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Self-activated luminescent material K3Dy(PO4)2: Crystal growth, structural analysis and characterizations
Optik 127 (2016) 10297–10302
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Self-activated luminescent material K3 Dy(PO4 )2 : Crystal growth, structural analysis and characterizations D. Zhao ∗ , F.X. Ma, Y.C. Fan ∗ , Hai-Yan Li, L. Zhang College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China
a r t i c l e
i n f o
Article history: Received 17 June 2016 Accepted 23 August 2016 Keywords: Phosphate Flux method Crystal structure Luminescence
a b s t r a c t A potassium dysprosium orthophosphate K3 Dy(PO4 )2 has been prepared using high temperature flux method and structurally characterized by single crystal X-ray diffraction (SC-XRD) analysis. Its structure features a two-dimensional (2D) layer structure that is composed of [Dy(PO4 )2 ]∞ anionic layers and K∞ cationic layers alternatively stacking along the a-axis. The diverse excitation and emission photoluminescence spectra, fluorescence lifetime and color CIE coordinates for K3 Dy(PO4 )2 were discussed. Although with high Dy3+ concentration, the emission spectrum excited at 350 nm shows strong greenyellow emission bands corresponding to the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+ ions, respectively. The results show that K3 Dy(PO4 )2 can be potentially used as a green-yellow phosphor for fields of near UV-excited white-light-emitting diode and optoelectronic devices. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Rare-earth photoluminescence (PL) materials for converting UV or near UV-blue radiation into visible light have a broad range of applications such as light emitting diodes (LEDs), cathode ray tubes, and field-emission displays [1–5]. Rare-earth ions can be excited resonantly through f → f transitions or f → d transitions or non-directly in charge transfer process or dipole−dipole energy transfer. Among rare-earth ions, the Dy3+ ion with 4f9 configuration is one of the excellent activators in PL materials. Dy3+ usually shows strong fluorescent transitions in the bluish (485 nm) region corresponding to the 6 4 6 4F 9/2 → H15/2 transition and yellowish (575 nm) region corresponding to the F9/2 → H13/2 , which is necessary for the development of white light emitting diodes (LEDs) as well as optical display systems. Among the PL materials, inorganic phosphates have their host absorption edge at rather short wavelengths which make them suitable as the host for active rare-earth ions. Moreover, phosphates have significant advantages for using as PL materials including low sintering temperature, low cost, broad band gap, high luminous efficiency and high chemical stability. Recently, study on phosphate based luminescent materials have produced a large amount of literatures, such as KLn(PO3 )4 (Ln = Ce, Eu) [6], Na2 SrMg(PO4 )2 : Eu3+ [7], KSrBP2 O8 : Dy3+ [8], KBaBP2 O8 :Tb3+ [9]. The study of phosphates with the general formula A3 Ln(PO4 )2 (Ln = alkali metals; Ln = rare-earth elements) was conspicuous and some inspiring results have been recently achieved. Jiang et al. has reported new PL material K3 Gd(PO4 )2 : (Tb3+ , Eu3+ ), whose color emission can be easily tuned from yellowish-green to reddish-orange by adjusting the Eu3+ concentration [10]. The other A3 Ln(PO4 )2 based phosphor include Na3 Ln(PO4 )2 : Yb3+ [11], Na3 Gd(PO4 )2 : (Dy3+ , Tm3+ ) [12], K3 Gd(PO4 )2 : Sm3+ [13], K3 Gd(PO4 )2 : (Ce3+ , Tb3+ ) [14]. In
∗ Corresponding authors. E-mail addresses: [email protected] (D. Zhao), [email protected] (Y.C. Fan). http://dx.doi.org/10.1016/j.ijleo.2016.08.057 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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Fig. 1. Experimental and simulated X-Ray powder diffraction pattern of K3 Dy(PO4 )2 .
order to explore new PL materials, we started an explore within the K2 O–Dy2 O3 –P2 O5 system and successfully prepared a ternary compound K3 Dy(PO4 )2 , which show good PL properties although with fully concentrated Dy3+ . Herein, we report the crystal growth, structural determination and self-activated PL properties of K3 Dy(PO4 )2 . 2. Experimental section 2.1. Preparation Raw chemicals KF, K2 CO3 , Dy2 O3 and NH4 H2 PO4 were purchased from the Shanghai Reagent Factory. Single crystal of K3 Dy(PO4 )2 was prepared by a molten salt method by using additional K2 O–KF–P2 O5 as the flux. The mixture of initial reagents K2 CO3 (1.482 g, 10.72 mmol), Dy2 O3 (1.000 g, 2.681 mmol), NH4 H2 PO4 (2.158 g, 18.77 mmol) and KF(0.779 g, 13.40 mmol) with the molar ratio of 4:1:7:5 was thoroughly ground in an agate mortar put into a platinum crucible. The mixture was heated in an oven at 900 ◦ C for 24 h and then cooled slowly to 700 ◦ C at a rate of 2 ◦ C h−1 before the furnace was switched off. The flux attached to the crystal was readily dissolved in hot water. After appropriate structural analysis, powder sample of K3 Dy(PO4 )2 was obtained quantitatively by the solid state reaction of a mixture of K2 CO3 /Dy2 O3 /NH4 H2 PO4 in the molar ratio of 3:1:4. The mixture was sintered in a platinum crucible for 24 h at 950 ◦ C, with several intermediate grinding stages to ensure complete solid state reaction. XRD powder diffraction studies demonstrated that the powder sample was successfully obtained as the single phase (Fig. 1). 2.2. X-ray diffraction analysis A suitable single crystal with dimensions of 0.20 × 0.05 × 0.05 mm was selected and mounted on a glass fibre for the single-crystal X-ray diffraction experiments. Data collection was performed using a Bruker Smart Apex2 CCD diffractometer with graphite-monochromated Mo-K␣ ( = 0.71073 Å) radiation in the ω/2 scan mode at a temperature of 293 K. Lorentz and polarization corrections were applied to all data, and an empirical absorption correction was applied using SADABS program [15]. The structure was solved by direct methods and refined by full-matrix least-squares fitting on F2 by Shelx-2014 [16]. All of the atoms were refined with anisotropic thermal parameters. The final refined solution obtained was checked with the ADDSYM algorithm in the program PLATON [17], and no higher symmetry was found. Crystallographic data and structural refinement was summarized in Table 1. 3. Results and discussion 3.1. Crystal structure Single crystal X-ray diffraction analysis revealed that compound K3 Dy(PO4 )2 crystallizes in the monoclinic space group P21 /m, and features a two-dimensional (2D) layer structure composed of [Dy(PO4 )2 ]∞ anionic layers and K∞ cationic layers alternatively stacking along the a-axis, as shown in Fig. 2a. In the asymmetric unit, there is one Dy atom, two P atom, three K atom, and six O atoms. P atoms are coordinated by three O atoms into PO4 tetrahedra with the P O bond distances ranging from 1.514(4) Å to 1.545(4) Å and O P O bond angles ranging from 105.5(2)◦ to 110.93(15)◦ (Table 2). These values are common values comparing with other reported rare-earth phosphates [18,19]. The PO4 tetrahedra are isolated and are further interconnected by Dy atoms into a 2D infinite anionic layer of [Dy(PO4 )2 ]∞ , as shown in Fig. 2b. The Dy atom has a DyO7 polyhedral geometry with the Dy O bond distances ranging from 2.257(3) Å to 2.523(4) Å.
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Table 1 Summary of crystal data and structure refinement for K3 Dy(PO4 )2 . Crystal data Chemical formula Mr Crystal system, space group Temperature (K) a, b, c (Å) ˇ (◦ ) V (Å3 ) Z Radiation type (mm−1 ) Crystal size (mm) Diffractometer Absorption correction No. of measured, independent andobserved [I > 2(I)] reflections Rint (sin /)max (Å−1 ) R[F2 > 2(F2 )], wR(F2 ), S No. of reflections No. of parameters rmax , min (e·Å−3 )
K3 DyP2 O8 469.74 Monoclinic, P21 /m 296 7.3994 (9), 5.6136 (7), 9.3944 (11) 90.866 (1) 390.17 (8) 2 Mo K␣ 11.60 0.20 × 0.05 × 0.05 Bruker Apex2 CCD multi-scan 4885, 1068, 1018 0.039 0.667 0.023, 0.056, 1.11 1068 79 3.58, −1.03
Fig. 2. (a) View of the crystal structure of K3 Dy(PO4 )2 down the c-axis; (b) View of the [Dy(PO4 )2 ]∞ layer on the bc-plane.
The K atoms reside among these [Dy(PO4 )2 ]∞ layers and join them through coulombic action of K+ and O2− ions to construct the structure of compound K3 Dy(PO4 )2 . It should be noted that three crystallographically different K atoms have three different coordination geometries, that is, K(1) O10 , K(2) O9 and K(3) O11 polyhedra. The K−O bond distances fall in the range of 2.614(4)–3.255(3) Å, which is comparable with other potassium oxides [20,21]. Bond valence calculations (Dy, 2.44; P, 4.94–4.97; K, 1.06–1.24) indicate that the Dy, P and K atoms are in reasonable oxidation states of +3, +5 and +1, respectively [22]. 3.2. Photoluminescence properties The self-activated photoluminescence (PL) properties of K3 Dy(PO4 )2 was performed on EDINBURGH FLS980 fluorescence spectrophotometer. The excitation spectrum was recorded when monitoring emission wavelength of 571 nm in the range of 300–420 nm. As shown in Fig. 3a, the excitation spectrum consist a series of sharp peaks located at 324, 350, 365, and 387 nm arising from the intrinsic 4f → 4f transitions of Dy3+ . They can be assigned to the transitions from the ground 6 H15/2 state to the excited 4 I11/2 , 6 P5/2 , 4 F7/2 and 6 H15/2 states, respectively [23,24]. Among all the excitation bands, the band at 350 nm corresponding to the 6 H15/2 → 6 P7/2 transition possesses the maximum intensity. The strong excitation bands ranging from 350–400 nm indicate that K3 Dy(PO4 )2 phosphor match well with the could be excited by near-ultraviolet (NUV) emitting of InGaN-based LED chip (350–410 nm), implying a potential application in NUV-pumped white-LEDs. As shown in Fig. 3b, the PL emission spectrum excited by 350 nm UV light consists of two emission regions ranging from 450 to 650 nm: blue and yellow regions. The blue emission band at 488 nm corresponds to the 4 F9/2 → 6 H15/2 transition and the yellow emission band at 571 nm corresponds to the hypersensitive 4 F9/2 → 6 H13/2 transition. It is well-known that if Dy3+ ions located in a site with inversion symmetry, the magnetic dipole transition 4 F9/2 → 6 H15/2 is frequently the strongest one, while in a site without inversion symmetry the 4 F9/2 → 6 H13/2 electronic transition usually becomes dominant. For K3 Dy(PO4 )2 phosphor, the yellow 4 F9/2 → 6 H13/2 emission is stronger than the blue 4 F9/2 → 6 H15/2 , which indicates that the
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Table 2 Selected bond distances (Å) and angles (◦ ) for K3 Dy(PO4 )2 . Dy1—O2i
2.257(3)
K1—O1xvi
3.096(4)
Dy1—O2ii Dy1—O5iii Dy1—O5iv Dy1—O6 Dy1—O1 Dy1—O4 P1—O3 P1—O1 P1—O2 P1—O2viii P2—O6 P2—O4 P2—O5 P2—O5viii K1—O5xiii K1—O5xiv K1—O2vi K1—O2xv K1—O1vi K1—O1v K1—O2xvi K1—O2xvii
2.257(3) 2.359(3) 2.359(3) 2.388(4) 2.421(4) 2.523(4) 1.514(4) 1.545(4) 1.545(3) 1.545(3) 1.531(4) 1.540(4) 1.542(3) 1.542(3) 2.755(3) 2.755(3) 2.802(3) 2.802(3) 2.8205(5) 2.8205(5) 3.030(3) 3.030(3)
K1—O3 K2—O3 K2—O4xvi K2—O5iv K2—O5iii K2—O3v K2—O3vi K2—O2vi K2—O2xv K2—O1 K3—O6xii K3—O4xvi K3—O4xviii K3—O5xvii K3—O5xviii K3—O3v K3—O4x K3—O2v K3—O2xv K3—O5x K3—O5iv
2.684(5) 2.808(4) 2.681(4) 2.875(3) 2.875(3) 2.9747(15) 2.9747(15) 3.007(3) 3.007(3) 2.711(4) 2.614(4) 2.8728(10) 2.8728(10) 2.938(3) 2.938(3) 2.943(4) 3.048(4) 3.087(3) 3.087(3) 3.255(3) 3.255(3)
O3—P1—O1 O3—P1—O2 O1—P1—O2 O3—P1—O2viii O1—P1—O2viii O2—P1—O2viii
111.1(2) 109.50(15) 109.10(14) 109.50(15) 109.09(14) 108.5(2)
O6—P2—O4 O6—P2—O5 O4—P2—O5 O6—P2—O5viii O4—P2—O5viii O5—P2—O5viii
105.5(2) 110.16(15) 110.93(15) 110.16(15) 110.93(14) 109.1(2)
Symmetry codes: (i) −x + 2, y + 1/2, −z + 1; (ii) −x + 2, −y, −z + 1; (iii) −x + 2, −y, −z; (iv) −x + 2, y + 1/2, −z; (v) −x + 1, −y + 1, −z + 1; (vi) −x + 1, −y, −z + 1; (vii) x + 1, y, z; (viii) x, −y + 1/2, z; (ix) x + 1, y, z − 1; (x) −x + 2, −y + 1, −z; (xi) x + 1, y − 1, z; (xii) −x + 1, −y + 1, −z; (xiii) x − 1, −y + 1/2, z + 1; (xiv) x − 1, y, z + 1; (xv) −x + 1, y + 1/2, −z + 1; (xvi) x − 1, y, z; (xvii) x − 1, −y + 1/2, z; (xviii) x − 1, y + 1, z.
Fig. 3. Excitation (a) and emission (b) spectra of K3 Dy(PO4 )2 . (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Dy3+ ions are located at low-symmetry sites without inversion centers. This result matches well with the crystal structure analysis as mentioned above, which demonstrates that all of the Dy3+ ions are located in the glide plane ‘m’ without the inversion center. Moreover, the Commission International de L’Eclairage (CIE) chromaticity coordinates for K3 Dy(PO4 )2 were calculated under the excitation wavelength of 350 nm as shown in Fig. 4. The coordinate was found to be (x = 0.395, y = 0.437) which fall in the green-yellow region of the spectrum, which indicates that the self-activated phosphor K3 Dy(PO4 )2 is a suitable green-yellow emitting phosphor under excitation of near UV LEDs. The decay curve of K3 Dy(PO4 )2 phosphor is measured, as shown in Fig. 5. It is found that after a rapid initial rise in the PL intensity, when irradiation at the wavelength of 350 nm was started, the PL intensity at an emission wavelength of 571 nm decayed slowly to a steady-state value of about microseconds. The decay curve cannot be well fitted with a singleexponential function but can be well fitted a biexponential function as the equation, I(t) = A1 exp(−t/ 1 ) + A2 exp(−t/ 2 ) + I(0) (where I(t) and I0 are the luminescence intensities at time 0 and t; A1 and A2 are fitting parameters; t is the time, 1 and 2
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Fig. 4. The CIE chromaticity diagram for K3 Dy(PO4 )2 with excitation at 350 nm. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Fig. 5. Fluorescent decay curves of K3 Dy(PO4 )2 at 293 K.
are rapid and slow lifetimes for exponential components, respectively). The average decay time of Dy3+ can be calculated by the equation, =
A1 2 +A2 2 1 2 , A1 1 +A2 2
and the value of is calculated to be 3.458 s for representing the lifetime.
4. Conclusions In conclusion, we reported the synthesis, crystal structure and PL properties of a potassium dysprosium orthophosphate K3 Dy(PO4 )2 . The crystal growth was fulfilled using high temperature flux method with additional K2 O–KF–P2 O5 as the flux. Single-crystal X-ray diffraction analysis revealed that this compound crystallizes in the monoclinic space group P21 /m, and the structure features a 2D layer structure containing [Dy(PO4 )2 ]∞ anionic layers and K∞ cationic layers alternatively stacked along the a-axis. Although with high Dy3+ concentration, the emission spectrum excited at 350 nm shows two strong emission bands in blue (488 nm) and yellow (571 nm) regions related with the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+ ions, respectively. The decay curve at 571 nm was measured and the decay time was fitted to be 3.458 s. We think K3 Dy(PO4 )2 can be potentially used as a green-yellow phosphor for fields of near UV-excited white-light-emitting diode and optoelectronic devices. Acknowledgment We are grateful to the National Natural Science Foundation of China (Project 21201056, 21307028).
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References [1] W.-L. Zhang, C.-S. Lin, Z.-Z. He, H. Zhang, Z.-Z. Luo, W.-D. Cheng, Syntheses of three members of A(II) M(IV) (PO4 )2 : luminescence properties of PbGe(PO4 )2 and its Eu3+ -doped powders, CrystEngComm 15 (2013) 7089–7094. [2] L. Cao, J. Liu, Z.-C. Wu, S.-P. Kuang, Study on the photoluminescence properties of a color-tunable Ca9 ZnK(PO4 )7 : Eu3+ phosphor, Optik 127 (2016) 4039–4042. [3] C. Litterscheid, S. Krueger, M. Euler, A. Dreizler, C. Wickleder, B. Albert, Solid solution between lithium-rich yttrium and europium molybdate as new efficient red-emitting phosphors, J. Mater. Chem. C 4 (2016) 596–602. [4] D. Zhao, R.-H. Zhang, F.-X. Ma, F.-F. Li, (3 + 1)-Dimensional incommensurately modulated structure and photoluminescence property of polyphosphate Ho(PO3 )3 , Mater. Lett. 157 (2015) 219–221. [5] M.K. Ekmekci, M. Erdem, A.S. Basak, O. Danis, Structural and optical properties of Nd3+ doped columbite SrNb2 O6 , Optik 127 (2016) 4123–4126. [6] J. Zhu, W.D. Cheng, H. Zhang, Y.D. Wang, Two potassium rare-earth polyphosphates KLn(PO3)4 (Ln = Ce, Eu): structural, optical, and electronic properties, J. Lumin. 129 (2009) 1326–1331. [7] A. Boukhris, M. Hidouri, B. Glorieux, M. Ben Amara, Correlation between structure and photoluminescence of the europium doped glaserite-type phosphate Na2 SrMg(PO4 )2 , Mater. Chem. Phys. 137 (2012) 26–33. [8] F. Zhang, T. Zhang, G. Li, W. Zhang, Single phase M+ (M = Li, Na K), Dy3+ co-doped KSrBP2 O8 white light emitting phosphors, J. Alloys Compd. 618 (2015) 484–487. [9] B. Han, J. Zhang, P. Li, H. Shi, Luminescence properties of novel yellowish-green-emitting phosphor KBaBP2 O8 :Tb3+ , Phys. Status Solidi A 211 (2014) 2483–2487. [10] T. Jiang, X. Yu, X. Xu, H. Yu, D. Zhou, J. Qiu, Realization of tunable emission via efficient Tb3+ -Eu3+ energy transfer in K3 Gd(PO4 )2 for UV-excited white light-emitting-diodes, Opt. Mater. 36 (2014) 611–615. [11] A. Matraszek, P. Godlewska, L. Macalik, K. Hermanowicz, J. Hanuza, I. Szczygiel, Optical and thermal characterization of microcrystalline Na3 RE(PO4 )2 :Yb orthophosphates synthesized by Pechini method (RE = Y, La Gd), J. Alloys Compd. 619 (2015) 275–283. [12] B.C. Jamalaiah, M. Jo, J. Zehan, J.J. Shim, S. Il Kim, W.Y. Chung, H.J. Seo, Luminescence, energy transfer and color perception studies of Na3 Gd(PO4 )2 :Dy3+ :Tm3+ phosphors, Opt. Mater. 36 (2014) 1688–1693. [13] P. Gupta, A.K. Bedyal, V. Kumar, Y. Khajuria, V. Sharma, O.M. Ntwaeaborwa, H.C. Swart, Energy transfer mechanism from Gd3+ to Sm3+ in K3 Gd(PO4 )2 :Sm3+ phosphor, Mater. Res. Express 2 (2015) 076202. [14] T.-M. Jiang, X. Yu, X.-H. Xu, D.-C. Zhou, H.-L. Yu, P.-H. Yang, J.-B. Qiu, A novel strong green phosphor: k3 Gd(PO4 )2 :Ce3+ Tb3+ for a UV-excited white light-emitting-diode, Chin. Phys. B 23 (2014) 028505. [15] G.M. Sheldrick, SADABS, University of Gttingen, Germany, 1996. [16] G.M. Sheldrick, A short history of SHELX, Acta Cryst. A 64 (112) (2008) 112–122. [17] A.L. Spek, Structure validation in chemical crystallography, Acta Crystallogr. Sect. D: Biol. Crystallogr. 65 (2009) 148–155. [18] Z. He, W. Guo, W. Cheng, M. Itoh, Anisotropic magnetic behaviors of monoclinic Li3 Fe2 (PO4 )3 , J. Solid State Chem. 215 (2014) 189–192. [19] M. Yang, S. Zhang, W. Guo, Z. He, Hydrothermal synthesis and magnetic properties of a new phase of SrCo2 (PO4 )2 , Solid State Sci. 52 (2016) 72–77. [20] G. Yang, G. Peng, N. Ye, J. Wang, M. Luo, T. Yan, Y. Zhou, Structural modulation of anionic group architectures by cations to optimize SHG effects: a facile route to new NLO materials in the ATCO3 F (A = K Rh; T = Zn, Cd) series, Chem. Mater. 27 (2015) 7520–7530. [21] S. Wang, N. Ye, G. Zou, A new alkaline beryllium borate KBe4 B3 O9 with ribbon alveolate Be2 BO5 (infinity) layers and the structural evolution of ABe4 B3 O9 (A = K, Rb and Cs), CrystEngComm 16 (2014) 3971–3976. [22] N.E. Brese, M. O’Keeffe, Bond-valence parameters for solids, Acta Cryst. B 47 (1991) 192–197. [23] B. Han, J. Zhang, P. Li, H. Shi, Opt. Spectrosc. 118 (2015) 135–141. [24] D. Zhao, R.-H. Zhang, F.-F. Li, J. Yang, B.-G. Liu, Y.-C. Fan, Luminescence properties of novel single-host white-light-emitting phosphor KBaBP2 O8 :Dy3+ , Dalton Trans. 44 (2015) 6277–6287.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Self-activated luminescent material K3 Dy(PO4 )2 : Crystal growth, structural analysis and characterizations D. Zhao ∗ , F.X. Ma, Y.C. Fan ∗ , Hai-Yan Li, L. Zhang College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China
a r t i c l e
i n f o
Article history: Received 17 June 2016 Accepted 23 August 2016 Keywords: Phosphate Flux method Crystal structure Luminescence
a b s t r a c t A potassium dysprosium orthophosphate K3 Dy(PO4 )2 has been prepared using high temperature flux method and structurally characterized by single crystal X-ray diffraction (SC-XRD) analysis. Its structure features a two-dimensional (2D) layer structure that is composed of [Dy(PO4 )2 ]∞ anionic layers and K∞ cationic layers alternatively stacking along the a-axis. The diverse excitation and emission photoluminescence spectra, fluorescence lifetime and color CIE coordinates for K3 Dy(PO4 )2 were discussed. Although with high Dy3+ concentration, the emission spectrum excited at 350 nm shows strong greenyellow emission bands corresponding to the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+ ions, respectively. The results show that K3 Dy(PO4 )2 can be potentially used as a green-yellow phosphor for fields of near UV-excited white-light-emitting diode and optoelectronic devices. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Rare-earth photoluminescence (PL) materials for converting UV or near UV-blue radiation into visible light have a broad range of applications such as light emitting diodes (LEDs), cathode ray tubes, and field-emission displays [1–5]. Rare-earth ions can be excited resonantly through f → f transitions or f → d transitions or non-directly in charge transfer process or dipole−dipole energy transfer. Among rare-earth ions, the Dy3+ ion with 4f9 configuration is one of the excellent activators in PL materials. Dy3+ usually shows strong fluorescent transitions in the bluish (485 nm) region corresponding to the 6 4 6 4F 9/2 → H15/2 transition and yellowish (575 nm) region corresponding to the F9/2 → H13/2 , which is necessary for the development of white light emitting diodes (LEDs) as well as optical display systems. Among the PL materials, inorganic phosphates have their host absorption edge at rather short wavelengths which make them suitable as the host for active rare-earth ions. Moreover, phosphates have significant advantages for using as PL materials including low sintering temperature, low cost, broad band gap, high luminous efficiency and high chemical stability. Recently, study on phosphate based luminescent materials have produced a large amount of literatures, such as KLn(PO3 )4 (Ln = Ce, Eu) [6], Na2 SrMg(PO4 )2 : Eu3+ [7], KSrBP2 O8 : Dy3+ [8], KBaBP2 O8 :Tb3+ [9]. The study of phosphates with the general formula A3 Ln(PO4 )2 (Ln = alkali metals; Ln = rare-earth elements) was conspicuous and some inspiring results have been recently achieved. Jiang et al. has reported new PL material K3 Gd(PO4 )2 : (Tb3+ , Eu3+ ), whose color emission can be easily tuned from yellowish-green to reddish-orange by adjusting the Eu3+ concentration [10]. The other A3 Ln(PO4 )2 based phosphor include Na3 Ln(PO4 )2 : Yb3+ [11], Na3 Gd(PO4 )2 : (Dy3+ , Tm3+ ) [12], K3 Gd(PO4 )2 : Sm3+ [13], K3 Gd(PO4 )2 : (Ce3+ , Tb3+ ) [14]. In
∗ Corresponding authors. E-mail addresses: [email protected] (D. Zhao), [email protected] (Y.C. Fan). http://dx.doi.org/10.1016/j.ijleo.2016.08.057 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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Fig. 1. Experimental and simulated X-Ray powder diffraction pattern of K3 Dy(PO4 )2 .
order to explore new PL materials, we started an explore within the K2 O–Dy2 O3 –P2 O5 system and successfully prepared a ternary compound K3 Dy(PO4 )2 , which show good PL properties although with fully concentrated Dy3+ . Herein, we report the crystal growth, structural determination and self-activated PL properties of K3 Dy(PO4 )2 . 2. Experimental section 2.1. Preparation Raw chemicals KF, K2 CO3 , Dy2 O3 and NH4 H2 PO4 were purchased from the Shanghai Reagent Factory. Single crystal of K3 Dy(PO4 )2 was prepared by a molten salt method by using additional K2 O–KF–P2 O5 as the flux. The mixture of initial reagents K2 CO3 (1.482 g, 10.72 mmol), Dy2 O3 (1.000 g, 2.681 mmol), NH4 H2 PO4 (2.158 g, 18.77 mmol) and KF(0.779 g, 13.40 mmol) with the molar ratio of 4:1:7:5 was thoroughly ground in an agate mortar put into a platinum crucible. The mixture was heated in an oven at 900 ◦ C for 24 h and then cooled slowly to 700 ◦ C at a rate of 2 ◦ C h−1 before the furnace was switched off. The flux attached to the crystal was readily dissolved in hot water. After appropriate structural analysis, powder sample of K3 Dy(PO4 )2 was obtained quantitatively by the solid state reaction of a mixture of K2 CO3 /Dy2 O3 /NH4 H2 PO4 in the molar ratio of 3:1:4. The mixture was sintered in a platinum crucible for 24 h at 950 ◦ C, with several intermediate grinding stages to ensure complete solid state reaction. XRD powder diffraction studies demonstrated that the powder sample was successfully obtained as the single phase (Fig. 1). 2.2. X-ray diffraction analysis A suitable single crystal with dimensions of 0.20 × 0.05 × 0.05 mm was selected and mounted on a glass fibre for the single-crystal X-ray diffraction experiments. Data collection was performed using a Bruker Smart Apex2 CCD diffractometer with graphite-monochromated Mo-K␣ ( = 0.71073 Å) radiation in the ω/2 scan mode at a temperature of 293 K. Lorentz and polarization corrections were applied to all data, and an empirical absorption correction was applied using SADABS program [15]. The structure was solved by direct methods and refined by full-matrix least-squares fitting on F2 by Shelx-2014 [16]. All of the atoms were refined with anisotropic thermal parameters. The final refined solution obtained was checked with the ADDSYM algorithm in the program PLATON [17], and no higher symmetry was found. Crystallographic data and structural refinement was summarized in Table 1. 3. Results and discussion 3.1. Crystal structure Single crystal X-ray diffraction analysis revealed that compound K3 Dy(PO4 )2 crystallizes in the monoclinic space group P21 /m, and features a two-dimensional (2D) layer structure composed of [Dy(PO4 )2 ]∞ anionic layers and K∞ cationic layers alternatively stacking along the a-axis, as shown in Fig. 2a. In the asymmetric unit, there is one Dy atom, two P atom, three K atom, and six O atoms. P atoms are coordinated by three O atoms into PO4 tetrahedra with the P O bond distances ranging from 1.514(4) Å to 1.545(4) Å and O P O bond angles ranging from 105.5(2)◦ to 110.93(15)◦ (Table 2). These values are common values comparing with other reported rare-earth phosphates [18,19]. The PO4 tetrahedra are isolated and are further interconnected by Dy atoms into a 2D infinite anionic layer of [Dy(PO4 )2 ]∞ , as shown in Fig. 2b. The Dy atom has a DyO7 polyhedral geometry with the Dy O bond distances ranging from 2.257(3) Å to 2.523(4) Å.
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Table 1 Summary of crystal data and structure refinement for K3 Dy(PO4 )2 . Crystal data Chemical formula Mr Crystal system, space group Temperature (K) a, b, c (Å) ˇ (◦ ) V (Å3 ) Z Radiation type (mm−1 ) Crystal size (mm) Diffractometer Absorption correction No. of measured, independent andobserved [I > 2(I)] reflections Rint (sin /)max (Å−1 ) R[F2 > 2(F2 )], wR(F2 ), S No. of reflections No. of parameters rmax , min (e·Å−3 )
K3 DyP2 O8 469.74 Monoclinic, P21 /m 296 7.3994 (9), 5.6136 (7), 9.3944 (11) 90.866 (1) 390.17 (8) 2 Mo K␣ 11.60 0.20 × 0.05 × 0.05 Bruker Apex2 CCD multi-scan 4885, 1068, 1018 0.039 0.667 0.023, 0.056, 1.11 1068 79 3.58, −1.03
Fig. 2. (a) View of the crystal structure of K3 Dy(PO4 )2 down the c-axis; (b) View of the [Dy(PO4 )2 ]∞ layer on the bc-plane.
The K atoms reside among these [Dy(PO4 )2 ]∞ layers and join them through coulombic action of K+ and O2− ions to construct the structure of compound K3 Dy(PO4 )2 . It should be noted that three crystallographically different K atoms have three different coordination geometries, that is, K(1) O10 , K(2) O9 and K(3) O11 polyhedra. The K−O bond distances fall in the range of 2.614(4)–3.255(3) Å, which is comparable with other potassium oxides [20,21]. Bond valence calculations (Dy, 2.44; P, 4.94–4.97; K, 1.06–1.24) indicate that the Dy, P and K atoms are in reasonable oxidation states of +3, +5 and +1, respectively [22]. 3.2. Photoluminescence properties The self-activated photoluminescence (PL) properties of K3 Dy(PO4 )2 was performed on EDINBURGH FLS980 fluorescence spectrophotometer. The excitation spectrum was recorded when monitoring emission wavelength of 571 nm in the range of 300–420 nm. As shown in Fig. 3a, the excitation spectrum consist a series of sharp peaks located at 324, 350, 365, and 387 nm arising from the intrinsic 4f → 4f transitions of Dy3+ . They can be assigned to the transitions from the ground 6 H15/2 state to the excited 4 I11/2 , 6 P5/2 , 4 F7/2 and 6 H15/2 states, respectively [23,24]. Among all the excitation bands, the band at 350 nm corresponding to the 6 H15/2 → 6 P7/2 transition possesses the maximum intensity. The strong excitation bands ranging from 350–400 nm indicate that K3 Dy(PO4 )2 phosphor match well with the could be excited by near-ultraviolet (NUV) emitting of InGaN-based LED chip (350–410 nm), implying a potential application in NUV-pumped white-LEDs. As shown in Fig. 3b, the PL emission spectrum excited by 350 nm UV light consists of two emission regions ranging from 450 to 650 nm: blue and yellow regions. The blue emission band at 488 nm corresponds to the 4 F9/2 → 6 H15/2 transition and the yellow emission band at 571 nm corresponds to the hypersensitive 4 F9/2 → 6 H13/2 transition. It is well-known that if Dy3+ ions located in a site with inversion symmetry, the magnetic dipole transition 4 F9/2 → 6 H15/2 is frequently the strongest one, while in a site without inversion symmetry the 4 F9/2 → 6 H13/2 electronic transition usually becomes dominant. For K3 Dy(PO4 )2 phosphor, the yellow 4 F9/2 → 6 H13/2 emission is stronger than the blue 4 F9/2 → 6 H15/2 , which indicates that the
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Table 2 Selected bond distances (Å) and angles (◦ ) for K3 Dy(PO4 )2 . Dy1—O2i
2.257(3)
K1—O1xvi
3.096(4)
Dy1—O2ii Dy1—O5iii Dy1—O5iv Dy1—O6 Dy1—O1 Dy1—O4 P1—O3 P1—O1 P1—O2 P1—O2viii P2—O6 P2—O4 P2—O5 P2—O5viii K1—O5xiii K1—O5xiv K1—O2vi K1—O2xv K1—O1vi K1—O1v K1—O2xvi K1—O2xvii
2.257(3) 2.359(3) 2.359(3) 2.388(4) 2.421(4) 2.523(4) 1.514(4) 1.545(4) 1.545(3) 1.545(3) 1.531(4) 1.540(4) 1.542(3) 1.542(3) 2.755(3) 2.755(3) 2.802(3) 2.802(3) 2.8205(5) 2.8205(5) 3.030(3) 3.030(3)
K1—O3 K2—O3 K2—O4xvi K2—O5iv K2—O5iii K2—O3v K2—O3vi K2—O2vi K2—O2xv K2—O1 K3—O6xii K3—O4xvi K3—O4xviii K3—O5xvii K3—O5xviii K3—O3v K3—O4x K3—O2v K3—O2xv K3—O5x K3—O5iv
2.684(5) 2.808(4) 2.681(4) 2.875(3) 2.875(3) 2.9747(15) 2.9747(15) 3.007(3) 3.007(3) 2.711(4) 2.614(4) 2.8728(10) 2.8728(10) 2.938(3) 2.938(3) 2.943(4) 3.048(4) 3.087(3) 3.087(3) 3.255(3) 3.255(3)
O3—P1—O1 O3—P1—O2 O1—P1—O2 O3—P1—O2viii O1—P1—O2viii O2—P1—O2viii
111.1(2) 109.50(15) 109.10(14) 109.50(15) 109.09(14) 108.5(2)
O6—P2—O4 O6—P2—O5 O4—P2—O5 O6—P2—O5viii O4—P2—O5viii O5—P2—O5viii
105.5(2) 110.16(15) 110.93(15) 110.16(15) 110.93(14) 109.1(2)
Symmetry codes: (i) −x + 2, y + 1/2, −z + 1; (ii) −x + 2, −y, −z + 1; (iii) −x + 2, −y, −z; (iv) −x + 2, y + 1/2, −z; (v) −x + 1, −y + 1, −z + 1; (vi) −x + 1, −y, −z + 1; (vii) x + 1, y, z; (viii) x, −y + 1/2, z; (ix) x + 1, y, z − 1; (x) −x + 2, −y + 1, −z; (xi) x + 1, y − 1, z; (xii) −x + 1, −y + 1, −z; (xiii) x − 1, −y + 1/2, z + 1; (xiv) x − 1, y, z + 1; (xv) −x + 1, y + 1/2, −z + 1; (xvi) x − 1, y, z; (xvii) x − 1, −y + 1/2, z; (xviii) x − 1, y + 1, z.
Fig. 3. Excitation (a) and emission (b) spectra of K3 Dy(PO4 )2 . (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Dy3+ ions are located at low-symmetry sites without inversion centers. This result matches well with the crystal structure analysis as mentioned above, which demonstrates that all of the Dy3+ ions are located in the glide plane ‘m’ without the inversion center. Moreover, the Commission International de L’Eclairage (CIE) chromaticity coordinates for K3 Dy(PO4 )2 were calculated under the excitation wavelength of 350 nm as shown in Fig. 4. The coordinate was found to be (x = 0.395, y = 0.437) which fall in the green-yellow region of the spectrum, which indicates that the self-activated phosphor K3 Dy(PO4 )2 is a suitable green-yellow emitting phosphor under excitation of near UV LEDs. The decay curve of K3 Dy(PO4 )2 phosphor is measured, as shown in Fig. 5. It is found that after a rapid initial rise in the PL intensity, when irradiation at the wavelength of 350 nm was started, the PL intensity at an emission wavelength of 571 nm decayed slowly to a steady-state value of about microseconds. The decay curve cannot be well fitted with a singleexponential function but can be well fitted a biexponential function as the equation, I(t) = A1 exp(−t/ 1 ) + A2 exp(−t/ 2 ) + I(0) (where I(t) and I0 are the luminescence intensities at time 0 and t; A1 and A2 are fitting parameters; t is the time, 1 and 2
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Fig. 4. The CIE chromaticity diagram for K3 Dy(PO4 )2 with excitation at 350 nm. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Fig. 5. Fluorescent decay curves of K3 Dy(PO4 )2 at 293 K.
are rapid and slow lifetimes for exponential components, respectively). The average decay time of Dy3+ can be calculated by the equation, =
A1 2 +A2 2 1 2 , A1 1 +A2 2
and the value of is calculated to be 3.458 s for representing the lifetime.
4. Conclusions In conclusion, we reported the synthesis, crystal structure and PL properties of a potassium dysprosium orthophosphate K3 Dy(PO4 )2 . The crystal growth was fulfilled using high temperature flux method with additional K2 O–KF–P2 O5 as the flux. Single-crystal X-ray diffraction analysis revealed that this compound crystallizes in the monoclinic space group P21 /m, and the structure features a 2D layer structure containing [Dy(PO4 )2 ]∞ anionic layers and K∞ cationic layers alternatively stacked along the a-axis. Although with high Dy3+ concentration, the emission spectrum excited at 350 nm shows two strong emission bands in blue (488 nm) and yellow (571 nm) regions related with the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions of Dy3+ ions, respectively. The decay curve at 571 nm was measured and the decay time was fitted to be 3.458 s. We think K3 Dy(PO4 )2 can be potentially used as a green-yellow phosphor for fields of near UV-excited white-light-emitting diode and optoelectronic devices. Acknowledgment We are grateful to the National Natural Science Foundation of China (Project 21201056, 21307028).
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