Structure and luminescence properties of La₆Ba₄(SiO₄)₆F₂:Dy³⁺ phosphor with apatite structure

Abstract

In this study, we investigated the structure, luminescence properties and morphology of La₆Ba₄(SiO₄)₆F₂:Dy³⁺ in detail using X-ray diffraction (XRD), photoluminescence spectroscopy, scanning electron microscopy (SEM) and decay kinetics measurements. The results indicate that La₆Ba₄(SiO₄)₆F₂:Dy³⁺ was well crystallized, and its structure is of apatite-type and belongs to the hexagonal system. The prepared samples exhibit two intense characteristic bands in the blue (484 nm) and yellow (579 nm) spectral ranges corresponding to the Dy³⁺ transitions ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2, respectively. These dominant photoluminescence bands are accompanied by a weak red band (670 nm) due to the ⁴F_9/2 → ⁶H_11/2 transition. The emission color of La₆Ba₄(SiO₄)₆F₂:Dy³⁺ phosphors are found to fall in the white light region. The Dy³⁺ optimal dopant concentration in the La₆Ba₄(SiO₄)₆F₂ host was found to be 0.12 (mol). In addition, the phosphors have high thermal stability. Hence, La₆Ba₄(SiO₄)₆F₂:Dy³⁺ may have an application in white light-emitting diodes.

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1. Introduction

In the present time, white light-emitting diodes (w-LEDs) are being widely applied in the field of lighting because of their excellent characteristics, such as good reliability, low power consumption, long lifetime, environmental friendliness and high luminous efficiency.^1–3 By combining a yellow emitting phosphor such as YAG:Ce³⁺ and a blue LED, or by combining several different phosphors, w-LEDs can generally be fabricated.^4–7 However, they may have disadvantages associated with complicated phosphors such as efficiency loss. Respectively, the exploration of developing single-phase phosphors is a relevant task. Energy transfer of a single-phase host is an effective way to obtain white light, for example, Sr₃Y₂(Si₃O₉)₂:Ce³⁺, Tb³⁺/Mn²⁺/Eu²⁺ phosphors⁸ and Sr₃Gd₂(Si₃O₉)₂:Ce³⁺, Tb³⁺/Mn²⁺ phosphors.⁹ In our study, we tried to use single Dy³⁺ doped phosphor to obtain white light in a more convenient manner than co-doped ones.

There is an abundance of f-block energy levels in Dy³⁺ with ⁴f₉ configuration. Strong emission is possible over the visible range,¹⁰ consisting of three predominant emission bands in the blue region of 470–500 nm, yellow region of 550–600 nm and red region of 600–700 nm generated by the ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 transitions, respectively.^11–13 Concentration of the Dy³⁺ ion as well as pump wavelength can have a pronounced impact on the intensity ratio of the yellow-blue emission (Y/B). This is due to the fact that the hypersensitive ⁴F_9/2 → ⁶H_13/2 transition has a strong intensity dependent on the host, whereas the ⁴F_9/2 → ⁶H_15/2 transition is less sensitive.¹⁴ Thanks to this feasibility, the emission near white light is possible by adjusting the Y/B value.¹⁵ Single-phased white-light-emitting phosphors doped by Dy³⁺ ions have been widely investigated, for instance, Ca₉La(PO₄)₅(SiO₄)F₂:Dy³⁺,¹⁶ K₂Gd(PO₄)(WO₄):Dy³⁺,¹⁷ Na₃Sc₂(PO₄)₃:Dy³⁺,¹⁸ Sr₃Y(PO₄)₃:Dy³⁺ (ref. 19) and Ca₃Si₂O₇:Dy³⁺.²⁰

Compounds with apatite structure are considered as practical host materials because they have exceptional thermal and chemical stability.^21,22 Apatite-type compounds crystallize in the space group of P6₃/m (Z = 2) and have a general chemical composition of A₁₀[XO₄]₆Z₂,²³ where A stands for divalent cations such as Ca²⁺, Mg²⁺, Ba²⁺ and Sr²⁺; XO₄ represents a tetrahedron group in which X can be P, Si, Ge, V; Z can be anions such as OH⁻, Cl⁻, O²⁻, F⁻.^24–26 By isomorphic substitution, the A site can be occupied by rare earth ions like Y³⁺, La³⁺, Gd³⁺ or alkali metal ions like Li⁺, Na⁺, K⁺.^27–29 Accordingly, we can skillfully design novel Dy³⁺-doped phosphors with more accomplished light output for solid state lighting applications.

Being a likely host for the generation of tunable solid-state lasers and ultra-short pulses, the apatite-type La₆Ba₄(SiO₄)₆F₂ (abbreviated here as LBSF) compound was considered carefully.³⁰ There are two cationic sites in this compound, i.e., C₃ point symmetry in 9-fold coordinated 4f sites and C_s point symmetry in 7-fold coordinated 6h sites.³¹ Besides, numerous species of rare earth ions can be accepted in both sites.³² In our previous studies, we have obtained emission-tunable (from blue to green) phosphor La₆Ba₄(SiO₄)₆F₂:Ce³⁺, Tb³⁺ (ref. 33) and reddish-orange phosphor La₆Ba₄(SiO₄)₆F₂:Sm³⁺,³¹ which further indicates that LBSF is an excellent host material for single-phased white light-emitting phosphors. However, LBSF:Dy³⁺ phosphors have not yet been reported. Hence, in this contribution, the systematic study of the synthesis, structure and luminescence properties of LBSF:Dy³⁺ phosphors are reported. We show that, having feasible application, La₆Ba₄(SiO₄)₆F₂:Dy³⁺ phosphors may act as a white-light-emitting phosphor for w-LEDs (white light-emitting diodes).

2. Experimental details

2.1. Synthesis

La_6−xBa₄(SiO₄)₆F₂:xDy³⁺ (x = 0.03, 0.09, 0.12, 0.15, 0.18 and 0.30) compounds were synthesized using a high-temperature solid-state reaction method. La₂O₃ (Aldrich, 99.995%), BaCO₃ (Aldrich, 99.9%), SiO₂ (Aldrich, 99.9%), NH₄HF₂ (Aldrich, 99.9%) and Dy₂O₃ (Aldrich, 99.995%) were used as starting materials. The reagents were mixed and then ground, based on the calculated stoichiometric ratio. NH₄HF₂ was used with 50% stoichiometric excess to compensate for fluorine loss at high temperatures. The mixtures were well ground in an agate mortar, inserted into the alumina crucibles and pre-heated at 750 °C for 1 h in the air atmosphere. Afterwards, the precursors were ground and annealed at 1350 °C for 4 h in the air atmosphere. As-synthesized samples were cooled to room temperature (RT) and ground to powders for structural and optical characterizations.

2.2. Characterization

The structural properties of synthesized samples were carried out by X-ray powder diffraction using an XD-3, PGENERAL, China, with Cu Kα radiation (λ = 0.15406 nm), in the 2θ range from 10° to 70°, operating at 40 kV and 40 mA. Using a GeminiSEM 500 instrument for morphological characterization, the La₆Ba₄(SiO₄)₆F₂:xDy³⁺ (x = 0.12 mol) sample was examined by SEM. The photoluminescence (PL) and excitation (PLE) spectra of the samples were recorded with an F-4600 fluorescence spectrophotometer (HITACHI, Japan) equipped with a photomultiplier tube operating at 500 V and a 150 W Xe lamp as the excitation source. Decay kinetics measurements were performed at RT using a Spectrofluorometer (Horiba, Jobin Yvon TBXPS) with a tunable pulse laser radiation as an excitation source.

3. Results and discussion

3.1. Structure

The crystal structure and phase purity of the as-prepared phosphors were analyzed by XRD. In Fig. 1(a), XRD patterns of La_6−xBa₄(SiO₄)₆F₂:xDy³⁺ (x = 0.03, 0.09, 0.12, 0.15, 0.18 and 0.30) phosphors are shown. The relative intensity and position of all diffraction peaks can be exactly matched with those of the La₆Ba₄(SiO₄)₆F₂ (ICSD no. 170852) standard,³⁴ revealing the formation of hexagonal La₆Ba₄(SiO₄)₆F₂-type compounds and showing that the small doped Dy³⁺ contents has not affected the pure phase composition. No other crystalline phases were detected. The view of La₆Ba₄(SiO₄)₆F₂ crystal structure along [001] axis is illustrated in Fig. 1(b). As shown, there are two nonequivalent cation sites, the seven-coordinated 6h (C_s) site [La³⁺/Ba²⁺(II)] and the nine-coordinated 4f (C₃) site [La³⁺/Ba²⁺(I)], which are connected by isolated [SiO₄]⁴⁻ tetrahedrons. The [SiO₄]⁴⁻ tetrahedrons, La/Ba(I) polyhedrons and La/Ba(II) polyhedrons form the framework structure of the La₆Ba₄(SiO₄)₆F₂ host.

Fig. 1 (a) XRD patterns of LBSF:xDy³⁺ (x = 0.30, 0.18, 0.15, 0.12, 0.09 and 0.03) samples, (b) crystal structure of the host LBSF compound viewed along the c axis, (c) the SEM micrograph of LBSF:0.12Dy³⁺ phosphor and the elemental mapping of LBSF:Dy³⁺ phosphors.

The SEM micrograph of the LBSF:0.12Dy³⁺ phosphor is shown in Fig. 1(c). The typical microstructure of the sample shows the inherent characteristic results from the solid-state reaction method. The particles are irregular in shape, and the synthesized and ground phosphor has a wide particle size distribution range from 1 to 10 μm. In Fig. 1(c), the elemental mapping of LBSF: 0.12Dy³⁺ phosphor is shown. In the final product, the constituent elements La, Ba, Si, O, F and Dy are uniformly distributed over the whole observed area. Based on these results, the phosphor mostly can meet the fabrication requirements for w-LEDs. Furthermore, it is noticed that the particles are closely packed, which helps to prevent light scattering. Therefore, more efficient light output can be obtained.³⁵ In addition, by using EDS for LBSF phosphor, we accomplished elemental analysis and have identified the elements, i.e., Ba, La, Si, O, F in the proportions as seen in the EDS spectrum, respectively. The elemental composition of the sample is shown in Table 1.

Table 1 The elemental composition of the host La₆Ba₄(SiO₄)₆F₂ sample determined by EDS analysis

Element	Weight%	Atomic%
O	19.86	58.72
F	2.24	5.60
Si	6.71	11.33
Ba	28.17	9.70
La	43.01	14.65
Totals	100.00

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3.2. Luminescence properties of LBSF:Dy³⁺

At different Dy³⁺ concentrations, we tested the luminescence properties of LBSF samples at RT. In Fig. 2(a) and (b), the emission (λ_em = 579 nm) and excitation spectra (λ_ex = 353 nm) are displayed, respectively. The excitation spectrum shown in Fig. 2(b) exhibited four intense bands centered at the wavelengths of 300, 327, 353 and 389 nm, corresponding to the transitions ⁶H_15/2 → ⁶P_3/2, ⁶H_15/2 → ⁶P_7/2, ⁶H_15/2 → ⁶P_5/2 and ⁶H_15/2 → ⁴I_13/2 (389 nm), one moderate band at 454 nm and one weak band at 427 nm, due to ⁶H_15/2 → ⁴I_15/2 and ⁶H_15/2 → ⁴G_11/2 transitions, respectively. The PLE peaks for Dy³⁺ doped La₆Ba₄(SiO₄)₆F₂ phosphor offer a great opportunity for WLED devices with excitation using blue InGaN/GaN LED chips.³⁶ In Fig. 2(b), at 353 nm excitation, when the Dy³⁺ concentration increases from 0.03 to 0.30 mol, the emission intensity initially increases and then decreases due to the concentration quenching (CQ) phenomena. In LBSF doped with Dy³⁺ ions, the optimal doping concentration was 0.12 mol.³⁷ Because of partial crystal field degeneracy around the Dy³⁺ ions, the blue emission bands are split into two components. Due to the Stark effect enhancement and increase in Dy³⁺ concentration, the crystal-field splitting increases. The main emission band at 579 nm corresponds to the ⁴F_9/2 → ⁶H_13/2 transition known as the electric dipole transition or hypersensitive transition, and the one at 484 nm originates due to the ⁴F_9/2 → ⁶H_15/2 transition known as the magnetic dipole transition, which usually shows lower intensity than the electric dipole transition.^38–40 The variation of the Y/B, yellow-to-blue luminescence, intensity ratio is applicable to white light emission with Dy³⁺ ions tuning the yellow and blue components. In Table 2, the Y/B values for LBSF with different dysprosium concentrations are listed. As seen, the Y/B ratios are practically insensitive to the concentration of Dy³⁺ ions. Evidently, larger Y/B values are observed for higher concentration of dysprosium. Based on this trend, we validate that the Y/B ratio increases at low concentrations of Dy³⁺ ions in the La₆Ba₄(SiO₄)₆F₂ system. The maximum value of Y/B is 1.03 for LBSF:0.15Dy³⁺, showing a good agreement with the concentration dependence of PL, as shown in Fig. 2(b). It can be assumed that the small changes in the Y/B ratios are caused by structural changes near the Dy³⁺ ions.^41–44 The results show that the Dy³⁺ ions occupy lower symmetry sites in the LBSF structure, as indicated by the XRD results.

Fig. 2 (a) The PL excitation (PLE) spectra (λ_em = 579 nm) of LBSF:xDy³⁺, (b) the PL emission spectra (λ_ex = 353 nm) of LBSF:xDy³⁺, and the inset in (b) shows the variable 579 nm peak intensity with increasing dopant concentration.

Table 2 The (Y/B) intensity ratios of luminescence spectra evaluated and the calculated chromaticity coordinates for the white light emitted from LBSF:xDy³⁺ phosphor (x = 0.03, 0.09, 0.12, 0.15, 0.18, 0.3)

Sample no.	C	484	579	Y/B ratio	(x, y)
1	0.03	901.3	764.8	0.85	(0.3032, 0.3371)
2	0.09	1860	1821	0.98	(0.3298, 0.3646)
3	0.12	1895	1902	1.00	(0.3206, 0.3496)
4	0.15	2675	2749	1.03	(0.3422, 0.3780)
5	0.18	2681	2659	0.99	(0.3391, 0.3765)
6	0.30	2345	2390	1.02	(0.3403, 0.3768)

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3.3. Decay characteristics

In Fig. 3, the fluorescence decay curves of the Dy³⁺ emission are shown for the LBSF:Dy³⁺ phosphors, as excited at 350 nm and monitored at 573 nm. The decay curves can be properly fitted by a second order exponential equation:⁴⁵in which I stands for the phosphorescent intensity. I₁, I₂ are intensity constants, τ represents the time, and τ₁, τ₂ are the fast and slow lifetimes for exponential components. The determined lifetime values and fitting parameters are given in Table 3. The ⁴F_9/2 luminescence lifetime depends on the concentration of Dy³⁺ ions. With the enhancement of Dy³⁺ dopant concentration, the typical values of lifetime shorten from about 0.76 to 0.4 ms, as expected. We find that the effective lifetimes decrease with increasing Dy³⁺ concentration, and energy transfer to Dy³⁺ ions through cross-relaxation processes and energy transfer between luminescence quenching centers and impurities are the probable reasons.^44,46

Fig. 3 Decay curves of Dy³⁺ emission for LBSF:xDy³⁺ phosphors under excitation 353 nm, monitored at 579 nm.

Table 3 Determined luminescence lifetimes and fitting parameters for the LBSF:xDy³⁺ samples

Sample no.	C (mol)	τ1 (ms)	A1 (%)	τ2 (ms)	A2 (%)	τaverage (ms)	R^2
1	0.03	0.7644	94.38%	0.1976	5.62%	0.76	0.9983
2	0.09	0.0740	10.95%	0.6620	89.05%	0.65	0.9982
3	0.12	0.5864	80.44%	0.0498	19.56%	0.58	0.9980
4	0.15	0.5767	75.88%	0.0585	24.12%	0.56	0.9977
5	0.18	0.0443	31.98%	0.5559	68.02%	0.54	0.9976
6	0.30	0.4400	42.03%	0.0343	57.97%	0.40	0.9967

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3.4. Energy transfer process

Generally, with increasing Dy³⁺ concentration, the relative distance between the Dy³⁺ luminescent centers decreases. This promotes nonradiative energy transfer between adjacent Dy³⁺ ions, leading to concentration quenching at high dopant concentrations. In the energy transfer process, there could be quenching sites where the excitation energy dissipates, resulting in luminescence quenching. To check the concentration quenching mechanism, it is necessary to determine the critical distance (R_C) between the Dy³⁺ ions. R_C can be evaluated on the basis of the Blasse equation:^47,48in which V is the volume belonging to the unit cell, x_c is the critical concentration of Dy³⁺, N represents the formula units per unit cell. Taking appropriate values of V, x_c, and N (10, 608.77 ų, 0.12, respectively) for LBSF:0.12Dy³⁺ phosphor, R_C is estimated to be about 9.89 Å. Consequently, it is deduced that the exchange interaction contribution to concentration quenching effect is inefficient in the LBSF system since exchange interaction occurs when R_C is smaller than 5 Å.⁴⁹ Therefore, in the case of Dy³⁺, the concentration quenching mechanism was dominated by the multipolar–multipolar interaction. This model of energy transfer between similar activators, as well as the relation between Dy³⁺ concentration and the luminescence intensity is pointed out on the basis of Dexter theory. As given by the equation below, there is a relation between activator concentration (x) and luminescence intensity (I):^50,51where x is the activator concentration, θ is a multipolar interaction constant equal to 3, 6, 8 or 10 corresponding to the nearest-neighbor ions, which means dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively,⁵² and K and β are constants for each interaction at the same excitation. Exceeding the quenching concentration, we chose the LBSF:0.15, 0.18, 0.27, 0.3Dy³⁺ samples for consistent emission intensity measurements at 484 nm. As seen in Fig. 4, by the log(I/x) − log(x) plot, the value of θ is determined to be 6.07 for the peak at 484 nm. Thus, for concentration quenching in the system under consideration, the dipole–dipole (d–d) interaction is dominant.

Fig. 4 Relationship between the log(I/x) and log(x) of Dy³⁺ content in LBSF:xDy³⁺ phosphors beyond the quenching concentration (x = 0.15, 0.18, 0.27, 0.3).

3.5. CIE chromaticity coordinate

As one of the important factors, the CIE, Commission International de I'Eclairage⁵³ 1931, is widely used to ascertain the chromaticity coordinates and evaluate the phosphor performance. The chromaticity coordinates of LBSF:Dy³⁺ phosphors with different dopant concentrations under UV excitation at 353 nm are listed in Table 2. The LBSF:Dy³⁺ samples on the CIE chromaticity diagram are shown in Fig. 5. As evident, all LBSF:Dy³⁺ samples are in the white region. Moreover, the point of LBSF:0.12Dy³⁺ phosphor, (x = 0.3206, y = 0.3496), is very close to standard white light (x = 0.334, y = 0.337).⁵⁴ Hence, the present results imply that the LBSF:0.12Dy³⁺ phosphor could have a likely application for white light-emitting diodes.

Fig. 5 The CIE chromaticity coordinates (x, y) LBSF:xDy³⁺ samples, and the corresponding images of samples luminescence under UV light irradiation.

3.6. Temperature-dependent luminescence properties

For long-term operation, w-LED chip temperatures could increase up to 150 °C, so, the thermal stability of the phosphors is one of the most vital issues for their applications in high-powered LEDs.⁵⁵ In Fig. 6, the temperature-dependent emission spectra of the LBSF:0.12Dy³⁺ phosphor excited at 353 nm is shown for the temperature range from RT to 250 °C. The PL relative intensity decreases gradually with increasing temperature and it remains above 60% at 150 °C, reflecting that the prepared phosphor has high thermal stability. A plot of the luminescence intensity vs. temperature is shown in the inset of Fig. 6. Currently, during the LED operation, due to the heat generated by the LED itself, the temperature is up to 150 °C for a long time and the phosphors should maintain their emission efficiency. It is observed that in LBSF:0.12Dy³⁺ phosphor the Dy³⁺ luminescence intensity related to the ⁴F_9/2 to ⁶H_15/2 and ⁴F_9/2 to ⁶H_13/2 transitions reaches 62.03% and 67.28%, respectively, when the temperature is elevated up to 150 °C, as compared to the room temperature parameters. Because of non-radiative relaxation through the excited state and its crossover to the ground state, the thermal quenching effect, where the probability of non-radiative transition strictly relies on the temperature causes PL quenching of Dy³⁺ emission at high temperatures. The relationship between the temperature and photoluminescence intensity could be described with a modified Arrhenius equation:^56–58where I(T) is the intensity at temperature T, I₀ is the initial intensity, c is a constant, E is the activation energy between the bottom of the excited state ⁴F_9/2 and its crossover point to the ground state ⁶H_13/2 and k is the Boltzmann constant. For the present LBSF:0.12Dy³⁺ phosphor, Fig. 7 gives evidence of the emission thermal quenching model fitting line plotted as ln(I₀/I) − 1 against 1/kT. As revealed by the fitting with eqn (4), the activation energy value E is equal to 0.154 eV for the ⁴F_9/2 to ⁶H_13/2 transition of Dy³⁺ ions. Therefore, the obtained LBSF:0.12Dy³⁺ phosphor shows good thermal stability, which offers an excellent possibility for application in w-LEDs. The chromaticity coordinates of LBSF:0.12Dy³⁺ phosphors at different temperatures ranging from RT to 250 °C under 353 nm excitation are shown in Table 4. As shown in Table 4, the chromaticity coordinates of LBSF:0.12Dy³⁺ phosphors at different temperature ranging from RT to 250 °C under 353 nm excitation are stable, and are all located in white region.

Fig. 6 Emission spectra of LBSF:0.12Dy³⁺ under 353 nm excitation at different temperatures, and the inset shows the PL intensity as a function of temperature.

Fig. 7 The fitting line of the emission thermal quenching model plotted as ln(I₀/I) − 1 against 1/kT for the present LBSF:0.12Dy³⁺ phosphor.

Table 4 The chromaticity coordinates of LBSF:0.12Dy³⁺ phosphor at different temperatures ranging from RT to 250 °C under 353 nm excitation

Sample no.	Temperature (°C)	(x, y)
1	RT	(0.3285, 0.3538)
2	75	(0.3296, 0.3473)
3	100	(0.3290, 0.3461)
4	125	(0.3277, 0.3437)
5	150	(0.3261, 0.3412)
6	175	(0.3241, 0.3397)
7	200	(0.3182, 0.3320)
8	250	(0.3043, 0.3161)

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4. Conclusion

In conclusion, by using the traditional solid-state reaction method, the new feasible single-phased white light-emitting La_6−xBa₄(SiO₄)₆F₂:Dy³⁺ phosphors were synthesized favorably. Subsequently, we reported on the systematic study of the crystal structure, morphology and luminescence performance of prepared samples. The XRD patterns confirmed their phase purity, and that the compounds belong to the apatite family. SEM measurement demonstrated a closely packed particle morphology. Under UV excitation, two intense characteristic bands i.e. blue bands at 484 nm, yellow bands at 579 nm, corresponding to transitions of Dy³⁺, ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2, respectively, dominate the photoluminescence spectra accompanied by a weak red band at 670 nm, contributing to ⁴F_9/2 → ⁶H_11/2. The concentration quenching mechanism of the Dy³⁺ ions is controlled by a dipole–dipole interaction, and the optimal dopant concentration of Dy³⁺ in the La_6−xBa₄(SiO₄)₆F₂ host was determined to be 0.12 (mol). The critical transfer distance between the Dy³⁺ ions is calculated to be 9.89 Å. In the CIE diagram, the emission color of La₆Ba₄(SiO₄)₆F₂:Dy³⁺ phosphors falls in the white light zone. Furthermore, thermal quenching measurements indicate excellent thermal stability. Combining all the results, we can indicate that La₆Ba₄(SiO₄)₆F₂:Dy³⁺ may serve as white light-emitting phosphors for white-light-emitting diodes (w-LEDs).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The study was supported by the National Natural Science Foundations of China 277 (Grant No. 51672257) and the Fundamental Research Funds for the Central Universities (Grant No. 2652017094 and 2652017091). Jialei Zhang also express thanks to the supporting from the College Student Research Innovation Program of China University of Geosciences, Beijing.

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