Luminescence properties and energy transfer of K₃LuF₆:Tb³⁺,Eu³⁺ multicolor phosphors with a cryolite structure

Abstract

In recent years, compounds with a cryolite structure have become excellent hosts for luminescent materials. In this paper, Tb³⁺ doped and Tb³⁺/Eu³⁺ co-doped K₃LuF₆ phosphors were prepared via a high temperature solid phase sintering method. The XRD, SEM, as well as photoluminescence excitation (PLE) and emission (PL) spectra were measured to investigate the structure and luminescence properties of the as-prepared samples. In the Tb³⁺/Eu³⁺ co-doped K₃LuF₆ samples, both characteristic emission spectra of Tb³⁺ and Eu³⁺ could be observed and the emission color of the K₃LuF₆:0.12Tb³⁺,xEu³⁺ phosphors could be adjusted from green to yellowish pink and the corresponding CIE values could be regulated from (0.2781, 0.5407) in the green area to (0.4331, 0.3556) in the yellowish pink area by controlling the concentration ratio of Eu³⁺/Tb³⁺. In addition, the energy transfer mechanism in Tb³⁺/Eu³⁺ co-doped K₃LuF₆ was calculated to be a quadrupole–quadrupole interaction from Tb³⁺ to Eu³⁺ based on the Dexter's equation.

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1. Introduction

Fluoride compounds are potential hosts for luminescent materials due to their suitable chemical stability and excellent luminous properties, and as such, have received a lot of attentions over the past few decades.^1–4 Cryolite is an important kind of fluoride compound, the chemical formula of which can be expressed as M₃NF₆, where M represents monovalent cations such as alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺) and NH₄⁺, and the sites of N can be occupied by trivalent cations, that is Al³⁺, Y³⁺, Sc³⁺, Ga³⁺, etc.^5,6 In addition, the N sites can also be occupied by rare earth ions by isomorphic replacement in the cryolite lattice, such as K₃GaF₆,⁷ K₃InF₆,⁸ and K₃LuF₆.⁹ Since the luminescence behavior of rare earth ions depends on the matrix, it is significant to explore novel cryolite luminescent materials.

On the other hand, multicolor single-phase phosphors occupy an important seat in the field of luminescent materials. In particular, rare-earth ion-doped luminescent materials mixed with blue and UV LEDs are fabricated to produce white light. However, a traditional single-color single-phase strategy usually requires a complicated synthetic process, multiple excitation wavelengths with different responses of the various dopants to the excitation wavelength and reabsorption of the dopant.¹⁰ Single excitation wavelength excited phosphors with multicolor emissions (cyan-emitting, blue-green-emitting, and blue-red-emitting phosphors) can efficiently avoid the above problems and make it easier to obtain a phosphor with high luminous efficiency, excellent color rendering index (CRI), good thermal and chromatic stability. Accordingly, multicolor single-phase phosphors have received attention over the past few decades.^11–13

Energy transfer is an important way to realize single-phase multicolor phosphors, which have been investigated in abundant hosts.¹⁴ As we know, energy transfer is achieved by ion pairs from a sensitizer to activator, such as Eu²⁺/Tb³⁺, Ce³⁺/Eu²⁺, Ce³⁺/Dy³⁺, Tb³⁺/Eu³⁺, Er³⁺/Sm³⁺, Tm³⁺/Dy³⁺ and so on.^15–20 Due to ⁵D₀ → ⁷F₂ electronic transitions, Eu³⁺-doped luminescent materials can emit strong red light.²¹ Tb³⁺ has been widely used together with Eu³⁺ to produce multicolor emission by means of energy transfer, such as in CaMoO₄:Tb³⁺/Eu³⁺,²² Ba₂La₃(SiO₄)₃F:Tb³⁺/Eu³⁺, and²³ Na₂MgSiO₄:Tb³⁺/Eu³⁺.²⁴ However, until now, there have been no reports on the crystal structure, luminescence properties and energy transfer investigations of multicolor-emitting K₃LuF₆:Tb³⁺,Eu³⁺ phosphors with a cryolite structure.

In the present work, Tb³⁺ and Tb³⁺/Eu³⁺ activated cryolite-type K₃LuF₆ compounds were synthesized in a simple way and the crystal structure and luminescence properties, as well as the energy transfer mechanism between Tb³⁺ and Eu³⁺ in K₃LuF₆ were thoroughly studied. Benefiting from the crystal field environment of the cryolite-type samples, the emission color of K₃LuF₆:Tb³⁺,Eu³⁺ could be continuously regulated from green to yellowish pink by changing the content ratio of Tb³⁺/Eu³⁺.

2. Experimental section

2.1. Materials and synthesis

K₃LuF₆:xTb³⁺ (x = 0.01, 0.03, 005, 0.07, 0.09, 0.10, 0.12 and 0.13) and K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0.03, 005, 0.07, 0.10 and 0.13) phosphors were prepared via a high temperature solid-state method. The raw materials included K₂CO₃ (AR), Lu₂O₃ (99.99%), NH₄HF₂ (AR), Eu₂O₃ (99.99%) and Tb₄O₇ (99.99%). Firstly, the raw materials were weighed according to the stoichiometric ratios of the reactions and were then ground in a mortar for nearly 10 min, resulting in a uniform mixture. Considering the loss of fluorine sources at high temperatures, a 30% excess of NH₄HF₂ was required during the weighing process. Secondly, the mixture was moved to a furnace and heated at heating rate of 4° min⁻¹ and was maintained at 800 °C for 3 h under an argon atmosphere. Finally, the systems were naturally cooled down to room-temperature and evenly re-ground prior to characterization.

2.2. Characterization

The powder X-ray diffraction (PXRD) measurements of the phosphors were performed using a Bruker Corporation D8 powder X-ray diffractometer (Germany) equipped with a Cu kα radiation source at 0.15406 nm, a tube voltage of 40 kV and a tube current of 40 mA. The step scanning rate used was 8° min⁻¹ over the 2θ range from 10° to 70°. Morphological analysis of the compounds was carried out using a scanning electron microscope (SEM, JSM-6701F, Hitachi, Japan). The photoluminescence (PL) and photoluminescence excitation behavior were measured using a Hitachi F-4600 fluorescence spectrophotometer at room temperature, with a xenon lamp (400 V, 150 W) acting as the light source.

3. Results and discussion

3.1. Crystal structure

The PXRD patterns of the as-prepared K₃LuF₆:xTb³⁺ (x = 0.01, 0.03, 005, 0.07, 0.09, 0.1, 0.12 and 0.13) and K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0.03, 005, 0.07, 0.1 and 0.13) phosphors are shown in Fig. 1(a) and (b). The standard PXRD pattern of K₃LuF₆ (JCPDS no. 27-467) is shown as a reference. As shown in Fig. 1(a) and (b), all of the PXRD patterns of the as-prepared K₃LuF₆:Tb³⁺ and K₃LuF₆:0.12Tb³⁺,Eu³⁺ samples can be accurately assigned to the standard card of K₃LuF₆, without the appearance of any impurity peaks, which indicates that the introduction of Tb³⁺ and Eu³⁺ in K₃LuF₆ does not bring about any significant changes in the crystal structure. Fig. 1(c) shows the crystal structure of K₃LuF₆, which belongs to a monoclinic phase with a space group of P2₁/n. As depicted in Fig. 1(c), the Lu atom coordinates to six F atoms forming [LuF₆] octahedra, with the Lu atom in the centre. Two nonequivalent sites for the K atoms can be found in the crystal structure of K₃LuF_6, that is, twelve-coordinated K and six-coordinated K. Based on the charge balance and effective ionic radii, Tb³⁺ and Eu³⁺ were thought to occupy the sites of Lu³⁺. Fig. 1(d) shows the SEM image of K₃LuF₆:0.12Tb³⁺,0.07Eu³⁺, which indicates the prepared sample with an irregular shape and the particle size ranges from a few hundred nanometers to dozens of microns (Fig. 2).

Fig. 1 (a) The PXRD patterns of the as-prepared K₃LuF₆:xTb³⁺ (x = 0.01, 0.03, 005, 0.07, 0.09, 0.10, 0.12 and 0.13) and (b) K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0.03, 005, 0.07, 0.10 and 0.13) phosphors, (c) the crystal structure of K₃LuF₆, and (d) the SEM image of the K₃LuF₆:0.12Tb³⁺,0.07Eu³⁺ sample.

Fig. 2 SEM image, elemental mapping images and EDX spectrum of the K₃LuF₆:0.12Tb³⁺,0.07Eu³⁺ phosphor.

The elemental composition and distribution of the prepared K₃LuF₆:0.12Tb³⁺,0.07Eu³⁺ were investigated and the results are shown in Fig. 3. In addition, the elemental composition and distribution of the prepared K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 0.05, 0.07, 0.10, and 0.13) are shown in Fig. S1–S6,† among which the Au element was introduced to enhance the conductivity of the material during testing. The particles in all the samples seem like blocks with aggregation. The energy-dispersive X-ray (EDX) spectrum illustrates the existence of elemental K, Lu, F, Tb, and Eu in the sample, except for in the Tb³⁺ single-doped sample. Moreover, the elemental mapping results confirmed that all of the elements in the sample were homogenously distributed over the whole area.

Fig. 3 (a) The photoluminescence excitation (PLE) (λ_em = 550 nm) and emission (PL) (λ_ex = 375 nm) spectra of the K₃LuF₆:0.12Tb³⁺ phosphor, and (b) the emission PL spectra (λ_ex = 375 nm) of the K₃LuF₆:xTb³⁺ (x = 0.01, 0.03, 005, 0.07, 0.09, 0.10, 0.12 and 0.13) phosphors. The inset depicts the relative emission-intensity trends for the 550 nm peak (⁵D₄–⁷F₃) in terms of the Tb³⁺ concentration.

3.2. Photoluminescence properties and energy transfer process

Fig. 3(a) shows the PLE (λ_em = 550 nm) and PL (λ_ex = 375 nm) spectra of the K₃LuF₆:0.12Tb³⁺ phosphor. As depicted in Fig. 3(a), the PLE spectrum of the K₃LuF₆:0.12Tb³⁺ sample shows typical transitions of Tb³⁺ ranging from 310 to 460 nm, attributed to ⁷F₆–⁵H₇ (315 nm), ⁷F₆–⁵D₂ (340 and 351 nm), ⁷F₆–⁵D₄ (357 nm), ⁷F₆–⁵D₃ (375 nm), and ⁷F₆–⁵G₆ (440 nm), respectively. The excitation peak at 375 nm (⁷F₆–⁵D₃) is the most intense, matching well with near ultraviolet light LED chips, indicating that this sample has the potential for application in LEDs. When the phosphor was monitored at 375 nm, the PL spectrum of the K₃LuF₆:0.12Tb³⁺ sample exhibited several peaks located at 488/499, 540/550, 586/599, and 624 nm, respectively. These peaks can be assigned as the characteristic emission of ⁵D₄–⁷F_n (n = 6, 5, 5 and 3). Fig. 3(b) shows the emission PL spectra of the K₃LuF₆:xTb³⁺ (x = 0.01, 0.03, 005, 0.07, 0.09, 0.1, 0.12 and 0.13) phosphors monitored at 375 nm, and the inset depicts the relative emission-intensity trends for the 550 nm peak (⁵D₄–⁷F₃) regarding the Tb³⁺ concentration. Upon 375 nm excitation, all of the PL spectra of the K₃LuF₆:xTb³⁺ samples showed characteristic Tb³⁺ emission peaks, attributed to J = 6, 5, 4, and 3. It is well known that the emission intensity of the ⁵D₃–⁷F_J (J = 6, 5, 4, and 3) transitions of Tb³⁺ are quenched upon an increase in the Tb³⁺ concentration, ascribed to the cross-relaxation effect regarding the ⁵D₃–⁷F_J levels.^25–27 Meanwhile, the emission intensity for the 550 nm (⁵D₄–⁷F₃) peak increased upon an increase in the concentration of the Tb³⁺ concentration up to K₃LuF₆:0.12Tb³⁺,and then it decreased due to concentration quenching.²⁸ Thus, the Tb³⁺ doping concentration was fixed at 0.12 (mol) for the co-doping of Tb³⁺ and Eu³⁺ in K₃LuF₆.

In order to further investigate the possibility of energy transfer behavior between Tb³⁺ and Eu³⁺ in K₃LuF₆, K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0.03, 005, 0.07, 0.1 and 0.13) phosphors were prepared. The PLE and PL spectra of the Tb³⁺ and Eu³⁺ single-doped, as well as Tb³⁺/Eu³⁺ co-doped K₃LuF₆ phosphors are shown in Fig. 4. Under 393 nm excitation, the as-prepared K₃LuF₆:0.1Eu³⁺ sample shows sharp peaks at 579, 590, 613, 624, and 656 nm in Fig. 4(a), which are due to the ⁵D₀–⁷F₀, ⁵D₀–⁷F₁, ⁵D₀–⁷F₂, ⁵D₀–⁷F₂, and ⁵D₀–⁷F₃ transitions of Eu³⁺, respectively. In addition, the emission intensity of the peak at 613 nm is the highest among the transitions of Eu³⁺, as shown in Fig. 4(b). When monitored at 613 nm, the PLE spectrum of K₃LuF₆:0.1Eu³⁺ exhibits a series of sharp excitation bands between 300 and 500 nm, centered at 318, 361, 379, 393, and 414 nm, which can be attributed to the ⁷F₀–⁵H₅, ⁷F₀–⁵D₄, ⁷F₀–⁵L₆, ⁷F₀–⁵L₇ and ⁷F₀–⁵D₃ transitions, respectively.^29,30 The strongest excitation peak can be obviously observed at 393 nm. The PLE and PL spectra of the K₃LuF₆:0.12Tb³⁺,0.1Eu³⁺ phosphor are illustrated in Fig. 4(b). As shown in Fig. 4(c), when the co-doped sample was excited by near ultraviolet light at 375 nm, both characteristic emission peaks of Tb³⁺ and Eu³⁺ could be observed in the PL spectrum, indicating that energy transfer may occur between Tb³⁺ and Eu³⁺ in K₃LuF₆. Moreover, when monitored by 613 nm light, which is the typical emission wavelength of Eu³⁺, the K₃LuF₆:0.12Tb³⁺,0.1Eu³⁺ phosphor showed a characteristic excitation peak (375 nm) of Tb³⁺, while the peak at 375 nm could not be found in the excitation spectrum. Therefore, energy transfer from Tb³⁺ to Eu³⁺ exists in the K₃LuF₆ host and the emission color for K₃LuF₆:Tb³⁺/Eu³⁺ can be regulated according to different energy transfer efficiencies by adjusting the ratio of Eu³⁺/Tb³⁺.³¹ However, when the monitoring light was changed to 550 nm, the excitation spectrum of the co-doped sample did not exhibit the typical Eu³⁺excitation peak. Therefore, it could be inferred that the energy transfer process is irreversible in this system.

Fig. 4 The PLE and PL spectra of (a) K₃LuF₆:0.12Tb³⁺, (b) K₃LuF₆:0.1Eu³⁺, and (c) K₃LuF₆:0.12Tb³⁺,0.1Eu³⁺.

To further investigate the energy transfer process, the K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 005, 0.07, 0.1 and 0.13) samples were synthesized using the same method. The PL spectra of K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 0.05, 0.07, 0.1 and 0.13) under the excitation of 375 nm were recorded and the results are shown in Fig. 5. The inset shows the relative emission intensity at 550, 590, and 613 nm, respectively. Upon 375 nm excitation and with increasing Eu³⁺ concentration, the emission intensity of Tb at 550 nm (⁵D₄–⁷F₃ transitions) decreases all the time and the emission intensity of Eu³⁺ at 590 nm (⁵D₀–⁷F₁ transitions) increases and reaches a maximum at x = 0.10, after which the emission intensity decreases, caused by the concentration quenching of Eu³⁺ itself.³² The results indicate that energy transfer occurs from Tb³⁺ to Eu³⁺ in the K₃LuF₆ host. The emission intensity of Eu³⁺ at 613 nm (⁵D₀–⁷F₂ transitions) increases and reaches a maximum at x = 0.03, then the emission intensity decreases upon an increase in the Eu³⁺ concentration, which can be ascribed to the concentration quenching of Eu³⁺.

Fig. 5 The PL spectra of K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 005, 0.07, 0.1 and 0.13) under excitation at 375 nm. The inset shows the relative emission intensities at 550, 590, and 613 nm.

Fig. 6 shows energy levels of the possible energy transfer mechanism between Tb³⁺ and Eu³⁺. As depicted in Fig. 6, the Tb³⁺ electrons in the ⁷F₆ ground state energy level can be excited by n-UV light and jump to the ⁵D₃ excited state level. Then, they relax to the ⁵D₄ transition state due to multi-phonon relaxation. As is known, the excited states are not stable, and Tb³⁺ can exhibit green light when the electrons leap back from ⁵D₄ to ⁷F_J (J = 6, 5, 4, and 3) with non-radiative processes from ⁵D₃ to ⁵D₄. On the other hand, some Tb³⁺ electrons transfer their energy from the ⁵D₄ (Tb³⁺) level to the excited state of Eu³⁺ (⁵D₁ and ⁵D₂) by cross-relaxation, and then the Eu³⁺ (⁵D₁ and ⁵D₂) electrons relax to the ⁵D₀ level in a non-radiative process, resulting in ⁵D₀–⁷F_J (J = 0, 1, 2, 3, and 4) transitions of Eu³⁺, which produce a red emission color.

Fig. 6 Energy levels of the possible energy transfer mechanism from Tb³⁺ to Eu³⁺.

In order to further understand the energy transfer mechanism of multi-polar interactions between the Tb and Eu ions in this system, the Dexter equation can be applied, as follows:^33,34

(η_s0/η_s) ∝ C^n/3 (1)

in which η_s0 is the quantum efficiency of Tb³⁺ in the absence of Eu³⁺and η_s in the presence of Eu³⁺, C represents the total concentration of Tb³⁺ and Eu³⁺, n is a constant value, 3, 6, 8 or 10, corresponding to the exchange, dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively.^35,36 The value of η₀/η_s can be approximately calculated from the ratio of the photoluminescence intensity (I_s0/I_S). Furthermore, Fig. 7 shows the I_s0/I_S vs. C^n/3 (n = 3, 6, 8, or 10) correlation diagram of these phosphors. When n = 10, an optical linear relationship was observed, which corresponds to the quadrupole–quadrupole interaction between Tb³⁺ and Eu³⁺ in K₃LuF₆.

Fig. 7 Dependence of I_s0/I_S of Tb³⁺ on (a) C, (b) C^6/3, (c) C^8/3 and (d) C^10/3 for the K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 005, 0.07, 0.1 and 0.13) samples.

The CIE chromaticity coordinate diagram of the K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 005, 0.07, 0.1 and 0.13) phosphors under 375 nm excitation was calculated on the basis of the corresponding emission spectra, which are depicted in Table 1 and Fig. 8. As shown in Table 1 and Fig. 8, the emission color of K₃LuF₆:0.12Tb³⁺,xEu³⁺ can change from green (0.2781, 0.5407) to yellowish pink (0.4331, 0.3556) upon an increase in the concentration of Eu³⁺. Therefore, K₃LuF₆:0.12Tb³⁺,xEu³⁺ can be efficiently excited by 375 nm as an adjustable luminescence material for use in n-UV LEDs.

Table 1 The CIE chromaticity coordinates of the K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 005, 0.07, 0.10 and 0.13) samples under 375 nm excitation

No.	K3LuF6:0.12Tb^3+,xEu^3+	(x, y)
1	x = 0	(0.2781, 0.5047)
2	x = 0.03	(0.3710, 0.4289)
3	x = 0.05	(0.3812, 0.4069)
4	x = 0.07	(0.3857, 0.4019)
5	x = 0.10	(0.3958,0.3750)
6	x = 0.13	(0.4331, 0.3556)

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Fig. 8 The CIE chromaticity coordinate diagram of the K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0, 0.03, 005, 0.07, 0.10 and 0.13) samples under 375 nm excitation.

4. Conclusions

In summary, single-phase K₃LuF₆:xTb³⁺ (x = 0.01, 0.03, 005, 0.07, 0.09, 0.1, 0.12 and 0.13) and K₃LuF₆:0.12Tb³⁺,xEu³⁺ (x = 0.03, 005, 0.07, 0.1 and 0.13) samples were successfully prepared via a high temperature solid-state phase sintering method. In order to determine the energy transfer mechanism between the Tb³⁺ and Eu³⁺ ions, the photoluminescence properties and decay lifetimes were measured, and the results showed an energy transfer mechanism from Tb³⁺ to Eu³⁺ in K₃LuF₆, determined as a quadrupole–quadrupole interaction. Characteristic emissions for both Tb³⁺ and Eu³⁺ can be observed in the PL spectrum of the co-doped phosphors at an excitation of 375 nm (Tb³⁺), while the luminescent colors of the K₃LuF₆:0.12Tb³⁺,xEu³⁺ samples can be regulated by changing the concentration of Eu³⁺. All of the above results show that the phosphor could be an ideal single-phase multicolor phosphor.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This present work is supported by the National Natural Science Foundation of China (Grant No. 41672044).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra10059d

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