ emission color (orangish-red to green) by crystal chemical substitution. RSC Adv. 2016, 6, 98652− 98662. (9) Yousif, A.; Swart, H. C. Colour tuneable emission from (Y1.995−xGax)2O3:Bi3+, phosphor prepared by a sol-gel combustion method. Mater. Lett. 2017, 186, 345−348. 8802


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Chemistry of Materials (30) Wei, R.; Ma, C.; Wei, Y.; Gao, J.; Guo, H. Tunable white luminescence and energy transfer in novel Cu+, Sm3+ co-doped borosilicate glasses for W-LEDs. Opt. Express 2012, 20, 29743−29750. (31) Chen, M.; Xia, Z.; Molokeev, M. S.; Wang, T.; Liu, Q. L. Tuning of Photoluminescence and Local Structures of Substituted Cations in xSr2Ca(PO4)2−(1−x)Ca10Li(PO4)7:Eu2+ Phosphors. Chem. Mater. 2017, 29, 1430−1438. (32) Ji, H.; Wang, L.; Molokeev, M.; Hirosaki, N.; Xie, R. J.; Huang, Z.; et al. Structure evolution and photoluminescence of Lu3(Al,Mg)2(Al,Si)3O12:Ce3+ phosphors: New yellow-color converter for blue LED-driven solid state lighting. J. Mater. Chem. C 2016, 4, 6855−6863. (33) Baur, W. H. The geometry of polyhedral distortions. Predictive relationships for the phosphate group. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1974, 30, 1195−1215. (34) Maggay, I. V. B.; Lin, P. C.; Liu, W. R. Enhanced luminescence intensity of novel red-emitting phosphor -Sr3Lu2(BO 3)4:Bi3+, Eu3+, via energy transfer. J. Solid State Light.. 2014, 1, 1−15. (35) Wang, L. J.; Guo, H.; Wei, Y. L.; Noh, H. M.; Jeong, J. H. White luminescence and energy transfer process in Bi3+, Sm3+, co-doped Ca3Al2O6, phosphors. Opt. Mater. 2015, 42, 233−236. (36) Dorenbos, P. 5d-level energies of Ce3+, and the crystalline environment. IV. Aluminates and “simple” oxides. J. Lumin. 2002, 99, 283−299. (37) Khan, W. U.; Li, J.; Li, X.; Wu, Q.; Yan, J.; Xu, Y.; Wu, M.; et al. Efficient energy transfer and luminescence properties of Ca3Y(GaO)3(BO3)4:Tb3+,Eu3+ as a green-to-red colour tunable phosphor under near-UV excitation. Dalton Trans. 2017, 46, 1885−1891. (38) Liu, Y.; Zhang, C.; Cheng, Z.; Zhou, Z.; Jiang, H.; Jiang, J. Origin and Luminescence of Anomalous Red-Emitting Center in Rhombohedral Ba9Lu2Si6O24:Eu2+ Blue Phosphor. Inorg. Chem. 2016, 55, 8628−8635. (39) Chen, L.; Yang, G.; Liu, J.; Shu, X.; Zhang, G.; Jiang, Y. Photoluminescence properties of Eu3+ and Bi3+ in YBO3 host under vacuum ultraviolet/ultraviolet excitation. J. Appl. Phys. 2009, 105, 013513. (40) Guo, H.; Wang, Y.; Li, G.; Liu, J.; Feng, P.; Liu, D. Cyan emissive super persistent luminescence and thermoluminescence in BaZrSi3O9:Eu2+,Pr3+ phosphors. J. Mater. Chem. C 2017, 5, 2844− 2851. (41) Kang, F.; Zhang, Y.; Peng, M. Controlling the energy transfer via multi luminescent centers to achieve white light/tunable emissions in a single-phased X2-type Y2SiO5:Eu3+, Bi3+ phosphor for ultraviolet converted LEDs. Inorg. Chem. 2015, 54, 1462−1473. (42) Zhang, X.; Xu, J.; Guo, Z.; Gong, M. Luminescence and energy transfer of dual-emitting solid solution phosphors (Ca, Sr)10Li(PO4)7:Ce3+, Mn2+ for ratiometric temperature sensing. Ind. Eng. Chem. Res. 2017, 56, 890−898. (43) Li, M.; Zhang, J.; Han, J.; Qiu, Z.; Zhou, W.; Yu, L.; Lian, S.; Li, Z. Changing Ce3+ Content and Codoping Mn2+ Induced Tunable Emission and Energy Transfer in Ca2.5Sr0.5Al2O6:Ce3+,Mn2+. Inorg. Chem. 2017, 56, 241−251.

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