Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (6): 661-666

#### The article information

Li-zhi Fang, Xiong Zhou, Hai-ping Xia, Jian-xu Hu, Jian-li Zhang, Bao-jiu Chen

Optical Spectroscopy of Pr3+ Ion Singly Doped LiLuF4 Single Crystal by Bridgman Method
Pr3+离子单掺LiLuF4单晶的坩埚下降法生长及光谱性能研究
Chinese Journal of Chemical Physics, 2019, 32(6): 661-666

http://dx.doi.org/10.1063/1674-0068/cjcp1902025

### Article history

Received on: February 3, 2019
Accepted on: April 15, 2019
Optical Spectroscopy of Pr3+ Ion Singly Doped LiLuF4 Single Crystal by Bridgman Method
Li-zhi Fanga , Xiong Zhoua , Hai-ping Xiaa , Jian-xu Hua , Jian-li Zhanga , Bao-jiu Chenb
Dated: Received on February 3, 2019; Accepted on April 15, 2019
a. Key laboratory of Photo-electronic Materials, Ningbo University, Ningbo 315211, China;
b. Department of Physics, Dalian Maritime University, Dalian 116026, China
Abstract: High quality LiLuF4 single crystals doped with various Pr3+ ions were synthesized by a vertical Bridgman method in completely sealed platinum crucibles. The excitation spectra spans from 420 nm to 500 nm. The prepared single crystals exhibit a blue band at 480 nm (3P03H4), a green band at 522 nm (3P13H5), and a red band at 605 nm (1D23H4) when excited at 446 nm; their corresponding average lifetimes are 38.5 μs, 37.3 μs, and 36.8 μs, respectively, which are much longer than those in oxide single crystals. The effects of excitation wavelength and doping concentration on emission intensities and chromaticity coordinates are investigated. The optimal Pr3+ concentration is confirmed to be 0.5%. The temperature dependent emission shows that the emission intensity constantly decreases with the increase of temperature from 298 K to 443 K due to the enhancement of non-radiative quenching at high temperature. The 3P03H4 transition is the most vulnerable to temperature, followed by the 3P13H5 transition and 1D23H4 transition.
Key words: Pr3+ ion    LiLuF4 single crystal    Optical spectroscopy
Ⅰ. INTRODUCTION

The trivalent praseodymium ion (Pr3+) in solids has an intricate energy level scheme with energy gaps of various magnitude and rich emission spectral lines in UV, visible, and infrared regions. In the past decades, various Pr3+-doped materials have been investigated thoroughly in relation with several potential applications, such as the fiber laser amplifier [1], quantum cutting [2], red-emitting phosphors [3], up-conversion laser [4], and solid-state lasers [5]. They are widely employed in scintillators [6], fluorescent lamps [7], optical amplifiers for fiber-optic communication [8], 3D displays [9] long persistent luminescence application [10, 11] and optical frequency converters. Very recently, Pr3+ doped Cs2NaYF6 was fabricated for X-ray-activated long persistent phosphors featuring strong (short-wavelength ultraviolet light) ultraviolet C after glow emissions [12]. It shows that the ultraviolet C persistent luminescence of this phosphor is strong enough to be used for sterilization. This work opens up new ways for a wide variety of practical applications of Pr3+ [13].

The optical properties of Pr3+ ion can be influenced by the host lattice, doping concentration, and environmental temperature [14]. Single crystals exhibit high transparence, good anti-light irradiation, good thermal, mechanical, and chemical stability, which are suitable hosts for luminescence application. LiLuF4 (LLF) is one of important fluoride compounds and firstly reported in 1960 [15]. In recent years, LiLuF4 single crystal has attracted much attention as potential laser materials because of low maximum phonon energy, high chemical stability, and high doping concentration for rare earth ions [16]. However, report on the preparation of Pr3+ doped LiLuF4 single crystal for above applications is scarce. In this work, we report the growth of Pr3+ doped LiLuF4 single crystal by Bridgman method.

Ⅱ. EXPERIMENTS

The crystal was grown from 99.99% pure raw materials of LiF, LuF3, and PrF3. Pr3+ doped LiLuF4 single crystals with the molar composition of 50.5LiF-(49.5-x)LuF3-xPrF3, where x=0.1, 0.5 and 1.5 (designated as LFP1, LFP2 and LFP3), were prepared by a Bridgman method. The specific details and processes of crystal growth were described in Ref. [17]. The typical sample with a size of ϕ 10 mm×55mm and polished slices are displayed in FIG. 1(b). It can be clearly observed that the synthesized crystal appears in light green color, and is highly transparent.

 FIG. 1 (a) XRD pattern for LiLuF4 single crystal doped with 0.5% Pr3+ measured by using the powders derived from grinding the bulk crystal. (b) Photograph of 0.5% Pr3+ doped LiLuF4 single crystal, the right is the boule of single crystal, the left is polished slices. (c) Standard line pattern of the LiLuF4 (JCPDS No.27-1251).

The X-ray diffraction (XRD) measurements to identify the phase composition of the crystals were recorded by a Bruker D8 Advance (Germany). The XRD data were collected within a 2θ range of 10°-80° at a scan speed of 2°/min. The absorption spectra measurements for the samples were conducted by a Cary 5000 UV/VIS/NIR spectrophotometer (Agilent Co. America). Emission spectra and excitation spectra of the samples were recorded by an F-4500 spectrophotometer (Hitachi High-Technologies Co., Tokyo, Japan). The decay lifetimes of the prepared material were characterized by a Horiba Fluorolog-3 spectrofluorometer with a 450 W xenon lamp. The high-temperature measurement was conducted by a TAP-02 high-temperature heating instrument (Tian Jin Orient-KOJI instrument Co., Ltd.) connecting to the Horiba Fluorolog-3 spectrofluorometer to investigate the thermal quenching behavior of the grown crystal. The recording temperature varied from 298 K to 443 K. The experimental Pr3+ concentrations in single crystals were measured by an inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Inc, Optima 3000). Table Ⅰ lists the measured concentrations of Pr3+ ions in single crystals and doping concentration in raw materials. All the measurements were performed in atmospheric conditions.

Table Ⅰ The doping concentrations of Pr3+ ions in raw materials and the number of Pr3+ ions (N) in LiLuF4 crystals.
Ⅲ. RESULTS AND DISCUSSION A. X-ray diffraction

FIG. 1(a) shows XRD pattern for the 0.5% Pr3+ doped sample. All the diffraction peaks can be assigned to the standard profile of LiLuF4 (JCPDS No.27-1251). No extra diffraction peak originated from impurities is detected. The Lu3+ sites are most probable to be replaced by Pr3+ ions due to the comparable ionic radius between Pr3+ (1.013 Å) and Lu3+ (0.85 Å). Hence, the introduction of Pr3+ ions to the LiLuF4 host does not significantly affect the crystal structure as confirmed from the XRD characterization. The lattice parameters calculated from the XRD pattern are a=b=0.5137 nm, c=1.0542 nm.

B. Absorption and emission spectra

FIG. 2 displays absorption spectra of Pr3+ doped LiLuF4 single crystal from visible to infrared region. As shown in FIG. 2, characteristic absorption bands originated from the 3H4 ground state to the excited energy states of Pr3+ can be observed. Specifically, the visible absorption bands centered at 441, 464, 475, and 587 nm correspond to the electronic transition from the 3H4 ground state to the 3P2, 3P1, 3P0, 1D2 excited levels, respectively. Meanwhile, transitions from the 3H4 to the 1G4, 3F4, 3F3, 3F2, 3H6 energy levels contribute to absorption peaks at 1010, 1447, 1523, 1881 and 2238 nm [18], respectively. Moreover, the absorption intensity increases almost linearly with the Pr3+ doping concentration, suggesting Pr3+ ions have replaced effectively the crystal lattice sites of the LiLuF4 single crystal.

 FIG. 2 Absorption spectra of Pr3+ doped LiLuF4 single crystal (a) in the visible and (b) in the infrared region.

FIG. 3 (a) and (b) depict the excitation spectra monitored at 605 nm and emission spectra excited at 446 nm for Pr3+ doped LiLuF4 single crystal, respectively. The excitation spectra in FIG. 3(a) show three sharp excitation peaks at 446, 468, and 480 nm, which correspond to the typical f-f transitions of 3H43P2, 3H43P1, 3H43P0 transitions, respectively [19], and the most intense excitation peak centered at 446 nm. Characteristic emission bands around 468, 480, 490, 522, 541, 586, 605, 638, 695, and 718 nm can be observed from the emission spectra of FIG. 3(b), corresponding to the 3P13H4, 3P03H4, 3P23H5, 3P13H5, 3P03H5, 3P13H6, 1D23H4, 3P03F2, 3P03F3 and 3P03F4 transitions, respectively [20, 21]. The strongest emission band lies at around 480 nm. Moreover, the emission intensity is significantly enhanced with Pr3+ concentration increasing from 0.1 % to 0.5%. Further increase of Pr3+ content leads to a dramatical reduction of emission intensity, which can be ascribed to the concentration quenching effect.

 FIG. 3 Excitation spectra of 0.5% Pr3+ doped LiLuF4 single crystal. (b) Emission spectra of LiLuF4 single crystal doped with different Pr3+ concentration.

Based on the emission spectral results shown in FIG. 3(b), the energy level diagram for the Pr3+ doped LiLuF4 crystal is shown in FIG. 4. The Pr3+ ions are excited by 446 nm photons from the ground state 3H4 to the 3P2 state. Part of the excited ions on the 3P2 state nonradiatively decay to the 3P1 state, then from 3P1 return to the lower 3H6, 3H5, and 3H4 states by emitting the 586, 522, and 468 nm light. Ions on the 3P2 state can also relax nonradiatively to the 3P0 state. Transitions from the 3P0 state to the 3H4, 3H5, 3F2, 3F3, and 3F4 states lead to emission at 480, 541, 638, 695 and 718 nm, respectively. Meanwhile, emission at 490 and 605 nm attribute to the 3P23H5 and 1D23H4 transitions, respectively [22, 23].

 FIG. 4 Partial energy diagram of Pr3+ in the LiLuF4 single crystal.

FIG. 5 exhibits the decay curves at 480 nm (3P03H4), 522 nm (3P13H5) and 605 nm (1D23H4) of Pr3+ doped in LiLuF4 single crystal excited by 446 nm light. The decay curves can be well fitted by a double exponential function as follows [24]:

 FIG. 5 Decay curves of 0.5% Pr3+ at different sites in LiLuF4.
 $I = {A_1}{{\rm{e}}^{ - t/{\tau _1}}} + {A_2}{{\rm{e}}^{ - t/{\tau _2}}}$ (1)

where I is the luminescence intensity, τ1 and τ2 are the fast and slow components of the luminescence lifetimes, A1 and A2 are the weighting parameters. The average lifetime can be calculated by the following expression [25]:

 $\langle \tau \rangle = \frac{{{A_1}\tau _1^2 + {A_2}\tau _2^2}}{{{A_1}{\tau _1} + {A_2}{\tau _2}}}$ (2)

The average lifetimes for the emissions at 480, 522, and 605 nm are determined to be 38.5 µs, 37.3 µs, and 36.8 µs, respectively. It can be seen from Table Ⅱ that the lifetimes are much longer than those in PbWO4 [26] and Bi2ZnOB2O6 [27] oxide single crystal of 3P03H4, 3P13H5 and 1D23H4 transitions, which is attributed to the fluoride host lattice of LiLuF4.

Table Ⅱ Comparison of the lifetime at different sites for Pr3+ doped oxide single crystal.
C. CIE chromatic coordinate

The CIE chromaticity coordinates for Pr3+ doped LiLuF4 single crystals under 446 nm excitation are illustrated in FIG. 6. The variation of CIE coordinates is not significant with Δ x < 0.006 and Δ y < 0.0135, suggesting the excellent color stability of the Pr3+ doped LiLuF4 single crystals. The correlated color temperature (Tc) can be estimated by the McCamy's empirical formula [28]:

 FIG. 6 CIE chromatic coordinates diagram of x % Pr3+ (x=0.1, 0.5, 1.5) doped LiLuF4 single crystal (λex}=446 nm).
 ${T_{\rm{c}}} = - 437{n^3} + 3601{n^2} - 6861n + 5514.31$ (3)

where $n = \frac{{x - {x_{\rm{e}}}}}{{y - {y_{\rm{e}}}}}$ and (xe, ye) is the chromaticity epicenter which lies at xe=0.3320, ye=0.1858 [28]. The Tc slightly increases from 5474 K to 5755 K with the Pr3+ doping concentration increase from 0.1% to 1.5%.

Thermal stability is a key factor to evaluate the potential of materials for applications, which significantly affects the performance of luminescence, such as the light output, lifetime, chromatic, and color rendering index [29]. The thermal quenching behavior of Pr3+ doped LiLuF4 single crystal is investigated by increasing temperature from 298 K to 443 K under 446 nm excitation. As shown in FIG. 7, the emission intensity constantly decreases with the increase of temperature due to the enhancement of non-radiative quenching at high temperature. The inset in FIG. 7 illustrates the integrated emission intensity for the 3P03H4, 3P13H5 and 1D23H4 transitions as a function of temperature. The 3P03H4 transition is the most vulnerable to temperature, followed by the 3P13H5 transition and 1D23H4 transition.

 FIG. 7 Temperature-dependence of the emission spectra of 0.5% Pr3+ doped LiLuF4 single crystal from 298 K to 493 K. Inset is the temperature-dependence of emission intensities.

The activation energy (Ea) as a crucial parameter to determine the thermal stability of the prepared materials can be calculated by the Arrhenius equation [30]:

 ${I_T} = \frac{{{I_0}}}{{1 + C{{\rm{e}}^{ - {E_{\rm{a}}}/{k_{\rm{B}}}T}}}}$ (4)

where I0 is the intensity at 298 K, IT is the intensity at temperature T, C is a constant and kB is the Boltzmann constant (8.617×10-5 eV/K). The Arrhenius equation can be modified to yield Ea by plotting ln[(I0/IT)-1] against 1/kBT. The activation energy is determined to be 0.31, 0.30 and 0.28 eV for the 3P03H4, 3P13H5 and 1D23H4 transitions from the optimal linear fitting to the experimental data, respectively, as illustrated in FIG. 8(b). Meanwhile, the CIE chromaticity coordinates are also influenced by the temperature. The emission color shifts to the blue region as the temperature increases, as shown in FIG. 8(a).

 FIG. 8 (a) Calculated color coordinates for the emissions of 0.5% Pr3+ doped LiLuF4 single crystal at 446 nm excitation at different temperature. (b) Activation energy plot at different sites of the corresponding to 0.5% Pr3+ doped LiLuF4 single crystal.
Ⅳ. CONCLUSION

In conclusion, Bridgman method in a completely closed environment is a favorable technical method to grow high quality Pr3+ doped LiLuF4 single crystals. The Pr3+ can be doped effectively in the crystal lattice sites of LiLuF4 single crystal inferred from the results of XRD and optical spectra. The emission spectra appears sharp emissions at 480, 522, and 605 nm when excited by 446 nm light. Increasing temperature leads to a reduction of the emission intensity and a blue-shift of the emission color for the synthesized material. The results indicate the Pr3+ doped LiLuF4 single crystal may be a potential candidate for optics applications such as UV laser and the ultraviolet C persistent luminescence.

Ⅴ. ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (No.51772159), the Natural Science Foundation of Zhejiang Province (No.LZ17E020001), and K. C. Wong Magna Fund in Ningbo University.

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Pr3+离子单掺LiLuF4单晶的坩埚下降法生长及光谱性能研究

a. 宁波大学光电子功能材料重点实验室，宁波 315211;
b. 大连海事大学物理系，大连 116026