Preparation, structure and up-conversion luminescence properties of novel cryolite K₃YF₆:Er³⁺, Yb³⁺

Abstract

Cryolite is a suitable host for up-conversion luminescent materials due to its low phonon energy and good optical transparency. In this work, a novel up-conversion material K₃YF₆:Yb³⁺, Er³⁺ with a cryolite structure was prepared successfully by a solid state method. The crystal structure, morphology, composition and up-conversion luminescence properties of the as-prepared sample were characterized by X-ray diffractometry (XRD), field emission scanning electron microscopy (SEM) and fluorescence spectrometer in detail. K₃YF₆:Er³⁺, Yb³⁺ has a cryolite structure. Under 980 nm excitation, the as-prepared sample can generate slight green emission at 524 and 546 nm (attributed to ²H_11/2 → ⁴I_15/2 transition, ⁴S_3/2→⁴I_15/2 transition of Er³⁺) and strong red emission at 661 and 672 nm (corresponding to ⁴F_9/2 → ⁴I_15/2 transition, ⁴I_9/2 → ⁴I_15/2 transition of Er³⁺). All the green and red up-conversion emission of K₃YF₆:Er³⁺, Yb³⁺ transfer and electronic transition process of the red and green light the sample emitted, the possible luminescence mechanism is discussed in this paper.

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1. Introduction

Under the excitation of ultraviolet light, visible light or infrared light source, a material with certain luminescent properties is called photoluminescent material. Photoluminescent materials are classified into down-conversion luminescent materials and up-conversion luminescent materials, which are widely used in illumination sources, plasma displays, luminescent coatings, etc.^1–3 The material that absorbs short-wave radiation and then emits long-wave radiation is called a down-converting luminescent material, while the material that first absorbs long-wave radiation and then emits short-wave radiation is called an up-converting luminescent material. The up-converting luminescent material can be excited with low energy near infrared (NIR) radiation, and emits the light of higher energy in a visible range, due to the multi-photon absorption followed by an anti-Stokes emission.^4–8 So far, rare earth doped up-conversion materials have aroused widespread attention due to their important applications^9–14 in the fields of the national economy and national defense construction, such as in infrared quantum counters, night vision systems, light-emitting diodes, other laser materials, etc.^15–18

The urgent problem to be solved by up-converting luminescent materials is to find a suitable host. Although the host does not constitute an excitation level, it provides a suitable crystal field environment for the sensitizer ions and activator ions to cause matching energy level splitting to produce energy transfer and up-conversion luminescence.^19,20 At present, there are four main types of up-conversion host materials: oxide systems, sulfide-containing systems, halide systems (excluding fluorine), and fluoride systems. Compared to the other systems, the fluoride system has the advantages of low phonon energy, wide light transmission range, and easy to form waveguides and so on. Recently, fluoride has attracted the attention of researchers as the most popular up-conversion host.^21,22

Cryolite, an important fluoride, has general formula of A₃BF₆ (A = Li, Na, K, NH₄, etc.; B = Al, Sc, V, Cr, Fe, Y, Ga, etc.), which is one of the most promising up-conversion host in the field of luminescent materials due to its low phonon energy, stable chemical composition and good optical transparency. Recently, compounds with cryolite structure have been widely used as host for luminescent materials, such as: K₃GaF₆:Mn⁴⁺,²³ Na₃GaF₆:Mn⁴⁺,²⁴ K₃LuF₆:Er³⁺,²⁵ K₃LuF₆:Ce³⁺.²⁶ Na₃AlF₆:Mn⁴⁺ red phosphor was prepared via the coprecipitation and hydrothermal methods, and the phosphor was a promising candidate for application in warm WLEDs.²⁷ By controlling the concentration ratio of Eu³⁺/Tb³⁺, K₃LuF₆:Tb³⁺, Eu³⁺ phosphors could be adjusted from green to yellowish pink.²⁸ K₃YF₆ is an a cryolite compound. Marek²⁹ reported the crystal structure and vibrational properties of K₃YF₆ solid solution and considered it suitable as luminescent materials. These works mainly focus on the down-conversion of cryolite structure. So far, there is little research on K₃YF₆ to be used as an up-conversion host. In addition, Er³⁺/Yb³⁺ doped monoclinic K₃YF₆ have not been reported.

Owing to the large absorption section and wide absorption region of Yb³⁺, the up-conversion luminescence intensities of Er³⁺ are largely enhanced in (Er³⁺, Yb³⁺) co-doped materials. Some works of Er³⁺/Yb³⁺ co-doped up-conversion luminescent material have been done, such as: K₂GdF₅:Yb³⁺/Er³⁺,³⁰ Y₂O₃:Er³⁺/Yb³⁺,³¹ Ba₂LaF₇:Er³⁺/Yb³⁺,³² Na₅Lu₉F₃₂:Er³⁺/Yb³⁺.³³

Herein, a novel cryolite up-conversion luminescent material K₃YF₆:Yb³⁺, Er³⁺ was successfully prepared by a solid state method. The crystal structure, elemental composition and up-conversion luminescence properties of the as-prepared samples were investigated in detail. Besides, the possible luminescence mechanism and electron transition process proposed were also discussed in this paper.

2. Experimental details

2.1 Synthesis

K₃YF₆:Er³⁺, Yb³⁺ was prepared by a solid state method. The raw materials of K₂CO₃ (A.R.), NH₄HF₂ (A.R.), Y₂O₃ (99.99%), Er₂O₃ (99.99%) and Yb₂O₃ (99.99%) were purchased from Aladdin. The starting materials were weighted with electronic balance according to stoichiometric ratio. To prevent the fluorine from volatilizing during heating, NH₄HF₂ is treated with an excess of 30%. The weighed raw materials were ground in an agate mortar until well mixed. Then, they were transferred to a muffle furnace and calcined at 750 °C for 3 h. After cooling to room temperature, the samples were taken out and grinded into powder again for a series of test followed.

2.2 Measurement

The structure of the prepared samples was characterized by X-ray diffraction (XRD) measurements using an X-ray powder diffractometer (D8 Advance, Bruker, Germany; Cu Kα radiation, λ = 0.15418 nm) at 40 kV and 100 mA, and the step width is 0.02° (2θ) with 2θ ranging from 10 to 70°. The morphology and energy dispersive X-ray (EDX) spectra of the as-prepared samples were obtained by a field emission scanning electron microscope (SEM, JSM-6701F, Hitachi, Japan) with 15 kV acceleration voltage. Under room temperature, the fluorescence emission spectra of samples were measured on a fluorescence spectrophotometer (Hitachi F4600) with an external tunable 980 nm infrared laser as excitation source.

3. Results and discussions

As we know, cryolite has two crystal forms, that is, mono-clinic and cubic. Fig. 1 shows the crystal structure for mono-clinic K₃YF₆ (space group P2₁/n). As shown in Fig. 1, Y³⁺ can occupy strongly distorted octahedral sites with centrosymmetric C_i local symmetry with six-fold coordinated by fluorine ions.³⁴ Since the electricity valences and ionic radii of Yb³⁺/Er³⁺ and Y³⁺ are close to each other, it is considered that Yb³⁺/Er³⁺ replaces the site for Y³⁺ during the doping process.

Fig. 1 Crystal structure of mono-clinic K₃YF₆.

The luminous efficiency of the up-converting luminescent material is not only related to the host structure, but also to the phase purity of the prepared material. Therefore, the structure of the prepared sample was analyzed by an X-ray powder diffractometer. Fig. 2 shows the XRD patterns of K₃YF₆:xEr³⁺ (x = 0.005, 0.01, 0.03, 0.05, 0.07, and 0.10), and the data for K₃YF₆ (JCPDF no. 27-467) is shown a reference. As illustrated in Fig. 2, the diffraction peak positions of the as-prepared samples match well with these of standard K₃YF₆ (JCPDF no. 27-467) without any impurity peak, indicating that all the as-prepared samples were cryolite compound. The increasing doping ratios of Er³⁺ did not cause K₃YF₆ structural changes.^35–37

Fig. 2 XRD patterns of K₃YF₆:xEr³⁺ (x = 0.005, 0.01, 0.03, 0.05, 0.07, and 0.10), and the data for K₃YF₆ (JCPDF no. 27-467) is shown a reference.

The emission spectra of as-prepared K₃YF₆:xEr³⁺ (x = 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) samples are shown in Fig. 3. The centers of the six emission peaks are 526 (²H_11/2 → ⁴F_15/2), 535 (²H_11/2 → ⁴F_15/2), 548 (⁴S_3/2 → ⁴F_15/2), 556 (⁴S_3/2 → ⁴F_15/2), 661 (⁴F_9/2 → ⁴F_15/2) and 672 nm (⁴I_9/2 → ⁴F_15/2) in the green and red regions, respectively. Remarkably, for K₃YF₆:0.03Er³⁺, the emission peak at 672 nm is the highest and the inset shows the dependence of luminescence intensity on the Er³⁺ doping ratio at 672 nm. With the increasing for Er³⁺ doping ratio from 0.005 to 0.10, the intensity of the emission peak of the samples firstly increased and then decreased and finally stabilized. When the Er³⁺ doping ratio was 0.03, the intensity of the emission peak at 672 nm was the highest. Fig. 4 shows the visual illumination of K₃YF₆:xEr³⁺ (x = 0.005, 0.01, 0.03, 0.05, 0.07, and 0.10) under 980 nm laser excite. As shown in Fig. 4, the emitted light changes from green to red, and the luminous intensity increases firstly and then decreases. When the Er³⁺ doping ratio is 0.03, the sample is the brightest and present red. This lays the foundation for the preparation of the up-conversion luminescent material with excellent performance.

Fig. 3 The emission spectra of K₃YF₆:xEr³⁺ (0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) excited by 980 nm infrared laser, with the doping ratio dependences of emission intensity at 672 nm shown in the inset.

Fig. 4 The luminescence of K₃YF₆:xEr³⁺ (x = 0.005 (a), 0.01 (b), 0.03 (c), 0.05 (d), 0.07 (e) and 0.10 (f)) under 980 nm laser irradiation.

A series of Er³⁺/Yb³⁺ co-doped K₃YF₆ up-conversion luminescent materials have been prepared. The microscopic, structures of the K₃YF₆:0.03Er³⁺/yYb³⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.12, 0.15, and 0.18) were determined. Fig. 5 shows the XRD patterns of K₃YF₆:0.03Er³⁺/yYb³⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.12, 0.15 and 0.18), and the data for K₃YF₆ (JCPDF no. 27-467) is shown a reference. It can be found that the diffraction peaks of the as-prepared samples match well with K₃YF₆ (JCPDS 27-467), indicating that Er³⁺/Yb³⁺ co-doping does not change the crystal structure significantly.

Fig. 5 XRD patterns of K₃YF₆:0.03Er³⁺/yYb³⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.12, 0.15 and 0.18), and the data for K₃YF₆ (JCPDF no. 27-467) is shown a reference.

Fig. 6 shows the SEM image and elemental mapping image of K₃YF₆:0.03Er³⁺, 0.12Yb³⁺ sample (a) and the EDX spectrum of K₃YF₆:0.03Er³⁺, 0.12Yb³⁺ (b). As shown in Fig. 6(a), the prepared sample appeared to be irregular particles with a grain size of about 10 μm. And the sample appear to partly agglomerate, which can be explained by the inhibition of grain growth due to the accumulation of vacancies on the grain boundaries when the Er³⁺/Yb³⁺ with higher valency occupies the K⁺ site. The element mapping results shown in Fig. 6(a) indicate that K, F, Y, Er, and Yb elements can be observed in K₃YF₆:0.03Er³⁺, 0.12Yb³⁺, and all the elements in the sample are uniformly distributed. The K (31.5%), F (36.5%), Y (29.5%), Er (0.4%), and Yb (2.0%) peaks were observed in the EDX spectrum of Fig. 6(b), further indicating that the measured lanthanide atomic ratios of K₃YF₆:0.03Er³⁺, 0.12Yb³⁺ are close to the calculated values.

Fig. 6 (a) The SEM image and elemental mapping image of K₃YF₆:0.03Er³⁺, 0.12Yb³⁺ sample; (b) the EDX spectrum of K₃YF₆:0.03Er³⁺, 0.12Yb³⁺.

The up-conversion emission spectra of K₃YF₆:0.03Er³⁺, yYb³⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.12, 0.15 and 0.18) are shown in Fig. 7. Four emission peaks at 524, 546, 661 and 672 nm in the green and red regions can be founded, which can be ascribed to the transitions from ²H_11/2, ⁴S^3/2, ⁴F_9/2, and ⁴I_9/2 states to ⁴I_15/2 state of Er³⁺, respectively.^38,39 The inset shows the effect of Yb³⁺ doping ratio on the luminescence intensity of the K₃YF₆:0.03Er³⁺, yYb³⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.12, 0.15 and 0.18). With the increasing for Yb³⁺ from 0.01 to 0.18, the emission intensity of the samples firstly increased and then decreased. When the Yb³⁺ doping ratio was 0.12, the intensity of the emission peak at 672 nm was the highest. Fig. 8 is the visual illumination of K₃YF₆:0.03Er³⁺, yYb³⁺ (y = 0, 0.01, 0.03, 0.07, 0.09, 0.12, 0.15 and 0.18) under 980 nm laser excite. As is shown in Fig. 8, the emitted light changes from light red to deep red, and the luminous intensity increases first and then decreases. When the Yb³⁺ doping ratio is 0.12, the sample is the brightest and present deep red.

Fig. 7 Up-conversion emission spectrum of K₃YF₆:0.03Er³⁺/yYb³⁺ (y = 0.01, 0.03, 0.07, 0.09, 0.12, 0.15 and 0.18) samples excited by 980 nm laser, with the doping ratio dependences of emission intensity at 672 nm shown in the inset.

Fig. 8 The luminescence of K₃YF₆:0.03Er³⁺, yYb³⁺ (y = 0 (a), 0.01 (b), 0.03 (c), 0.07 (d), 0.09 (e), 0.12 (f), 0.15 (g) and 0.18 (h)) under 980 nm laser irradiation.

The pump power dependent UC emission spectra of K₃YF₆:0.05Er³⁺, 0.12Yb³⁺ are shown in Fig. 9. With the increase of pump power from 257 to 300 mW, the up-conversion emission intensity shows an upward trend. The relationship between up-conversion intensity and excitation pump power can be applied as follow:

I∝Pⁿ

In which I and P are the intensity of the up-conversion emission and the power of the excitation pump, respectively. n represents the number of phonons that need to populate the upper excited state energy level.³⁰ In order to obtain the value of n, the double logarithmic figure of the emission intensity of green and red up-conversion emission with pump power for K₃YF₆:0.05Er³⁺, 0.15Yb³⁺ were depicted in inset of Fig. 9 The green and red dots represent up-conversion emission peaks at 546 and 672 nm, respectively. The experimental data are fitted in a straight line. The n values of green and red up-conversion lines are calculated to be 2.53 and 2.04, respectively. The number of n is near to 2, indicating that the green and red up-conversion emission of K₃YF₆:0.03Er³⁺, 0.12Yb³⁺ all belong to two-photon process.⁴⁰

Fig. 9 Pump power dependence UC emission spectra of K₃YF₆:0.03Er³⁺, 0.12Yb³⁺; and the inset shows lnarithmic plots of green and red UC emission intensity versus pump power for K₃YF₆:0.03Er³⁺, 0.12Yb³⁺.

The possible up-conversion luminescence mechanism and electron transition process for the sample of K₃YF₆:0.01Er³⁺, 0.12Yb³⁺ to emit red and green light are shown in Fig. 10. Under 980 nm laser excitation, the electron of the sensitizer Yb³⁺ absorbs a 980 nm photon from the energy level ²F_7/2 to the ²F_5/2 level. When the excited state is unstable, the electrons of Yb³⁺ will return to the ²F_7/2 level through the non-radiative transition, and the energy released during the transition will be transferred to the activator Er³⁺. The electrons in the ground state of Er³⁺ absorb the energy transmitted by Yb³⁺ and transited to the ⁴I_11/2 level. Since the Er³⁺ and Yb³⁺ levels match very well, the electrons of the activator Er³⁺ can also directly absorb a photon of 980 nm from the ground state level ⁴I_15/2 to the ⁴I_11/2 excited state level. The specific process is as follows:

²F_7/2 (Yb³⁺) + hν (980 nm) → ²F_5/2 (Yb³⁺)

²F_5/2 (Yb³⁺) + ⁴I_15/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺)

⁴I_15/2 (Er³⁺) + hν (980 nm) → ⁴I_11/2 (Er³⁺)

Fig. 10 K₃YF₆:0.03Er³⁺, 0.12Yb³⁺ up-conversion luminescence mechanism.

Some electrons at the ⁴I_11/2 level of Er³⁺ directly accept the energy released from the electronic transition in the excited state of Yb³⁺ back to the ground state, and jump up to the ⁴F_7/2 level. Since the energy gap between ⁴F_7/2 and ²H_11/2, ²H_11/2 and ⁴S_3/2 is small, electrons at the ⁴F_7/2 level will quickly relax to the ²H_11/2/⁴S_3/2 level without radiation. When the electron absorption energy at the ⁴I_13/2 level shifts to the ⁴F_9/2 level, the electrons of the ⁴F_9/2 level will undergo a non-radiative transition to ⁴I_9/2, and the electron will go back to the ground state of the ⁴I_9/2 and ⁴F_9/2 levels.

4. Conclusions

A series of up-conversion luminescent materials K₃YF₆:Er³⁺, Yb³⁺ with cryolite structure were prepared by a solid state method for the first time. XRD results indicated that all prepared K₃YF₆:Er³⁺, Yb³⁺ samples were pure phase, belonging to monoclinic system, space group P2₁/n. Under the 980 nm light exhibition, the K₃YF₆:Er³⁺, Yb³⁺ shows typical transition of Er³⁺, which composed of two parts in the visible region that green and red. When the doping ratio of Er³⁺is 0.03 and the doping ratio of Yb³⁺is 0.12, the luminous intensity reaches the maximum. According to the relation between the up-conversion intensity and the excitation pump power at 980 nm laser excitation, the two-photon process dominated in the red and green up-conversion processes of Er³⁺ ions. All the results indicated K₃YF₆:0.03Er³⁺, 0.12Yb³⁺ is an excellent up-conversion luminescent material.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The current work has been supported by the National Natural Science Foundation of China project (Grant No. 41672044).

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