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Hydrothermal synthesis and spectral properties of Ce3+ and Eu2+ ions doped KMgF3 phosphor
Optics & Laser Technology 81 (2016) 162–167
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Hydrothermal synthesis and spectral properties of Ce3 þ and Eu2 þ ions doped KMgF3 phosphor Guoxian Zhu a,b, Mubiao Xie a,n, Qu Yang b, Yingliang Liu b,nn a b
School of Chemistry and Chemical Engineering, Institute of Physical Chemistry, Lingnan Normal University, Zhanjiang 524048, PR China Department of Chemistry and Institute of Nanochemistry, Jinan University, Guangzhou 510632, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2015 Accepted 23 September 2015
Phase-pure Ce-/Eu-doped and co-doped KMgF3 phosphors are synthesized by hydrothermal techniques at 200 °C for 5 days. The crystal structure, particle size, morphologies and the energy band structure of the as-synthesized products are investigated by X-ray powder diffraction (XRD), environment scanning electron microscopy (ESEM) and X-ray photoelectron spectroscopy (XPS). The excitation and emission spectra of the rare earth ions doped KMgF3 are measured by the fluorescence spectrophotometer and the effects of Ce3 þ ions molar fraction on the luminescence of Eu2 þ ions are investigated. In the co-doped Eu2 þ and Ce3 þ system, the emission intensity of Ce3 þ ion gradually increases with the increasing Ce3 þ concentration, and the enhancement of Ce3 þ fluorescence is due to an efficient energy transfer from Eu2 þ to Ce3 þ in the host. In addition, the mechanism of energy transfer has been discussed in detail. These results suggest that the phosphors of KMgF3: Ce3 þ , Eu2 þ would become promising tunable laser materials. & 2016 Published by Elsevier Ltd.
Keywords: KMgF3 Rare earth Optical properties Hydrothermal synthesis
1. Introduction The application of complex metal fluorides as active media in the most efficient up-conversion luminescent materials [1], laser photosources [2] and neutron scintillation detectors [3] is well established. Among these complex fluorides, the best example is KMgF3 because of several advantages: better optical homogeneity, low melting, high thermal stability, isotropy, and high optic diaphaneity. It is considered that KMgF3, with a typical cubic perovskite, is a host of ideal optical function materials for searching for a new solid-state laser [4] and has great potential adhibitions in thermoluminescent dosimeter and window materials in the ultra-violet (UV) and vacuum-ultra-violet (VUV) wavelength region [5]. It is well known that pure and doped complex fluorides crystals can be prepared by conventional high-temperature solidstate reactions [6], Bridgman–Stockbarger method [7] and high temperature (4 400 °C), high-pressure (4 100 MPa) hydrothermal technique [8], Sol–Gel [9], mild hydrothermal and solvothermal process [10–12]. Among the various synthesis methods, the hydrothermal route is of considerable interest. The hydrothermal method, benefiting from a relatively low growth temperature and an approximate thermodynamic equilibrium growth condition, n
Corresponding author. Fax: þ86 0759 3183510. Corresponding author. Fax: þ 86 20 85221697. E-mail address: [email protected] (M. Xie).
nn
http://dx.doi.org/10.1016/j.optlastec.2015.09.027 0030-3992/& 2016 Published by Elsevier Ltd.
can fabricate the formation of phase-pure and -homogeneous materials crystals with the well-structured, the uniform size distribution and high crystallinity [13]. In this paper, phase-pure Ce-/Eu-doped and co-doped KMgF3 phosphors are prepared by a facile hydrothermal method and their luminescent properties of the as-synthesized particles are investigated. Energy transfer mechanism in the co-doped KMgF3 system is also analyzed. It is hoped to develop a novel feature of the rare-earth luminous or laser materials in a single matrix doped with different rare earth ions.
2. Experimental A series of complex fluorides KMgF3 doped Eu or/and Ce were prepared by hydrothermal method, which was carried out in a 20 mL Teflon-lined stainless steel autoclave under autogenous pressure, using K2CO3 (A.R.), MgCO3 (A.R.), NH4HF2 (A.R.), EuF3 (99.99%) and CeF3 (99.99%) as raw materials. Here we took the fabrication of KMgF3: 0.02 Ce (mol fraction) as an example to illustrate the process of synthesis. The molar ratios of initial mixtures were 1.0 K2CO3: 1.0 MgCO3: 0.02 CeF3. The typical synthesis procedure was as follows: 0.1382 g K2CO3 (0.01 mol), 0.0843 g MgCO3 (0.01 mol) and 0.0394 g CeF3 (0.0002 mol) were mixed and homogenized thoroughly (all the grinding was done with agate pestle and mortar) and the de-ionized water was added into the mixture with slow heating and stirring. After dissolved, the
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mixture was quickly transferred into a Teflon-lined stainless-steel autoclave of 20 mL capacity. Then 2.8525 g NH4HF2 (0.05 mol) was carefully added into the autoclave under stirring. The autoclave was filled with appropriate de-ionized water up to 70–75% of the total volume. Subsequently hydrofluoric acid (40 mass%, A.R.) was used to adjust the pH to 4–5. The autoclave was sealed into a stainless-steel tank and heated in an oven at 200 °C under autogenous pressure for 5 days. After being cooled to room temperature naturally, the resultant precipitate was centrifuged and washed with distilled water several times to pH ca. 7, and then dried in air at ambient temperature. Excess ions were removed during washing. For the synthesis of KMgF3: Eu and KMgF3: Ce, Eu, the mole ratios of initial mixtures were 1.0 K2CO3: 1.0 MgCO3: 0.02 EuF3 and 1.0 K2CO3: 1.0 MgCO3: 0.02 CeF3: 0.02 EuF3. The other operations were the same as the synthesis process of KMgF3: 0.02 Ce (mol fraction). All products were characterized by X-ray powder diffraction (XRD), using a Japan Rigaku D/max-IIB diffractometer with CuKα1 radiation (λ ¼0.1541 nm). The XRD data for index and cell-parameter calculations were collected by a scanning mode with a step of 0.02° in the 2θ range from 10° to 100° and a scanning rate of 4.0 °min 1 with silicon used as an internal standard. Particle-size and morphology were performed on a Hitachi S-570 environment scanning electron microscopy (ESEM). Gold was used to coat the particles as a means to reduce charging effects. X-ray photoelectron spectra (XPS) were recorded on a VG Scientific MAR-II X-ray photoelectron spectroscopy, using non-monochromated MgKα radiation as the excitation source. Binding energy values (Eb) were all referenced to carbon 1s line taken as 285.00 eV. Detecting vacuum was 1.33 101 Pa. The luminescence spectra were measured using a Hitachi F-4500 fluorescence spectrometer equipped with a monochromator (resolution: 0.2 nm) and 150 W Xe lamp as the excitation source. All measurements were carried out at room temperature.
3. Results and discussion
Fig. 2. ESEM images of KMgF3: Eu2 þ (a), KMgF3: Ce3 þ (b) and KMgF3: Eu2 þ , Ce3 þ (c).
3.1. Description of the structure Fig. 1shows the XRD pattern of the as-prepared KMgF3: 0.02 Ce, 0.02 Eu powder. All the peaks in Fig. 1 can be in good agreement with the standard JCPDS card (No. 18-1033). No other peaks or impurities are detected. Therefore, XRD confirms the sample obtained under mild hydrothermal conditions is in a pure cubic phase (space group: Pm3m [221]) of KMgF3. It can also be seen from the XRD patterns of Eu or Ce-doped KMgF3 at the dopant of 0.02 (mol fraction) that the crystal structures are still cubic, which is the same as that of KMgF3: 0.02 Ce, 0.02 Eu. The result shows that at the dopant concentration of 0.02 Eu or/and 0.02 Ce the
obtained products are free from impurities and the structure can not be changed by a low doping concentration of rare-earth ions. 3.2. Shape and size The morphology of the samples is examined by ESEM at room temperature. Fig. 2 shows the environment scanning electron micrograph (ESEM) images of the rare-earth ion-doped KMgF3. As can be seen from these figures, the powders are cubic with good shape, which can be obtained via a crystallization–dissolution– recrystallization–self-assembly growth process [14]. Under mild hydrothermal synthesis, the reactant ions can move freely in the solution and make contact with each other directionally and completely, indicating that the pure phase can be fabricated. Uniform grain texture of KMgF3 doped with rare-earth ions can be observed with grain sizes about 2.5 μm, 130 nm and 2.2 μm, respectively. 3.3. X-ray photoelectron spectra (XPS) of KMgF3: Eu2 þ
Fig. 1. XRD pattern of KMgF3: 0.02 Ce, 0.02 Eu.
It is well known that the content of oxygen in complex fluorides synthesized by hydrothermal method is lower than that of the corresponding complex fluorides synthesized by high temperature solid-state reaction [13,15]. In the hydrothermal synthesis system, OH is present. Because the OH ionic radius is similar to that of F ion, complex fluorides containing oxygen may be obtained.
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Fig. 4. The excitation (a: λem ¼ 287 or 300 nm) and emission spectra (b: λexc ¼260 nm) of KMgF3: 0.02 Ce3 þ powder. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Furthermore, the peak shape is in correspondence with that of MgF2 as reference. The result indicates that O–F bond has not been formed in KMgF3: 0.02 Eu2 þ . No distinct difference between Mg (2p), K (2p3/2) of KMgF3: 0.02 Eu2 þ and the references Mg (2p), K(2p3/2) is observed. Therefore, the typical Mg–F and K–F had been present for both obtained and standard. The result shows that the content oxygen of KMgF3: 0.02 Eu2 þ synthesized by hydrothermal method is very low. The binding energy of the elements in KMgF3: 0.02 Eu2 þ is listed in Table 1. Fig. 3. XPS spectra of KMgF3: 0.02 Eu2 þ .
3.4. Spectral properties of KMgF3: Ce
Table 1 The binding energy of the elements in KMgF3: 0.02 Eu2 þ (eV). Sample
F 1s
K 2p3/2
Mg 2p
KMgF3 KF MgF2
685.4
292.8 292.8
51.1
685.4
50.9
How about the oxygen content of the samples? It is considered that X-ray photoelectron spectroscopy (XPS) is widely used in studies of the energy band structure of solids and may be an effective method for detecting oxygen content. If the samples prepared by hydrothermal method contained oxygen, the electron binding energy (Eb) of the atom inner shell would change with the change of the chemical environment, thus the Eb would change too. The F (ls), Mg (2p) and K (2p3/2) XPS of KMgF3: 0.02Ce3 þ synthesized by hydrothermal method is shown in Fig. 3(a)–(c). It can be seen from Fig. 3(a) that the F (ls) binding energy is 685.4 ev, and the symmetrization of the peak shape is very good.
The spectroscopic characters of Ce3 þ ion greatly depend on the host crystal-field strength and the symmetry of the crystal site. The excitation (a) and emission (b) spectra for KMgF3: 0.02Ce3 þ phosphor are shown in Fig. 4 at room temperature. The excitation spectra of KMgF3: 0.02 Ce3 þ powder monitored at λem ¼287 nm is shown on the red line (A) of Fig. 4a. There is a broadband in a range of 200–275 nm with a maximum center located at 260 nm, which corresponds to the electronic transitions from the 4f ground state to the 5d excited state of Ce3 þ . In addition, the feature of the excitation spectrum remains unchanged when recorded at λexc ¼287 or 300 nm as labeled on the black line (B) of Fig. 4a except for the difference in intensity. The emission spectrum of KMgF3: 0.02 Ce3 þ sample, recorded at λexc ¼260 nm, is shown in Fig. 4(b). From Fig. 4(b), it can be clearly observed that the emission spectrum presents as a broad range of 265–350 nm with the maximums locating at 287 and 300 nm, and the former is dominant band, which may be attributed to the 4f°5d1-2F5 and 2F7 electric dipole-allowed transition of Ce3 þ , respectively. It is apparent that the double broad emission is not obvious.
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Fig. 6. The excitation spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ powders (λem ¼287 or 360 nm). (a. x ¼0.01; b. x ¼0.02; c. x¼ 0.04; d. x ¼0.06; e. x¼ 0.08; f. x ¼0.10).
Fig. 5. The excitation (a: λem ¼360 nm) and emission spectra (b: λexc ¼258 nm) of KMgF3: 0.02 Eu2 þ powder.
3.5. Spectral properties of KMgF3: Eu The divalent europium ion shows the narrow-band and broadband emissions due to their parity-forbidden f–f transitions (lying around 360 nm) and dipole-allowed f–d transitions. Fig. 5 describes the excitation (a) and emission (b) spectra of KMgF3: 0.02 Eu2 þ . The excitation spectrum obtained by monitoring the emission of the Eu2 þ 6P7/2-8S7/2 transition at 360 nm is shown in Fig. 5 (a). It can be seen that there is the broadband range of 200– 280 nm with the maximums locating at 257 nm, which is ascribed to the 4f7–4f65d transition of Eu2 þ . Upon excitation at the 4f8–4f75d transition at 257 nm, the emission spectrum of KMgF3: 0.02 Eu2 þ presents the characteristic line emission at 360 nm from the 6P7/2-8S7/2 transition of Eu2 þ ion as shown in Fig. 5(b). The sharp line located at 360 nm is arising from the substitution of K þ sites by Eu2 þ ions in the host lattice, which act partly as luminescent centers. Based on previous reference [7], the emission about 420 nm of KMgF3: Eu2 þ single crystal risen from the trace oxygen and color center not appears in KMgF3: Eu2 þ , which fully illustrates that the oxygen content of those products is quite low. 3.6. Optical spectroscopy properties of KMgF3: Ce, Eu The luminescence characteristics of KMgF3: x Ce3 þ , 0.02 Eu2 þ phosphors with different Ce-doping contents are explored in the completely same synthesizing conditions. Fig. 6 shows the excitation spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ with different Ce3 þ concentration. The excitation spectra, monitored by the 360 nm emission (6P7/2-8S7/2) of Eu2 þ and 287 nm emission (4f05d1-2F5) of Ce3 þ , are nearly identical to that of Ce3 þ singly doped KMgF3 phosphor as Fig. 4(a) except for intensity, indicating that this excitation is from 4f–5d transition of Ce3 þ exclusively. The emission spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ under the excitation of 260 nm are shown in Fig. 7. It can be seen clearly that there are the stronger emission broadband with the maximums locating at 287 and 300 nm of Ce3 þ ion, which obviously increase
Fig. 7. The emission spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ powders (λexc ¼ 260 nm).(a. x¼ 0.01; b. x¼ 0.02; c. x ¼0.04; d. x¼ 0.06; e. x¼ 0.08; f. x ¼0.10).
compared with that of the KMgF3: Ce3 þ in Fig. 4(b), and the weaker shoulder peak at 360 nm of Eu2 þ ion, which markedly decreases than that of the Eu2 þ singly doped KMgF3 phosphor as Fig. 5(b). As the concentration of Ce3 þ ion increases, the excitation and emission intensity gradually increases. Concentration quenching is not found in the scope of this experimental research. It is can be seen from Figs. 4(a) and 5(a) that the scope of the excitation spectra of KMgF3: Ce3 þ almost superimposes on the broad excitation band of KMgF3: Eu2 þ , with a maximum center located around 260 nm. From Figs. 4(b) and 5(b), it can be seen, under 257 nm excitation, the sharp line emission at 360 nm of Eu2 þ should overlap in the longer wavelength side of Ce3 þ emission band. That is to say in co-doped with Ce3 þ and Eu2 þ system of KMgF3, both emissions will have a “synergistic” effect. And from the luminous intensity of excitation band of Figs. 4 (a) and 5(a), it can be clearly seen that in the double-doped system, Ce3 þ ions have advantages in the process of Ce3 þ and Eu2 þ to the excitation energy of competing absorption. Therefore “coordination emission” in the co-doped system will lead to the emission intensity of Ce3 þ greater enhancement. The point of view can be proved by the emission spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ powders, which the strong broadband emission of Ce3 þ and the weaker line emission (360 nm) of Eu2 þ can be observed as shown in Fig. 7. It is good to explain that the energy can be transferred from Eu2 þ to Ce3 þ , as well as the emission intensity of Ce3 þ enhances dramatically, indicating that there is an effective energy transfer process from Eu2 þ to Ce3 þ , which is contrary to the experimental results through high-temperature solid-state reactions method according to previous literature [16]. In the codoped Eu2 þ and Ce3 þ system prepared by high-temperature solid-state reactions, the energy can be transferred from Ce3 þ to Eu2 þ , as well as the emission intensity of Eu2 þ enhances significantly, indicating that there is an effective energy transfer process from Ce3 þ to Eu2 þ .
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(A
(
C 4v
50
E
Eu
Mg 2
2+
+
40
(103cm-1)
4f5d 4f 65d
30
6P5/2 6P 7/2
20 Fig. 8. Crystal structure of KMgF3.
10 3.7. Energy transfer mechanism from Eu
2þ
to Ce
3þ
In Ce3 þ or Eu2 þ single doped KMgF3 system, Whether Ce3 þ or Eu ions substitute for the K þ ion sites, and the excess of positive charges in the lattice can be compensated by the K þ ion vacancies. In Ce3 þ and Eu2 þ co-doped KMgF3 system, the charge number and ionic radius of Eu2 þ (0.112 nm) are all closer to the charge number and ionic radius (0.133 nm) of K þ than those of Ce3 þ (0.103 nm). Moreover the charge number of Eu2 þ is low, and the charge compensation only needs a cation vacancy. Thus Eu2 þ ions will give priority to entering into the K þ site of octahedral space, and quickly reach saturation. For Ce3 þ , due to perturbation by Eu2 þ vacancy, the only substitutional site available for Ce3 þ is the C4v site (Point A shown in Fig. 8). This makes the substituted sites of Ce3 þ and Eu2 þ in the neatest adjacent position, resulting in the formation of pairs of close neighbor Ce3 þ and Eu2 þ ions in Ce–Eu co-doped KMgF3 [17], which makes process of mutual energy exchange between d–d levels more easily, and thus the Eu2 þ ion can transfer the energy to the Ce3 þ ion. That is to say there exists the energy transfer from Eu2 þ -Ce3 þ . In fact, whether phosphors of KMgF3: Eu, Ce are synthesized by high temperature solid state reaction, or by hydrothermal method, there are two kinds of energy transfer processes including Ce3 þ -Eu2 þ and Eu2 þ -Ce3 þ . The former, because of the emission spectrum of cerium more overlapping with the excitation spectrum of europium, conforms to the condition of Dexter’s energy transfer theory [18], so that Ce3 þ passes the energy to Eu2 þ , so the energy transfer process of Ce3 þ -Eu2 þ is dominant. As for the latter, the energy transfer process of Eu2 þ -Ce3 þ is preponderant, because there are pairs of close neighbor Ce3 þ ―Eu2 þ ions in co-doped system, the oscillator strength of Eu2 þ is high, and the absorption and emission intensity of Eu2 þ is better than that of Ce3 þ , meanwhile the emission spectrum of cerium overlaps very slightly with the excitation spectrum of europium, which the stronger d–d mutual interaction among ions will make energy transfer from Eu2 þ to Ce3 þ [19]. Therefore, in the Ce and Eu doubly doped system of KMgF3 synthesized by hydrothermal method, the energy transfer process of Eu2 þ -Ce3 þ is caused by the d–d mutual interaction between the Ce and Eu ions. Fig. 9 shows the schematic energy levels of Eu2 þ and Ce3 þ in KMgF3 according to the absorption spectra. The 6PJ energy level of Eu2 þ is lower than the 4f5d energy level of Ce3 þ as can be seen from Fig. 9, after Eu2 þ is excited to the 6P5/2 state, some activation energy must be absorbed and the particle must be reversed to the 4f5d state, the energy can be delivered to the 4f5d state of Ce3 þ . By calculation, the activation energy (i.e. difference between the
8S7/2
2þ
Eu 2+
2F 7/2 2F 5/2
Ce 3+
Fig. 9. Schematic energy levels of Eu2 þ and Ce3 þ in KMgF3 and the processes of energy transfer and luminescence decay.
lowest 4f5d level of Ce3 þ and 6P5/2 energy level of Eu2 þ ) is very small, about 1900 cm 1. Thus the system only needs absorb a certain amount of heat from the environment, resulting in the thermal population can be inversed from 6P5/2 level of Eu2 þ to 4f5d level of Ce3 þ , which the energy transfer must occur from Eu2 þ to Ce3 þ in the co-doped system. The energy transfer mechanism can be interpreted as arising from thermal population of the Eu2 þ 4f65d states following excitation into the 6P5/2 manifold upon 260 nm laser excitation, following by energy transfer from the 6P5/2 state of Eu2 þ to the 4f5d state of Ce3 þ via the d–d interaction. Therefore, this energy transfer mechanism undoubtedly will lead to developing a tunable short-wave solid state laser.
4. Conclusions In summary, a convenient method for the synthesis of KMgF3 doped with Ce or/and Eu by hydrothermal proceeding at 200 °C for 5 days was presented. The characterization of XRD and ESEM of samples revealed that the as-synthesized products were pure cubic phase with the even size distribution. X-ray photoelectron spectroscopy (XPS) analysis indicated the oxygen content of the product was very low. The excitation and emission spectra of the KMgF3 doped with rare-earth ions were discussed based on fluorescence spectrum. The excitation and emission spectra appeared as a broad range in the KMgF3: Ce3 þ . In the KMgF3: Eu2 þ sample, there was only one sharp line emission located at 360 nm arising from 6P7/2-8S7/2 transition of Eu2 þ in the host lattice and the broad bands appearing at 420 nm arising from Eu2 þ ’O could not be observed. The results showed that the oxygen content was low. In Ce3 þ and Eu2 þ co-doped system of KMgF3, the excitation band of the Ce3 þ ion and the strong emission peak of the Ce3 þ ion could be observed. The emission intensity of Ce3 þ ion increases was due to an efficient energy transfer from Eu2 þ to Ce3 þ in the matrix. In addition, energy transfer way of co-doped system has also been analyzed at length. It was expected that the phosphor of KMgF3: Ce3 þ , Eu2 þ could be utilized as a tunable laser material.
G. Zhu et al. / Optics & Laser Technology 81 (2016) 162–167
Acknowledgments This present work was financially supported by the National Natural Science Foundation of China (Grant nos. U0734005, 20906037 and 21401165), the Fundamental Research Funds for the Central Universities (No. 21610102), and Natural Science Foundation of Guangdong Province (Grant no. 2014A030307040), and the Doctor Special Foundation of Zhanjiang Normal University (No. ZL1104).
References [1] K.W. Krämer, D. Biner, G. Frei, H.U. Güdel, M.P. Hehlen, S.R. Lüthi, Hexagonal sodium yttrium fluoride based green and blue emitting upconversion phosphors, Chem. Mater. 16 (2004) 1244–1249. [2] J.H. Yan, Z.T. Li, L.K. Wang, L.J. Cao, S.Q. Wang, C.S. Shi, Preparation and characterization of BaLiF3: Er3 þ nanoparticles by the hydrothermal microemulsion synthesized method, J. Rare Earths 26 (2008) 48–50. [3] C. Dujardin, C. Pedrini, N. Garnier, A.N. Belsky, K. Lebbou, J.M. Ko, T. Fukuda, Spectroscopic properties of CeF3 and LuF3: Ce3 þ thin films grown by molecular beam epitaxy, Opt. Mater. 16 (2001) 69–74. [4] P. Hagenmuller, Inorganic Solid Fluoride Chemistry and Physics, Academy Press Inc., New York (1985), p. 477–483. [5] H. Sato, A. Bensalah, N. Solovieva, A. Beitlerova, A. Vedda, M. Martini, M. Nikl, T. Fukuda, X-ray damage characterization in BaLiF3, KMgF3 and LiCaAlF6 complex fluorides, Radiat. Meas. 38 (2004) 463–466. [6] Y. Tan, C. Shi, Optical spectroscopy properties and charge compensation of BaLiF3 doped with Ce3 þ , J. Solid State Chem. 150 (2000) 178–182. [7] H.Q. Su, Z.H. Jia, C.S. Shi, Study of color centers and trace-oxygen in KMgF3: Eu
167
single crystal, Chem. Mater. 14 (2002) 310–312. [8] S. Somiya, S. Hirano, M. Yoshimura, K. Yanagisawa, Hydrothermal crystal growth of perovskite-type fluorides, J. Mater. Sci. 16 (1981) 813–816. [9] B. Damien, M. Rachid, Powders and coatings of LiYF4: Eu3 þ obtained via an original way based on the Sol Gel process, Chem. Mater. 16 (2004) 2518–2521. [10] R.N. Hua, B.F. Lei, D.M. Xie, C.S. Shi, Synthesis of the complex fluoride LiBaF3 and optical spectroscopy properties of LiBaF3: M (M ¼Eu,Ce) through a solvothermal process, J. Solid State Chem. 175 (2003) 284–288. [11] G.X. Zhu, Q. Yang, X.B. Shi, W.Z. Zheng, Y.L. Liu, Solvothermal fabrication and luminescent properties of Eu2 þ /Ce3 þ doped barium lithium fluoride, J. Rare Earth 30 (2012) 985–989. [12] G.X. Zhu, Q. Yang, X.B. Shi, W.Z. Zheng, Y.L. Liu, Hydrothermal synthesis and optical properties of Tb/Ce doped barium lithium fluoride, Opt. Mater. 35 (2013) 1309–1314. [13] X.T. Liu, Y.L. Zhang, C.S. Shi, D.M. Xie, Hydrothermal synthesis and luminescent properties of BaBeF4: RE (RE ¼Eu, Tb), J. Solid State Chem. 178 (2005) 2167–2174. [14] Z.L. Fu, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, La2Sn2O7: Eu3 þ micronanospheres: hydrothermal synthesis and luminescent properties, Cryst. Growth Des. 9 (2009) 616–621. [15] C. Zhao, S. Feng, Z. Chao, C. Shi, R. Xu, J. Ni, Hydrothermal synthesis of the complex fluorides LiBaF3 and KMgF3 with perovskite structures under mild conditions, Chem. Commun. (1996) 1641–1642. [16] G.X. Zhu, J.H. Yan, F.S. Mo, C.S. Shi, Synthesis and fluorescence properties of rare-earth doped complex fluoride nanoparticles, Chem. J. Chin. Univ. 27 (2006) 401–405. [17] U.G. Caldino, C. Gruz, H.G. Munoz, O. Rubio, Ce3 þ -Eu2 þ energy transfer in CaF2, Solid State Commun. 69 (1989) 347–351. [18] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 837–850. [19] H.Q. Su, Z.H. Jia, C.S. Shi, X. Ju, Study of transfer in KMgF3: Eu-X(X ¼Gd, Cr, Ce) by the decay model of 6P7/2 exited state of Eu2 þ , Chem. Phys. Lett. 335 (2001) 17–22.
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Full length article
Hydrothermal synthesis and spectral properties of Ce3 þ and Eu2 þ ions doped KMgF3 phosphor Guoxian Zhu a,b, Mubiao Xie a,n, Qu Yang b, Yingliang Liu b,nn a b
School of Chemistry and Chemical Engineering, Institute of Physical Chemistry, Lingnan Normal University, Zhanjiang 524048, PR China Department of Chemistry and Institute of Nanochemistry, Jinan University, Guangzhou 510632, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2015 Accepted 23 September 2015
Phase-pure Ce-/Eu-doped and co-doped KMgF3 phosphors are synthesized by hydrothermal techniques at 200 °C for 5 days. The crystal structure, particle size, morphologies and the energy band structure of the as-synthesized products are investigated by X-ray powder diffraction (XRD), environment scanning electron microscopy (ESEM) and X-ray photoelectron spectroscopy (XPS). The excitation and emission spectra of the rare earth ions doped KMgF3 are measured by the fluorescence spectrophotometer and the effects of Ce3 þ ions molar fraction on the luminescence of Eu2 þ ions are investigated. In the co-doped Eu2 þ and Ce3 þ system, the emission intensity of Ce3 þ ion gradually increases with the increasing Ce3 þ concentration, and the enhancement of Ce3 þ fluorescence is due to an efficient energy transfer from Eu2 þ to Ce3 þ in the host. In addition, the mechanism of energy transfer has been discussed in detail. These results suggest that the phosphors of KMgF3: Ce3 þ , Eu2 þ would become promising tunable laser materials. & 2016 Published by Elsevier Ltd.
Keywords: KMgF3 Rare earth Optical properties Hydrothermal synthesis
1. Introduction The application of complex metal fluorides as active media in the most efficient up-conversion luminescent materials [1], laser photosources [2] and neutron scintillation detectors [3] is well established. Among these complex fluorides, the best example is KMgF3 because of several advantages: better optical homogeneity, low melting, high thermal stability, isotropy, and high optic diaphaneity. It is considered that KMgF3, with a typical cubic perovskite, is a host of ideal optical function materials for searching for a new solid-state laser [4] and has great potential adhibitions in thermoluminescent dosimeter and window materials in the ultra-violet (UV) and vacuum-ultra-violet (VUV) wavelength region [5]. It is well known that pure and doped complex fluorides crystals can be prepared by conventional high-temperature solidstate reactions [6], Bridgman–Stockbarger method [7] and high temperature (4 400 °C), high-pressure (4 100 MPa) hydrothermal technique [8], Sol–Gel [9], mild hydrothermal and solvothermal process [10–12]. Among the various synthesis methods, the hydrothermal route is of considerable interest. The hydrothermal method, benefiting from a relatively low growth temperature and an approximate thermodynamic equilibrium growth condition, n
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http://dx.doi.org/10.1016/j.optlastec.2015.09.027 0030-3992/& 2016 Published by Elsevier Ltd.
can fabricate the formation of phase-pure and -homogeneous materials crystals with the well-structured, the uniform size distribution and high crystallinity [13]. In this paper, phase-pure Ce-/Eu-doped and co-doped KMgF3 phosphors are prepared by a facile hydrothermal method and their luminescent properties of the as-synthesized particles are investigated. Energy transfer mechanism in the co-doped KMgF3 system is also analyzed. It is hoped to develop a novel feature of the rare-earth luminous or laser materials in a single matrix doped with different rare earth ions.
2. Experimental A series of complex fluorides KMgF3 doped Eu or/and Ce were prepared by hydrothermal method, which was carried out in a 20 mL Teflon-lined stainless steel autoclave under autogenous pressure, using K2CO3 (A.R.), MgCO3 (A.R.), NH4HF2 (A.R.), EuF3 (99.99%) and CeF3 (99.99%) as raw materials. Here we took the fabrication of KMgF3: 0.02 Ce (mol fraction) as an example to illustrate the process of synthesis. The molar ratios of initial mixtures were 1.0 K2CO3: 1.0 MgCO3: 0.02 CeF3. The typical synthesis procedure was as follows: 0.1382 g K2CO3 (0.01 mol), 0.0843 g MgCO3 (0.01 mol) and 0.0394 g CeF3 (0.0002 mol) were mixed and homogenized thoroughly (all the grinding was done with agate pestle and mortar) and the de-ionized water was added into the mixture with slow heating and stirring. After dissolved, the
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mixture was quickly transferred into a Teflon-lined stainless-steel autoclave of 20 mL capacity. Then 2.8525 g NH4HF2 (0.05 mol) was carefully added into the autoclave under stirring. The autoclave was filled with appropriate de-ionized water up to 70–75% of the total volume. Subsequently hydrofluoric acid (40 mass%, A.R.) was used to adjust the pH to 4–5. The autoclave was sealed into a stainless-steel tank and heated in an oven at 200 °C under autogenous pressure for 5 days. After being cooled to room temperature naturally, the resultant precipitate was centrifuged and washed with distilled water several times to pH ca. 7, and then dried in air at ambient temperature. Excess ions were removed during washing. For the synthesis of KMgF3: Eu and KMgF3: Ce, Eu, the mole ratios of initial mixtures were 1.0 K2CO3: 1.0 MgCO3: 0.02 EuF3 and 1.0 K2CO3: 1.0 MgCO3: 0.02 CeF3: 0.02 EuF3. The other operations were the same as the synthesis process of KMgF3: 0.02 Ce (mol fraction). All products were characterized by X-ray powder diffraction (XRD), using a Japan Rigaku D/max-IIB diffractometer with CuKα1 radiation (λ ¼0.1541 nm). The XRD data for index and cell-parameter calculations were collected by a scanning mode with a step of 0.02° in the 2θ range from 10° to 100° and a scanning rate of 4.0 °min 1 with silicon used as an internal standard. Particle-size and morphology were performed on a Hitachi S-570 environment scanning electron microscopy (ESEM). Gold was used to coat the particles as a means to reduce charging effects. X-ray photoelectron spectra (XPS) were recorded on a VG Scientific MAR-II X-ray photoelectron spectroscopy, using non-monochromated MgKα radiation as the excitation source. Binding energy values (Eb) were all referenced to carbon 1s line taken as 285.00 eV. Detecting vacuum was 1.33 101 Pa. The luminescence spectra were measured using a Hitachi F-4500 fluorescence spectrometer equipped with a monochromator (resolution: 0.2 nm) and 150 W Xe lamp as the excitation source. All measurements were carried out at room temperature.
3. Results and discussion
Fig. 2. ESEM images of KMgF3: Eu2 þ (a), KMgF3: Ce3 þ (b) and KMgF3: Eu2 þ , Ce3 þ (c).
3.1. Description of the structure Fig. 1shows the XRD pattern of the as-prepared KMgF3: 0.02 Ce, 0.02 Eu powder. All the peaks in Fig. 1 can be in good agreement with the standard JCPDS card (No. 18-1033). No other peaks or impurities are detected. Therefore, XRD confirms the sample obtained under mild hydrothermal conditions is in a pure cubic phase (space group: Pm3m [221]) of KMgF3. It can also be seen from the XRD patterns of Eu or Ce-doped KMgF3 at the dopant of 0.02 (mol fraction) that the crystal structures are still cubic, which is the same as that of KMgF3: 0.02 Ce, 0.02 Eu. The result shows that at the dopant concentration of 0.02 Eu or/and 0.02 Ce the
obtained products are free from impurities and the structure can not be changed by a low doping concentration of rare-earth ions. 3.2. Shape and size The morphology of the samples is examined by ESEM at room temperature. Fig. 2 shows the environment scanning electron micrograph (ESEM) images of the rare-earth ion-doped KMgF3. As can be seen from these figures, the powders are cubic with good shape, which can be obtained via a crystallization–dissolution– recrystallization–self-assembly growth process [14]. Under mild hydrothermal synthesis, the reactant ions can move freely in the solution and make contact with each other directionally and completely, indicating that the pure phase can be fabricated. Uniform grain texture of KMgF3 doped with rare-earth ions can be observed with grain sizes about 2.5 μm, 130 nm and 2.2 μm, respectively. 3.3. X-ray photoelectron spectra (XPS) of KMgF3: Eu2 þ
Fig. 1. XRD pattern of KMgF3: 0.02 Ce, 0.02 Eu.
It is well known that the content of oxygen in complex fluorides synthesized by hydrothermal method is lower than that of the corresponding complex fluorides synthesized by high temperature solid-state reaction [13,15]. In the hydrothermal synthesis system, OH is present. Because the OH ionic radius is similar to that of F ion, complex fluorides containing oxygen may be obtained.
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Fig. 4. The excitation (a: λem ¼ 287 or 300 nm) and emission spectra (b: λexc ¼260 nm) of KMgF3: 0.02 Ce3 þ powder. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Furthermore, the peak shape is in correspondence with that of MgF2 as reference. The result indicates that O–F bond has not been formed in KMgF3: 0.02 Eu2 þ . No distinct difference between Mg (2p), K (2p3/2) of KMgF3: 0.02 Eu2 þ and the references Mg (2p), K(2p3/2) is observed. Therefore, the typical Mg–F and K–F had been present for both obtained and standard. The result shows that the content oxygen of KMgF3: 0.02 Eu2 þ synthesized by hydrothermal method is very low. The binding energy of the elements in KMgF3: 0.02 Eu2 þ is listed in Table 1. Fig. 3. XPS spectra of KMgF3: 0.02 Eu2 þ .
3.4. Spectral properties of KMgF3: Ce
Table 1 The binding energy of the elements in KMgF3: 0.02 Eu2 þ (eV). Sample
F 1s
K 2p3/2
Mg 2p
KMgF3 KF MgF2
685.4
292.8 292.8
51.1
685.4
50.9
How about the oxygen content of the samples? It is considered that X-ray photoelectron spectroscopy (XPS) is widely used in studies of the energy band structure of solids and may be an effective method for detecting oxygen content. If the samples prepared by hydrothermal method contained oxygen, the electron binding energy (Eb) of the atom inner shell would change with the change of the chemical environment, thus the Eb would change too. The F (ls), Mg (2p) and K (2p3/2) XPS of KMgF3: 0.02Ce3 þ synthesized by hydrothermal method is shown in Fig. 3(a)–(c). It can be seen from Fig. 3(a) that the F (ls) binding energy is 685.4 ev, and the symmetrization of the peak shape is very good.
The spectroscopic characters of Ce3 þ ion greatly depend on the host crystal-field strength and the symmetry of the crystal site. The excitation (a) and emission (b) spectra for KMgF3: 0.02Ce3 þ phosphor are shown in Fig. 4 at room temperature. The excitation spectra of KMgF3: 0.02 Ce3 þ powder monitored at λem ¼287 nm is shown on the red line (A) of Fig. 4a. There is a broadband in a range of 200–275 nm with a maximum center located at 260 nm, which corresponds to the electronic transitions from the 4f ground state to the 5d excited state of Ce3 þ . In addition, the feature of the excitation spectrum remains unchanged when recorded at λexc ¼287 or 300 nm as labeled on the black line (B) of Fig. 4a except for the difference in intensity. The emission spectrum of KMgF3: 0.02 Ce3 þ sample, recorded at λexc ¼260 nm, is shown in Fig. 4(b). From Fig. 4(b), it can be clearly observed that the emission spectrum presents as a broad range of 265–350 nm with the maximums locating at 287 and 300 nm, and the former is dominant band, which may be attributed to the 4f°5d1-2F5 and 2F7 electric dipole-allowed transition of Ce3 þ , respectively. It is apparent that the double broad emission is not obvious.
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Fig. 6. The excitation spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ powders (λem ¼287 or 360 nm). (a. x ¼0.01; b. x ¼0.02; c. x¼ 0.04; d. x ¼0.06; e. x¼ 0.08; f. x ¼0.10).
Fig. 5. The excitation (a: λem ¼360 nm) and emission spectra (b: λexc ¼258 nm) of KMgF3: 0.02 Eu2 þ powder.
3.5. Spectral properties of KMgF3: Eu The divalent europium ion shows the narrow-band and broadband emissions due to their parity-forbidden f–f transitions (lying around 360 nm) and dipole-allowed f–d transitions. Fig. 5 describes the excitation (a) and emission (b) spectra of KMgF3: 0.02 Eu2 þ . The excitation spectrum obtained by monitoring the emission of the Eu2 þ 6P7/2-8S7/2 transition at 360 nm is shown in Fig. 5 (a). It can be seen that there is the broadband range of 200– 280 nm with the maximums locating at 257 nm, which is ascribed to the 4f7–4f65d transition of Eu2 þ . Upon excitation at the 4f8–4f75d transition at 257 nm, the emission spectrum of KMgF3: 0.02 Eu2 þ presents the characteristic line emission at 360 nm from the 6P7/2-8S7/2 transition of Eu2 þ ion as shown in Fig. 5(b). The sharp line located at 360 nm is arising from the substitution of K þ sites by Eu2 þ ions in the host lattice, which act partly as luminescent centers. Based on previous reference [7], the emission about 420 nm of KMgF3: Eu2 þ single crystal risen from the trace oxygen and color center not appears in KMgF3: Eu2 þ , which fully illustrates that the oxygen content of those products is quite low. 3.6. Optical spectroscopy properties of KMgF3: Ce, Eu The luminescence characteristics of KMgF3: x Ce3 þ , 0.02 Eu2 þ phosphors with different Ce-doping contents are explored in the completely same synthesizing conditions. Fig. 6 shows the excitation spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ with different Ce3 þ concentration. The excitation spectra, monitored by the 360 nm emission (6P7/2-8S7/2) of Eu2 þ and 287 nm emission (4f05d1-2F5) of Ce3 þ , are nearly identical to that of Ce3 þ singly doped KMgF3 phosphor as Fig. 4(a) except for intensity, indicating that this excitation is from 4f–5d transition of Ce3 þ exclusively. The emission spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ under the excitation of 260 nm are shown in Fig. 7. It can be seen clearly that there are the stronger emission broadband with the maximums locating at 287 and 300 nm of Ce3 þ ion, which obviously increase
Fig. 7. The emission spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ powders (λexc ¼ 260 nm).(a. x¼ 0.01; b. x¼ 0.02; c. x ¼0.04; d. x¼ 0.06; e. x¼ 0.08; f. x ¼0.10).
compared with that of the KMgF3: Ce3 þ in Fig. 4(b), and the weaker shoulder peak at 360 nm of Eu2 þ ion, which markedly decreases than that of the Eu2 þ singly doped KMgF3 phosphor as Fig. 5(b). As the concentration of Ce3 þ ion increases, the excitation and emission intensity gradually increases. Concentration quenching is not found in the scope of this experimental research. It is can be seen from Figs. 4(a) and 5(a) that the scope of the excitation spectra of KMgF3: Ce3 þ almost superimposes on the broad excitation band of KMgF3: Eu2 þ , with a maximum center located around 260 nm. From Figs. 4(b) and 5(b), it can be seen, under 257 nm excitation, the sharp line emission at 360 nm of Eu2 þ should overlap in the longer wavelength side of Ce3 þ emission band. That is to say in co-doped with Ce3 þ and Eu2 þ system of KMgF3, both emissions will have a “synergistic” effect. And from the luminous intensity of excitation band of Figs. 4 (a) and 5(a), it can be clearly seen that in the double-doped system, Ce3 þ ions have advantages in the process of Ce3 þ and Eu2 þ to the excitation energy of competing absorption. Therefore “coordination emission” in the co-doped system will lead to the emission intensity of Ce3 þ greater enhancement. The point of view can be proved by the emission spectra of KMgF3: x Ce3 þ , 0.02 Eu2 þ powders, which the strong broadband emission of Ce3 þ and the weaker line emission (360 nm) of Eu2 þ can be observed as shown in Fig. 7. It is good to explain that the energy can be transferred from Eu2 þ to Ce3 þ , as well as the emission intensity of Ce3 þ enhances dramatically, indicating that there is an effective energy transfer process from Eu2 þ to Ce3 þ , which is contrary to the experimental results through high-temperature solid-state reactions method according to previous literature [16]. In the codoped Eu2 þ and Ce3 þ system prepared by high-temperature solid-state reactions, the energy can be transferred from Ce3 þ to Eu2 þ , as well as the emission intensity of Eu2 þ enhances significantly, indicating that there is an effective energy transfer process from Ce3 þ to Eu2 þ .
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(A
(
C 4v
50
E
Eu
Mg 2
2+
+
40
(103cm-1)
4f5d 4f 65d
30
6P5/2 6P 7/2
20 Fig. 8. Crystal structure of KMgF3.
10 3.7. Energy transfer mechanism from Eu
2þ
to Ce
3þ
In Ce3 þ or Eu2 þ single doped KMgF3 system, Whether Ce3 þ or Eu ions substitute for the K þ ion sites, and the excess of positive charges in the lattice can be compensated by the K þ ion vacancies. In Ce3 þ and Eu2 þ co-doped KMgF3 system, the charge number and ionic radius of Eu2 þ (0.112 nm) are all closer to the charge number and ionic radius (0.133 nm) of K þ than those of Ce3 þ (0.103 nm). Moreover the charge number of Eu2 þ is low, and the charge compensation only needs a cation vacancy. Thus Eu2 þ ions will give priority to entering into the K þ site of octahedral space, and quickly reach saturation. For Ce3 þ , due to perturbation by Eu2 þ vacancy, the only substitutional site available for Ce3 þ is the C4v site (Point A shown in Fig. 8). This makes the substituted sites of Ce3 þ and Eu2 þ in the neatest adjacent position, resulting in the formation of pairs of close neighbor Ce3 þ and Eu2 þ ions in Ce–Eu co-doped KMgF3 [17], which makes process of mutual energy exchange between d–d levels more easily, and thus the Eu2 þ ion can transfer the energy to the Ce3 þ ion. That is to say there exists the energy transfer from Eu2 þ -Ce3 þ . In fact, whether phosphors of KMgF3: Eu, Ce are synthesized by high temperature solid state reaction, or by hydrothermal method, there are two kinds of energy transfer processes including Ce3 þ -Eu2 þ and Eu2 þ -Ce3 þ . The former, because of the emission spectrum of cerium more overlapping with the excitation spectrum of europium, conforms to the condition of Dexter’s energy transfer theory [18], so that Ce3 þ passes the energy to Eu2 þ , so the energy transfer process of Ce3 þ -Eu2 þ is dominant. As for the latter, the energy transfer process of Eu2 þ -Ce3 þ is preponderant, because there are pairs of close neighbor Ce3 þ ―Eu2 þ ions in co-doped system, the oscillator strength of Eu2 þ is high, and the absorption and emission intensity of Eu2 þ is better than that of Ce3 þ , meanwhile the emission spectrum of cerium overlaps very slightly with the excitation spectrum of europium, which the stronger d–d mutual interaction among ions will make energy transfer from Eu2 þ to Ce3 þ [19]. Therefore, in the Ce and Eu doubly doped system of KMgF3 synthesized by hydrothermal method, the energy transfer process of Eu2 þ -Ce3 þ is caused by the d–d mutual interaction between the Ce and Eu ions. Fig. 9 shows the schematic energy levels of Eu2 þ and Ce3 þ in KMgF3 according to the absorption spectra. The 6PJ energy level of Eu2 þ is lower than the 4f5d energy level of Ce3 þ as can be seen from Fig. 9, after Eu2 þ is excited to the 6P5/2 state, some activation energy must be absorbed and the particle must be reversed to the 4f5d state, the energy can be delivered to the 4f5d state of Ce3 þ . By calculation, the activation energy (i.e. difference between the
8S7/2
2þ
Eu 2+
2F 7/2 2F 5/2
Ce 3+
Fig. 9. Schematic energy levels of Eu2 þ and Ce3 þ in KMgF3 and the processes of energy transfer and luminescence decay.
lowest 4f5d level of Ce3 þ and 6P5/2 energy level of Eu2 þ ) is very small, about 1900 cm 1. Thus the system only needs absorb a certain amount of heat from the environment, resulting in the thermal population can be inversed from 6P5/2 level of Eu2 þ to 4f5d level of Ce3 þ , which the energy transfer must occur from Eu2 þ to Ce3 þ in the co-doped system. The energy transfer mechanism can be interpreted as arising from thermal population of the Eu2 þ 4f65d states following excitation into the 6P5/2 manifold upon 260 nm laser excitation, following by energy transfer from the 6P5/2 state of Eu2 þ to the 4f5d state of Ce3 þ via the d–d interaction. Therefore, this energy transfer mechanism undoubtedly will lead to developing a tunable short-wave solid state laser.
4. Conclusions In summary, a convenient method for the synthesis of KMgF3 doped with Ce or/and Eu by hydrothermal proceeding at 200 °C for 5 days was presented. The characterization of XRD and ESEM of samples revealed that the as-synthesized products were pure cubic phase with the even size distribution. X-ray photoelectron spectroscopy (XPS) analysis indicated the oxygen content of the product was very low. The excitation and emission spectra of the KMgF3 doped with rare-earth ions were discussed based on fluorescence spectrum. The excitation and emission spectra appeared as a broad range in the KMgF3: Ce3 þ . In the KMgF3: Eu2 þ sample, there was only one sharp line emission located at 360 nm arising from 6P7/2-8S7/2 transition of Eu2 þ in the host lattice and the broad bands appearing at 420 nm arising from Eu2 þ ’O could not be observed. The results showed that the oxygen content was low. In Ce3 þ and Eu2 þ co-doped system of KMgF3, the excitation band of the Ce3 þ ion and the strong emission peak of the Ce3 þ ion could be observed. The emission intensity of Ce3 þ ion increases was due to an efficient energy transfer from Eu2 þ to Ce3 þ in the matrix. In addition, energy transfer way of co-doped system has also been analyzed at length. It was expected that the phosphor of KMgF3: Ce3 þ , Eu2 þ could be utilized as a tunable laser material.
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Acknowledgments This present work was financially supported by the National Natural Science Foundation of China (Grant nos. U0734005, 20906037 and 21401165), the Fundamental Research Funds for the Central Universities (No. 21610102), and Natural Science Foundation of Guangdong Province (Grant no. 2014A030307040), and the Doctor Special Foundation of Zhanjiang Normal University (No. ZL1104).
References [1] K.W. Krämer, D. Biner, G. Frei, H.U. Güdel, M.P. Hehlen, S.R. Lüthi, Hexagonal sodium yttrium fluoride based green and blue emitting upconversion phosphors, Chem. Mater. 16 (2004) 1244–1249. [2] J.H. Yan, Z.T. Li, L.K. Wang, L.J. Cao, S.Q. Wang, C.S. Shi, Preparation and characterization of BaLiF3: Er3 þ nanoparticles by the hydrothermal microemulsion synthesized method, J. Rare Earths 26 (2008) 48–50. [3] C. Dujardin, C. Pedrini, N. Garnier, A.N. Belsky, K. Lebbou, J.M. Ko, T. Fukuda, Spectroscopic properties of CeF3 and LuF3: Ce3 þ thin films grown by molecular beam epitaxy, Opt. Mater. 16 (2001) 69–74. [4] P. Hagenmuller, Inorganic Solid Fluoride Chemistry and Physics, Academy Press Inc., New York (1985), p. 477–483. [5] H. Sato, A. Bensalah, N. Solovieva, A. Beitlerova, A. Vedda, M. Martini, M. Nikl, T. Fukuda, X-ray damage characterization in BaLiF3, KMgF3 and LiCaAlF6 complex fluorides, Radiat. Meas. 38 (2004) 463–466. [6] Y. Tan, C. Shi, Optical spectroscopy properties and charge compensation of BaLiF3 doped with Ce3 þ , J. Solid State Chem. 150 (2000) 178–182. [7] H.Q. Su, Z.H. Jia, C.S. Shi, Study of color centers and trace-oxygen in KMgF3: Eu
167
single crystal, Chem. Mater. 14 (2002) 310–312. [8] S. Somiya, S. Hirano, M. Yoshimura, K. Yanagisawa, Hydrothermal crystal growth of perovskite-type fluorides, J. Mater. Sci. 16 (1981) 813–816. [9] B. Damien, M. Rachid, Powders and coatings of LiYF4: Eu3 þ obtained via an original way based on the Sol Gel process, Chem. Mater. 16 (2004) 2518–2521. [10] R.N. Hua, B.F. Lei, D.M. Xie, C.S. Shi, Synthesis of the complex fluoride LiBaF3 and optical spectroscopy properties of LiBaF3: M (M ¼Eu,Ce) through a solvothermal process, J. Solid State Chem. 175 (2003) 284–288. [11] G.X. Zhu, Q. Yang, X.B. Shi, W.Z. Zheng, Y.L. Liu, Solvothermal fabrication and luminescent properties of Eu2 þ /Ce3 þ doped barium lithium fluoride, J. Rare Earth 30 (2012) 985–989. [12] G.X. Zhu, Q. Yang, X.B. Shi, W.Z. Zheng, Y.L. Liu, Hydrothermal synthesis and optical properties of Tb/Ce doped barium lithium fluoride, Opt. Mater. 35 (2013) 1309–1314. [13] X.T. Liu, Y.L. Zhang, C.S. Shi, D.M. Xie, Hydrothermal synthesis and luminescent properties of BaBeF4: RE (RE ¼Eu, Tb), J. Solid State Chem. 178 (2005) 2167–2174. [14] Z.L. Fu, H.K. Yang, B.K. Moon, B.C. Choi, J.H. Jeong, La2Sn2O7: Eu3 þ micronanospheres: hydrothermal synthesis and luminescent properties, Cryst. Growth Des. 9 (2009) 616–621. [15] C. Zhao, S. Feng, Z. Chao, C. Shi, R. Xu, J. Ni, Hydrothermal synthesis of the complex fluorides LiBaF3 and KMgF3 with perovskite structures under mild conditions, Chem. Commun. (1996) 1641–1642. [16] G.X. Zhu, J.H. Yan, F.S. Mo, C.S. Shi, Synthesis and fluorescence properties of rare-earth doped complex fluoride nanoparticles, Chem. J. Chin. Univ. 27 (2006) 401–405. [17] U.G. Caldino, C. Gruz, H.G. Munoz, O. Rubio, Ce3 þ -Eu2 þ energy transfer in CaF2, Solid State Commun. 69 (1989) 347–351. [18] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 837–850. [19] H.Q. Su, Z.H. Jia, C.S. Shi, X. Ju, Study of transfer in KMgF3: Eu-X(X ¼Gd, Cr, Ce) by the decay model of 6P7/2 exited state of Eu2 þ , Chem. Phys. Lett. 335 (2001) 17–22.