Luminescence properties and energy transfer of co-doped Ba₃GdNa(PO₄)₃F:Ce³⁺,Tb³⁺ green-emitting phosphors

Abstract

Phase pure Ce³⁺ and Tb³⁺ singly doped and Ce³⁺/Tb³⁺ co-doped Ba₃GdNa(PO₄)₃F samples have been synthesized via the high temperature solid-state reaction. The crystal structures, photoluminescence properties, fluorescence lifetimes, thermal properties and energy transfer of Ba₃GdNa(PO₄)₃F:Ce³⁺,Tb³⁺ were systematically investigated. Rietveld structure refinement indicates that Ba₃GdNa(PO₄)₃F crystallizes in a hexagonal crystal system with the space group P-6. For the co-doped Ba₃GdNa(PO₄)₃F:Ce³⁺,Tb³⁺ samples, the emission color can be tuned from blue to green by varying the doping concentration of the Tb³⁺ ions. The intense green emission was realized in the Ba₃GdNa(PO₄)₃F:Ce³⁺,Tb³⁺ phosphors on the basis of the highly efficient energy transfer from Ce³⁺ to Tb³⁺. Also the energy transfer mechanism has been confirmed to be quadrupole–quadrupole interaction, which can be validated via the agreement of critical distances obtained from the concentration quenching (13.84 Å). These results show that the developed phosphors may possess potential applications in near-ultraviolet pumped white light-emitting diodes.

1 Introduction

In the past decade, more and more interest has been focused on the GaN-based light-emitting diode (LED) technology, especially the developing advanced solid-state lighting sources [1]. The LED-based lighting have gained significant attention over conventional lighting sources due to their advantages such as significant power saving, longer lifetime, higher luminous efficiency, and brightness, etc [2, 3]. Nowadays the commonly used white emitting LEDs are based on the combination of a blue InGaN LED chip and a yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce). However, these systems have a low color rendering index (CRI, Ra < 80) and a high color temperature due to the lack of red-emitting component [4]. Therefore, it is crucial to develop the new phosphors with high chemical stability and strong absorption in ultraviolet (UV) or near-ultraviolet (NUV) region with high conversion efficiency [5].

The Tb³⁺ ion, a well-known activator, emits a green color as a result of its main ⁵D₄–⁷F₅ transition with an emission peak around 545 nm [6]. The absorption peaks of Tb³⁺ in the NUV region above 290 nm are rather weak and narrow because these transitions belong to the 4f–4f forbidden transition. Nevertheless, the Ce³⁺ ion can be used as an important sensitizer for Tb³⁺ ions by transferring part of its excitation energy to Tb³⁺ as a result of its strong 4f–5d absorption in NUV region [7]. As a promising sensitizer for Tb³⁺ ions, Ce³⁺ ions have been extensively applied in many Tb³⁺-doped hosts, such as Ba₃Lu(PO₄)₃, Sr₂La₈(SiO₄)₆O₂, and Sr₃Lu(PO₄)₃ [8,9,10]. It was found that Tb³⁺ can be sensitized by Ce³⁺ in these lattices due to the spectral overlap between the emission band of Ce³⁺ and the excitation band of Tb³⁺. Consequently, the tunable emission from blue to green can be produced in the type of Ce³⁺/Tb³⁺ co-doped BGNPF single-composition host.

As is well known, an appropriate host matrix is crucial in the generation of white light. Apatite is a structural type for compounds with the general formula M₁₀(XO₄)₆L₂ (M = Ca, Sr, Ba; X = P, As, V, Mn, Cr, Si and Ge; L = F, Cl, OH). It has been known since Naray-Szabo determined the structure of Ca₁₀(PO₄)₆F₂ in 1930 and was widely studied due to its biological and industrial importance [11, 12]. Recently, a variety of phosphates with the formula BGNPF have been reported in the literatures, such as Ba₃NaGd(PO₄)₃F:Eu²⁺, Ba₃KGd(PO₄)₃F:Tb³⁺,Eu³⁺, and Ba₃NaGd(PO₄)₃F:Eu²⁺,Tb³⁺ [13,14,15]. And these results show that the host material BGNPF possess the advantages of low synthetic temperature and high thermal/chemical stability. To the best of our knowledge, the luminescence properties of BGNPF:Ce³⁺,Tb³⁺ have not been investigated yet. We therefore co-doped the rare earth ions Ce³⁺ and Tb³⁺ into the BGNPF host. In the present work, we reported the crystal structure refinement of BGNPF and preferred crystallographic sites for activators. BGNPF crystallizes in a hexagonal unit cell with the space group P-6 and lattice constants of a = 9.80587 Å, c = 7.33931 Å, and cell volume = 611.164 Å³ based on the powder diffraction Rietveld refinement. Moreover, we have demonstrated a novel blue–green tunable BGNPF:Ce³⁺,Tb³⁺ phosphor by varying the relative ratios of Ce³⁺/Tb³⁺. Their detailed luminescence properties and the energy transfer between Ce³⁺ and Tb³⁺ have been discussed, and the energy transfer mechanism between Ce³⁺ and Tb³⁺ ions has been investigated systematically.

2 Experimental

2.1 Materials and synthesis

BaCO₃ (A.R. (Analytical Reagent)), BaF₂ (A.R.), Na₂CO₃ (A.R.), (NH₄)₂HPO₄ (A.R.), Gd₂O₃ (99.99%), CeO₂ (99.99%), and Tb₄O₇ (99.99%) were employed as the raw materials. In addition, 20% excess of BaF₂ was added to compensate the loss of fluorine. These samples were weighted with designed chemical compositions and thoroughly mixed in an agate mortar for 25 min. Then the mixtures were taken in an alumina crucible and sintered at 1050 °C for 4 h in a corundum crucible imbedded in active carbon. Finally, after natural cooling to room temperature, grounding powder for the following analysis.

2.2 Characterization methods

The X-ray Powder diffraction spectroscopy of the as-prepared phosphor was identified by a Bruker D2 Focus diffractometer with Cu Kα radiation (λ = 1.5406 Å). The morphology of the samples was determined by a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were obtained using a fluorescence spectrophotometer (F-7000, Hitachi, Japan) equipped with a 150 W Xe lamp used as an excitation source. The luminescence decay curve was obtained using a spectrofluorometer (HORIBA, JOBIN YVON FL3-21) with xenon flash lamp as the source. The temperature-dependence PL spectra were measured on the same spectrophotometer with a self-made heating attachment (Tianjin Orient KOJI Co. Ltd, TAP-02).

3 Results and discussion

3.1 Crystal structures and phase analysis

As a representative, the XRD patterns of BGNPF host, BGNPF:0.22Ce³⁺, BGNPF:0.16Tb³⁺, BGNPF:0.22Ce³⁺,0.16Tb³⁺, and the reference diffraction lines based on the JCPDS card with no. 71-1317 are shown in Fig. 1. The results of XRD analysis confirm that the XRD patterns of all the samples are in good agreement with the reported BGNPF phase without any impurity phase. This fact suggests that Ce³⁺ and Tb³⁺ ions have been successfully built into the BGNPF host lattice and do not destroy the lattice structure of BGNPF host. The inset of Fig. 1 represents the SEM micrograph of the BGNPF:0.22Ce³⁺,0.16Tb³⁺ phosphors. It can be seen that the obtained powders consist of particles with size around several microns which usually occurs in the high temperature solid state reaction. Meantime, it is clear that the sizes of the BGNPF:0.22Ce³⁺,0.16Tb³⁺ aggregations are in the range of 1–6 µm. This is a suitable size for fabrication of solid-state lighting devices [16].

Fig. 1

The XRD patterns of BGNPF host, BGNPF:0.22Ce³⁺, BGNPF:0.16Tb³⁺, BGNPF:0.22Ce³⁺,0.16Tb³⁺ samples and the standard pattern of JCPDS card file of 71-1317. Inset: the SEM micrograph of BGNPF:0.22Ce³⁺,0.16Tb³⁺ phosphor

To further understand the crystal structure of BGNPF, Fig. 2 presents the experimental, calculated, background and difference results of the XRD profiles for the Rietveld refinement of BGNPF host and the corresponding refined parameters are listed in in Table 1. Rietveld refinement was performed using General Structure Analysis System (GSAS) software, and it was found that almost all peaks were indexed to a hexagonal cell with parameters close to those of the BGNPF phase [17]. The phosphor was found to crystallize in the hexagonal crystal system with the space group of P-6 and its cell parameters were a = b = 9.80587 Å, c = 7.33931 Å and V = 611.164 Å³. The refinement finally converged to R_p = 3.8%, R_wp = 4.89%, and χ² = 1.84, indicating that all the observed peaks satisfy the reflection conditions and our prepared phosphor is of single phase.

Fig. 2

Rietveld refinement of the XRD profile of BGNPF

Table 1 Rietveld refinement results and crystal data for the BGNPF host

Furthermore, Fig. 3a, b show the unit cell structure of BGNPF sample viewed from the c axis. In the host, the cations are connected by the [PO₄]³⁻ tetrahedral formed by the P and O atoms. In this crystal structure, there exist two kinds of Gd³⁺ ions sites, which are named Gd1 and Gd2 here for identification. Gd1 is coordinated by two fluorine atoms and five oxygen atoms; Gd2 is coordinated by nine oxygen atoms. The coordination environment around the four cationic sites Gd1 and Gd2 are given in Fig. 3c. Compared with the ionic radii and valence state, the doped Ce³⁺ ions (r = 0.102 nm) and Tb³⁺ (r = 0.092 nm) are easy to substitute the Gd³⁺ (r = 0.094 nm) ions [18].

Fig. 3

Schematic illustration of the crystal structure of BGNPF viewed from the c axis, and the coordination environment around Gd1 and Gd2

3.2 Luminescent properties of Ce³⁺, Tb³⁺ and Ce³⁺/Tb³⁺ co-doped in BGNPF

The PL as well as PLE spectra of BGNPF:0.22Ce³⁺ are depicted in Fig. 4a. The PLE spectrum consists of three absorption bands that peaking at about 224, 273 and 314 nm, which correspond to the transitions from the ground state of the Ce³⁺ ions to the field splitting levels of the 5d state [19]. Upon the excitation of 273 nm, the PL spectrum shows an asymmetric broad band peaking at 351 nm. The asymmetric emission band can be decomposed into two Gaussian bands (dash lines) peaking at 362 and 379 nm, which correspond to the Gd1/Gd2 independent sites. The energy difference is close to the energy separation between the ²F_7/2 and ²F_5/2 levels [20]. Figure 4b shows the PL spectra of BGNPF:xCe³⁺ (x = 0.03, 0.06, 0.09, 0.12, 0.18, 0.22, 0.24, 0.26, 0.30 and 0.32). The PL intensity of Ce³⁺ increases with its increasing concentration (x) and the intensity reaches a maximum value at x = 0.22. However, when above the optimal Ce³⁺ amount, the emission intensity is found to decline because of the concentration quenching effect.

Fig. 4

a PLE and PE spectra of BGNPF:0.22Ce³⁺; b PL spectra of BGNPF:xCe³⁺ (x = 0.03, 0.06, 0.09, 0.12, 0.18, 0.22, 0.26, 0.30)

According to the theory of Blasse, the concentration quenching among the same activator ions is mainly ascribed to the non-radiative energy migration. The critical distance (R_c) can be calculated by the following equation [21]:

$${R_c} \approx 2\left[ {\frac{{3V}}{{4\pi {x_c}N}}} \right]$$

(1)

where V is the volume of one unit cell, x_c is the critical concentration and N is the number of the cationic sites occupied by activators in one unit cell. For BGNPF host, V = 611.164 Å³, N = 2, x_c = 0.22. As a result, the critical distance of energy transfer can be quickly obtained to be 13.84 Å by using Eq. (1). Based on the theory of Van Uitert, if the critical distance between the sensitizer and activator is shorter than 5 Å, the exchange interaction is dominant in the process of energy transfer [22]. Therefore the electric multipolar interaction will take place for quenching mechanism in Ce³⁺ doped samples. According to the energy transfer theory of Van Uiter, the relationship between the luminescent intensity (I) and the doping concentration (x) follows the equation [23]:

$$\frac{I}{x}=K{\left( {1+\beta {{\left( x \right)}^{Q/3}}} \right)^{ - 1}}$$

(2)

where I is the integral intensity of emission spectra, x is the activator concentration beyond the optimal concentration, K and β are constants, Q represents the interaction type between rare earth ions, here Q = 6, 8 or 10, indicating the dipole–dipole, dipole–quadrupole or quadrupole–quadrupole interaction, respectively. As shown in Fig. 5, the relation between log (I/x) and log (x) is found to be an approximately linear and the slope is about − 2.86. The value of Q, thus, turns out to 8.58, which is approximately equal to 8. This indicates that the dipole–quadrupole interaction is the major mechanism for concentration quenching in BGNPF:xCe³⁺ phosphors.

Fig. 5

The relationship of log(x) versus log(I/x) for BGNPF:xCe³⁺ (x = 0.22, 0.24, 0.26, 0.30, 0.32) phosphor; the inset shows the variation of emission intensity as a function of doped Ce³⁺ molar concentration

The excitation and emission spectra of BGNPF:0.22Ce³⁺ and BGNPF:0.16Tb³⁺ samples are shown in Fig. 6a, b, respectively. The excitation spectrum monitored at 545 nm for BGNPF:0.16Tb³⁺ sample exhibits a shoulder and a broad band centered at 223 and 276 nm, which correspond to the spin-allowed 4f⁸–4f⁷5d¹ (⁷F₆–⁷D) and spin-forbidden 4f⁸–4f⁷5d (⁷F₆–⁹D) transition of Tb³⁺ ions [24]. When the sample is excited at 276 nm, the emission spectrum exhibits four sharp peaks located at 489, 545, 589 and 624 nm which are assigned to the ⁵D₄–⁷F_J (J = 6, 5, 4 and 3) ascribed to the characteristic transition of Tb³⁺, respectively [25]. The phosphor exhibits green emitting hue due to the maximum peak at 545 nm. The PLE and PL spectra of the BGNPF:0.22Ce³⁺,0.16Tb³⁺ phosphor are shown in Fig. 6c. At the irradiation of 273 nm, the PL spectrum exhibits both the Ce³⁺ and the typical Tb³⁺ emissions. In addition, to avoid the existence of the Ce³⁺ emission, we chose the emission peak at 545 nm as monitoring wavelength which is beyond the emission range of the Ce³⁺ ions. From Fig. 6c, we can see that the PLE spectrum is similar to that monitored at 351 nm. So the energy transfer from Ce³⁺ to Tb³⁺ ions can be achieved in the BGNPF system. To further confirm the phenomenon, Fig. 6d illustrates the PL spectra of BGNPF:0.22Ce³⁺ and PLE spectra of BGNPF:0.16Tb³⁺ phosphors. A notable spectral overlap between the PL spectrum of BGNPF:0.22Ce³⁺ and the PLE spectrum of BGNPF:0.16Tb³⁺ is observed. The above analysis on the PL and PLE spectra of the phosphors proves the occurrence of the energy transfer from the Ce³⁺ to Tb³⁺ ions [26].

Fig. 6

PLE and PL spectra of a BGNPF:0.22Ce³⁺, b BGNPF:0.16Tb³⁺, and c BGNPF:0.22Ce³⁺,0.16Tb³⁺ phosphors; d PLE spectra of BGNPF:0.16Tb³⁺ and PL spectra of BGNPF:0.22Ce³⁺ phosphors

3.3 Energy transfer of Ce³⁺/Tb³⁺ co-doped BGNPF phosphors

As mentioned above, energy transfer can be expected from the spectral overlap of the singly doped samples. In order to investigate the energy transfer process in the BGNPF host lattice, the fluorescence decay curves of Ce³⁺ with different Tb³⁺ doping contents are illustrated in Fig. 7a. According to the decay behavior of Ce³⁺, their corresponding luminescence decay curves can be best fitted with a typical second-order exponential c the following equation [27]:

Fig. 7

a Decay curves of Ce³⁺ emission monitored at 351 nm for BGNPF:0.22Ce³⁺,yTb³⁺ (y = 0, 0.04, 0.08, 0.12, 0.16, 0.24). b The fluorescence lifetime of Ce³⁺ ions and energy transfer efficiencies from Ce³⁺ to Tb³⁺ as a function of Tb³⁺ concentration

$$I\left( t \right)={A_1}\exp \left( { - t/{t_1}} \right)+{A_2}\exp \left( { - t/{t_2}} \right)$$

(3)

where I is the luminescence intensity at time t, A₁ and A₂ are constants, t is the time, and t₁ and t₂ are the decay time for the exponential components. Then, the average decay lifetime τ can be calculated as follows:

$$\tau =\frac{{{A_1}{t_1}^{2}+{A_2}{t_2}^{2}}}{{{A_1}{t_1}+{A_2}{t_2}}}$$

(4)

According to the Eq. (4), and decay curve fitting date in Fig. 7a, the average decay lifetimes τ are calculated to be 37.35, 32.41, 27.50, 24.93, 19.32 and 7.6 ns for BGNPF:0.22Ce³⁺,yTb³⁺ with y = 0, 0.04, 0.08, 0.12, 0.16 and 0.24, respectively. These phenomena strongly demonstrated the energy transfer from Ce³⁺ to Tb³⁺. In addition, the energy transfer efficiency η between the Ce³⁺ and Tb³⁺ ions can be obtained from the following equation [28]:

$${\eta _T}=1 - \tau /{\tau _0}$$

(5)

where τ and τ₀ are the decay times of the sensitizer Ce³⁺ in the presence and absence of the activator Tb³⁺, respectively. As shown in Fig. 7b, the energy transfer efficiencies increase gradually with increasing Tb³⁺ concentration. The value of η_T reaches the maximum of 80% for the emission center monitored at 545 nm when y = 0.24, indicating that the energy transfer from the Ce³⁺ to Tb³⁺ is efficient.

To further analyze the energy transfer behaviors from Ce³⁺ to Tb³⁺, Fig. 8 illustrates a series of emission spectra for BGNPF:0.22Ce³⁺,yTb³⁺ (y = 0, 0.04, 0.08, 0.12, 0.16, 0.24) excited at 273 nm. With increasing Tb³⁺ concentration, it is found that the emission colors of this series of phosphors varied from blue to green. In addition, one can see that the emission intensity of Ce³⁺ ions decreased monotonously with the increasing Tb³⁺ content in the insert of Fig. 8. Conversely, the emission intensity of the Tb³⁺ ions increased greatly as y changed from 0 to 0.24. These results reflect the behaviors of the energy transfer from Ce³⁺ to Tb³⁺.

Fig. 8

PL spectra of BGNPF:0.22Ce³⁺,yTb³⁺ (y = 0, 0.04, 0.08, 0.12, 0.16, 0.24) phosphors; the inset shows relative emission intensity of Ce³⁺ and Tb³⁺ in BGNPF:0.22Ce³⁺,yTb³⁺ phosphor

According to Dexter’s energy transfer formula for exchange and multipolar interactions and Reisfeld’s approximation, the following relation can be acquired [29]:

$$\left( {{I_{s0}}/{I_s}} \right) \propto {C^{n/3}}$$

(6)

where I_S0 and I_S are emission intensity of Ce³⁺ in the absence and presence of Tb³⁺, respectively. C is the total doping concentration of the Ce³⁺ and Tb³⁺ ions. The relationship of I_S0/I_S ∝ C^n/3 corresponds to the exchange interaction, and n = 6, 8, and 10 are ascribed to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The plots of (I_S0/I_S) − C^n/3 are illustrated in Fig. 9a–c. The linear relationship reaches the optimal one for I_S0/I_S ∝ C^n/3 by comparing the fitting factors of R² values in Fig. 9c, implying that energy transfer from Ce³⁺ to Tb³⁺ occurs via the quadrupole–quadrupole interaction.

Fig. 9

Dependence of I_S0/I_S of Ce³⁺ on a C^6/3, b C^8/3, and c C^10/3

In an effort to understand the energy transfer process, Fig. 10 shows the energy level model for energy transfer process of Ce³⁺ → Tb³⁺. When excited at 273 nm, the excited electrons shift to the excited state (4f⁶5d) of Ce³⁺ and non-radiatively (NR) relaxes to the lowest 5d crystal field splitting state, and then the electrons of the Ce³⁺ ions shift from the excited state (4f⁶5d) to its ground state at ²F_5/2 and ²F_7/2 levels with a consequent broad band emission. Subsequently, some decay to the ²F_5/2 and ²F_7/2 levels occurs via a radiative process. As a consequence of the matched energy levels of Ce³⁺ and Tb³⁺, other photons transfer the excitation energy to the high excited levels of Tb³⁺ (4fⁿ). Finally, the ⁵D₄ level gives the strong emission of Tb³⁺ (⁵D₄ → ⁷F_J, J = 3, 4, 5 and 6) [30].

Fig. 10

The schematic of energy transfer in BGNPF:Ce³⁺,Tb³⁺

3.4 CIE chromaticity coordinates and thermal stability of BGNPF:Ce³⁺,Tb³⁺

The x and y values of the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates for different samples in BGNPF:0.22Ce³⁺,yTb³⁺ phosphors were measured based on the corresponding PL spectra upon 273 nm excitation, and the results are summarized in Fig. 11 and Table 2, respectively. From the luminescence photographs of the samples in Fig. 11, it can be seen that the colour tone of the phosphors can be easily modulated from blue (0.252, 0.133) to green (0.290, 0.507) with an increase in the doping content with Tb³⁺, confirming that the CIE chromaticity coordinates are tunable. Based on these findings, it is clear that these bluish green BGNPF:Ce³⁺,Tb³⁺ phosphors can be efficiently excited in the NUV range. This suggests that BGNPF:Ce³⁺,Tb³⁺ can act as a promising bluish green tunable phosphor for possible applications in solid-state lighting and displays [31].

Fig. 11

CIE chromaticity coordinates and the luminescence photographs of BGNPF:0.22Ce³⁺,yTb³⁺ (y = 0, 0.04, 0.08, 0.12, 0.16 and 0.24) under excitation at 273 nm

Table 2 Comparison of CIE chromaticity coordinates for BGNPF:Ce³⁺,nTb³⁺ phosphors

In general, the thermal stability of luminescence intensity of phosphors has a great significance for the practical application of w-LEDs [32]. Figure 12a showed the relative emission intensities of the BGNPF:0.22Ce³⁺,0.16Tb³⁺ phosphor measured under 273 nm excitation and Fig. 12b presented the detailed tendency to decrease for the emission intensities under different temperatures. From the Fig. 12a, b, we can see the PL intensities just decreased to 94% of the initial intensity (50 °C) at 150 °C. That shows its excellent temperature stability of the BGNPF:0.22Ce³⁺,0.16Tb³⁺ phosphor. Generally, the decrease of emission intensity is ascribed to the thermal quenching of emission intensity via phonon interaction. And the configuration coordinate diagram as shown in Fig. 12c can be used to explain the temperature quenching mechanism [33]. With the increase in temperature, electrons at lower excited levels C could jump to higher excited levels E, assisted by thermal phonons. Electrons at state E can go back to the 4f ground state A in a non-radiative way, making the number of electrons which can go back to the ground state through radiation decrease.

Fig. 12

a The temperature-dependent PL spectra of the BGNPF:0.22Ce³⁺,0.16Tb³⁺ under the 273 nm excitation; b normalized intensity as a function of temperature; c the schematic configuration coordinate diagram for the explanation of temperature quenching; and d the plots of ln[(I₀/I_T) − 1] versus 1/kT and the thermal activation energy

Furthermore, the activation energy from the thermal quenching can be calculated using the Arrhenius equation [34]:

$$I\left( T \right)=\frac{{{I_o}}}{{1+c\exp \left( { - \left( {\Delta E/KT} \right)} \right)}}$$

(7)

where I₀ is the initial intensity, I(T) is the intensity at a given temperature T, ΔE is the activation energy for thermal quenching, c is a constant for a certain host, and K is the Boltzmann constant (8.629 × 10⁻⁵ eV). And, the plot of ln[(I₀/I_T) − 1] versus 1/(kT) are shown in Fig. 12d. The activation energy (ΔE) was deduced to be 3.22 eV by the fitting process, which is much higher than commercial phosphor YAG:Ce³⁺ (ΔE = 0.136 eV). It can be concluded that the relatively high activation energy ΔE resulting in the excellent thermal stability of our phosphors.

In general, for the practical application of phosphors for LEDs, the quantum efficiency (QE) of the phosphor is an important factor to be considered. We have also measured the internal QE of BGNPF:0.22Ce³⁺,yTb³⁺ phosphor according to the reported method [35, 36]. The internal QE value can be calculated by Eq. (8):

$${\eta _{{\text{QE}}}}=\frac{{\int {{L_S}} }}{{\int {{E_R}} - \int {{E_S}} }}$$

(8)

where L_S is the luminescence emission spectrum of the sample; E_S is the spectrum of the light used for exciting the sample; E_R is the spectrum of the excitation light without the sample in the sphere; and all the spectra were collected using the sphere on the FL3-21 fluorescence spectrophotometer. Under the excitation of 273 nm, the recorded quantum yield values of BGNPF:0.22Ce³⁺,yTb³⁺ are 35.9, 42.4, 49.7, 56, and 65.5%, for y = 0.04, 0.08, 0.12, 0.16, and 0.24 respectively, and it is found that the value increases with increasing Tb³⁺ content. These results indicate that BGNPF:0.22Ce³⁺,yTb³⁺ phosphors can act as potential blue–green tunable phosphors for the possible applications in w-LEDs.

4 Conclusions

In summary, a blue and green double-color emitting phosphor BGNPF:Ce³⁺,Tb³⁺ phosphor was successfully prepared by the solid-state reaction. The phase purity, PL properties, energy transfer mechanism, thermal stability and chromaticity coordinates were investigated. The BGNPF:Ce³⁺,Tb³⁺ phosphor shows both a blue emission (351 nm) from Ce³⁺ and a green emission (545 nm) from Tb³⁺ with considerable intensity under NUV excitation. The energy transfer from Ce³⁺ to Tb³⁺ ions takes place in the BGNPF:Ce³⁺,Tb³⁺ phosphor on the basis of the analysis of the luminescence spectra. The energy transfer mechanism from Ce³⁺ to Tb³⁺ ions was proved to be quadrupole–quadrupole interaction. The energy transfer from Ce³⁺ to Tb³⁺ in the phosphor is also demonstrated by luminescence spectra and lifetime values. This indicates that BGNPF:Ce³⁺,Tb³⁺ can act as a potential blue–green tunable phosphor for application in white LEDs.