Unusual luminescence and temperature dependent decay behavior of divalent europium ion in KBaBP2O8

Journal of Luminescence 192 (2017) 616–619

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Unusual luminescence and temperature dependent decay behavior of divalent europium ion in KBaBP2O8

MARK

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Luyi Wua, Xinmin Zhanga, , Jiaxin Yanga, Yang Xua, Hongzhi Zhanga, Mengdong Heb, Guangming Yuana, Hyo Jin Seoc a

School of Materials Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China Institute of Mathematics and Physics, Central South University of Forestry and Technology, Changsha 410004, China c Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea b

A R T I C L E I N F O

A B S T R A C T

Keywords: Phosphors Photoluminescence Decay KBaBP2O8:Eu2+

The unusual photoluminescence (PL) properties and temperature dependent decay behavior of Eu2+ in KBaBP2O8 are reported and discussed. The emission spectrum is so complicate that it can be deconvoluted into three fitting peaks. The emission bands peaking at 380 and 444 nm can be assigned to the 4f65d1→4f7 transition of Eu2+ centers substituting for K+ and Ba2+ site, while the emission band at 497 nm may be attributed to the Eu2+ center on K+ site with a nearby charge-compensated vacancy. The decay time of the 4f65d emission increases with temperature. This increase can be explained by either thermal population of the 4f7 (6P7/2) level or thermal population of higher energetic 4f65d states.

1. Introduction The photoluminescence (PL) properties of Eu2+ in different host lattices have been investigated widely in the past [1–3]. In most cases, Eu2+ emission arises from 4f65d→4f7 transitions and broader band emission is observed. Moreover, the peak position of emission band depends strongly on the Eu2+ sites and the crystal field environment around them. Usually the full-width of half-maximum (FWHM) is less than 100 nm if only one site is available for Eu2+ ion occupation or only one kind of crystal field environment felt by Eu2+ ion [4,5]. Recently, Dai et al. showed that in Sr5(PO4)3-x(BO3)xCl:Eu2+ two distinct Eu2+ centers can appear with changing the value x, resulting in a yellow emission (550 nm) and a blue emission (446 nm) [6]. The 4f65d→4f7 transition of Eu2+ is a parity-allowed transition. The decay time is in the microsecond range and usually decreases with increasing temperature [7]. The reason is nonradiation processes become more efficient at higher temperature. However, some abnormal examples have been reported in the literature [8–11]. In this paper, the PL properties of Eu2+ in KBaBP2O8 are reported and discussed. The PL properties of Eu2+ in KBaBP2O8 doped with Eu2+ have been investigated due to their potential application in the field of white LEDs [12]. First, we will discuss the PL properties of KBaBP2O8:6%Eu2+ at room temperature. Some unusual luminescence properties were observed compared to the results reported by other

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Corresponding author. E-mail address: [email protected] (X. Zhang).

http://dx.doi.org/10.1016/j.jlumin.2017.07.057 Received 17 January 2017; Received in revised form 21 July 2017; Accepted 28 July 2017 Available online 29 July 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

groups. Then the temperature dependence of the emission spectrum and decay time of the 4f65d→4f7 transition of Eu2+ is reported. Just as for BaFBr:Eu2+, Ba5SiO4Cl6:Eu2+ and other host lattices doped with Eu2+, the decay time increases with increasing temperature. It will be shown that the increase for different emission bands can be explained by different models. 2. Experimental The measurements were carried out on KBaBP2O8:6%Eu2+ powder sample, which was prepared by solid state reactions as described elsewhere [12]. Stoichiometric amounts of K2CO3 (A.R.), BaCO3 (A.R.), H3BO3 (A.R.), NH4H2PO4 (A.R.), and Eu2O3 (99.99%) were ground and pre-heated at 600 °C for 3 h in air. After ground, the mixtures were fired at a tube furnace under reducing atmosphere (95% N2/5% H2) at 850 °C for 6 h with heating rate of 5 °C/min. X-ray powder diffraction (XRD) analysis was carried out on a Beijing Puxi XD-2 diffractometer with Cu Kα ( λ = 1.5405 Å) operated at 36 kV and 30 mA. PL spectra at room temperature were performed on a fluorescence meter (Shimadzu, RF 5301PC) with a 150 W Xe lamp as an excitation source. The higher and lower temperature PL spectra (9–525 K) and the decay curves were recorded by the 500 MHz digital storage oscilloscope (LeCroy 9350 A) in which the signal was fed from PMT. The excitation source was 266 nm UV laser (originating from the

Journal of Luminescence 192 (2017) 616–619

2. λex=268 nm

(231)

400

40

50

(235)

Intensity (arb. units)

30

1. λex=254 nm

(a)

2+

KBaBP2O8:6% Eu

(134) (233) (042)

(121)

(020, 004) 20

(022)

(112)

Intensity (arb. units)

10

(123, 015) (024) (031) (130) (116,033,125)

L. Wu et al.

60

70

KBaBP2O8

5

3. λex=282 nm 4. λex=330 nm

3

300

5. λex=371 nm 6. λex=381 nm

6

2 200

7. λex=391 nm

7

1

8. λex=401 nm 8

9. λex=411 nm

4

100

9 0

10

20

30

40

50

60

300

70

350

400

450

500

550

600

650

700

Wavelength (nm)

2 θ (degree) Fig. 1. XRD pattern of KBaBP2O8:6%Eu2+. The standard pattern of KBaBP2O8 crystal calculated from CIF file is depicted for comparison.

500

1. λem=361 nm

(b)

2. λem=369 nm

6 400

Intensity (arb. units)

fourth harmonic of a Quanta-ray DCR YAG:Nd laser). For the lower temperature measurement (9–300 K), the sample was placed at cold finger in a He gas recycled cryostat [13]. For the higher temperature measurement (300–525 K), the setup was equipped with a homemade heating cell connected to a temperature controller.

3. Results and discussion

4. λem=402 nm

5

5. λem=429 nm

300

8

6. λem=448 nm

3

7. λem=470 nm

2

200

9

8. λem=492 nm

4 100

Phase purity of KBaBP2O8:6%Eu2+ sample was checked by means of X-ray powder diffraction. The crystal structure of KBaBP2O8 has been investigated by Zhao et al. [14]. It represents a new kind of noncentrosymmetric borophosphate with three dimensional diamond-like framework and crystallizes in tetragonal system with space group I 42d with lattice parameters a = 7.202, c = 14.300 Å. It is impossible to separated K+ and Ba2+ cations in this structure and each surrounded by eight oxygen atoms. Namely, there is only one type of site for the Ba2+/K+ ions. The XRD pattern of the KBaBP2O8 doped with Eu2+ sample is shown in Fig. 1. It shows similarities to the pattern assigned to KBaBP2O8 in Ref. [14], indicating that the doped Eu2+ ions are incorporated into the cation ions sites and forms solid solution. PL emission spectra of KBaBP2O8:6%Eu2+ are shown in Fig. 2(a). It is clear that the emission spectra are super-wide bands extending from 350 to 600 nm when excited by photons of 254, 268, 282, 330, 371, 381, 391, 401 and 411 nm wavelengths. The emission spectra consist of two sub-bands peaking at about 380 and 450 nm, and the ration of them depends on excitation wavelength strongly. The emission bands are assigned to the 4f65d1→4f7 transitions on different Eu2+ centers. Moreover, the peak at 380 nm is a symmetrical band with a FWHM of 50 nm, which is a regular emission of d→f transition of Eu2+ [4,5], whereas the peak at 450 nm present an asymmetrical band with a long tail at the low-energy side. We carried out excitation measurements to confirm which absorption bands are associated with each emission band. So we choose different emission wavelengths (λem = 361, 369, 380, 402, 429, 448, 470, 492 and 529 nm) as the monitoring wavelengths. The results are presented in Fig. 2(b). The excitation spectra of these bands are clearly different, although all of them are broad band absorption originating from 4f7→4f65d1 transition of Eu2+. The excitation spectra (curves 1, 2, 3 and 4) peaking at about 280 nm correspond to one kind of Eu2+ center's absorption and the others (curves 5, 6, 7, 8 and 9) peaking at 335 nm are similar to each other corresponding to the other Eu2+ centers’ absorption. The optical properties of Eu2+ doped KBaBP2O8 have been reported in the literatures [12,15]. However, some different results are found when compared with their works. For example, they did not observe the emission band peaking at

3. λem=380 nm

7

9. λem=529 nm

1

0 250

300

350

400

450

500

Wavelength (nm) Fig. 2. PL emission spectra (a) and excitation spectra (b) of KBaBP2O8:6%Eu2+ sample. The excitation and monitoring wavelengths are presented in the figures.

380 nm under UV excitation and the difference between excitation spectra of these different emission bands. As discussed above, the PL emission spectrum of KBaBP2O8:6%Eu2+ system presents super-wide band. We try to deconvolute the emission spectra into multi-peaks using Gauss function and one of the results is shown in the inset of Fig. 3. The emission spectrum can be deconvoluted into three sub-Gaussian peaks. We observe peaks at 380, 444 and 497 nm, which indicates that two or more Eu2+ centers are available in the KBaBP2O8 crystal. It is well known that the crystal field effect and nephelauxetic effect have great influence on the luminescent properties of Eu2+. In this case, the doped Eu2+ ions can incorporate into not only the Ba2+ sites but also K+ sites according to the ionic radius (rBa(II)=1.42 Å, rK(I)=1.51 Å, rEu(II)=1.25 Å, CN=8) [16]. The luminescence properties of KBaBP2O8 and KSrBP2O8 activated with Eu2+ were reported by Sun and Wen [12,17]. KBaBP2O8 and KSrBP2O8 are isostructural and crystallize in the tetragonal system with space group I 42d [14]. The observed emission bands are situated at 445 and 462 nm for KBaBP2O8:Eu2+ and KSrBP2O8:Eu2+, respectively. The crystal field splitting of the Eu2+ 5d level will become smaller when the radius of the cation increases. So a smaller crystal field splitting of the 5d level in KBaBP2O8 crystal will shift the Eu2+ emission band to higher energy. Therefore, the emission band peaking at 444 nm can be assigned to the 4f65d1→4f7 transition of Eu2+ center substituting for Ba2+ site, while the emission band peaking at 380 nm can be assigned to the 4f65d1→ 4f7 transition of Eu2+ center substituting for K+ site. In the crystal structure of KBaBP2O8, both Ba2+ and K+ have only one 617

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λem = 380 nm, τ = 0.86 μs

1E-3

λem = 420 nm, τ = 0.75 μs λem = 445 nm, τ = 0.86 μs

Relative Intensity (arb. units)

λem = 515 nm, τ = 1.08 μs λem = 560 nm, τ = 1.15 μs

1E-4

350 300

1E-5

Intensity (arb. units)

Intensity (arb. units)

λem = 475 nm, τ = 0.98 μs

Experimental Data Fit Peak 1 Fit Peak 2 Fit Peak 3 Cumulative Fit Peak

380 nm 444 nm

250 200

497 nm

150 100 50 0 300

1E-6

400

500

600

9K 300 K 500 K

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Wavelength (nm)

0

1

2

3

4

5

6

7

Time (μs)

300

Fig. 3. Decay curves of KBaBP2O8:6%Eu2+ for different emission wavelengths under UV excitation (The inset shows the Gaussian fitting results of emission spectrum, λex = 282 nm).

500

600

700

Wavelength (nm) Fig. 4. Temperature dependence of emission spectra of KBaBP2O8:6%Eu2+ sample. (The inset shows the temperature dependence of decay time for different emission wavelengths).

crystallographic site for Eu2+ to be substituted, corresponding to the 444 and 380 nm emission bands. As discussed above, another deconvoluted emission band peaking at 497 nm is observed in the emission spectrum. Which Eu2+ center does this band originate from? The optical properties of Eu2+ in the host crystals containing alkali metal ions have been studied widely [18–21]. The doped Eu2+ ions could substitute for alkali metal ions sites due to their similar radii [16]. However, the replacement of monovalent alkali metal ions by Eu2+ requires charge compensation, which could result in multiple Eu2+ sites with different luminescence properties due to variations in local charge compensation [22,23]. The presence of different charge-compensated sites was observed in NaMgF3:Eu2+, KMgF3:Eu2+ and NaCl:Eu2+ [20,21,24]. The mechanism of charge compensation can be expressed by Kröger-Vink notation:

2Na×Na → Eu•Na + V′Na , 2K×K → Eu•K + V′K

400

because of the increase of the number of vibronic transitions [27]. In addition, the emission bands shift towards the higher energy as the increasing temperature. Usually the shift of the emission band towards short wavelengths with increasing temperature can be explained by a simple model in which the crystalline field strength changes with temperatures as a result of lattice distortion (thermal expansion). The luminescence decay dependence on the observation temperature was carried out. All the decay curves can be described by a simple exponential. The derived lifetime dependence on the temperature for the different emission wavelengths is depicted in the inset of Fig. 4. It can be seen clearly that the lifetimes show an unusual increase with increasing temperatures. This unusual phenomenon has been reported in many systems [8–10,28–33]. In order to explain the increase of the decay time with temperature, two possible reasons are available in the literature. One is thermal population of the 4f7 (6P7/2) level proposed by Spoonhower and Burberry when they investigated the luminescence properties of Eu2+ in BaFBr:Eu2+ system [9]. The 4f7 (6P7/2) level is located at about 27 600 cm−1, independent of the host lattice [34]. The value for the energy gap between the 4f65d state and the 4f7 (6P7/2) state found by Spoonhower and Burberry from the temperature dependence of the decay time is about 480 cm−1 in BaFBr:Eu2+. So they suggest that the decay time of the 4f65d emission of Eu2+ increasing with temperature can be explained by a model based on thermal population of the 4f7 (6P7/2) level due to the small energy gap. The high energy emission band (380 nm) in KBaBP2O8:6%Eu2+ sample is close to that of BaFBr:Eu2+ (390 nm). Therefore, the energy difference of 4f65d state and the 4f7 (6P7/2) state in KBaBP2O8:6%Eu2+ system should also be small. We argue that the decay time of the 4f65d emission of Eu2+ (380 nm) increasing with temperature could be attributed to the thermal population of the 4f7 (6P7/2) level. Another is thermal population of higher energetic 4f65d states [10]. The value for the energy gap between the 4f65d state and the 4f7 (6P7/2) state will increase if the emission band shifts towards long wavelength. For example, in Ba5SiO4×6 (X = Cl, Br) systems the emission spectrum consists of a single band peaking at ~440 nm [10]. The value for the energy gap between the 4f65d state and the 4f7 (6P7/2) state are about 4 400 cm−1. The derived lifetimes also show an unusual increase with increasing temperature. Meijerink and Blasse think this increase cannot be explained by the thermal population of the 4f7 (6P7/2) state due to the large energy gap, but be explained by thermal population of higher energetic 4f65d states for which the transition probability to the 4f7 (8S7/2) ground state is smaller than for the lowest 4f65d state. The

(1)

so compensating cation vacancies (V′Na and V′K ) will generate. In this case, we think two different sites are possible as a consequence of differences in charge compensation through K+ vacancy, namely Eu2+ sites with zero and one closely positioned K+ vacancy [25]. The charge compensation by a nearby K+ vacancy could lower the symmetry of the Eu2+ ion and then result in a larger crystal field splitting of its 5d level [26]. Therefore, Eu2+ occupied in this kind of site will give an emission at longer wavelengths. So the emission band peaking at 497 nm may be ascribed to Eu2+ ions associated with a K+ vacancy, while that peaking at 380 nm can be ascribed to Eu2+ ions on K+ site without a nearby charge-compensated vacancy [26]. The decay time of Eu2+ is short due to the f-d transition is parity allowed. The typical values are in the order of microsecond [7]. The decay curves of the different emission wavelengths obtained upon 266 nm UV excitation are depicted in Fig. 3. All decay curves exhibit a single exponential behavior in the recorded time range. The decay curves can be described as I = I0 e−t / τ . The decay times determined from the decay curves are 0.86, 0.75, 0.86, 0.98, 1.08 and 1.15 μs for 380, 420, 445, 475, 515 and 560 nm emission wavelengths, respectively, which are close to typical value for the f-d transition of Eu2+. It has been shown that the radiative lifetime increases as the emission shifts to lower energies. The similar experimental result was reported by Capelletti et al. in investigation of NaCl:Eu2+ system [8]. Temperature dependence of emission spectra of KBaBP2O8:6%Eu2+ sample are carried out. For the sake of clarity, only three curves at 9, 300 and 500 K are presented in Fig. 4. As can be seen from Fig. 4 the emission intensity decreases with increasing temperature due to temperature quenching. At the same time, the emission peaks broaden 618

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theoretical splitting of the 4f65d configuration was studied by Weakliem [35]. The exchange interaction between 4f6 core and 5d electron will result in further splitting besides the spin-orbit splitting of 4f6 core and the crystal-field splitting of 5d electron. This exchange interaction leads to octet states and sextet states. The sextet states are at higher energies than the octet states according to Hund's rule. At low temperature, the lifetime is short since the transition from the lowest 4f65d state (octet state) to the ground state 4f7 (8S7/2) is spin allowed. At high temperature, the higher energetic sextet states will be thermally populated. The transition from the sextet states to the 8S7/2 ground state is spin forbidden, resulting in an increase of the lifetime. In the present case, the low energy emission bands situate at 444 and 497 nm, which are longer than those of Ba5SiO4×6:Eu2+ (X = Cl, Br) (440 nm). So the energy gaps between the 4f65d state and the 4f7 (6P7/2) state in these emission bands should be larger than those of Ba5SiO4×6:Eu2+ (X = Cl, Br). Therefore, for the low energy emission bands peaking at 444 and 497 nm, the increase of the lifetime with increasing temperature in KBaBP2O8:6%Eu2+ system may be due to the thermal population of higher energetic 4f65d states. In addition, the decrease of the lifetime above ~300 K can be attributed to thermal quenching.

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4. Conclusions The PL of Eu2+ in KBaBP2O8 is reported and discussed. The superwide emission bands extending from 350 to 600 nm are attributed to d→f transitions of different Eu2+ centers. The emission bands peaking at 380 and 444 nm should be attributed to the 4f65d1→4f7 transition of Eu2+ centers substituting for K+ and Ba2+ site, while the emission band at 497 nm could originate from the Eu2+ center on K+ site with a nearby charge-compensated vacancy. All the decay times of different emission bands increase with temperature. For the 380 nm emission band, the increase can be explained by thermal population of the 4f7 (6P7/2) level as suggested by Spoonhower and Burberry for BaFBr:Eu2+, while for the 444 and 497 nm emission bands, the increase can be explained by thermal population of higher energetic 4f65d states as suggested by Meijerink and Blasse for Ba5SiO4×6:Eu2+ (X = Cl, Br) for which the transition from the higher energetic sextet states to the 8S7/2 ground state is spin forbidden, resulting in an increase of the lifetime. Acknowledgements The project was sponsored by the Technology Program of Environmental Protection Department of Hunan (No. 2013-312) and Special Fund for Forest Scientific Research in the Public Welfare (No. 201504503). References [1] Z.G. Xia, S.H. Miao, M.S. Molokeev, M.Y. Chen, Q.L. Liu, Structure and luminescence properties of Eu2+ doped LuxSr2−xSiNxO4−x phosphors evolved from chemical unit cosubstitution, J. Mater. Chem. C. 4 (2016) 1336–1344. [2] T. Wang, Q.C. Xiang, Z.G. Xia, J. Chen, Q.L. Liu, Evolution of structure and photoluminescence by cation cosubstitution in Eu2+-Doped (Ca1−xLix)(Al1−xSi1+x)N3 solid solutions, Inorg. Chem. 55 (2016) 2929–2933. [3] W.L. Zhou, J. Han, X.J. Zhang, Z.X. Qiu, Q.J. Xie, H.B. Liang, S.X. Lian, J. Wang, Synthesis and photoluminescence properties of a cyan-emitting phosphor Ca3(PO4)2:Eu2+ for white light-emitting diodes, Opt. Mater. 39 (2015) 173–177. [4] V. Bachmann, C. Ronda, O. Oeckler, W. Schnick, A. Meijerink, Color point tuning for (Sr,Ca,Ba)Si2O2N2:Eu2+ for white light LEDs, Chem. Mater. 21 (2009) 316–325. [5] G. Blasse, A. Bril, J. Devries, Luminescence of alkaline-earth borate-phosphates activated with divalent europium, J. Inorg. Nucl. Chem. 31 (1969) 568–570.

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