Synthesis, Luminescence Property and Thermal Quenching Investigation of Niobate Phosphors Ba₃ZrNb₄O₁₅:Eu³⁺ under Multiple Excitations

Samples of BZN:xEu³⁺ (0 ≤ x ≤ 0.04) are prepared by solid state method with BaCO₃ (99.9%), Zr(NO₃)₄·5H₂O (99.9%), Nb₂O₅ (99.9%) and Eu₂O₃ (99.99%) from Sinopharm Chemical Reagent Co. Ltd. The stoichiometric starting materials are uniformly ground by adding ethanol in agate mortar, then put in an alumina crucible and calcined at 1523 K in air for 6 h. Finally, the samples are slowly cooled to room temperature with at 5 K/min.

The crystal structure and phase purity was determined by using a Rigaku Ultima IV X-ray diffractometer (XRD). The morphologies and microstructure were investigated by scanning electron microscope (SEM, S4800) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30, operated at 300 kV). FS5-MCS fluorescence spectrophotometer was employed to examine the photoluminescence spectra and the fluorescence decay times. The thermal quenching property was investigated by a heat booster (TCB1402C) equipped on the fluorescence spectrophotometer.

Figure 1a illustrates the XRD Rietveld refinement results of BZN host, which shows that the prepared BZN power crystallizes in a noncentrosymmetric crystal structure with a space group of P4bm and the cell parameters are calculated to be a = b = 12.6815 Å, c = 4.0135 Å. The goodness-of-fit parameters (R_wp = 11.85%, R_p = 9.16%) guarantee the pure phase of BZN host. In each BZN unit cell, there are four kinds of available cationic sites, including Ba1, Ba2, Zr/Nb1 and Zr/Nb2, of which Zr/Nb atoms adopt octahedron groups with six coordination, while the larger Ba²⁺ cations embed rigidity to the host framework to form the tetragonal structure. The series XRD patterns of BZN:xEu³⁺ (0 ≤ x ≤ 0.04) is plotted in Fig. 1b. No detectable secondary phases can be found in the series XRD patterns, showing the addition of Eu³⁺ has no significant change in the BZN structure. The main peak is found to slightly shift toward the larger angle direction, which is in virtue of the smaller radius of Eu³⁺ ions compared with that of Ba²⁺ ions in BZN host lattice and indicates that Eu³⁺ ions has successfully entered BZN host to occupy Ba²⁺ sites.¹⁹ Due to the low doping contents of Eu³⁺ ions, the charge can be balanced by generating some defects during solid state reaction process. Thus, the charge compensator is not introduced in the following discussion. Figs. 1c and 1d illustrate the SEM and HR-TEM images of BZN host. It could be seen that the obtained BZN presents irregular form with particle sizes around 2–5 μm. HRTEM image clearly reveals that the as-prepared BZN sample is well crystallized. The interplanar distance d of the selective area is measured to be 1.279 Ǻ, which is assigned to the (223) crystal plane of BZN.

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Figure 2a illustrates the excitation spectra of BZN:xEu³⁺ (0.005 ≤ x ≤ 0.04) monitored at 613 nm. Generally, the excitation spectrum of Eu³⁺ presents sharp line absorptions due to the intra-4f forbidden transition excitations and is mainly dominated by line absorptions from either ⁷F₀→⁵L₆ transition around 396 nm or ⁷F₀→⁵D₂ transition around 468 nm.^20,21 However, most Eu³⁺ doped phosphors could be effectively excited by only one of the above two excitation wavelengths. It is interesting that in BZN:Eu³⁺, the ⁷F₀→⁵L₆ transition excitation at 396 nm and the ⁷F₀→⁵D₂ transition excitation at 468 nm are both dominant and have similar excitation intensity, indicating BZN:Eu³⁺ has versatile luminescence properties and could be pumped by either UV or blue light excitation. With increasing Eu³⁺ contents, the excitation intensity of ⁷F₀→⁵L₆ transition reaches maximum when x is 0.02. However, for the ⁷F₀→⁵D₂ transition, it reaches maximum when x increases to 0.03. The different contents quenching may be attributed to the different cross relaxation processes under different excitations.²² To further investigate it, versatile emission spectra under different excitations are investigated as shown in Figs. 2b and 2c. It can be seen that the emission spectra mainly consist of several sharp lines ranging from 500 to 750 nm, which are associated with the transitions from the excited state ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3, 4) levels of Eu³⁺. In particular, the highest red emission at 613 nm is due to the ⁵D₀→⁷F₂ electric dipole transition, indicating that Eu³⁺ ions occupy an anti-inversion symmetry site in BZN host based on the Judd-Ofelt theory, which is consistent with the previous analysis.²³ Conversely, a reddish orange emission will be dominated ascribed to the magnetic transition ⁵D₀→⁷F₁ if Eu³⁺ occupies an inversion symmetry site. With increasing Eu³⁺ contents, the emission intensity under 396 and 468 nm excitation are found to be gradually increased and reach the maximum when x is 0.02 and 0.03, respectively, as shown in Figs. 2b and 2c. The different emission quenching behaviors in BZN:Eu³⁺ is reasonable and should be ascribed to the different selective excitation intensities around 396 and 468 nm under different Eu³⁺ contents, as illustrated in Fig. 2a and the inset. The CIE chromaticity coordinates of BZN:0.02Eu³⁺ and BZN:0.03Eu³⁺ are calculated to be (0.643,0.352) and (0.656,0.343), respectively, which approaches to those of the commercial red phosphor Y₂O₂S:Eu³⁺ (0.647, 0.343).²⁴ The dominant wavelength and color purity as compared to the 1931 CIE Standard Source C [illuminant C = (0.3101, 0.3162)] are determined based on the emission spectrum in Fig. 2. The dominant wavelength of a color is the single monochromatic wavelength of the spectrum whose chromaticity is on the same straight line as the sample point (x_s, y_s) and the illuminant point (x_i, y_i). The color purity can be estimated using the formula,

where (x_d, y_d) is the color coordinate of the dominant wavelength. Based on the spectra data in Fig. 2, the color purity of BZN:0.02Eu³⁺ under 396 nm excitation and BZN:0.03Eu³⁺ under 468 nm excitation was calculated to be 91.7% and 95%, respectively. What's more, the quantum efficiency (QE) of BZN:Eu³⁺ phosphors is measured by the integrating sphere method as discussed in other works.²⁵ The results show that the QE of BZN:0.02Eu³⁺ under 396 nm excitation and BZN:0.03Eu³⁺ under 468 nm excitation are measured to be 39.8% and 42.5%, respectively, which can be further enhanced by subsequent improvement such as charge compensation or adding some fluxing agent.

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To investigate the influence of temperature on the luminescence properties, the temperature dependent emission spectra of BZN:0.02Eu³⁺ and BZN:0.03Eu³⁺ (the optimal doping contents under different excitation wavelength) are measured ranging from room temperature to 205°C under 396 and 468 nm, respectively. Interestingly, the temperature dependent emission spectra exhibit totally different emission quenching properties. As shown in Fig. 3a, when excites it at 396 nm, the emission intensity of BZN:0.02Eu³⁺ drops rapidly as the temperature arises. A linear changed thermal quenching behavior can be observed as illustrated in the inset of Fig. 3a, indicating that the phosphor may be suitable to be applied in some temperature sensitive luminescent devices such as optical temperature sensor.²⁶ In Fig. 3b, when excites it at 468 nm excitation, BZN:0.03Eu³⁺ shows excellent thermal stability. When the temperature is increased to 205°C, the emission intensity of BZN:0.03Eu³⁺ still keeps 51.9% of its initial intensity. Much attention has been focused on the thermal quenching mechanism of ⁵D₀-⁷F₂ emission of Eu³⁺ ions, which demonstrates that the crossover process, in which the electrons on the ⁵D₀ level could overcome the energy barrier and migrate to the Eu-O charge transition band (CTB) and finally relax to ⁷F_J levels, is responsible for the temperature quenching of ⁵D₀ emission,^27,28 as illustrated by the configurational coordinate diagram in Fig. 3c. In order to try to explain the interesting thermal quenching behavior under different excitation wavelength, a possible quenching mechanism based on the configurational coordinate diagram is inferred and illustrated. When excites it through 396 nm excitation, the electrons are excited to higher energy level. On one hand, some of them at higher energy level are relaxed by cross-relaxation to the ⁵D₀ state (through wave line) and overcome the activation energy ΔE₁ assisted by phonons as the temperature increases and feeds to the ⁷F_J state through Eu-O CTB, which provides the non-raditative process and results in the emission quenching (via ① to ③).²⁹ The remaining electrons are contributed to the ⁵D₀→⁷F₂ emission at 613 nm through path ④. On the other hand, some of the electrons at higher energy level might overcome the activation energy ΔE₂ under thermal activation effect and also provide the non-raditative process through pathway ⑤. In other words, there are two pathways of non-raditative process when excited the phosphors at 396 nm. However, when excites it by 468 nm, most of the electrons may be excited to the ⁵D₀ level (only few or small part of electrons can reach higher energy level), thus the non-raditative transitions pathways are reduced (pathway ⑤ is blocked). More electrons return to the ground state through raditative transitions pathway ④ and finally leads to the better thermal stability than that excited by 396 nm. Anyway, the thermal stability is satisfactory and here we only discuss one of the possible causations for the observed thermal quenching behavior.

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Additionally, in order to further investigate the temperature quenching process, the temperature dependent luminescence decay curves of BZN:Eu³⁺ under 396 and 468 nm excitations are measured from room temperature to 205°C, as illustrated in Fig. 4. Upon 396 nm excitation, it can be seen that the decay behavior of BZN:Eu³⁺ strongly depends on the temperature, indicating that the decrease of the Eu³⁺ lifetime is also caused by the thermal vibration. All the decay curves can be well fitted to a double exponential model and the average decay time τ can be determined by the formula,³⁰

where I(t) is the luminescence intensity; A₁ and A₂ are constants; t is the time; and τ₁ and τ₂ are rapid and slow lifetimes for exponential components. Using these parameters, the average decay times (τ) can be determined by the formula given in the following,³⁰

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On the basis of Eq. 2 and the measured decay curves, the average lifetime of Eu³⁺ in BZN can be calculated to be 752.64, 731.15, 694.56, 669.53, 622.58, 551.16 and 542.64 μs, respectively. Such significant lifetime shortening presents evidence that there is the intrinsic severe quenching in BZN:Eu³⁺ upon 396 nm excitation.³¹ The shortening in lifetime is accompanied by the decrease of emission intensity, which can also be seen from the decay curves where t equals to 0 in Fig. 4a, corresponding to the initial emission intensity I. The suddenly shorter decay times indicate that there are more non-radiative transition pathways for the electrons to return to ground state, which is consistent with the configurational coordinate diagram as illustrated in Fig. 3c. But unlike the decay behavior of BZN:Eu³⁺ upon 396 nm excitation, the decay behavior upon 468 nm excitation changes slightly as shown in Fig. 4b, which can also be seen at t = 0, the initial intensity decreased tardily along with increasing the temperature. Finally, the average lifetimes of BZN:Eu³⁺ upon 468 nm excitation can also be well fitted by double exponential model and are calculated to be 679.89, 669.70, 644.88, 612.46, 574.32, 546.49 and 533.31 μs, respectively. Many works have focused on the calculation of activation energy through the emission intensity by employing Eq. 4,³²

where I₀ is the intensity at 300 K and I_T is the intensity at temperature T, c is a constant, k is the Boltzmann constant and ΔE is the activation energy. Here we evaluate the activation energy the temperature-dependent relative lifetime rather than emission intensity by a modified Arrhenius equation,³³

where τ_t is the emission decay life time at temperature(K), τ₀ is the initial emission life time, C is a rate constant for the process. The decay lifetimes are more reliable than the emission intensity because it is not affected by the testing conditions.³¹ According to Eq. 5, the activation energy (ΔE) can be calculated to be 0.27 eV and 0.30 eV under 396 and 468 nm excitations, respectively, and the fitting results are illustrated in Fig. 5.

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In summary, a series of novel red emission phosphor BZN:Eu³⁺ was synthesized for the first time. XRD Rietveld structural refinement indicated that the obtained BZN was single phased and well crystallized with space group P4bm. The site occupation situation of Eu³⁺ in BZN was investigated through series XRD analysis and revealed that Eu³⁺ was favor of occupying Ba²⁺ sites in BZN. The excitation spectra indicated that BZN:Eu³⁺ had versatile luminescence which could be efficiently excited by both UV and blue light. Upon 396 and 468 nm excitation, the phosphors could exhibit red emission with dominated peak at 613 nm due to ⁵D₀→⁷F₂ transitions of Eu³⁺, but showed different contents quenching. The interesting thermal quenching properties under different excitation wavelengths were studied and the mechanism was discussed. Finally, the temperature dependent luminescence decay curves of BZN:Eu³⁺ were discussed in detail to further investigate the thermal quenching process. The activation energy (ΔE) was calculated to be 0.27 eV and 0.30 eV under 396 and 468 nm excitations, respectively.

This work is supported by the Doctoral Research Fund of Liaoning Province (Nos. 201601351, 20170520277), the National Natural Science Fund (Nos. 117040435, 1702020) and the Special Foundation for theoretical physics Research Program of China (Nos. 11747113, 11747117).

Ge Zhu 0000-0002-2109-4857