Color tunability and energy transfer studies of a new Eu3+ doped langbeinite-type phosphate phosphor for lighting applications

Journal of Solid State Chemistry 279 (2019) 120965

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Color tunability and energy transfer studies of a new Eu3þ doped langbeinite-type phosphate phosphor for lighting applications Shi-Rui Zhang, Dan Zhao *, Yun-Chang Fan **, Zhao Ma, Ya-Li Xue, Ya-Nan Li College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan, 454000, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Crystal structure Phosphate Eu3þ Dy3þ Luminescence

Phosphate compounds have been widely studied in solid luminescence due to their variable structure types and excellent performance. In this work, we reported a new langbeinite-type phosphate, K2DyHf(PO4)3, in which the octahedral sites are mixed occupied by Dy3þ and Hf4þ ions. For further confirm the proportion of Dy/Hf, we carried out scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis. Meanwhile, in order to investigate the effect of Eu3þ doping on the luminous properties, a series of Eu3þ doped solid solutions K2Dy1-xEuxHf(PO4)3 (x ¼ 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared and investigated by powder X-ray diffraction, IR, UV–Vis. The photoluminescence spectra demonstrated that energy transfer between Dy3þ and Eu3þ happens. The results suggest that the emiting light can be tuned from whitish, yellowish to orangish in K2Dy1-xEuxHf(PO4)3 phosphors by modulating x values.

1. Introduction White light-emitting diode (LED) light source has become the main lighting source because of its advantages of environmental protection, energy saving and high efficiency, long work life and so on [1]. A typical LED light source is comprised of a PN junction chip and a phosphor to generate the white light emission. The current popular phosphors are mainly comprised of an activated ion and a host matrix, including oxides, rare earth sulfur oxides, rare earth halogen oxides and so forth. An excellent phosphor should involve characteristics of rich spectral lines [2], narrow luminous bands and concentrated luminous energy in the visible region [3]. It is well known that single Dy3þ doped luminescent materials exhibit two characteristic emission in the blue (4F9/2→6H15/2 transition) and yellow (4F9/2→6H13/2 transition) regions lack of red component. In order to solve this problem, a red light-emitting center is introduced in the light-emitting material doped with the Dy3þ ion. Because the special structure of Eu3þ ion is red in near ultraviolet emission, it is a good choice to add red light component. Therefore, the Eu3þ and Dy3þ are co-doped in the light-emitting matrix, so that better color light performance can be obtained, for example SrGd2O4:Dy3þ/Eu3þ [4], Ca5(PO4)3F:Dy3þ, Eu3þ [5] and Na3Sc2(PO4)3:Dy3þ, Eu3þ [6]. Phosphate is an oxysalt host material and have attract much

attentions [7] because of its simple synthesis condition, low cost and good chemical stability. The basic building block of phosphates is the PO4 tetrahedron, which is flexible and can inhabit various coordination environments by altering the P─O bond lengths in the a wide range of 1.45─1.65 Å [8]. In addition, combining MO6 (M ¼ rare-earth metal elements or transition) with PO4 groups may construct various structures in which the MO6 and PO4 gourps are interconnected via common O atoms. A typical example is the phosphate, such as KSrSc2(PO4)3:Ce3þ/Eu2þ/Tb3þ [9], KEu(PO3)4 [10], K2YHf(PO4)3 [11], NaAlP2O7:Pr3þ [12]. However, there was no reports on langbeinite-type phosphate containing both Dy3þ and Hf4þ which we think, they can enter into M sites to form a disordered occupancy. For this purpose we designed a synthetic exploration the quaternary system K2O─Dy2O3─HfO2─P2O5, and a new phosphate compound K2DyHf(PO4)3 was successfully obtained. Herein, we will report the crystal structure, and Eu3þ doped fluorescent performance. 2. Experimental section 2.1. Materials and instrumentation All reacting ingredients KH2PO4 (99.5 %), KF.2H2O (99.0 %),

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Zhao), [email protected] (Y.-C. Fan). https://doi.org/10.1016/j.jssc.2019.120965 Received 13 July 2019; Received in revised form 11 September 2019; Accepted 17 September 2019 Available online 18 September 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.

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Dy2O3 (99.99 %), Eu2O3 (99.99 %), HfO2 (99.99 %) and NH4H2PO4 (99.0 %) were purchased from Sinopharm Chemical Reagent Shanghai limited company. All chemicals were used as received without further purification. The X-ray powder diffraction (XRD) analysis were performed by Rigaku SmartLab 9 KW dffractormeter with graphitemonochromated CuKα characteristic radiation, operating at 150 mA, 40 KV; scanning speed, step length and diffraction range were 10 min1, 0.02 and 5─75 and the Rietveld structure calculation was carried out by the GSAS-EXPGUI program to further clarify the crystal structure. The microstructures of the products were characterized using scanning electron microscopy, and the compositions atomic percentage was obtained from energy dispersive X-ray spectroscopy. Infrared (IR) data were recorded on a Bruker Tensor 27 spectrophotometer (Bruker Optics, Munich, Bavaria, Germany) with KBr pellets in the region of 3000─400 cm1. Ultraviolet–visible (UV─Vis) spectrum was measured using a UH4150 (HITACHI company) spectro-photometer in the range of 240─700 nm. Photoluminescence excitation (PLE) and emission (PL) spectra were carried out by FLS1000 Edinburgh Analytical Instrument. A microsecond pulsed xenon flash lamp μF900 is available to record the decay curves for lifetimes with an average power of 60 W.

Table 1 Experimental details for the data collection and structural refinement details of K2DyHf(PO4)3.

2.2. Synthetic procedures

Chemical formula

DyHfK2O12P3

Mr Crystal system, space group Temperature (K) a (Å) V (Å3) Z Radiation type μ (mm1) Crystal size (mm) Diffractometer Absorption correction No. of measured, independent and observed [I > 2σ(I)] reflections Rint (sin θ/λ)max (Å1) R[F2 > 2σ(F2)], wR(F2), S No. of reflections No. of parameters Δρmax, Δρmin (e∙Å3) Absolute structure

704.10 Cubic, P213 296 10.2733 (5) 1084.25 (16) 4 Mo Kα 17.68 0.18  0.15  0.14 Bruker Apex2 CCD Multi-scan 7283, 910, 905

Absolute structure parameter Computer program

Single crystal of K2DyHf(PO4)3 was cultured by molten salt method with the additional K2O–P2O5 as flux. A solute-flux mixture, KH2PO4 (2.02 g, 14.84 mmol), HfO2 (0.160 g, 0.75 mmol), Dy2O3 (0.128 g, 0.34 mmol), KF.2H2O (0.6437 g, 6.84 mmol) with appreciate ratio of K/  Hf/Dy/P/F ¼ 33/1/1/224/10 was heat treated at 860 C for 5 h to ensure   the best homogeneity, cooled at 5 C /h to 450 C and finally cooled to ambient temperature by the furnace turn off. Finally, wash the product by hot water, and then some block shaped colorless crystals of K2DyHf(PO4)3 were obtained with the size of about 0.16 mm. The polycrystalline sample of K2Dy1-xEuxHf(PO4)3 (x ¼ 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) can be prepared via solid state reaction of a mixture of KH2PO4, Dy2O3, Eu2O3, HfO2 and NH4H2PO4 in the stoichiometry ratio. The mixture was put into an agate mortar and  was calcined in a 20 ml platinum crucible for 40 h at 1000 C, with fourth intermediate grinding stages every 10 h to make sure a complete solidstate reaction.

0.027 0.666 0.015, 0.036, 1.23 910 59 0.55, 0.95 Flack x determined using 376 quotients 0.008 (7) SHELXL2017/1

for Dy) whereas Hf is enriched on the Dy|Hf(2) atom site [58 % for Hf]. In order to further confirm the proportion of Dy/Hf, we carried out scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, and the results are shown in Figs. S1 and S2 (see supporting file). The experimental results show that the atomic ratio of Dy/Hf is 4.69/4.85, very close to 1:1. The small dispersion might be due to the generally insufficient of EDS method or the poor morphology of selected single crystal. Thus we believe that the prepared K2DyHf(PO4)3 is stoichiometry to achieve electric neutrality. Table 1 presents the experimental details for data collection and structural refinement details of compound K2DyHf(PO4)3, important bond lengths and bond angles were given in Table 2, atomic coordinates and isotropic displacement coefficients are listed in Table S1, fractional atomic anisotropy displacement parameters (Å2) are listed in Table S2, and the detailed crystal data was embraced in a CIF file deposited in Inorganic Crystal Structure Database (No. 1939089).

2.3. Crystallography A suitable single crystal with dimensions of 0.18  0.15  0.14 mm was selected for the single-crystal X-Ray diffraction (SC-XRD) experiments. A set of intensity data was collected using a Bruker Smart Apex2 CCD diffractometer system equipped with a graphite-monochromated Mo-Kα radiation source (λ ¼ 0.71073 Å) with a tube power of 50 kV and 20 mA. The frames were collected at ambient temperature of 293 K with a scan width of 0.5 in ω and integrated with the Bruker software package. The unit cell was determined and refined by least square method upon the refinement of XYZ centroid of reflections above 20 σ (I). No weak satellite reflections were observed and thus no structure modulation was considered in the structure model. Then the date was scaled for absorption using the Sadabs program of Apex2 package [13]. Intensities of all measured reflections were corrected for Lorentz–polarization (Lp) and crystal absorption effects. The structure solution was fulfilled by direct method using software SHELX-2017 [14]. Crystal data, data collection and structure refinement details are summarized in Table 1.It should be mentioned that there exists two Dy|Hf substitution disorder sites in the structure. In the course of structural refinement, the atomic position and anisotropic displacement parameters of Dy|Hf(1) and Dy|Hf(2) atoms in these sites were both constrained to be identical, and the relative occupancies were set freely. Moreover, a ‘Sump’ constraint instruction was used to restraint Dy and Hf atoms in the two mixed sites to get the total molar ratio of 1:1 to balance the charge. The final refined results show that Dy is enriched on the Dy|Hf(1) site (58 %

3. Results and discussion 3.1. Synthesis For title compound K2DyHf(PO4)3, prepared single crystals was not pure phase, mixed with some impurities in amorphous or glass phase. In order to obtain pure phase for property studies, we used traditional solid state method to prepare K2Dy1-xEuxHf(PO4)3 (x ¼ 0–1.0). The reaction can be shown by the following equation: 4 KH2PO4 þ (1-x) Dy2O3 þ x Eu2O3 þ 2 HfO2 þ 2 NH4H2PO4 → 2 K2Dy1xEuxHf(PO4)3 þ 2 NH3 þ 7H2O The purity of the polycrytallline samples could be confirmed by XRD analysis, as shown in Fig. 1a and Fig. S3 (see supporting information). It can be observed that all experimental diagrams are agreement well with the simulated pattern that was obtained using JANA2006 software [15]. Thus we confirmed that all prepared samples were pure phases. Specifically, the typical diffraction peak at 27 was observed to move to small angle with increasing Eu3þ concentration, which further confirm the Eu3þ was successfully doped into K2DyHf(PO4)3 crystal. Moreover, Rietveld refinement for K2Dy0.95Eu0.05Hf(PO4)3 was performed, as shown in Fig. 1b and Table 3. The final obtained Rwp and Rp are determined to be reasonable values of 10.16 and 7.51%, respectively. The

2

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Journal of Solid State Chemistry 279 (2019) 120965

Table 2 Select bond distances of K2DyHf(PO4)3. K1—O4

2.954 (7) i

K1—O4 K1—O4ii K1—O1iii K1—O1iv K1—O1v K1—O3i K1—O3ii K1—O3 K1—O3v K1—O3iv K1—O3iii K2—O2vi K2—O2vii K2—O2 K2—O1viii K2—O1ix K2—O1x K2—O3x

2.954 2.954 3.102 3.102 3.102 3.238 3.238 3.238 3.265 3.265 3.265 2.968 2.968 2.968 3.172 3.172 3.172 3.284

(7) (7) (8) (8) (8) (9) (9) (9) (9) (9) (9) (7) (7) (7) (8) (8) (8) (8)

O3v—Dy1—O3ii O3v—Dy1—O3xi O3ii—Dy1—O3xi O3v—Dy1—O4vii O3ii—Dy1—O4vii O3xi—Dy1—O4vii O3v—Dy1—O4vi O3ii—Dy1—O4vi O3xi—Dy1—O4vi O4vii—Dy1—O4vi O3v—Dy1—O4 O3ii—Dy1—O4 O3xi—Dy1—O4 O4vii—Dy1—O4 O4vi—Dy1—O4 O1—Dy2—O1xii O1—Dy2—O1ix O1xii—Dy2—O1ix

90.0 (3) 90.0 (3) 90.0 (3) 171.3 (3) 94.0 (3) 82.3 (3) 82.3 (3) 171.3 (3) 94.0 (3) 94.2 (3) 94.0 (3) 82.3 (3) 171.3 (3) 94.2 (3) 94.2 (3) 88.1 (3) 88.1 (3) 88.1 (3)

K2—O3viii ix

Table 3 Refined crystallographic parameters of K2Dy0.95Eu0.05Hf(PO4)3 sample.

3.284 (8)

K2—O3 Dy1—O3v Dy1—O3ii Dy1—O3xi Dy1—O4vii Dy1—O4vi Dy1—O4 Dy2—O1 Dy2—O1xii Dy2—O1ix Dy2—O2xiii Dy2—O2vii Dy2—O2xiv P1—O4 P1—O1 P1—O2 P1—O3

3.284 2.114 2.114 2.114 2.131 2.131 2.131 2.085 2.085 2.085 2.129 2.129 2.129 1.503 1.513 1.515 1.521

(8) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (7)

O1—Dy2—O2xiii O1xii—Dy2—O2xiii O1ix—Dy2—O2xiii O1—Dy2—O2vii O1xii—Dy2—O2vii O1ix—Dy2—O2vii O2xiii—Dy2—O2vii O1—Dy2—O2xiv O1xii—Dy2—O2xiv O1ix—Dy2—O2xiv O2xiii—Dy2—O2xiv O2vii—Dy2—O2xiv O4—P1—O1 O4—P1—O2 O1—P1—O2 O4—P1—O3 O1—P1—O3 O2—P1—O3

175.7 (3) 92.2 (3) 87.7 (3) 87.7 (3) 175.7 (3) 92.2 (3) 92.1 (2) 92.2 (3) 87.7 (3) 175.7 (3) 92.1 (2) 92.1 (2) 109.9 (4) 110.6 (4) 109.6 (4) 108.5 (4) 107.6 (4) 110.6 (4)

Formula

K2Dy0.95Eu0.05Hf(PO4)3

Crystal system Space group a ¼ b ¼ c, Å α ¼ β ¼ γ, ( ) V (Å3) 2θ-interval Z Rwp, % Rp, %

cubic P 21 3 10.330071(1562) 90 1102.236 5─75 4 10.15% 7.51%

structure with space group P213 (193#). The crystal structure can be thought as a three-dimensional (3D) anionic network of [DyHf(PO4)3]∞ interconnected by K(I) ions (Fig. 2) [18]. There are two K atoms, two disordered Dy|Hf atoms, one P atom and four O atoms in the asymmetric unit of K2DyHf(PO4)3. In this structure, P(1) atom is coordinated by four O atoms to form a distorted PO4 tetrahedron with P–O bond distances of 1.503(6)─1.521(7) Å and O–P–O bond angles of 107.6(4)─110.6(4) Å (Table 2), which is commonly comparing with other reported phosphate compounds [19]. Dy|Hf(1) and Dy|Hf(2) are both coordinated by six oxygen atoms to generate Dy|Hf(1)O6 and Dy|Hf(2)O6 octahedra with the Dy|Hf‒O bond distances ranging from 2.085(6) to 2.131(6) Å. Because the ionic radius of Dy3þ (1.052 Å for CN ¼ 6) is significantly longer than that of Hf4þ (0.85 Å for CN ¼ 6), the Dy|Hf‒O bond distances are generally shorter than Dy‒O but longer than Hf‒O bonds for other reported Dy3þ and Hf4þ contained oxide compounds [20]. PO4 groups are isolated from each other and are bridged by Dy|Hf atoms into a [DyHf(PO4)3]∞ anionic 3D network. In this structure, Dy| HfO6 octahedron is coordinated by six PO4 tetrahedra via corner-sharing O atoms, whereas each PO4 tetrahedron is surrounded by four Dy|HfO6 octahedra, as shown in Fig. 3. Taking the P and Dy|Hf atoms as the nodes, the [DyHf(PO4)3]∞ network can be described as a 4,6,6-connected 3D topological net. The Schlafli symbols are 466683 for six-connected Dy| Hf(1)O6 groups, 4669 for six-connected Dy|Hf(2)O6 groups and 4363 for four-connected PO4 groups. The anionic [DyHf(PO4)3]∞ network delimits large cavities in which K(I) ions locate to sustain the framework and compensate the negative charges of the anionic network into the total structure of compound K2DyHf(PO4)3. K(1) and K(2) atoms are tweleve- or nine-coordinated by oxygen atoms with K–O bond distances of 2.954(7)─3.284(8) Å, which falls in the common values of inorganic potassium oxide compounds.

Symmetry codes: (i) z, x, y; (ii) y, z, x; (iii) yþ1, z1/2, xþ1/2; (iv) z1/2, xþ1/2, yþ1; (v) xþ1/2, yþ1, z1/2; (vi) zþ1/2, xþ1, y1/2; (vii) yþ1, zþ1/2, xþ1/2; (viii) x1/2, yþ3/2, zþ1; (ix) y1/2, zþ3/2, xþ1; (x) z1/2, xþ3/2, yþ1; (xi) zþ1, xþ1/2, yþ1/2; (xii) zþ1, xþ1/ 2, yþ3/2; (xiii) z, xþ1, y; (xiv) xþ1/2, yþ3/2, zþ1.

results further confirmed the formation of a good solid solutions of K2Dy1-xEuxHf(PO4)3 (x ¼ 0–1.0) [16]. 3.2. Crystal structure

3.3. IR and UV–Vis spectra As far as our knowledge goes, compound K2DyHf(PO4)3 is the first compound in the quaternary K2O‒Dy2O3‒HfO2‒P2O5 system. SC-XRD analysis reveals that it features langbeinite [K2Mg2(SO4)3] [17] type

IR spectra of phosphate K2DyHf(PO4)3 are supplied in Fig. 4. The peaks at 555, 586, 625, 975 and 1107 cm1 are ascribed to the symmetric

Fig. 1. (a) PXRD patterns of powder samples K2Dy1-xEuxHf(PO4)3 (x ¼ 0, 0.05, 0.2, 0.4, 0.8, 1.0) comparing with simulated one of K2DyHf(PO4)3. (b) Rietveld refinements of the XRD files for K2Dy0.95Eu0.05Hf(PO4)3. 3

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Fig. 5. Experimental UV–Vis absorption spectrum of K2DyHf(PO4)3 ranging from 240 to 700 nm.

Fig. 2. View of the crystal structure of K2DyHf(PO4)3.

Fig. 3. The coordination environments of the (Dy|Hf)O6 and PO4 groups.

Fig. 6. Excitation and Emission spectra of K2DyHf(PO4)3.

Fig. 4. The IR spectrum of K2DyHf(PO4)3 in the range of 3000─400 cm1

stretching mode of PO4 group, because the IR absorption bands of PO4 generally locate at 1120─940 and 650─540 cm1 scopes, such as Cu3(PO4)2 [21], Ca9Al1-xYx(PO4)7:Eu2þ [22] and NaBaFe2(PO4)3 [23]. A weak peak at 1624 cm1 band may be explained by interaction between P─O and (Dy|Hf)─O bonds. The UV–Vis diffuse reflectance (UV-DR) spectrum of K2DyHf(PO4)3 was measured and given in Fig. 5. At room temperature, it shows a strong band absorption appear at 265 nm, with no effect on the 4f→4f transition of Dy3þ. A series peaks of medium intensity are observed in the wavelength scope of 300─500 nm, due to the characteristic 4f→4f transitions

Fig. 7. Excitation and Emission spectra of K2EuHf(PO4)3.

of Dy3þ: 6H15/2→4K15/2 (324 nm), 6H15/2→6K7/2 (351 nm), 6H15/2→6K5/2 (365 nm), 6H15/2→4I13/2 (388 nm), 6H15/2→4G11/2 (423 nm) and 6H15/ 4

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highly hypersensitive 4F9/2→6H13/2 transition that is hinge on the crystal field strength of the surrounding environment of Dy3þ [27]. If the Dy3þ sites has an inversion center, the blue peak is frequently the stronger one for its insensitivity to the local environment. Otherwise, the yellow one becomes dominant because it is more sensitivity to the host lattice. Herein, Dy3þ occupy non-centrosymmetry crystallographic sites according to SC-XRD analysis, matching well with the luminescent features [28]. Fig. 7 shows the PLE and PL spectra of doped 100% Eu3þ substituted sample K2EuHf(PO4)3. By monitoring 611 nm emission, the PLE spectrum exhibits several sharp peaks and abroad band ranging from 200─300 nm, which is attributed to the charge transfer band of O2→Eu3þ [29]. The peaks at 319 nm, 363 nm, 382 nm, 393 nm, 416 nm and 464 nm are ascribed to the 7F0→5H3, 7F0→5D4, 7F0→5L7, 7F0→5L6, 7 F0→5D3 and 7F0→5D2 transitions of Eu3þ, respectively [30,31]. The 7 F0→5L6 transition at 393 nm has the highest intensity, which is in accord with other Eu3þ activated phosphors [32]. By using 393 nm light as the exciting source, the emission spectrum displays its many Eu3þ characteristic peaks at about 580, 594, 611, 654, and 704 nm, which are assigned to the 5D0→7FJ (J ¼ 0, 1, 2, 3, 4) transitions [33]. In addition, we observed that the intensity of the 5D0→7F1 magnetic-dipole transition (594 nm) is much lower than that of the 5D0→7F2 electric-dipole transition (611 nm). It illustrates that Eu3þ ions mainly occupy the inversion symmetry sites in the host lattice, which is consistently with above SC-XRD analysis [34]. The PLE spectrum of K2Dy0.98Eu0.02Hf(PO4)3 by monitoring at 577 nm of Dy3þ: 4F9/2→6H13/2 emission and at 611 nm of Eu3þ: 5D0→7F2 emission are drawn in Fig. 8 for comparation. Fig. 8a only shows the characteristic bands of Dy3þ, whereas characteristic peaks of both Dy3þ and Eu3þ coexist in Fig. 8b. This suggests that energy transfer from Dy3þ to Eu3þ occurs but the contrary transfer from Eu3þ to Dy3þ do not happen. It is worth nothing that the intensity of Dy3þ bands at 350 nm (Fig. 8b) is significantly lower than these of Eu3þ bands, showing the in Dy3þ→Eu3þ charge transfer is inefficient [35]. The other important thing is, the 393 nm photons can effectively excite both Dy3þ and Eu3þ, respectively. Therefore, we may expect a muti-color light emission can be achieved under 393 nm radiation [36]. Fig. 9 depicts energy levels of Dy3þ, Eu3þ and possible Dy3þ→Eu3þ energy transfer mechanism [37]. Under 350 nm excitation, the electrons

Fig. 8. (a) The PLE spectrum of K2Dy0.98Eu0.02Hf(PO4)3 phosphors by monitoring at 577 nm; (b) The PLE spectrum of K2Dy0.98Eu0.02Hf(PO4)3 and K2EuHf(PO4)3 phosphors by monitoring at 611 nm. 4 2→ I15/2

(452 nm) [24].

3.4. Steady-state fluorescence The PL excitation spectra and emission spectra of K2DyHf(PO4)3 are shown in Fig. 6. Four strong excitation peaks were observed at 350 nm, 364 nm, 386 nm and 453 nm, which corresponded to the 6H15/2→6K7/2, 6 H15/2→6K5/2, 6H15/2→4I13/2 and 6H15/2→4I15/2 transitions of Dy3þ, respectively [25]. Besides, the excitation spectra contain a broad band (240─315 nm) attributed to the O2→Dy3þ charge transfer band (CTB). These results demonstrate that the K2DyHf(PO4)3 phosphor was readily excited by UV light [26]. The emission spectra excited by 350 nm light, two strong emission peaks were observed at 482 nm and 577 nm, which were ascribed to the blue 4F9/2→6H15/2 and yellow 4F9/2→6H13/2 transitions of Dy3þ, respectively. The highest intensity at 577 nm is due to

Fig. 9. The schematic for the energy transfer from Dy3þ to Eu3þ. 5

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on Dy3þ are excited from the ground state (6H15/2) to the excited state (4I11/2) [38]. Then 4I11/2 state may undergo a non–radiative process relaxing to 4F9/2 state resulting in the Dy3þ transition emission, or Dy3þ transfers the energy to 5D2 state of Eu3þ to give the characteristic emissions of Eu3þ [39]. Then the electrons can nonradiatively relax to the lowest excited level 5D0 (Eu3þ), where the Eu3þ emissions (5D0→7F1, 2) take place [40]. A series of K2Dy1-xEuxHf(PO4)3 (x ¼ 0  0.8) has been prepared to achieve multi-color light emission. The variations of emission spectra at 393 nm and corresponding intensities are plotted in Fig. 10 and Fig. 11. It can be observed that the emission profiles own both Dy3þ and Eu3þ emission peaks under 393 nm excitation. The position of all peaks does not change with the change concentration of Eu3þ, but the intensity of peaks undergo tremendous changes [41]. When x < 0.05, the emission intensity of Dy3þ decreases monotonously with the augment of x value and Eu3þ emission intensity reaches the maximum at x ¼ 0.05. Afterward, the Eu3þ emission intensity weakens sharply along with the increment of Eu3þ, which can be attributed to concentration quenching effect [42]. On the other hand, the relatively stable luminescent intensity of Dy3þ with increasing Eu3þ concentrations range from 00.05 again proves the inefficiency of Dy3þ→Eu3þ charge transfer. In this case, we can obtain a hybrid luminescent emission of both Dy3þ and Eu3þ and modulate the emitting color by simply adjusting the Eu3þ concentration [43].

Fig. 11. Emission spectra of doped phosphors K2Dy1-xEuxHf(PO4)3 (x ¼ 0.05, 0.1, 0.02, 0.03, 0.04, 0.05) by monitoring at 393 nm. Table 4 The CIE chromaticity coordinates (x, y) and correlated color temperature (CCT) (K) of K2DyHf(PO4)3 and K2Dy1-xEuxHf(PO4)3 (x ¼ 0.01─0.05).

3.5. Emission color analysis The Commission International de l’Eclairage (CIE) and correlated color temperature (CCT) of K2Dy1-xEuxHf(PO4)3 (x ¼ 0─0.05) phosphors were calculated from the experimental data, and the results were summarized in Table 4 and Fig. 12. With increasing concentration of Eu3þ, the value of CIE increased and CCT value decreased. The chromaticity coordinates for the K2DyHf(PO4)3 under excitation at 350 nm are (0.342, 0.345), which fall in the white region of the spectrum. The emitted color of K2Dy1-xEuxHf(PO4)3 phosphors can be gradually change from whitish through yellowish, and eventually to orangish by adjusting the concentration of Eu3þ. All of these results suggest that multicolor luminescence can be achieved by modulating the concentration of Eu3þ dopant [44, 45].

No. of the point in the CIE diagram

λex (nm)

Concentration of Eu3þ (mol)

CIE (x, y) x

y

1 2 3 4 5 6

350 393 393 393 393 393

0 0.01 0.02 0.03 0.04 0.05

0.342 0.395 0.416 0.429 0.469 0.472

0.345 0.376 0.367 0.358 0.371 0.365

CCT (K) 5095 3578 2979 2602 2116 2034

3.6. Fluorescence lifetime The photoluminescent decay curves of Eu3þ doped series K2Dy1xEuxHf(PO4)3 (x ¼ 0─0.05) phosphors were studied at their emission peak wavelengths 577 nm with 393 nm excitation (Dy3þ: 4F9/2→6H13/2)

Fig. 12. The chromaticity coordinates of K2DyHf(PO4)3 (1) and K2Dy1(x ¼ 0.01─0.05) (2─6).

xEuxHf(PO4)3

[46]. All decay curves can be well fitted by a second-order exponential function and the corresponding decay time can be calculated by the following equation [47]: IðtÞ ¼ A1 expð  t1 =τ1 Þ þ A2 expð  t2 =τ2 Þ þ I0 Fig. 10. Emission spectra of doped phosphors K2Dy1-xEuxHf(PO4)3 (x ¼ 0, 0.01, 0.02, 0.03, 0.04, 0.05) by monitoring at 393 nm. The inset shows emission intensity of Dy3þ ions at 577 nm and Eu3þ ions at 611 nm as a function of Eu3þ concentration.

(2)

in which I(t) and I0 represents the luminescence intensity at time t and t ¼ 0, respectively; τ1 and τ2 are the luminescence lifetime, A1 and A2 are constants. The average decay lifetime can be calculated by the following 6

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Journal of Solid State Chemistry 279 (2019) 120965

Fig. 13. Luminescent decay curves of Dy3þ: 4F9/2→6H13/2 emission of phosphors K2Dy1-xEuxHf(PO4)3, (x ¼ 0.01, 0.02, 0.03, 0.04, 0.05) (λex ¼ 393 nm, λem ¼ 577 nm).

Universities of Henan Province (NSFRF170301), China; Henan Postdoctoral Foundation and Program for Innovative Research Team (in Science and Technology) in the University of Henan Province (16IRTSTHN005), China.

equation [48]:

τave ¼

ðA1 τ1 2 Þ þ ðA2 τ2 2 Þ ðA1 τ1 þ A2 τ2 Þ

(3)

As shown in Fig. 13, the effective lifetime values were determined to be 2.65, 2.60, 2.55, 2.54, 2.52 and 2.40 μs for K2Dy1-xEuxHf(PO4)3 samples with x ¼ 0, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively. In the case of the energy transfer, the luminescent lifetime of a sensitizer is shortened [49], because there are additional decay channels that shorten the lifetime of the excited state. In this work, the lifetimes decrease gradually with the increasing of Eu3þ concentrations from 0─0.05, which verifies the feasibility of energy transfer from Dy3þ to Eu3þ [50].

Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.120965. References [1] S. Jiang, X. Luo, Y. Liu, Y. Zhang, C. Huang, Y. Wang, X. Luo, G. Xiang, X. Tang, L. Li, X. Zhou, Warm white light emission of apatite-type compound Ca4Y6O(SiO4)6 doped with Dy3þ, Mater. Res. Bull. 106 (2018) 428–432. [2] Y. Xue, D. Zhao, S. Zhang, Y. Li, Y. Fan, A new disordered langbeinite-type compound, K2Tb1.5Ta0.5P3O12, and Eu3þ-doped multicolour light-emitting properties, Acta Crystallogr. C: Struct. Chem. 75 (2019) 213–220. [3] G.J. Barbosa Junior, A.M. Sousa, S.M. de Freitas, R.D.S. Santos, M.V.D.S. Rezende, Investigation of Europium dopant in the orthophosphate KMPO4 (M ¼ Ba and Sr) compounds, J. Phys. Chem. Solids 130 (2019) 282–289. [4] X. Sun, T. Han, D. Wu, F. Xiao, S. Zhou, Q. Yang, J. Zhong, Investigation on luminescence properties of Dy3þ, Eu3þ-doped, and Eu3þ/Dy3þ-codoped SrGd2O4 phosphors, J. Lumin. 204 (2018) 89–94. [5] M. Yu, W. Zhang, S. Qin, J. Li, K. Qiu, Synthesis and luminescence properties of single-component Ca5(PO4)3F:Dy3þ, Eu3þ white-emitting phosphors, J. Am. Ceram. Soc. 101 (2018) 4582–4590. [6] R. Vijayakumar, H. Guo, X. Huang, Energy transfer and color-tunable luminescence properties of Dy3þ and Eu3þ co-doped Na3Sc2(PO4)3 phosphors for near-UV LEDbased warm white LEDs, Dyes Pigments 156 (2018) 8–16. [7] Z. Xu, Q. Zhu, X. Li, X. Sun, J. Li, White-light emitting (Y, Gd)PO4:Dy3þ microspheres: Gd3þ mediated morphology tailoring and selective energy transfer and correlation of photoluminescence behaviors, Mater. Res. Bull. 110 (2019) 149–158. [8] S. Han, H. Li, Z. Yang, H.H. Yu, S. Pan, Three new phosphates, Cs8Pb4(P2O7)4, CsLi7(P2O7)2 and LiCa(PO3)3: structural comparison, characterization and theoretical calculation, Dalton Trans. 48 (2019) 8948–8954. [9] M. Jiao, W. Lv, W. Lue, Q. Zhao, B. Shao, H. You, Optical properties and energy transfer of a novel KSrSc2(PO4)3:Ce3þ/Eu2þ/Tb3þ phosphor for white light emitting diodes, Dalton Trans. 44 (2015) 4080–4087. [10] M. Malinowski, M. Kaczkan, Absorption intensity analysis and emission properties KEu(PO3)4 and KEuxY1-x(PO3)4 crystals, J. Lumin. 211 (2019) 138–143. [11] I.V. Ogorodnyk, I.V. Zatovsky, N.S. Slobodyanik, Rietveld refinement of langbeinite-type K2YHf(PO4)3, Acta Crystallogr. E: Crystallogr Commun. 65 (2009) I63–U191.

4. Conclusions In general, a new langbeinite-type phosphate K2DyHf(PO4)3 was successfully synthesized by molten salt method. The crystal structure can be thought as a 3D anionic network of [DyHf(PO4)3]∞ interconnected by K(I) ions. The XRD, IR spectrum, UV–Vis spectrum and photoluminescence properties were studied. A series of Eu3þ-doped phosphors K2Dy1-xEuxHf(PO4)3 (x ¼ 0.0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) were synthetized via traditional high temperature solid state reaction method. The near-UV light (393 nm) can both excite Dy3þ and Eu3þ ions, resulting in a mixed color emission. The Dy3þ→Eu3þ process happens, with increasing Eu3þ concentration, the emission intensity of Dy3þ decreases monotonously contrary to the Eu3þ emission intensity in the current concentration interval. In this case, a multi-color emission properties can be generated with NUV light excitation for phosphors K2Dy1-xEuxHf(PO4)3, from whitish through yellowish, and eventually to orangish by adjusting the concentration of Eu3þ. The K2Dy1xEuxHf(PO4)3 material may find potential applications as a phosphor in the field of LED light illuminator. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21201056); Plan for Scientific Innovation Talent of Henan Province (194200510019), China; Fundamental Research Funds for the 7

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