Synthesis and luminescence properties of novel Eu^2+/3+, Ce³⁺ ion single- and co-doped BaZn₂(PO₄)₂ phosphors for white-light applications

Abstract

A series of novel Eu^2+/3+, Ce³⁺ ion single- and co-doped BaZn₂(PO₄)₂ samples were prepared via a high-temperature solid-state reaction. XRD powder diffraction results indicated that all of the products were pure phases. The photoluminescence properties of BaZn₂(PO₄)₂:Eu showed that Eu²⁺ and Eu³⁺ coexist in the system and Eu³⁺ can be self-reduced to Eu²⁺ in an air atmosphere. In addition, the strongest emission peak of Eu³⁺ ions at 593 nm implied that Eu³⁺ ions occupy the inversion symmetry lattice and also the site of Zn in BaZn₂(PO₄). We used the theoretical method of bond energy to explain why the self-reduction of Eu³⁺ to Eu²⁺ can occur in the BaZn₂(PO₄)₂ system. The calculation results indicated that the bond energy change value is smaller than , indicating that Eu²⁺ ions are more likely to occupy the Zn site and more stable than Eu³⁺ ions in BaZn₂(PO₄). Furthermore, the energy transfer process between Ce³⁺ and Eu²⁺ ions in the photoluminescence spectrum and the decay lifetime were observed, and the energy transfer mechanism was determined to be a dipole–dipole interaction. In this work, by adjusting the ratio of Ce and Eu ions, the emission color can be changed from blue to white, implying that the phosphor can be used as a promising candidate in the manufacture of white LEDs.

---

1. Introduction

Rare-earth doped luminescent materials have wide applications in the fields of illumination, display, laser, anti-counterfeiting, fiber-optic communication and biomedicine, and they have become a hotspot in many interdisciplinary research studies. The rare-earth doped white LED materials have drawn extensive attention due to their advantages of luminous efficiency, energy saving, environmental protection, long life, and simple structure.^1–5 Among most rare-earth ions, Eu³⁺ ions have been extensively studied. It is a typical red-emitting rare-earth ion with an electronic structure of [Xe]4f⁶, and its emission spectrum is caused by an electronic transition from the excited state ⁵D₀ high-energy level to the ⁷F_J (J = 0, 1, 2⋯6) ground-state energy level in the 4f configuration. If the Eu³⁺ ion is located at the strict inversion center, its ⁵D₀ → ⁷F₁ electronic transition is the strongest at around 590 nm, emitting orange-red light. When the Eu³⁺ ion is deviated from the inversion center position, the ⁵D₀ → ⁷F₂ electric dipole transition in the emission spectrum is the strongest at about 612 nm with red light.^6–8 In recent years, the reduction from Eu³⁺ to Eu²⁺ has been found to occur in the same crystal in an air atmosphere. For example, in aluminosilicates⁹, aluminate,^10,11 silicate,¹² phosphate,¹³ borate,^14,15 the reduction from Eu³⁺ to Eu²⁺ have appeared. The Eu²⁺ spectral position changes with the strength of the crystal field, and ultraviolet, blue, and red light are generated according to the degree of splitting.¹⁶ Ce³⁺ is also an important luminescent ion, and its transition between 4f and 5d is the allowed electric dipole transition; hence, it has high excitation and emission peaks. The interaction of the exposed d-electrons with the crystal field broadens the excitation and emission of Ce³⁺, and the emission of light can be adjusted from the near-ultraviolet to red region depending on the matrix. This property allows Ce³⁺ ions to spectrally overlap with most common luminescent ions in a specific matrix, greatly increasing the probability and efficiency of energy transfer. When the Ce³⁺ ion is in an excited state, it can transfer energy to the co-doped rare-earth ion by resonance according to the ion spacing, which is a commonly used and excellent sensitizer.^17–19 Ce³⁺ ions can be co-doped with many ions such as Ce³⁺, Tb³⁺/Mn²⁺ in Ca₁₉Ce(PO₄)₁₄,²⁰ CaSc₂O₄:Ce³⁺, Yb³⁺,²¹ LiYSiO₄:Ce³⁺, Eu³⁺,²² BaY₂Si₃O₁₀:Ce³⁺, Pr³⁺,²³ K₂Ba₃Si₈O₂₀:Ce³⁺, and Eu²⁺ (ref. 24) to generate energy transfer behavior.

The bond energy theory is used to analyze and explain the priority occupancy for doping ions entering the crystal lattice. The change in bond energy is smaller as per the bond energy theory when the doping ions replace the cation sites in the crystal lattice, in which the easier the doping ions preferentially occupy. At present, this theory has been effectively used in the CaAl₂Si₂O₈:Eu³⁺,⁹ Ca₁₀M(PO₄)₇ (M = Li, Na, K):Eu²⁺/Eu³⁺,²⁵ and Sr₅(BO₃)₃F:Eu³⁺(ref. 26) samples.

MZn₂(PO₄)₂ (M = Mg, Ca, Sr, Ba) is an important host with the advantages of low cost, high luminescence efficiency, low synthesis temperature and good thermal stability. Based on these characteristics, BaZn₂(PO₄)₂ has also become one of the hot topics of discussion. In the previous reports, the properties of BaZn₂(PO₄)₂ phosphors have been studied. For example, the photoluminescence properties of BaZn₂(PO₄)₂:Ce³⁺, Tb³⁺/Dy³⁺ (ref. 27 and 28) and red phosphor BaZn₂(PO₄)₂:Sm³⁺ (ref. 29) have been studied. However, the lattice occupancy and the self-reduction phenomenon of Eu ion doping in BaZn₂(PO₄)₂ and the energy transfer behavior of Ce³⁺, Eu²⁺ co-doping in BaZn₂(PO₄)₂ have not been discussed.

In this work, a series of Eu^2+/3+ and Ce³⁺ single- and co-doped BaZn₂(PO₄)₂ products were synthesized via a high-temperature solid-state reaction. First, the self-reduction phenomenon was found from Eu³⁺ to Eu²⁺ in the BaZn₂(PO₄)₂ system. Next, we use the bond energy method to analyze the self-reduction reaction from Eu³⁺ to Eu²⁺ in the lattice position of Zn in BaZn₂(PO₄)₂. At the same time, we use this method to explain why Eu³⁺ can be self-reduced to Eu²⁺. Finally, the photoluminescence properties and energy transfer mechanisms of Ce³⁺–Eu²⁺ ion co-doped BaZn₂(PO₄)₂ are studied in detail. In addition, based on the energy transfer, the emission color can be adjusted from blue to white by adjusting the relative proportions of Ce and Eu ions.

2 Experimental

2.1 Preparation of phosphors

A series of Eu²⁺/³⁺, Ce³⁺ ion single- and co-doped BaZn₂(PO₄)₂ phosphors were synthesized by a traditional high-temperature solid-state method in an air atmosphere. The raw materials selected include BaCO₃, (NH₄)₂HPO₄, ZnO, CeO₂ and Eu₂O₃. The exact amount of raw materials of each component was calculated, and the analytical balance was accurately and effectively weighed. The raw materials were thoroughly ground and mixed evenly in an agate mortar, and transferred to an alumina crucible of appropriate size. Then the mixtures were pre-sintered at 850 °C for 4 h, and further calcined at 1000 °C for 4 h. Finally, all the as-synthesized products were naturally cooled to room temperature and ground to a powder.

2.2 Characterization methods

The phase purity characterization of the phosphors was performed using an X-ray diffractometer. Excitation source information: 40 kV, 40 mA on a Bruker D8 Advance X-ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation. The scanning mode is continuous, the scanning speed is 2° per minute, and the test range is 5–80°. The morphologies of the samples were characterized by scanning electron microscopy (SEM) on an instrument of JSM 6510 LV Electronics, Japan; the resolution is 3 nm. The spectrometer FLS980 was used to test the emission and excitation as well as decay curves of the samples. A 450 W xenon lamp was used as a steady-state light source in the test and a 60 W μF flash lamp was used as the decay curve test source.

3. Results and discussion

3.1 Phase analysis and crystal structure

Fig. 1 depicts the XRD patterns of Eu²⁺/³⁺, Ce³⁺ ion single- and co-doped BaZn₂(PO₄)₂ phosphors. Compared with the standard card JCPDS#16-0554, it can be seen that the diffraction peaks of the synthesized samples corresponded well to the standard card, and no other diffraction peaks appeared. Therefore, the prepared samples were all pure phases and no impurities were formed. This indicates that rare-earth ions enter the BaZn₂(PO₄)₂ lattice, and no other impurity phases are formed; hence, the doping of these ions does not cause any change in the lattice structure of BaZn₂(PO₄)₂. However, the XRD patterns of Eu^2+/3+, Ce³⁺ ion single- and co-doped BaZn₂(PO₄)₂ phosphors are light-shifted compared with the pure BaZn₂(PO₄)₂ compound. This result could be attributed to the larger ionic radii of Eu³⁺ [r = 1.09 Å and 1.101 Å for CN = 4 and 7, respectively] and Ce³⁺ [r = 1.07 Å for CN = 7] with respect to those of Ba²⁺ (r = 1.38 Å for CN = 7, respectively) and Zn²⁺ (r = 0.60 Å for CN = 4, respectively). Moreover, as the radius of the dopant ions increase, the lattice gets distorted, and hence, the position of the diffraction peak shifts slightly.

Fig. 1 XRD patterns of Eu²⁺/³⁺, Ce³⁺ ion single- and co-doped BaZn₂(PO₄)₂ phosphors and standard card (JCPDS#16-0554).

The unit cell of BaZn₂(PO₄)₂ is exhibited in Fig. 2. BaZn₂(PO₄)₂ belongs to a monoclinic crystal structure of space group P2₁/c. The unit cell contains five kinds of cation coordination environments, namely Ba1, Zn1, Zn2, P1 and P2. Ba1 form isolated BaO polyhedra, which has seven coordination sites with an asymmetric polyhedron surrounded by oxygen atoms. Both Zn1 and Zn2 are tetrahedrons connected to oxygen. At the same time, there are two environments of tetrahedrons, one with vertices connected to a polyhedron centered on Ba and the other with a co-edge. P1 and P2 are also two four-coordinated sites. The parameters of the unit cell are a = 8.598 Å, b = 9.761 Å, c = 9.159 Å, and V = 768.45 ų, respectively.

Fig. 2 Schematic of the crystal structure of BaZn₂(PO₄)₂ and coordination environments for Ba, Zn and P sites.

The SEM micrograph, EDS mapping images and elemental distribution of Eu/Ce doped in BaZn₂(PO₄)₂ synthesized under ambient conditions are displayed in Fig. 3. According to the SEM image of the two samples, we can see that all the samples have a block structure with inconsistent size. According to the EDS scanning structure and elemental distribution image, the atomic distribution patterns of Ba, O, P, Zn, Eu or Ce can be obtained very clearly in the Eu- or Ce-doped BaZn₂(PO₄)₂ samples. These indicate that the Ce and Eu elements are indeed present in the BaZn₂(PO₄)₂ sample.

Fig. 3 (a and b) Scanning electron micrographs and EDS of BaZn₂(PO₄)₂:Eu/Ce. (c) Elemental distribution map of BaZn₂(PO₄)₂:Ce.

3.2 Photoluminescence properties of BaZn₂(PO₄)₂:Eu

The excitation and emission spectra of the undoped BaZn₂(PO₄)₂ sample are shown in Fig. 4. At an excitation wavelength of 240 nm, the emission spectrum consists of a broad peak at a maximum wavelength position of 345 nm in the wavelength range of 300–425 nm (Fig. 4a). In the excition spectrum (Fig. 4b), the maximum wavelength position of the excitation spectrum is at 240 nm.

Fig. 4 (a and b) PLE (λ_{ex=240 nm}) and PL (λ_{em=345 nm}) spectra of the BaZn₂(PO₄)₂ sample.

Fig. 5 displays the photoluminescence spectra of BaZn₂(PO₄)₂:Eu phosphors. It can been found that a series of sharp peaks with an emission spectrum between 550–750 nm are due to the ⁵D₀–⁷F_J (J = 0, 1, 2, 3, 4) transition of Eu³⁺ upon the 258 nm excitation, whose maximum wavelength position belongs to the ⁵D₀–⁷F₁ transition at 593 nm. By monitoring the emission at 593 nm, it was found that the excitation spectrum in the range of 200–550 nm consists of a broadband excitation with a central wavelength at 258 nm, which is attributable to the charge transition of O²⁻–Eu³⁺ and some sharp peaks can be ascribed to the characteristic excitation of Eu³⁺ in Fig. 5(a). We know that when the Eu³⁺ ion is in a position with a strict inversion center, it will be dominated by the allowable ⁵D₀–⁷F₁ magnetic dipole transition, and the emission peak is around 590 nm. Therefore, it can be judged that in the BaZn₂(PO₄)₂:Eu sample we prepared, the Eu³⁺ ion occupies the inversion symmetry position. According to the asymmetrical structure centered on the Ba atom, it is presumed that the Eu³⁺ ion enters a symmetric tetrahedral structure centered on the Zn atom.

Fig. 5 (a and b) Excitation (λ_em = 593, 532 nm) and emission spectra (λ_ex = 258, 283 and 375 nm) of the BaZn₂(PO₄)₂:Eu sample. (c and d) Gaussian fitting excitation (λ_em = 532 nm) and emission band (λ_ex = 365 nm) of BaZn₂(PO₄)₂:Eu.

The spectrum of the phosphor BaZn₂(PO₄)₂:Eu at different excitation and monitoring wavelengths is shown in Fig. 5(b). Upon monitoring the wavelength at 532 nm, it was found that the excitation spectrum of BaZn₂(PO₄)₂:Eu consists of a narrow band absorption ranging from 200 to 450 nm with the dominant absorption peak at 374 nm, which comes from the 4f → 5d transition of Eu²⁺ ions. Under the excitation of 283 and 375 nm, BaZn₂(PO₄)₂:Eu phosphors exhibited broad emission bands in the range of 350–750 nm with a maximum at 532 nm, which originates from the 5d → 4f transitions of Eu²⁺ ions. At the same time, the emission sharp peaks belonging to the ⁵D₀–⁷F₁ transition of Eu³⁺ at 593 nm can also be clearly observed. Therefore, the existence of Eu²⁺ was proved. From Fig. 5(a), it can be observed that Eu²⁺ and Eu³⁺ coexist in the system, indicating that the self-reduction process from Eu³⁺ to Eu²⁺ occurred in the BaZn₂(PO₄)₂ crystal. We also made a Gaussian fitting to the excitation spectrum monitored at 532 nm wavelength and the emission spectrum at 365 nm wavelength, as shown in Fig. 5(c and d). The center positions of the three sub-excitation peaks obtained by Gaussian fitting are at 282, 344, and 375 nm, respectively. This is due to the splitting of the excitation peak broadband. Similarly, two sub-emission peaks of the emission spectrum are obtained with center positions at 500 and 544 nm. From this, we conclude that Eu²⁺ ions occupy two sites in the BaZn₂(PO₄)₂ lattice.

In BaZn₂(PO₄)₂ crystals, there are five kinds of cationic sites that can be occupied by the dopant, namely Ba1, Zn1, Zn2, P1 and P2. From the photoluminescence spectrum, we know that the strongest position of the emission peak of Eu³⁺ ions is at 593 nm, indicating that Eu³⁺ ions are in the inversion symmetry position and occupy the position of Zn. We use the theoretical method of bond energy to explain the reduction of Eu³⁺ to Eu²⁺ in the air and the priority of Eu²⁺ and Eu³⁺ ions in the lattice position of Zn in the system. It can be used by the following expression:

where V_N is the valence state of Zn²⁺ cations in the system and V_M is the number of valence states of Eu²⁺ and Eu³⁺. For Eu³⁺ ions taken as an example, if Eu³⁺ occupies the Zn site, then V_N/V_M = 2/3; if the doping ion Eu²⁺ is at the Zn site, then V_N/V_M = 1/1. This indicates that the valence state has some effect on the bond energy of the crystal. J represents the intrinsic standard atomization energy and d₀ is equal to a constant for a given pair of atoms. The J and d₀ of the compounds involved are listed in Table 1. E_M–O is the bond energy value. According to formula (1), the bond energy values E_{Zn²⁺–O²⁻}, E_{Eu²⁺–O²⁻} and E_{Eu³⁺–O²⁻} are calculated and shown in Table 2. The variation of bond energy reflects the preferential occupancy of the dopant ions. Its calculation method is as follows:where is the variation in bond energy, that is, a change of bond energy caused when Eu²⁺ and Eu³⁺ ions enter the lattice to replace the Zn1 and Zn2 sites. The smaller the change in bond energy, the easier it is to be occupied by dopant ions. Here, if the bond energy change value is smaller than , then the Eu²⁺ ion is more likely to occupy the position of Zn, and vice versa. From eqn (2), we calculated the variation in bond energy when Eu^2+/3+ ions occupy the Zn1 and Zn2 sites in the BaZn₂(PO₄)₂ system. The results are shown in Table 2. For Eu^2+/3+ ions, we can see that the bond energy variation values of and are 28.6619, 29.0296, 57.1607 and 57.8942, respectively. It is clear that the bond energy difference of Eu²⁺ at the Zn site is smaller than that of , so Eu²⁺ is more likely to occupy this site than the Eu³⁺ ion. At the same time, this also implies that Eu²⁺ is more stable in the Zn site and it provides conditions that Eu³⁺ can be reduced to Eu²⁺ under non-reducing condition.

Table 1 The value of J and d₀ for Zn²⁺–O²⁻, Eu³⁺–O²⁻ and Eu²⁺–O²⁻

Ions	J (kcal mol^−1)	d0 (Å)
Zn^2+–O^2−	86.900	1.704
Eu^3+–O^2−	109.400	2.074
Eu^2+–O^2−	56.180	2.049

---

Table 2 Bond energy of Zn–O bonds in BaZn₂(PO₄)₂ (E_{Zn²⁺–O²⁻}) and bond energy of Eu–O bond (E_{Eu²⁺–O²⁻}, E_{Eu³⁺–O²⁻}) and variation in bond energy when Eu^2+/3+ is at the sites of Zn1 and Zn2 in BaZn₂(PO₄)₂ . The units are kcal mol⁻¹

Central atom	Coordination atom	Count	dm–o	EM–O	EEu^2+–O^2−	EEu^3+–O^2−
Zn1	O2	1×	1.9168	48.8924	80.3071	111.5430	28.6619	57.1607
	O1	1×	1.9429	45.5623	74.8373	103.9458
	O3	1×	1.959	43.6223	71.6507	99.5197
	O6	1×	1.9878	40.3556	66.2851	92.0672
Zn2	O7	1×	1.9065	50.2726	82.5740	114.6917	29.0296	57.8942
	O4	1×	1.9491	44.8052	73.5938	102.2185
	O5	1x	1.9654	42.8742	70.4220	97.8131
	O8	1x	1.9663	42.7700	70.2509	97.5755

---

In summary, we calculated that when Eu²⁺ and Eu³⁺ ions are into the BaZn₂(PO₄)₂ matrix, the bond energy change values of Eu²⁺ and Eu³⁺ ions at the Zn1 and Zn2 sites by using the theoretical method of bond energy. The bond energy change value is smaller than , indicating that Eu²⁺ is more likely to occupy the Zn site than Eu³⁺, which also implies that Eu³⁺ can be self-reduced to Eu²⁺. All theoretical calculations are consistent with the PL spectrum phenomenon.

Based on the principle of charge compensation mechanism,³⁰ when Eu³⁺ replaces the site of Zn²⁺, the charge in the environment is unbalanced. In order to maintain the overall charge exhibiting electrical neutrality, the formation of the cationic vacancy defect will generate two negative electrons to compensate for the cation vacancy defects induced by . Then form a dipole complex , where represents an electron donor and an electron acceptor. When the negative charge generated on is transferred to , Eu³⁺ will be reduced to Eu²⁺:

3.3 Photoluminescence properties of BaZn₂(PO₄)₂:Ce, Eu

Fig. 6 depicts the excitation and emission spectra of different concentrations of Ce³⁺ doped in BaZn₂(PO₄)₂. For Fig. 6(a), under the monitoring of 336 nm, the excitation spectrum is in the range of 230–310 nm, and the strongest absorption of these excitation bands is about 287 nm, which is due to the transition of the Ce³⁺ ion from the ground state to crystal field splitting level of the 5d state. In general, the emission spectrum of Ce³⁺ ion sample has a dual characteristic due to the ground state ²F_5/2 and ²F_7/2 spin orbital splitting, which indicates that Ce³⁺ is successfully doped into BaZn₂(PO₄)₂. As shown in Fig. 6(b), the emission spectra have two broad bands centered at 334 and 410 nm in the range of 300–500 nm, which can be clearly observed under the excitation at 287 nm.

Fig. 6 (a) Excitation spectra of BaZn₂(PO₄)₂:x% Ce under 336 nm monitoring wavelength (x = 0.5, 1, 2, 4, 6, 8, 10); (b) emission (λ_ex = 287 nm) spectra of BaZn₂(PO₄)₂:x% Ce under 287 nm excitation (x = 0.5, 1, 4, 6, 8, 10); (c–e) the excitation spectra of BaZn₂(PO₄)₂:x% Ce (x = 0.5, 1, 2, 4, 6, 8, 10) phosphors at the monitoring wavelength of 336, 356, and 410 nm, respectively; (f–h) normalized excitation spectra monitored at 336, 356 and 410 nm, respectively.

The excitation spectra of the BaZn₂(PO₄)₂:x% Ce sample monitored at 336, 356 and 410 nm are shown in Fig. 6(c–e). We can see that the photoluminescence intensity reaches the maximum when the doping concentration of Ce³⁺ is 0.5%. At the same time, we normalized the excitation spectra under these different monitoring wavelengths, as shown in Fig. 6(f–h). At the monitoring wavelength of 336 nm, the broadband center of the excitation spectrum was located at 286 nm; it is worth noting that when the monitoring wavelength is 356 nm, the central position of the excitation spectrum changes with different concentrations of Ce³⁺ doping, that is, the central position of the broad excitation bands are 290 and 305 nm, respectively. In addition, the excitation spectrum exhibited a red shift phenomenon when monitored at 410 nm which is due to the influence of the crystal field.^31,32 By comparing the excitation spectra at different monitoring wavelengths in Fig. 6, it can be seen that in addition to the difference in luminescence intensity, the peak shape and the dominated peak position also have significant differences.

The emission spectrum of the BaZn₂(PO₄)₂:4% Ce, x% Eu series sample at an excitation wavelength of 304 nm for Ce³⁺ is shown in Fig. 7(a). There are two distinct broadband emission peaks in the range of 300–800 nm, which are the 5d–4f transitions corresponding to Ce³⁺ and Eu²⁺ at 428 and 609 nm, respectively. In the BaZn₂(PO₄)₂:4% Ce, x% Eu co-doped sample, not only the emission of Ce³⁺ but also the emission of Eu²⁺ was observed by the 304 nm characteristic excitation of Ce³⁺. At the same time, it can be clearly seen that as the concentration of Eu ions increases, the luminescence intensity of Ce³⁺ ions gradually decreases, and that of Eu²⁺ ions gradually increases. This strongly proves that energy transfer occurs between Ce³⁺ and Eu²⁺. Fig. 7(b) is the excitation spectrum obtained under the monitoring of 609 nm. It shows that there are three strong broad bands in the range of 225–550 nm, and their central positions at 248, 303 and 376 nm belong to the charge transition of O²⁻–Eu³⁺, the f–d energy level transition of Ce³⁺ and the 4f-5d transition of Eu²⁺, respectively. Therefore, we found that under the monitoring of Eu²⁺, the excitation spectrum exhibits the excitation peak of Ce³⁺, which further proves the phenomenon of Ce³⁺–Eu²⁺ energy transfer.

Fig. 7 (a and b) The emission spectrum of a series of BaZn₂(PO₄)₂:4% Ce, x% Eu (x = 3, 4, 6, 8, 10) samples excited at 304 nm and the excitation spectrum monitored at 428 nm, respectively; (c) decay lifetime of Ce³⁺ in BaZn₂(PO₄)₂:4% Ce, x% Eu; (d) variations of the decay lifetime for Eu²⁺ with the increasing Eu ion concentration in BaZn₂(PO₄)₂:4% Ce, x% Eu (λ_ex = 376 nm, λ_em = 600 nm).

3.4 Energy transfer mechanism

In order to further study the energy transfer process, we tested the lifetime of Ce³⁺ ions under 320 nm excitation (λ_ex = 302 nm, λ_em = 428 nm) and Eu²⁺ ions under the corresponding excitation and emission wavelengths, as shown in Fig. 7(c). The decay curves are better fitted by the second-order exponential decay. The formula is as follows:³³where I(t) represents the luminescence intensity; A₁ and A₂ are the amplitude constants; t is the decay time; and τ₁ and τ₂ are the exponential components of fast and slow decay times, respectively. The average decay time (τ*) is calculated using the following formula:

τ* = (A₁τ²₁ + A₂τ²₂)/(A₁τ₁ + A₂τ₂) (4)

According to eqn (3) and (4), the decay time of Ce³⁺ in BaZn₂(PO₄)₂:4% Ce, x% Eu samples are approximately 1.5, 1.47, 1.42, 1.31 and 1.30 μs corresponding to x = 3, 4, 6, 8, 10 in Fig. 7(c). We can observe that as the concentration of Eu ions increases, the lifetime of Ce³⁺ ions decreases, whereas the lifetime of Eu²⁺ ions increases, which is in good agreement with the spectral characteristics of BaZn₂(PO₄)₂:4% Ce, x% Eu. This also strongly indicates the energy transfer behavior between Ce³⁺ and Eu²⁺ ions.

Based on Dexter's theory, the critical distance of energy transfer is defined as the distance at which the transition probability is equal to the probability of Ce³⁺ radiation emission. The critical distance R_c for energy transfer between Ce³⁺ and Eu²⁺ in BaZn₂(PO₄)₂ is discussed by the following formula:²⁸

where V represents the volume of the unit cell and X_c stands for the critical concentration of the dopant, and N is the number of available sites per unit cell. In the BaZn₂(PO₄)₂ crystal structure, the values of V, X_c and N are 768.45 ų, 0.1 and 4, respectively. The critical distance R_c is calculated to be 15.426 Å by eqn (5). We know that when the distance between two luminescence centers is less than the critical distance of 5 Å, the energy transfer between the two luminescence centers is carried out by the exchange interaction mechanism, otherwise it is through electric multipolar interactions. Therefore, it can be seen that energy transfer from Ce³⁺ to Eu²⁺ in BaZn₂(PO₄)₂ belongs to electrical multipole interaction. Based on Dexter's multipole interaction energy transfer formula and Reisfeld's approximation, the mechanism in the present system is applied through the following relationship:where I_S0 indicates the luminescence intensity of Ce³⁺ ions when Eu³⁺ ions are not doped; I_S is equal to the luminescence intensity of Ce³⁺ ions with the presence of Eu²⁺ ions; and C stands for the sum of the concentrations when Eu²⁺ and Ce³⁺ coexist. When the value of n is 6, 8, or 10, it means dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively.³⁴ The I_S0/I_S–C^n/3 plots are exhibited in Fig. 8. Obviously, when n = 6, 8 and 10, the linear fitting R² values of I_S0/I_S–C^n/3 are 0.9920, 0.9826 and 0.9667, respectively, that is, when n = 6, linear behavior is the best. Therefore, the energy transfer of Ce³⁺–Eu²⁺ in this system follows the electric dipole–dipole interaction mechanism. The quantum efficiency was determined to be 24.58%, 29.12%, 24.16%, 32.52%, and 29.02% for BaZn₂(PO₄)₂:4% Ce, x% Eu with x = 3, 4, 6, 8, 10, respectively.

Fig. 8 Dependence of I_S0/I_S of Ce³⁺ on C^n/3 (n = 6, 8, 10) in the BaZn₂(PO₄)₂:4% Ce, x% Eu samples.

The chromaticity calculation was performed based on the photoluminescence spectra of the BaZn₂(PO₄)₂:4% Ce, x% Eu phosphor at 304 nm excitation, and the calculation results of the CIE coordinates are shown in Fig. 9. Detailed CIE chromaticity coordinates are listed in Table 3. In a series of samples of BaZn₂(PO₄)₂:4% Ce, x% Eu, as the doping concentration of Eu ions is from 0.03 to 0.1, we can see that the CIE coordinates of the samples change from the blue region to the white region. It is worth noting that the CIE chromaticity coordinates of the BaZn₂(PO₄)₂:4% Ce, 8% Eu sample in the white light region (0.3195, 0.2959) is very close to the ideal white luminescence chromaticity coordinates (0.33, 0.33). It is suggested that the BaZn₂(PO₄)₂:4% Ce, 8% Eu phosphor has suitable color coordinates as white phosphors in the field of lighting. The results show that the light emission of white phosphors can be achieved by appropriately adjusting the ratio of Ce/Eu ions.

Fig. 9 CIE chromaticity diagram for BaZn₂(PO₄)₂:4% Ce, x% Eu under 304 nm excitation (x = 3, 4, 6, 8, 10).

Table 3 CIE chromaticity coordinates of BaZn₂(PO₄)₂:4% Ce, x% Eu (x = 3, 4, 6, 8, 10)

Point	Sample	CIE (x, y)	Peak	Peak intensity
A	BZPO:4% Ce, 3% Eu	(0.2318, 0.1939)	430	1256820
B	BZPO:4% Ce, 4% Eu	(0.2450, 0.2238)	429	1201980
C	BZPO:4% Ce, 6% Eu	(0.2404, 0.2226)	428	1146960
D	BZPO:4% Ce, 8% Eu	(0.3195, 0.2959)	429	910659
E	BZPO:4% Ce, 10% Eu	(0.3718, 0.3562)	608	1208720

---

4. Conclusions

A series of novel Eu²⁺/³⁺, Ce³⁺ ion single- and co-doped BaZn₂(PO₄)₂ samples have been synthesized via a high-temperature solid-state reaction. It was confirmed by XRD powder diffraction that our target products are all pure phases. The self-reduction from Eu³⁺ to Eu²⁺ has been found in the BaZn₂(PO₄)₂ system. The photoluminescence spectra show that Eu²⁺ and Eu³⁺ ions coexist in the BaZn₂(PO₄) crystal. At the same time, the strongest emission peak of Eu³⁺ ions belongs to the ⁵D₀–⁷F₁ transition of Eu³⁺ at 593 nm, which indicates that Eu³⁺ ions occupy the inversion symmetry lattice and occupy the site of Zn in BaZn₂(PO₄). According to the bond energy method, the priority of Eu²⁺ and Eu³⁺ ions in the lattice position of Zn and the self-reduction of Eu³⁺ to Eu²⁺ were explained in BaZn₂(PO₄). The results indicate that the value is smaller than , indicating that Eu²⁺ is more likely to occupy the Zn site and is more stable than Eu³⁺ in BaZn₂(PO₄), which also explains why Eu³⁺ ion can reduce to Eu²⁺ ion. For BaZn₂(PO₄)₂:4% Ce, x% Eu phosphors, the energy transfer from Ce³⁺ to Eu²⁺ was deduced by excitation and emission spectra, which was further confirmed by the decrease in the decay lifetime of Ce³⁺ as the concentration of Eu ions gradually increases. The energy transfer mechanism between Ce³⁺ and Eu²⁺ proved to be a dipole–dipole interaction. Moreover, the CIE chromaticity coordinates show that the emission color of BaZn₂(PO₄)₂:4% Ce, x% Eu phosphors can be changed from blue to white. In particular, the chromaticity coordinates (0.3195, 0.2959) of BaZn₂(PO₄)₂:4% Ce, 8% Eu are very close to the ideal white luminescence (0.33, 0.33), indicating that the phosphor has potential applications in white LEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundations of China (Grant no. 21301053) and Hubei Natural Science Foundations from Science and Technology Department of Hubei Province (2018CFB517), Ministry-of-Education Key Laboratory for the Synthesis and Applications of Organic Functional Molecules (KLSAOFM1804).

Notes and references

1. K. Biswas, S. Balaji, D. Ghosh, A. D. Sontakke and K. Annapurna, J. Alloys Compd., 2014, 608, 266–271.
2. X. Zhang, L. Huang, F. Pan, M. Wu, J. Wang, Y. Chen and Q. Su, ACS Appl. Mater. Interfaces, 2014, 6, 2709–2717.
3. X. Zhang, J. Wang, L. Huang, F. Pan, Y. Chen, B. Lei, M. Peng and M. Wu, ACS Appl. Mater. Interfaces, 2015, 7, 10044–10054.
4. X. Zhang, J. Yu, J. Wang, C. Zhu, J. Zhang, R. Zou, M. Wu, B. Lei and Y. Liu, ACS Appl. Mater. Interfaces, 2015, 7, 28122–28127.
5. S. K. Jaganathan, A. J. Peter, V. Mahalingam and R. Krishnan, J. Mater. Sci.: Mater. Electron., 2019, 30, 2037–2044.
6. K. H. Jang, N. M. Khaidukov, V. P. Tuyen, S. I. Kim, Y. M. Yu and H. J. Seo, J. Alloys Compd., 2012, 536, 47–51.
7. M. Keskar, S. K. Gupta, R. Phatak, S. Kannan and V. Natarajan, J. Photochem. Photobiol., A, 2015, 311, 59–67.
8. M. Xia, Z. Ju, H. Yang, Z. Wang, X. Gao, F. Pan and W. Liu, J. Alloys Compd., 2018, 739, 439–446.
9. L. Li, Y. Pan, W. Wang, Y. Zhu, W. Zhang, H. Xu, L. Zhou and X. Liu, J. Alloys Compd., 2018, 731, 496–503.
10. M. V. d. S. Rezende, M. E. G. Valerio and R. A. Jackson, Mater. Res. Bull., 2015, 61, 348–351.
11. Y. Pan, W. Wang, Y. Zhu, H. Xu, L. Zhou, H. M. Noh, J. H. Jeong, X. Liu and L. Li, RSC Adv., 2018, 8, 23981–23989.
12. Y. Chu, Z. Liu, Q. Zhang, H. Fang, Y. Li and H. Wang, J. Alloys Compd., 2017, 728, 307–313.
13. H. Li and Y. Wang, Inorg. Chem., 2017, 56, 10396–10403.
14. T. Li, P. Li, Z. Wang, S. Xu, Q. Bai and Z. Yang, Phys. Chem. Chem. Phys., 2017, 19, 4131–4138.
15. W. Chen, X. Chen, F. Sun and X. Wang, J. Alloys Compd., 2017, 698, 565–570.
16. H. J. Lee, S. H. Choi, K. P. Kim, H. H. Shin and J. S. Yoo, Jpn. J. Appl. Phys., 2010, 49, 102101.
17. C. Cao, H. K. Yang, J. W. Chung, B. K. Moon, B. C. Choi, J. H. Jeong and K. H. Kim, J. Mater. Chem., 2011, 21, 10342–10347.
18. F. Huang, J. Cheng, X. Liu, L. Hu and D. Chen, Opt. Express, 2014, 22, 20924–20935.
19. L. Zhang, Z. Fu, Z. Wu, Y. Wang, X. Fu and T. Cui, Mater. Res. Bull., 2014, 56, 65–70.
20. M. Shang, S. Liang, H. Lian and J. Lin, Inorg. Chem., 2017, 56, 6131–6140.
21. J. Li, L. Chen, Z. Hao, X. Zhang, L. Zhang, Y. Luo and J. Zhang, Inorg. Chem., 2015, 54, 4806–4810.
22. R. Shi, G. Liu, H. Liang, Y. Huang, Y. Tao and J. Zhang, Inorg. Chem., 2016, 55, 7777–7786.
23. R. Shi, Y. Huang, Y. Tao, P. Dorenbos, H. Ni and H. Liang, Inorg. Chem., 2018, 57, 8414–8421.
24. X. Kang, W. Lu, H. Wang and D. Ling, Mater. Res. Bull., 2018, 108, 46–50.
25. W. Wang, Y. Pan, Y. Zhu, H. Xu, L. Zhou, H. M. Noh, J. H. Jeong, X. Liu and L. Li, Dalton Trans., 2018, 47, 6507–6518.
26. Y. Zhu, Y. Pan, W. Wang, Z. Xu, Q. Luo, L. Zhou, X. Liu and L. Li, Ceram. Int., 2019, 45, 14360–14365.
27. Y. Huang, K. Jang, H. S. Lee, E. Cho, J. Jeong, S.-S. Yi, J. H. Jeong and J. H. Park, Phys. Procedia, 2009, 2, 207–210.
28. L. Wang, M. Xu, H. Zhao and D. Jia, New J. Chem., 2016, 40, 3086–3093.
29. Z. Wang, P. Li, Z. Yang and Q. Guo, J. Lumin., 2012, 132, 1944–1948.
30. G. Zhu, Y. Shi, M. Mikami, Y. Shimomura and Y. Wang, Mater. Res. Bull., 2014, 50, 405–408.
31. P. Dorenbos, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 15640–15649.
32. P. Dorenbos, J. Lumin., 2000, 91, 155–176.
33. M. Jiao, N. Guo, W. Lu, Y. Jia, W. Lv, Q. Zhao, B. Shao and H. You, Inorg. Chem., 2013, 52, 10340–10346.
34. G. Li, D. Geng, M. Shang, Y. Zhang, C. Peng, Z. Cheng and J. Lin, J. Phys. Chem. C, 2011, 115, 21882–21892.

---