Color-tunable emission in Ce³⁺, Tb³⁺ co-doped Ca₅(BO₃)₃F phosphor

Abstract

In this paper, Ce³⁺ doped and Ce³⁺, Tb³⁺ co-doped Ca₅(BO₃)₃F phosphors were synthesized by a high-temperature solid-state reaction. Upon excitation at 360 nm, the emission spectra of Ce³⁺ doped phosphors exhibit a broad emission band peaking at 392 nm, which originates from the 5d to 4f transition of Ce³⁺. The Ce³⁺, Tb³⁺ co-doped phosphors show strong energy transfer from Ce³⁺ to Tb³⁺, and the emission color can be tuned from purplish blue to green by changing the Tb³⁺ content. The excitation band in the 300–400 nm region broadens when monitored at 541 nm compared to that monitored at 392 nm. Furthermore, the co-doping of Tb³⁺ facilitates the appearance of green emitting Ce³⁺, which originates from Ce³⁺ on the Ca site other than that for purplish-blue Ce³⁺. The relationship between the luminescence properties of Ce³⁺ and its coordination environments, namely, different Ca sites, is discussed based on the calculations of centroid shift and crystal field splitting of 5d energy levels of Ce³⁺. Results suggest that Ce³⁺, Tb³⁺ co-doped Ca₅(BO₃)₃F phosphors may be a candidate for near-UV chip based white light-emitting diodes.

---

1. Introduction

White light-emitting diodes (w-LEDs) have been regarded as the new generation of illumination source due to numerous advantages, such as long lifetime, energy saving properties, environmental friendliness, etc., when they compare with traditional incandescent and fluorescent lamps.^1–3 However, the commonly used w-LEDs at present, which are based on the combination of a blue InGaN LED chip and a yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce), have a low color rendering index (CRI, Ra < 80) and a high color temperature due to the lack of red-emitting component.^4,5 These problems can be overcome by combining a near-UV LED chip with a white-emitting phosphor or blue, green and red emitting phosphors.^2,6 Therefore, it is important to search for phosphors that could be excited efficiently by near-UV LEDs.

Ce³⁺ ion is a common activator for phosphors with intense and broad excitation and emission bands due to the allowed 4f–5d and 5d–4f transitions, respectively, which is similar to the Eu²⁺ ion. Furthermore, the excitation and emission bands can be adjusted by changing hosts or crystallographic sites in a host.^7,8 Therefore, it is quite convenient to obtain phosphors that are suitable for near-UV LEDs by selecting a proper host. In the past year, there were several new Ce³⁺ doped phosphors reported, such as, BaLa₂Si₂S₈:Ce³⁺,⁹ γ-Ca₂SiO₄:Ce³⁺,¹⁰ MSiAl₂O₃N₂:Ce³⁺ (M = Sr, Ba),¹¹ novel garnet Ca₂GdZr₂(AlO₄)₃:Ce³⁺,¹² and Ca_3−xSr_xAl₂O₆:Ce³⁺ (x = 0, 1 and 2),^8,13,14 etc.

Tb³⁺ ion is used as a green-emitting component of phosphors, which usually has an intense emission peak at ∼540 nm originating from ⁵D₄–⁷F₅ transition. However, the excitation bands in near-UV region are usually very weak due to the forbidden 4f–4f transitions. Therefore, Ce³⁺ and Eu²⁺ ions are utilized to enhance the emission intensity of Tb³⁺ by energy transfer.^15–26

In the present work, we are going to report the luminescent properties of Ce³⁺ doped and Ce³⁺, Tb³⁺ co-doped Ca₅(BO₃)₃F phosphors. The emission color of the phosphors can be tuned from purplish blue to green by energy transfer from Ce³⁺ to Tb³⁺. Furthermore, the excitation and absorption bands of the Ce³⁺, Tb³⁺ co-doped phosphors broaden compared to the single-doped ones, which show potential application as phosphor for near-UV LEDs.

2. Experimental section

Ca₅(BO₃)₃F:Ce³⁺/Tb³⁺ phosphors were prepared by a high-temperature solid-state reaction under a reductive atmosphere. In a typical synthesis procedure, raw materials CaCO₃ (A.R.), CaF₂ (A.R., excess 10%), H₃BO₃ (A.R., excess 3%), CeO₂ (4N) and Na₂CO₃ (A.R.) were thoroughly mixed with a hypothetical composition of Ca_5−2x(BO₃)₃F:xCe³⁺, xNa⁺ (CBOF:xCe³⁺, xNa⁺), where Na⁺ acted as charge compensator. After ground thoroughly, the mixed raw materials were calcined in a tube furnace at 1000 °C for 6 h under a gas flow of 5% H₂ plus 95% N₂. The as-synthesized phosphors were ground and subjected to phase characterization and luminescent study. Raw material Tb₄O₇ (4N) is used as the source of Tb³⁺ ion. The amount of CaCO₃ is reduced for the substitution of luminescent centers. One should notice that the molar concentrations for Ce³⁺ and Tb³⁺ are 0.2x and 0.2y, respectively.

X-ray powder diffraction (XRD) patterns were collected by a Shimadzu X-ray Diffractometer XRD-6100 with Cu Kα radiation at 40 kV and 30 mA with a scan speed of 8° min⁻¹. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the phosphors were collected on a Hitachi F4500 spectrophotometer with a 150 W xenon lamp. Diffuse reflectance (DR) spectra were performed on a Hitachi U-3310 Spectrophotometer using BaSO₄ as a standard reference. Decay curves were recorded on an EDINBURGH FLS920 combined Fluorescence Lifetime & Steady State Spectrometer with a 450 W xenon lamp. The morphology and elemental mapping of samples were observed with a thermal field emission environmental SEM-EDS-EBSD (Quanta 400F). Ce and Tb elements on the surface of phosphors are detected by X-ray photoelectron spectroscopy (Thermo Scientific ESCALAB 250, USA). Transmission electron microscopic (TEM) micrographs and high resolution TEM (HR-TEM) images were obtained on a Tecnai G2 F20 S-TWIN electron microscope (America, FEI, 200 kV). All measurements were carried out at room temperature.

3. Results and discussion

3.1 Phase characterization, crystal structure and morphology

The XRD patterns of selected CBOF:xCe³⁺, xNa⁺ and CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺ phosphors are shown in Fig. 1 together with standard PDF cards. When the doping contents of Ce³⁺ or Tb³⁺ are low, the samples contain mainly Ca₃(BO₃)₃F phase (PDF#80-1702) and an impurity phase, namely, CaO (PDF#75-0264). The diffraction intensity of CaO peaks weakens gradually and finally disappears as the increase of Ce³⁺ or Tb³⁺ content (see Fig. 1d). We attempted to avoid the formation of CaO by reducing the amount of CaCO₃ raw material. The diffraction peaks of CaO weaken indeed, however, additional peaks that belong to Ca₃(BO₃)₂ appear. Fortunately, the luminescent properties of CaO:Ce³⁺, Na⁺ can be distinguished from that of CBOF:Ce³⁺, Na⁺, which will be discussed below. The crystal structure of Ca₅(BO₃)₃F is illustrated in Fig. 1e. Ca₅(BO₃)₃F crystallizes in a monoclinic phase. There are three Ca crystalline sites, which are denoted as Ca(1), Ca(2) and Ca(3). Ca(1) coordinates with 5 O and 1 F, Ca(2) coordinates with 6 O, and Ca(3) coordinates with 4 O and 2 F. The numbers of Ca(1), Ca(2) and Ca(3) atoms in a unit cell are 4, 4 and 2, respectively.

Fig. 1 XRD patterns of (a) stand PDF cards, (b) CBOF:xCe³⁺, xNa⁺ and (c) CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺, (d) the magnified XRD patterns in 31–33° and 37–38° regions, (e) crystal structure of Ca₅(BO₃)₃F.

The XRD analysis implies that Ce³⁺, Tb³⁺ and Na⁺ ions have entered into the lattice of the host. XPS and elemental mapping are also conducted to confirm the above assumption. Fig. 2 shows the XPS results of Ce and Tb in CBOF:0.03Ce³⁺, 0.15Tb³⁺, 0.18Na⁺. The binding energies of Ce 3d_5/2, Ce 3d_3/2 and Tb 4d are inconsistence with the reference data,²⁷ which indicates the existence of Ce and Tb in the final products. Elemental mapping was conducted by using SEM and EDS, in order to get the information of the distribution of these doping elements in the particles. The elemental maps of Ca, Ce, Tb and Na for corresponding particles of selected Ce³⁺-doped, Ce³⁺ and Tb³⁺ co-doped phosphors are shown in Fig. 3. Results indicate that the doping ions are well distributed in the particles.

Fig. 2 XPS spectra of (a) Ce 3d and (b) Tb 4d in CBOF:0.03Ce³⁺, 0.15Tb³⁺.

Fig. 3 Elemental mapping of Ca, Ce, Tb and Na for corresponding SEM images of (a) CBOF:0.03Ce³⁺, 0.03Na⁺, (b) CBOF:0.03Ce³⁺, 0.05Tb³⁺, 0.08Na⁺ and (c) CBOF:0.03Ce³⁺, 0.15Tb³⁺, 0.18Na⁺.

The SEM images of three representative phosphors are shown in Fig. 4. The morphologies of the phosphor particles are irregular with a size in micrometer region. Furthermore, the particle size increases with the amount of Na⁺, which implies that Na₂CO₃ acts as not only the supplier of charge compensator (Na⁺), but also fluxing agent. TEM and HRTEM images of selected Ce³⁺ doped and Ce³⁺, Tb³⁺ co-doped phosphors are illustrated in Fig. 5. The interplanar distances of (110) plane for these two phosphors are near the same, indicating the doping ions make little change to the crystal lattice of the host.

Fig. 4 SEM images of selected phosphors. (a) CBOF:0.03Ce³⁺, 0.03Na⁺, (b) CBOF:0.03Ce³⁺, 0.05Tb³⁺, 0.08Na⁺ and (c) CBOF:0.03Ce³⁺, 0.15Tb³⁺, 0.18Na⁺.

Fig. 5 TEM and HRTEM images of phosphors: (a–c) CBOF:0.03Ce³⁺, 0.03Na⁺ and (d–f) CBOF:0.03Ce³⁺, 0.15Tb³⁺, 0.18Na⁺ (FFT: Fast Fourier Transform).

3.2 Luminescence properties

The DR spectra of CBOF host, Ce³⁺/Tb³⁺ single-doped and Ce³⁺, Tb³⁺ co-doped phosphors are illustrated in Fig. 6. There is a weak absorption band around 250 nm for the host. There are two obvious absorption bands peaking at ∼340 and ∼420 nm for CBOF:Ce³⁺, Na⁺. Furthermore, a weak band around 470 nm is deduced from the asymmetric band of 420 nm. For Tb³⁺ doped phosphor, there are several absorption bands in 200–330 nm region. While, the absorption bands for Ce³⁺, Tb³⁺ co-doped phosphors seem the combination of those for Ce³⁺ and Tb³⁺ single-doped ones. Furthermore, the absorption intensities for Ce³⁺, Tb³⁺ co-doped phosphors increase with Tb³⁺ content.

Fig. 6 Diffuse reflectance spectra of host, Ce³⁺/Tb³⁺ single-doped and Ce³⁺, Tb³⁺ co-doped phosphors.

The PL and PLE spectra of CBOF:0.03Ce³⁺, 0.03Na⁺ are illustrated in Fig. 7. Firstly, the PL spectrum excited by a 360 nm light contains an asymmetric band peaking at 392 nm originating from 5d → 4f transition of Ce³⁺ (Fig. 7a), suggesting a purplish-blue emission. The corresponding PLE spectrum in the 200–380 nm near UV region is composed of four bands at 250, 290, 338 and 360 nm. The strongest excitation band locates at 360 nm. Secondly, the PL spectra excited by 400 and 465 nm are also collected and shown in Fig. 7b according to the DR spectra. There is no emission band when excited at 400 nm. While a broad emission band at 550 nm is observed when excited by a blue light around 460 nm, which is the same as that of CaO:Ce³⁺, Na^{+ 28,29}. This PLE and PL spectra, indeed, belong to CaO:Ce³⁺, Na⁺, which will be approved below by the luminescent properties of Ce³⁺, Tb³⁺ co-doped samples. Fig. 7c–f show the PL spectra excited by 250, 290, 338 and 360 nm lights, which indicate that the resulted emission bands peak at a same wavelength. All the emission bands contain two Gaussian fitted bands, with energy differences of 1539–1806 cm⁻¹, which are in consistent with the energy differences of 4f sublevels of Ce³⁺ due to spin–orbital coupling. Therefore, these four excitation bands, which are monitored at 392 nm, originate from the 4f → 5d transitions of Ce³⁺ ions on a same crystalline site. The curves of PL intensity versus Ce³⁺ content is illustrated in Fig. 7a, which suggests that the optimal doping content (x) of Ce³⁺ is 0.03, namely, Ca_4.94(BO₃)₃F:0.03Ce³⁺, 0.03Na⁺. The difference between DR and PLE spectra of Ce³⁺ doped phosphor suggests that the absorption of a light is not necessarily accompanied by emission of a light.

Fig. 7 (a) PL (λ_ex = 360 nm) and PLE (λ_em = 392 nm) spectra of CBOF:0.03Ce³⁺, 0.03Na⁺, and curve of PL intensity versus Ce³⁺ content; (b) PL (λ_ex = 400, 465 nm) and PLE (λ_em = 550 nm) spectra of CBOF:0.03Ce³⁺, 0.03Na⁺; (c–f) PL spectra under different UV lights and corresponding Gaussian fitted curves.

The PLE spectrum of CBOF:Tb³⁺, Na⁺ monitored at 541 nm contains five bands at 241, 261, 272, 284 and 324 nm and two weaker peaks at 379 and 485 nm (see Fig. 8a). DR spectrum of CBOF:Tb³⁺, Na⁺ also exhibits 5 absorption bands in 200–350 nm region, one of which is on the shoulder of the band at 300 nm. By comparing the DR spectra of the host and CBOF:Tb³⁺, Na⁺, and the PLE spectrum of CBOF:Tb³⁺, Na⁺, it could be deduced that the five excitation bands of CBOF:Tb³⁺, Na⁺ originate from 4f → 5d transitions of Tb³⁺. While the excitation peaks at 379 and 485 nm belong to ⁷F₆ → ⁵D₃ and ⁷F₆ → ⁵D₄ of Tb³⁺, respectively. Fig. 8b shows again the PLE and PL spectra of CBOF:Ce³⁺, Na⁺ for comparison. There is overlap between the PL spectrum of Ce³⁺ and the PLE spectrum of Tb³⁺, suggesting the existence of energy transfer from Ce³⁺ to Tb³⁺. The PLE spectrum of Ce³⁺, Tb³⁺ co-doped CBOF phosphor monitored at 541 nm contains not only excitation bands from 4f → 5d and intra-4f transitions of Tb³⁺, but also two broad bands in about 330–450 nm region as shown in Fig. 8d, which are not the same as that monitored at 392 nm (Fig. 8b and c). The Gaussian fitting result of the 300–450 nm band for CBOF:0.03Ce³⁺, 0.30Tb³⁺, 0.33Na⁺ is illustrated in Fig. 8d, which contains five Gaussian peaks at 324, 341, 362, 379 and 400 nm. Therefore, the 300–385 nm PLE band of Ce³⁺, Tb³⁺ co-doped CBOF phosphors is the overlap of bands at 324 nm (Tb³⁺), 338 nm (Ce³⁺) and 360 nm (Ce³⁺). The PLE band of Ce³⁺, Tb³⁺ co-doped phosphors at ∼400 nm exists neither in Tb³⁺ single-doped nor in Ce³⁺ single-doped CBOF phosphors.

Fig. 8 Comparison in PLE spectra of (a) Tb³⁺ single-doped CBOF phosphor (λ_em = 541 nm), (b) PL (λ_ex = 360 nm) and PLE (λ_em = 392 nm) of Ce³⁺ single-doped phosphor and (c and d) PLE of Ce³⁺–Tb³⁺ co-doped CBOF phosphors.

The PL spectra of CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺ excited by 360 nm light contain both the purplish-blue emission band of Ce³⁺ and the typical narrow emission bands of Tb³⁺ as shown in Fig. 9a. The emission intensities of 392 nm band decrease with the increase of Tb³⁺ content (y), while those of Tb³⁺ bands increase firstly, reaching a maximum at y = 0.20, and then decrease. Fig. 9b is the PLE spectra of Ce³⁺, Tb³⁺ co-doped samples monitored at 541 nm. Intensities of all the PLE bands belonging to Ce³⁺ and Tb³⁺ in CBOF have similar tendencies as that of Tb³⁺ PL bands in co-doped samples. Fig. 9c illustrates the PL intensities of Ce³⁺ (392 nm) and Tb³⁺ (541 nm) versus Tb³⁺ content in CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺ for clarity. Fig. 9d illustrates the CIE chromaticity diagram of CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺ (y = 0 − 0.30) under 360 nm excitation and the photographs of corresponding phosphors under 365 nm lamp, indicating that the emission color of the phosphors can change from purplish blue to green with the increase of Tb³⁺ content. These results indicate that there is energy transfer from Ce³⁺ to Tb³⁺.

Fig. 9 (a) PL (λ_ex = 360 nm) and (b) PLE (λ_em = 541 nm) spectra of CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺; (c) PL intensities of Ce³⁺ (392 nm) and Tb³⁺ (541 nm) versus Tb³⁺ content (y); (d) CIE chromaticity diagram for CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺ (y = 0 − 0.30) under 360 nm excitation, inset: photographs of phosphors under 365 nm lamp.

Decay time curves of the Ce³⁺ single-doped and Ce³⁺, Tb³⁺ co-doped samples monitored at 392 nm are illustrated in Fig. 10. The decay time curve of CBOF:Ce³⁺, Na⁺ is single exponential, while it changes to non-exponential for Ce³⁺, Tb³⁺ co-doped ones, and the decay time shortens with the increase of Tb³⁺ content. The decay curves can be well fitted by the following equations³⁰

Fig. 10 Decay curves of Ce³⁺ in CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺ phosphors monitored at 392 nm and excited by a 360 nm light. Insert shows the decay times and efficiencies of energy transfer from purplish-blue Ce³⁺ to Tb³⁺.

For Ce³⁺ single-doped one,

I = A₁ exp(−t/τ₁) (1)

For Ce³⁺, Tb³⁺ co-doped ones,

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I is the luminescence intensity, t is the time, τ₁ and τ₂ are lifetimes, and A₁ and A₂ are constants. The average decay times (τ) can be calculated by the following formula³⁰

τ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (3)

The decay times decrease with the increase of Tb³⁺ as shown in Table 1 and Fig. 10. These phenomena suggest the existence of energy transfer from purplish-blue emitting Ce³⁺ to Tb³⁺. The energy transfer efficiency (η_T) from Ce³⁺ to Tb³⁺ can be calculated by the following equation³¹

where τ₀ and τ are the lifetimes of Ce³⁺ in the absence and presence of Tb³⁺, respectively. The energy transfer efficiency can reach 45% when Tb³⁺ content (y) is 0.30 (Table 1).

Table 1 Decay times of purplish-blue Ce³⁺ obtained from the decay curves

Tb^3+ content	A1	τ1/ns	A2	τ2/ns	τ/ns	η (%)
0	918.56	26.0			26.0
0.05	670.27	22.2	298.15	6.0	20.5	21.2
0.10	611.17	21.0	410.14	5.9	18.6	28.5
0.15	605.29	17.1	401.84	4.2	15.3	41.2
0.20	549.65	17.6	445.70	4.6	15.3	41.2
0.25	528.34	16.6	429.66	4.8	14.4	44.6
0.30	588.88	16.2	405.43	4.4	14.3	45.0

---

On the basis of Dexter's energy-transfer expressions of multi-polar interactions and Reisfeld's approximation, the energy transfer behavior from Ce³⁺ to Tb³⁺ can be deduced by the following formula^32,33

η_S0/η_S ∝ C^α/3 (5)

where η_S0 and η_S are the luminescence quantum efficiencies of Ce³⁺ in the absence and presence of Tb³⁺, respectively; C is the total concentration of Ce³⁺ and Tb³⁺; and the α values 3, 6, 8 and 10 correspond to exchange interaction, electric dipole–dipole (d–d), dipole–quadrupole (d–q) and quadruople–quadrupole (q–q) interactions, respectively. η_S0/η_S can be estimated approximately by the ratio of relative emission intensities (I_S0/I_S), where I_S0 and I_S are the emission intensity of Ce³⁺ in the absence and presence of Tb³⁺, respectively. Fig. 11 shows the dependence of I_S0/I_S of purplish-blue Ce³⁺ on C^α/3 in CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺. The linear relationship for I_S0/I_S–C^8/3 and I_S0/I_S–C^10/3 is better than the other two. However, q–q interaction is generally not expected to play an important role in solids in view of the very short interaction range.³⁴ Therefore, d–q interaction is responsible for the energy transfer from purplish-blue Ce³⁺ to Tb³⁺.

Fig. 11 Dependence of I_S0/I_S of purplish-blue Ce³⁺ on C^α/3 in CBOF:0.03Ce³⁺, yTb³⁺, (0.03 + y)Na⁺.

The critical distance (R_c) of energy transfer from purplish-blue Ce³⁺ to Tb³⁺ can be calculated by using the equation given by Blasse³⁵

R_c = 2[3V/(4πX_cN)]^1/3 (6)

where V is the volume of the unit cell, N is the number of host cations in the unit cell. X_c is the total concentration of Ce³⁺ and Tb³⁺, where the emission of Tb³⁺ in the co-doped phosphor reaches the maximum. By introducing the values of V (453.08 ų), N (10) and X_c (0.046, 0.03 × 0.2 + 0.20 × 0.2), the critical distance is calculated to be 12.3 Å, which is an effective distance for d–q interaction.

The PLE spectra in Fig. 9 may suggest that the introduction of Tb³⁺ ion forces Ce³⁺ to occupy another Ca site and results in additional PLE and PL spectra. The PL spectra of Ce³⁺, Tb³⁺ co-doped phosphors excited at 400 nm are illustrated in Fig. 12a, which contain not only the emission peaks of Tb³⁺, but also a broad band in 450–700 nm region peaking at ∼530 nm. The emission intensities of both the broad band and Tb³⁺ peaks increase with Tb³⁺ content from 0.05 to 0.20, and then decrease. However, the PL intensities are much weaker than those based on purplish-blue emitting Ce³⁺. The PL spectra of Ce³⁺, Tb³⁺ co-doped samples excited by a 465 nm light are shown in Fig. 12b, which also contain a broad band at ∼540 nm. The broad band weakens and disappears with the increase of Tb³⁺ content. When monitored by 520 nm as containing little emission intensity from Tb³⁺, the PLE spectra exhibit a strong band peaking at ∼400 nm, a band at ∼465 nm and several other weaker bands in the region of 200–370 nm (Fig. 12c). The profile of the weaker bands is similar as that monitored at 541 nm (Fig. 9b), which belongs to 4f → 5d transition of Tb³⁺ and purplish-blue Ce³⁺. Sharp peaks on the 465 nm band are due to the interference of Xe lamp. The PLE bands at 400 and 465 nm have similar tendencies as those of the corresponding PL spectra. Fig. 12d shows the PLE intensities versus Tb³⁺ content. All the luminescent properties shown before indicate that the co-doping of Tb³⁺ ions forces Ce³⁺ ions to occupy another Ca site, which results in a broad emission band at ∼530 nm excited by a light around 400 nm. The PLE band at 465 nm and corresponding PL band at ∼540 nm belong to CaO:Ce³⁺, as these bands decrease with the increase of Tb³⁺, which are in accordance with the XRD results. Co-doping of Tb³⁺ ions facilitates the disappearance of CaO phase.

Fig. 12 PL spectra of CBOF:Ce³⁺, Tb³⁺, Na⁺ excited by (a) 400 nm and (b) 465 nm light, (c) PLE spectra monitored at 520 nm, (d) PLE intensity at 400 and 465 nm versus Tb³⁺ content.

The above discussion indicates that Ce³⁺ ions can occupy two Ca sites of CBOF and exhibit two different luminescent properties. It is well known that the excitation and emission spectra of Ce³⁺ doped phosphors are related to nephelauxetic effect, crystal field splitting effect and Stokes shift.^36–38 The nephelauxetic effect will result in a shift of the energy barycenter (centroid shift, ε_c) of 5d levels relative to the free ion value. The crystal field splitting (ε_cfs) is the energy difference between the highest and lowest 5d levels of Ce³⁺ in a certain crystalline site. While Stokes shift is the energy difference between the peaks values of the lowest excitation band and the highest emission band.

The 5d centroid shift of Ce³⁺ (in eV, relative to the free ion value of 6.2 eV) can be expressed by the following equation^39,40

where R_i is the distance (pm) between Ce³⁺ and anion i in the undistorted lattice. The summation is over all N anions that coordinate Ce³⁺. 0.6ΔR is a correction for lattice relaxation around Ce³⁺, and ΔR is the difference between the radii of Ce³⁺ and cation sites that Ce³⁺ ions occupy. α_spⁱ (in units of 10⁻³⁰ m⁻³) is the spectroscopic polarizability of anion i. For O, α^O_sp = 0.33 + 4.8/χ_av². For F, α^F_sp = 0.15 + 0.96/χ_av². χ_av is the electronegativity of the cations. χ_av = (10χ_Ca + 9χ_B)/19 = 1.47 in Ca₅(BO₃)₃F, and one obtains α^O_sp = 2.55 × 10⁻³⁰ m⁻³ and α^F_sp = 0.59 × 10⁻³⁰ m⁻³. If Ce³⁺ can occupy all the three Ca sites, the ε_c values for Ce³⁺(1), Ce³⁺(2) and Ce³⁺(3) are calculated to be 1.52, 1.44 and 1.27 eV, respectively.

The energy of the lowest 5d excited level of Ce³⁺ is influenced by crystal field splitting and centroid shift. The crystal field splitting of the 5d levels can be expressed as⁴¹

ε_cfs = β_poly^QR_av⁻² (8)

where β_poly^Q is a constant that depends on the type of the coordination polyhedron, Q is 3 + for Ce³⁺, and R_av is close to the average distance between anions and cation that is replaced by Ce³⁺. Three Ca sites are all coordinated by six anions. To simplify the comparison, the β_poly^Q values are assumed to be the same, viz. β_octa^Q = 1.35 × 10⁹ pm² cm⁻¹.⁴² R_av values for Ca(1), Ca(2) and Ca(3) in Ca₅(BO₃)₃F are 233.9, 241.5 and 235.6 pm, respectively. Therefore, the ε_cfs values for Ce³⁺(1), Ce³⁺(2) and Ce³⁺(3) are 3.06, 2.87 and 3.02 eV, respectively. The ε_c and ε_cfs values for Ce³⁺(1), Ce³⁺(2) and Ce³⁺(3) are in the following order:

ε_c: Ce³⁺(1) > Ce³⁺(2) > Ce³⁺(3)

ε_cfs: Ce³⁺(1) > Ce³⁺(3) > Ce³⁺(2)

Ca(2) is coordinated by six O atoms, forming an octahedron which is the most distorted one among the three Ca polyhedra. Ce³⁺ on an octahedron with a more distorted form may have more 5d sublevels. Therefore, it could be deduced that the PLE (360 nm)–PL (392 nm) spectra belong to Ce³⁺(2). While PLE (400 nm)–PL (∼530 nm) spectra may originate from Ce³⁺(1) based on the comparison of ε_c and ε_cfs. Different Stokes shift values are related to the difference in size and morphology of the Ca sites occupied by Ce³⁺. The 4f and 5d energy levels of Ce³⁺ on the two Ca sites and energy transfer from Ce³⁺ to Tb³⁺ are illustrated roughly in Fig. 13.

Fig. 13 Schematic diagram of 4f and 5d energy levels for Ce³⁺ and Tb³⁺, and Ce³⁺ → Tb³⁺ energy transfer in CBOF host.

4. Conclusion

In summary, Ce³⁺ doped and Ce³⁺, Tb³⁺ co-doped Ca₅(BO₃)₃F phosphors have been synthesized successfully by a high-temperature solid-state reaction. Ca₅(BO₃)₃F:Ce³⁺, Na⁺ phosphor exhibits a purplish-blue emission peaking at 392 nm with a strongest excitation band at 360 nm, which originates from Ce³⁺ on Ca(2) site. The excitation energy of Ce³⁺ can transfer to Tb³⁺ efficiently, showing a broader excitation band in 300–400 nm region for Tb³⁺. Furthermore, the co-doping of Tb³⁺ ion results in the appearance of green emitting Ce³⁺ on Ca(1) site. Ce³⁺, Tb³⁺ co-doped Ca₅(BO₃)₃F shows potential application as green emitting phosphor for near-UV LEDs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (grant no. 51402105 and 21471055), Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20134306120009 and 20124306120005), Hunan Provincial Natural Science Foundation of China (grant no. 12JJ2029) and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.

References

1. L. Chen, C.-C. Lin, C.-W. Yeh and R.-S. Liu, Materials, 2010, 3, 2172–2195.
2. C. C. Lin and R.-S. Liu, J. Phys. Chem. Lett., 2011, 2, 1268–1277.
3. P. F. Smet, A. B. Parmentier and D. Poelman, J. Electrochem. Soc., 2011, 158, R37–R54.
4. V. Bachmann, C. Ronda and A. Meijerink, Chem. Mater., 2009, 21, 2077–2084.
5. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma and Q. Y. Zhang, Mater. Sci. Eng., R, 2010, 71, 1–34.
6. M. Shang, C. Li and J. Lin, Chem. Soc. Rev., 2014, 43, 1372–1386.
7. Z. Xia, Y. Zhang, M. S. Molokeev, V. V. Atuchin and Y. Luo, Sci. Rep., 2013, 3, 3310.
8. Y. He, J. Zhang, W. Zhou, J. Han, Z. Qiu, L. Yu, C. Rong and S. Lian, J. Am. Ceram. Soc., 2014, 97, 1517–1522.
9. S.-P. Lee, C.-H. Huang, T.-S. Chan and T.-M. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 7260–7267.
10. A. Kalaji, M. Mikami and A. K. Cheetham, Chem. Mater., 2014, 26, 3966–3975.
11. W.-Y. Huang, F. Yoshimura, K. Ueda, Y. Shimomura, H.-S. Sheu, T.-S. Chan, C.-Y. Chiang, W. Zhou and R.-S. Liu, Chem. Mater., 2014, 26, 2075–2085.
12. X. Gong, J. Huang, Y. Chen, Y. Lin, Z. Luo and Y. Huang, Inorg. Chem., 2014, 53, 6607–6614.
13. M. Jiao, Y. Jia, W. Lu, W. Lv, Q. Zhao, B. Shao and H. You, Dalton Trans., 2014, 43, 3202–3209.
14. Y. Li, Y. Shi, G. Zhu, Q. Wu, H. Li, X. Wang, Q. Wang and Y. Wang, Inorg. Chem., 2014, 53, 7668–7675.
15. X. Zhang, L. Zhou, Q. Pang, J. Shi and M. Gong, J. Phys. Chem. C, 2014, 118, 7591–7598.
16. X. Zheng, H. Luo, J. Liu, P. Liu and X. Yu, J. Mater. Chem. C, 2013, 1, 7598–7607.
17. Y. Zhang, G. Li, D. Geng, M. Shang, C. Peng and J. Lin, Inorg. Chem., 2012, 51, 11655–11664.
18. Z. Xia and R.-S. Liu, J. Phys. Chem. C, 2012, 116, 15604–15609.
19. G. Li, Y. Zhang, D. Geng, M. Shang, C. Peng, Z. Cheng and J. Lin, ACS Appl. Mater. Interfaces, 2012, 4, 296–305.
20. H. Liu, Y. Luo, Z. Mao, L. Liao and Z. Xia, J. Mater. Chem. C, 2014, 2, 1619–1627.
21. D. Geng, M. Shang, Y. Zhang, H. Lian, Z. Cheng and J. Lin, J. Mater. Chem. C, 2013, 1, 2345–2353.
22. J. Chen, Z. Xia and Q. Liu, J. Mater. Chem. C, 2015, 3, 4197–4204.
23. Z. Xia, J. Zhuang, A. Meijerink and X. Jing, Dalton Trans., 2013, 42, 6327–6336.
24. G. Zhu, S. Xin, Y. Wen, Q. Wang, M. Que and Y. Wang, RSC Adv., 2013, 3, 9311–9318.
25. Y. Wang, M. G. Brik, P. Dorenbos, Y. Huang, Y. Tao and H. Liang, J. Phys. Chem. C, 2014, 118, 7002–7009.
26. M. Jiao, N. Guo, W. Lü, Y. Jia, W. Lv, Q. Zhao, B. Shao and H. You, Inorg. Chem., 2013, 52, 10340–10346.
27. C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation and Physical Electronics Division, Eden Prairie, 1979.
28. Z. Hao, X. Zhang, Y. Luo, L. Zhang, H. Zhao and J. Zhang, J. Lumin., 2013, 140, 78–81.
29. Z. Hao, X. Zhang, X. Wang and J. Zhang, Mater. Lett., 2012, 68, 443–445.
30. C.-H. Huang and T.-M. Chen, J. Phys. Chem. C, 2011, 115, 2349–2355.
31. P. I. Paulose, G. Jose, V. Thomas, N. V. Unnikrishnan and M. K. R. Warrier, J. Phys. Chem. Solids, 2003, 64, 841–846.
32. D. L. Dexter, J. Chem. Phys., 1954, 22, 1063–1070.
33. R. Reisfeld, E. Greenberg, R. Velapoldi and B. Barnett, J. Chem. Phys., 1972, 56, 1698–1705.
34. C. Ronda, Luminescence: From Theory to Applications, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008.
35. G. Blasse, Philips Res. Rep., 1969, 24, 131–144.
36. V. Bachmann, C. Ronda, O. Oeckler, W. Schnick and A. Meijerink, Chem. Mater., 2009, 21, 316–325.
37. G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994.
38. R.-J. Xie, Y. Q. Li, N. Hirosaki and H. Yamamoto, Nitride Phosphors and Solid-State Lighting, Taylor & Francis, Boca Raton, 2011.
39. P. Dorenbos, Phys. Rev. B, 2000, 62, 15640–15649.
40. P. Dorenbos, J. Lumin., 2003, 105, 117–119.
41. P. Dorenbos, J. Phys.: Condens. Matter, 2003, 15, 4797–4807.
42. P. Dorenbos, Phys. Rev. B, 2001, 64, 125117.

---