High thermal stability and colour saturation red-emitting Ba2AGe2O7: Eu3+ (A = Mg, Zn) phosphors for WLEDs

Journal of Luminescence 216 (2019) 116734

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

High thermal stability and colour saturation red-emitting Ba2AGe2O7: Eu3+ (A = Mg, Zn) phosphors for WLEDs

T

Changyan Jia,∗, Ting-Hong Huangb,∗, Zhi Huangc, Jin Wena,∗, Wei Xied, Xiuying Tiana, Tengyan Wua, Hengping Hec, Yangxi Penga a Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials, School of Materials and Environmental Engineering, Hunan University of Humanities, Science and Technology, Loudi, 417000, China b School of Chemical Engineering, Sichuan University of Science & Engineering, Zigong, 643000, China c National Electronic Ceramics Product Quality Supervision and Inspection Center (Hunan), Loudi, 417000, China d School of Physical Science and Technology, Lingnan Normal University, Zhanjiang, 524048, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: WLEDs Thermal stability Colour saturation Ba2AGe2O7: Eu3+

Two series of Eu3+ ions activated high performance red-emitting phosphors Ba2(1-x)AGe2O7: 2xEu3+ (A = Mg, Zn, 0 ≤ x ≤ 0.2) were synthesized by high-temperature solid-state procedure. The research results showed that the host lattice Ba2MgGe2O7 possess higher band gap than that of Ba2ZnGe2O7, and all the Ba2(1-x)MgGe2O7: 2xEu3+ (BMGO: 2 xEu3+, 0 ≤ x ≤ 0.2) and Ba2(1-x)ZnGe2O7: 2xEu3+ (BZGO: 2 xEu3+, 0 ≤ x ≤ 0.2) phosphors crystallized in a tetragonal unit cell with the space group of P421 m (113). Compared with other phosphors in the same series, the samples BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ presented the highest absorption intensity when monitoring at 616 nm under 395 nm excited, respectively. Moreover, the samples BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ not only exhibited high color purity of 90% and 90.3%, respectively, but also possess excellent thermal stability. Unsurprisingly, the WLED devices based on BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ as the red-emitting components displayed good luminescence properties, especially the color rendering index (CRI, Ra) R9 represented the saturation of colour of illuminated objects exhibited a high value of 89.6 and 82.3, respectively. In contrast, the WLED device based on BMGO: 0.30 Eu3+ as red-emitting phosphors presented better electroluminescence performances than those of BZGO: 0.25 Eu3+ on account of its much favorable crystal structure, PL performance and thermal stability.

1. Introduction White light-emitting diodes (WLEDs) deemed as the most promising lighting technologies have attracted more and more attention for their advantages of high brightness, reliability, lower power consumption, long lifetime, and environmentally friendly [1–4]. Nowadays, most of ways to fabricate the WLEDs are based on phosphors. Therefore, the properties of phosphors have an important influence on their application performance of the WLEDs. Currently, the commercial WLEDs is fabricated by the combination of a blue-emitting GaN LED chip and Y3Al5O12:Ce3+ (YAG:Ce) yellow phosphors [5–7]. However, the YAG:Ce3+ converted WLEDs usually present the color rendering index (CRI, Ra) less than 80 on account of the deficiency of red-emitting components. Though the Mn4+ activated fluoride phosphors and rare earth ions Eu2+ doped nitride phosphors were developed to overcome the issue [8–11], the weak luminous intensity and poor stability caused

∗

by low Mn4+ doping content and some harsh reaction conditions of Eu2+ doped nitride phosphors restrict their application. In this connection, the WLEDs were fabricated with near ultraviolet LED chips and blue/green/red tricolor with a view to achieving high performance in their practical application. Therefore, new phosphors are highly desirable and are sought after through choosing suitable active ions and host material. Recently, great Interest has been gained in rare-earth ions activated inorganic phosphor materials for their special 4f electron configuration [12–18]. Among the rare-earth ions, Eu3+ is regarded as a promising red-emitting activator for its particular PL red emissions. As is known that the Eu3+ ions activated red-emitting phosphors usually present red luminescence in the range between 570 nm and 750 nm originated from the 5D0→7FJ (J = 0–4) transitions [19–21]. The featured narrow red emission located at about 616 nm strongly depends on its local environment and can be utilized to investigate the site symmetry of Eu3+ ions in the host lattice. In addition,

Corresponding authors. E-mail addresses: [email protected] (C. Ji), [email protected] (T.-H. Huang), [email protected] (J. Wen).

https://doi.org/10.1016/j.jlumin.2019.116734 Received 24 June 2019; Received in revised form 26 August 2019; Accepted 6 September 2019 Available online 07 September 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

Fig. 1. The calculated electronic band structures of Ba2MgGe2O7 (a) and Ba2ZnGe2O7 (b); The partial density of state of host groups Ba2MgGe2O7 (c) and Ba2ZnGe2O7 (d).

solid state reaction method. All the raw materials were purchased and used without further purification. The compounds BaCO3 (99.99%), GeO2 (99.999%), Eu2O3 (99.999%), MgO (99.99%) or ZnO (99.99%) were weighted out according to the required nominal chemical compositions and mixed thoroughly for 30 min in an agate mortar with appropriate amount of ethanol. Then the mixture were transferred to a 5 mL corundum crucible and sintered at 1100 °C for 4 h in the air. After that the red emitting phosphors Ba2(1-x)AGe2O7: 2xEu3+ (A = Mg, Zn, 0 ≤ x ≤ 0.2) were obtained.

the representative absorption located at about 395 nm matched well with the ultraviolet LED chips. Thus the Eu3+ ions activated inorganic materials can be act as excellent red-emitting components in the WLEDs. Such as Katelnikovas and coworkers reported that the phosphors K2Bi(PO4)(MoO4):Eu3+ exhibited high quantum efficiency close to 100% and high color purity [22]. Recently, Huang et al. synthesized a novel red phosphors Ca3Gd(AlO)3(BO3)4: Eu3+ exhibits high-brightness and thermal-stable with high colour purity for near ultraviolet pumped WLEDs [23]. Therefore, we believe that Eu3+ ions activated phosphors will be an excellent red emission candidates on the part of improving the WLEDs performance, especially the CRI R9. In this work, two series of Eu3+ ions activated red-emitting phosphors BMGO: 2xEu3+ (0 ≤ x ≤ 0.2) and BZGO: 2 xEu3+ (0 ≤ x ≤ 0.2) were synthesized successfully through high-temperature solid-state procedure. Motivation for choosing tetragonal Ba2MgGe2O7 and Ba2ZnGe2O7 as the host lattice was attributed to the good crystal structure and excellent thermal stability [24]. In addition, we reported the detailed discussions of BMGO: 2 xEu3+ (0 ≤ x ≤ 0.2) and BZGO: 2 xEu3+ (0 ≤ x ≤ 0.2) on theoretical and experimental study to investigated their potential application in WLEDs. This work shows that BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ can be act as excellent redemitting phosphors for WLEDs with high thermal stability and CRI R9.

2.2. Measurements and characterization The Cambridge Serial Total Energy Package (CASTEP) code were performed to calculate the band structure of the host lattice Ba2MgGe2O7 and Ba2ZnGe2O7 [25]. The diffuse reflectance and ultraviolet visible absorption spectra were measured on a UV–Vis–NIR spectrophotometer (UV-2700, Shimadzu Corp.). The X-ray diffraction diffractometer (Shimadzu 6100) with Cu Kα radiation (λ = 1.5405 Å) was used to characterize the final products of BMGO:2x Eu3+ and BZGO:2x Eu3+ (0 ≤ x ≤ 0.2). The fluorescence spectrophotometer (Hitachi F7000) was employed to examine the photoluminescence properties. The spectrophotometer (Otsuka Photal Electronics QE-2100, Japan) was adopted to investigate the thermal quenching properties. The decay curves were recorded on an FLS920 fluorescence spectrophotometer equipped with a Xe lamp. The auto-temperatured LED optoelectronic analyzer (ATA-1000, Everfine) was recorded on the electroluminescence performance of the WLED based on BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ as red emitting components.

2. Experimental section 2.1. Synthesis The red-emitting phosphors Ba2(1-x)AGe2O7: 2xEu3+ (A = Mg, Zn, 0 ≤ x ≤ 0.2) were synthesized by the conventional high temperature 2

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

cards are shown in Fig. 2. From Fig. 2(a) and fig2(c), it can be seen that there is a well match between the XRD patterns of BMGO: 2x Eu3+ or BZGO: 2x Eu3+ ((0 ≤ x ≤ 0.2) and their corresponding standard diffraction cards. This result indicating that the active ions Eu3+ were successfully doped into the host lattice Ba2MgGe2O7 and Ba2ZnGe2O7 without any impurity phase. Fig. 2(b) and Fig. 2(d) are the enlarged XRD patterns of BMGO: 2x Eu3+ and BZGO: 2x Eu3+ (0 ≤ x ≤ 0.2) in the range of 28.4–29.4°. In comparison with their JCPDS cards Ba2MgGe2O7 and Ba2ZnGe2O7, the diffraction positions were slightly shifted to larger angle with the increasement of Eu3+ ions contents. This can be rationally explained by Bragg's law: 2d sin θ = nλ, where d is the interplanar distance, θ is the diffraction angle, and λ is the X-ray wavelength. Clearly, the replacement of Ba2+ ions (r = 1.35 Å) by Eu3+ ions (r = 1.17 Å) with smaller radius lead to a decrease of the lattice expands and interplanar distance, and thus bringing about the larger diffraction angle θ. In addition, the radiation intensity of BMGO: 2x Eu3+ was slightly higher than that of BZGO: 2x Eu3+ when with same Eu3+ con tents, which maybe attribute to the much favorable crystal structure of BMGO: 2x Eu3+ (0 ≤ x ≤ 0.2) than BZGO: 2x Eu3+ (0 ≤ x ≤ 0.2) counterparts. To further explore the structure variation of BMGO:2x Eu3+ and BZGO:2x Eu3+ (0 ≤ x ≤ 0.2), the XRD Rietveld refinement were performed on the host lattice of Ba2MgGe2O7 and Ba2ZnGe2O7, and the exemplary sample of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ by using GSAS program [26–28]. The patterns and refinement results are shown in Fig. 3, the crystallographic data and structure refinement parameters are listed in Table 1. It can be seen that all the observed diffraction peaks of Ba2MgGe2O7, BMGO: 0.30 Eu3+, Ba2ZnGe2O7, and BZGO: 0.25 Eu3+ are in well match with the calculated results as shown in Fig. 3. Moreover, the reliability factors are Rwp = 13.03%, Rp = 9.11% and χ2 = 1.74 for Ba2MgGe2O7, Rwp = 13.88%, Rp = 9.71% and χ2 = 1.75 for BMGO: 0.30 Eu3+, Rwp = 13.14%, Rp = 9.71% and χ2 = 1.76 for Ba2ZnGe2O7, Rwp = 12.69%, Rp = 8.82% and χ2 = 1.67 for BZGO: 0.25 Eu3+, respectively, which means that the refinement results is credible. In contrast, the Eu3+ doped red emitting phosphors BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ possess relatively larger refined lattice parameters of a, b, c and V relative to their corresponding host lattice Ba2MgGe2O7 and Ba2ZnGe2O7, respectively, which can be rationally attributed to the replacement of Ba2+ by small-radius Eu3+ ions. Above all, the XRD Rietveld refinement results fully demonstrate that the single-phased BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ can be successfully obtained by doping Eu3+ ions. According the Rietveld refinement results, the crystal structure of

2.3. Details of the WLEDs device fabrication The WLED device was fabricated with a structure of “near ultraviolet LED chips + blue/green/red tricolor phosphors”, in which the commercial (Ba, Sr)2SiO4: Eu2+ and (Ba, Sr)2SiO4: Eu2+ were chosen as the blue and green phosphors, respectively. The blue/green/red phosphors were weighted in a certain proportion, and then mixed with organic silica gel thoroughly. The mixture were coated on an InGaN LED chip of 395 nm. After dried at 120 °C for 1.0 h, the obtained WLED device was used for the follow-up test. 3. Results and discussion 3.1. Theoretical calculation The band gap of the host lattice is an important parameter influencing on the luminous property of Eu3+ ions. Therefore, the density functional theory (DFT) were used to calculate the electronic band structures and the partial density of states (PDOS) of Ba2MgGe2O7 and Ba2ZnGe2O7 by using Material Studio 2017 software at the GGA-PBE with a CASTEP basis. The theoretically calculated results displayed in Fig. 1(a) and Fig. 1(b). As a result, the sample Ba2MgGe2O7 possess an indirect band gap of 3.68 eV and the sample Ba2ZnGe2O7 exhibits a direct band gap of 2.30 eV. In contrast, the relatively large K-space between the top of valence bands (VB) and the bottom of conduction bands (CB) of Ba2MgGe2O7 can help to avoid the absorption of stimulating light by the host lattice which is beneficial for enhancing the PL performance. Fig. 1(c) and Fig. 1(d) display the total density of states (DOS) of Ba2MgGe2O7 and Ba2ZnGe2O7, and the PDOS of the corresponding elements of Ba, Mg/Zn, Ge and O. Notably, the VB are dominated by O-2s2p, Ge-4p4d, Ba-6s6p states for Ba2MgGe2O7 and O2s2p, Ge-4s4p4d, Zn-4p4d, Ba-6s6p6 states for Ba2ZnGe2O7, respectively, and their CB are principally composed of O-2p, Ge-4p4d, Mg3s3p3d, Ba-6s6d states and O-2s2p, Ge-4s4p, Zn-4s4p, Ba-6s6p6d states, respectively. These results indicate that the consisted of DOS and PDOS of Ba2MgGe2O7 and Ba2ZnGe2O7 is difference, which will lead to apparent distinctions in the PL properties between them. 3.2. Crystal structure The XRD patterns of the synthetic samples BMGO: 2x Eu3+ and BZGO: 2x Eu3+ (0 ≤ x ≤ 0.2) as well as their corresponding JCPDS

Fig. 2. The XRD patterns and JCPDS card of (a) BMGO: 2x Eu3+ and (c) BZGO: 2x Eu3+ (0 ≤ x ≤ 0.2), respectively, The magnified XRD patterns for (b) BMGO: 2× Eu3+ and (d) BZGO:2x Eu3+ (0 ≤ x ≤ 0.2) in 28.3–29.4°. 3

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

Fig. 3. The Rietveld refinement of powder XRD patterns of Ba2MgGe2O7 (a), BMGO: 0.30 Eu

Ba2MgGe2O7, BMGO: 0.30 Eu3+, Ba2ZnGe2O7 or BZGO: 0.25 Eu3+ is shown in Fig. 4. Obviously, the Ba2MgGe2O7, BMGO: 0.30 Eu3+, Ba2ZnGe2O7, and BZGO: 0.25 Eu3+ samples crystallized in a tetragonal unit cell with the space group of P421 m (113). The M2+ (M = Mg, Zn) and Ge4+ are also coordinated with four oxygen atoms which form the MO4 (M = Mg, Zn) and GeO4 tetrahedrons.

3+

(b), Ba2ZnGe2O7 (c) and BZGO: 0.25 Eu

3+

(d), respectively.

Eu and Mg or Zn peaks are all in the corresponding EDS spectra, which further conformed the construction of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+.

3.4. Diffuse reflectance spectrum of host lattice The diffuse reflection spectra of the host lattice Ba2MgGe2O7 and Ba2ZnGe2O7 is shown in Fig. 6. It can be seen that the host lattice Ba2MgGe2O7 and Ba2ZnGe2O7 show high reflection from the range between 390 nm and 700 nm, which is consistent with their excellent PL emission in this wavelength region. In combination with the diffuse reflection and absorption spectrum of Fig. 6(a) and Fig. 6(b), it is interesting to note that Ba2MgGe2O7 and Ba2ZnGe2O7 possess an intense absorption in the range of 200–390 nm, which can be due to the electron transfer from valence band to unoccupied orbitals of Eu3+ and Ba, Ge states and Ba, Ge, Zn states, respectively. In addition, the optical band gap (Eg) of the host lattice Ba2MgGe2O7 and Ba2ZnGe2O7 were also extrapolated from

3.3. Size and morphology The representative morphologies of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu 3+ phosphor provided by SEM analysis are depicted in Fig. 5(a) and Fig. 5(b). The micrograph indicated that BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ consist of aggregated and irregular crystal shapes. These results illustrate that both of them exhibit good crystallization properties by synthesized with high temperature solid state reaction method. In the light of SEM image, the EDS was employed to detect the element constitution of the BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ phosphors, as shown in Fig. 5(c) andFig. 5(d). Obviously, the Ba, Ge, O,

Table 1 Crystallographic data and structure refinement parameters for the Ba2MgGe2O7, BMGO: 0.30 Eu3+, BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ phosphor, respectively. Compound

2θ range

Crystal symmetry

Space group

Z

a = b (Å)

c (Å)

V (Å3)

RWP (%)

RP (%)

χ2

Ba2MgGe2O7 BMGO:0.30 Eu3+ Ba2ZnGe2O7 BZGO: 0.25 Eu3+

5-120° 5-120° 5-120° 5-120°

tetragonal tetragonal tetragonal tetragonal

P421 m P421 m P421 m P421 m

1 1 1 1

8.3785 8.3794 8.3553 8.3595

5.5520 5.5535 5.5629 5.5644

389.75 389.94 388.35 388.85

13.03% 13.88% 13.14% 12.69%

9.11% 9.71% 9.71% 8.82%

1.74 1.75 1.76 1.67

(113) (113) (113) (113)

4

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

Fig. 4. The crystal structure of Ba2MgGe2O7, BMGO: 0.30 Eu3+, BMGO: 0.30 Eu3+or BZGO: 0.25 Eu 3+ obtained from refinement results.

their corresponding diffuse reflection spectra with follow equation [29,30].

(α hv) n/2 = A(hv − Eg)

Fig. 6. Diffuse reflection and absorption spectrum of the host lattice Ba2MgGe2O7 (a) and Ba2ZnGe2O7 (b); Inset: the relationship between the absorption coefficient and the photon energy of Ba2MgGe2O7 and Ba2ZnGe2O7, respectively.

(1)

where α refers to the absorption coefficient, A is a proportional constant, hν represents the incident photo energy, n = 1 for a direct band gap and n = 4 for an indirect band gap. In line with the relationship between the absorption coefficient α and the photon energy hν of Ba2MgGe2O7 and Ba2ZnGe2O7 shown in the insets of Fig. 6, the value of Eg can be estimated to be about 4.71eV and 3.75 eV, respectively. The slight deviation between the calculated and measured band gaps mainly attributes to the discontinuity of exchange correlation terms in PBE functional.

3.5. Photoluminescence properties Fig. 7 gives the PLE and PL spectra of the representative samples BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+. From Fig. 7(a) andFig. 7(b), it can be seen that the excitation spectra of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ cover from 200 nm to 550 nm. The relatively strongest band in the range of 200–300 nm can be rationally assigned to the band gap transitions of the host lattice Ba2MgGe2O7 and Ba2ZnGe2O7,

Fig. 5. The SEM image of BMGO: 0.30 Eu3+ (a) and BZGO: 0.25 Eu3+ (b) phosphors; The EDS spectrum of BMGO: 0.30 Eu3+ (c) and BZGO: 0.25 Eu3+ (d) phosphors. 5

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

Fig. 7. The PLE spectrum monitoring at 593 nm, 616 nm and 624 nm for BMGO: 0.30 Eu3+ (a) and BZGO: 0.25 Eu3+ (b); (c) The PL emission spectra of sample BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ phosphors excited at 395 nm; (d) the CIE chromaticity coordinates of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ phosphors excited at 395 nm. Fig. 8. The PLE spectra monitoring at 616 nm and PL spectra excited at 395 nm of (a) BMGO:2x Eu3+ (0 ≤ x ≤ 0.2) and (b) BZGO:2x Eu3+ (0 ≤ x ≤ 0.2) phosphors; The dependence of PLE and PL intensity on content of Eu3+ ions for (c) BMGO:2x Eu3+ (0 ≤ x ≤ 0.2) and (d) BZGO:2x Eu3+ (0 ≤ x ≤ 0.2).

respectively. Moreover, those sharp peaks located at about 321 nm, 363 nm, 382 nm, 395 nm, 416 nm, 465 nm and 536 nm in the wavelength between 300 nm and 550 nm can be attributed to the 7F0→5H6, 7 F0→5D4, 7F0→5G2,3, 7F0→5L6, 7F0→5D3, 7F0→5D2 and 7F0→5D1 transitions of Eu3+ ions, respectively. Among them, the excitation peaks located at 395 nm exhibited the strongest absorption intensity. In addition, the BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ phosphors show a relatively higher absorption intensity when monitoring at 616 nm than those of monitored at 593 nm and 624 nm, as shown in Fig. 7(a) and Fig. 7(b). Therefore, the 395 nm and 616 nm is chosen as the excitation and monitoring wavelength of BMGO: 2xEu3+ and BZGO: 2x Eu3+ (0 ≤ x ≤ 0.2) phosphors, respectively. In comparison with Fig. 7(b), the PLE intensity of BMGO: 0.30 Eu3+ presented in Fig. 7(a) is significantly higher than that of BZGO: 0.25 Eu3+. These results possibly related to the goodness crystallization of BMGO: 0.30 Eu3+. Furthermore, the PL emission spectra of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ covering the wavelength from 550 nm to 750 nm, and the corresponding emission peaks located at about 593 nm, 616 nm, 624 nm, 655 nm and 706 nm, which can be assigned to the 5D0→7F1, 5D0→7F2, 5 D0→7F3, 5D0→7F4 transitions of Eu3+, respectively. In accordance of magnetic dipole (MD) transition rule, the 5D0→7F1 transition belongs to the MD transition, while the 5D0→7F2 transition should be geared to the electric dipole (ED) transition. When excited at 395 nm, the PL emission peak with the highest intensity is located at 616 nm, showing that the Eu3+ occupied the low symmetry sites in the host lattice Ba2MgGe2O7 and Ba2ZnGe2O7, respectively. The Commission Internationale De I'eclairage (CIE) chromaticity coordinates of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ phosphors excited at 395 nm are shown in Fig. 7(d). The coordinates (x, y) of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ based on the corresponding PL spectra excited by 395 nm are found to be (0.6401, 0.3596) and (0.6416, 0.358), respectively, which located at red region. To further evaluate the quality of the red emission, the color purity was analyzed and it can be defined by follow equation [31].

Color purity =

(x − x i )2 + (y − yi )2 (x d − x i )2 + (yd − yi )2

× 100% (1)

where the (x, y), (xi, yi) and (xd, yd) represents the CIE coordinates of the synthesized samples, white illumination and dominate wavelength, respectively. In this work, the CIE coordinates (xi, yi) = (0.310, 0.316), (xd, yd) ≈ (0.6801, 0.316), the (x, y) of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ is (0.644, 0.356) and (0.644, 0.356), respectively. As a result, the color purity of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ is calculated to be about 90% and 90.3%, respectively. These results indicate that BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ possess excellent CIE chromaticity coordinate and high color purity which can be propitious to their potential application as red-emitting components in WLED. Fig. 8(a) presents the PLE and PL spectra of series Eu3+ doping phosphors BMGO:2x Eu3+ (0 ≤ x ≤ 0.2). It can be seen that there are none obviously PLE and PL band when monitored at 616 nm emission of Eu3+ ions for the host lattice of Ba2MgGe2O7 excited at 395 nm. The relative intensity in PLE and PL spectra varies with the Eu3+ doping concentrations, with a peak value at x = 0.15. The dependence of PLE and PL intensity on content of Eu3+ ions for BMGO: 2x Eu3+ (0 ≤ x ≤ 0.2) is shown in Fig. 8(c). The variations of the photoluminescence properties for BZGO:2x Eu3+ (0 ≤ x ≤ 0.2) are similar with BMGO:2x Eu3+ (0 ≤ x ≤ 0.2), as shown in Fig. 8(b). Moreover, the optimal value of Eu3+ content in BZGO: 2x Eu3+ (0 ≤ x ≤ 0.2) is 0.25, and Fig. 8(d) depicts the dependence of PLE and PL intensity on content of Eu3+ ions for BZGO:2x Eu3+ (0 ≤ x ≤ 0.2). In contrast, the relatively PLE and PL intensity of BMGO: 2x Eu3+ (0 ≤ x ≤ 0.2) with same Eu3+ contents are much higher than that of BZGO: 2x Eu3+ (0 ≤ x ≤ 0.2), which probably due to their much favored crystal structure. As descripted in Fig. 7, with the contents of Eu3+ ions exceed the 6

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

optimum value at x = 0.15 in BMGO:2x Eu3+ (0 ≤ x ≤ 0.2) and x = 0.125 in BZGO:2x Eu3+ (0 ≤ x ≤ 0.2), the PL emission intensity decreased on account of the concentration quenching effect which could be attributed to the non-radiative energy transfer between Eu3+ ions. The concentration quenching mechanism can be certified by the critical distance (Rc) between the neighboring rare-earth Eu3+ ions according to the Blass equation [28,32].

resulting in a relatively smaller lifetime and lower PL emission intensity than those of BMGO: 0.30 Eu 3+. This is consistent with results observed in photoluminescence properties. 3.7. Judd-Ofelt analysis The theoretical calculation of Judd-Ofelt (J-O) intensity parameters ΩJ (J = 2, 4) by J-O theory was performed to get a deeper understanding of the local structure environment around the Eu3+ ions in the host lattice of Ba2MgGe2O7 and Ba2ZnGe2O7. On the basis of J-O theory, the relationship between the total relative integrated intensity I and the total radiative transition rate estimated from the PL emission spectra can be defined as follows [36–38].

1/3

3V ⎤ Rc ≈ 2 ⎡ ⎢ 4 πx cZ ⎥ ⎣ ⎦

(2)

In which the V is the volume of the unit cell, xc represents the critical concentration of Eu3+ ions and Z refers to the number of cation sites in the unit cell. The value of V = 389.75 Å3, xc = 0.30, Z = 1 and V = 388.85 Å3, xc = 0.25, Z = 1 for the host lattice of Ba2MgGe2O7 and Ba2ZnGe2O7, respectively. As a result, the Rc was estimated to be 13.54 Å and 14.38 Å, respectively, clarifying that the electric multipolar interactions may be play a leading role in the concentration quenching between Eu3+ions for red-emitting phosphors BMGO:2x Eu3+ (0.15 ≤ x ≤ 0.2) and BZGO:2x Eu3+ (0.125 ≤ x ≤ 0.2), respectively.

∑

I7FJ = a

J = 0,1,2,3,4

A7FJ

(4)

J = 0,1,2,3,4

I7FJ

A7FJ

in which and represents the relative integrated emission intensities and radiative transition rate for the 5D0→7FJ (J = 0, 1, 2, 3, 4) transitions, respectively, a is a constant. In addition, the total radiative transition rate is inversely proportional to the value of lifetime τ, and their relationship can be expressed as follows:

3.6. PL decay

∑

Fig. 9 shows the decay curves monitoring at 616 nm emission of Eu3+ ions for BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ phosphors excited at 395 nm. Moreover, the decay curve of BZGO: 0.30 Eu3+ phosphors is also given in Fig. 9 to fully investigated the effect of Eu3+ on the decay times of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+. It is evident that the recorded decay curves can be well fitted by a single exponential decay pattern, as follows [33–35]. I = I0 exp(-t/τ)+A

∑

I=

J = 0,1,2,3,4

A7FJ =

1 τ

(5)

The lifetime value of Ba2MgGe2O7 and Ba2ZnGe2O7 is 1.02 ms and 0.92 ms, respectively. According to equation (5), the constant a can be calculated to be about 50.7 and 30.3 for Ba2MgGe2O7 and Ba2ZnGe2O7, respectively. Additionally, the A7F1, A7F2, A7F4 of Ba2MgGe2O7 is 306.5 s−1, 588.2 s−1and 29.9 s−1, respectively, and the A7F1, A7F2, A7F4 of Ba2ZnGe2O7 is 330.4 s−1, 674 s−1and 42.9 s−1, respectively. In contrast, the corresponding A7FJ of Ba2MgGe2O7 is slightly higher than that of Ba2ZnGe2O7, this can be rationally ascribed to the interrelationship of radiative transition rate, non-radiative transition rate and energy transfer rate of Eu3+ in different host lattices. On the other hand, the radiative transition rate of 5D0→7F1 transi3+ tion ( AJMD can be given by equation (6): − J ′ ) for Eu

(3)

where I(t) and I0 are the luminescence intensity at time t and t = 0, A is fitting constants; τ represents the lifetime for the exponential components. As a result, the PL lifetimes for BMGO: 0.30 Eu3+, BZGO: 0.25 Eu3+ and BZGO: 0.30 Eu3+ are estimated to be 1.02 ms, 0.91 ms and 0.73 ms, respectively. Obviously, the PL lifetimes value of BZGO: 0.25 Eu 3+ is higher than that of BZGO: 0.30 Eu3+, which can be rationally assigned to the relatively larger distance between Eu3+ in BZGO: 0.25 Eu +. In contrast, the PL lifetimes value of BMGO: 0.30 Eu 3+ is about 1.4 times higher than that of BZGO: 0.30 Eu3+, indicating that the doped Eu 3+ ions exhibit much stronger non-radiative energy migration in phosphors BZGO: 0.30 Eu 3+ and BZGO: 0.25 Eu 3+, and thus

AJMD −J ′ =

64π 4ν3 n3SMD 3h (2J + 1)

(6)

where ν is the center wavenumber for J→J0 transition, h is Planck's constant (h = 6.626 × 10−27), 2J + 1 is the degeneracy of the initial state, n represents the refractive index of host lattice, and SMD refers to the MD line strength (SMD = 7.83 × 10−42). Therefore, the refractive index n of Ba2MgGe2O7 and Ba2ZnGe2O7 can be estimated to be about 2.97 and 3.03, respectively. Meanwhile, the electronic transition rate of the 5D0→7F2, 4 transi3+ tion AJED can be defined as: − J ′ for Eu

AJED −J ′ =

64π 4e 2νJ3 n (n2 + 2)2 3h (2J + 1) 9

∑

Ωt ψJU tψ′J ′2 (7)

J = 2,4 −10

Fig. 9. The decay curves of BMGO: 0.30 Eu3+, BZGO: 0.25 Eu 0.30 Eu 3+ phosphors, respectively.

3+

where e is the electronic charge (4.80 × 10 esu), the VJ is the center wave number (cm−1) of the 5D0→7F2,4. Moreover, it is well known that the value of reduced matrix elements ⟨5D0 ‖U 2‖7 F2⟩2 is 0.0032 and the value of ⟨5D0 ‖U4‖7 F4 ⟩2 is 0.0023. As a result, the J-O intensity parameters Ω2 and Ω4 of Ba2MgGe2O7 were calculated to be about 1.54 × 10−20 and 0.16 × 10−20 cm2, respectively, and the Ω2 and Ω4 of Ba2ZnGe2O7 were estimated to be about 1.76 × 10−20 and 0.23 × 10−20 cm2, respectively. Unsurprisingly, the value of J-O intensity parameters Ω2 of Ba2MgGe2O7 and Ba2ZnGe2O7 is much larger than those of their corresponding Ω4, indicating that the Eu3+ ions occupied the low symmetry sites. This phenomenon is well consistent with the results obtained from the PL emission spectra. The fluorescence branch ratios β of 5D0 level can be calculated by following equation:

and BZGO:

7

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

Fig. 10. The temperature-dependent PL emission spectrum from room temperature to 473 K for Ba1.70MgGe2O7: 0.30 Eu3+ (a) and Ba1.75ZnGe2O7: 0.25 Eu3+ (b) phosphors when excited at 395 nm; The relative temperature-dependent PL intensity of Ba1.70MgGe2O7: 0.30 Eu3+ (c) and Ba1.75ZnGe2O7: 0.25 Eu3+ (d).

β=

Fig. 11. The EL spectrum and photographs of the fabricated WLEDs devices based on BMGO: 0.30 Eu3+ (a) and BZGO: 0.25 Eu3+ (b) as red phosphors driven by 20 mA. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.9. Performance of LED device

AJ − j′ ∑J = 0,1,2,3,4 AJ − J ′

In order to verify the application possibility of the synthesized redemitting phosphors BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+, the WLED devices were fabricated by combining a 395 nm near ultraviolet chips with the mixture of synthesized red emitting phosphors BMGO: 0.30 Eu3+ or BZGO: 0.25 Eu3+ as well as commercial green and blue emission phosphors. The EL spectrum and photographs of the fabricated WLEDs devices based on BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ as red phosphors driven by 20 mA are shown in Fig. 11, the related chromaticity parameters of device under different driven currents from 20 mA to 120 mA are listed in Table 2. White light can clearly be generated for BMGO: 0.30 Eu3+ or BZGO: 0.25 Eu3+ by combining the blue emission BaMgAl10O17:Eu2+ and the green emission (Ba, Sr)2SiO4: Eu2+. In accordance with expectation, a continuous electroluminescence (EL) spectra can be obtained in the wavelength range of 380 nm–780 nm together with clearly red, green and blue emission band.

(8)

As a result, the fluorescence branch ratios β of 5D0→7FJ (J = 0, 1, 2, 3, 4) are calculated to be about 2.2%, 31.3%, 61.4%, 1.9% and 3.1% for Ba2MgGe2O7, respectively, and about 2.8%, 30.0%, 61.2%, 2.2% and 3.9% for Ba2ZnGe2O7, respectively. Significantly, the 5D0→7F2 is the mainly transitions in Ba2MgGe2O7 and Ba2ZnGe2O7, respectively, which further confirming that Ba2MgGe2O7 and Ba2ZnGe2O7 can be act as excellent red-emitting phosphors in their WLED applications.

3.8. Thermal quenching properties The temperature stable property is closely related to the performance of WLED device, especially the light output and CRI. The thermal quenching phenomenon of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ was observed from room temperature to 473 K under 395 nm excitation, and the temperature-dependent PL emission spectra were shown in Fig. 10. The results showed that the PL emission intensity of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ gradually decreased with the temperature rises on account of the thermal quenching effect, as shown in Fig. 10(a) andFig. 10(b). Unsurprisingly, the phosphors BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ exhibit low thermal quenching behavior, and their PL emission intensity maintain 96% and 93.6% of the initial intensity at 298 K when the temperature up to 373 K, respectively, as descript in Fig. 10(c) andFig. 10(d). Significantly, as the temperature increases above 473 K, the whole PL emission intensity for BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ only drops to 88.74% and 81.8% of their values at room temperature. In comparison with BZGO: 0.25 Eu3+, the phosphors BMGO: 0.30 Eu3+ shows relatively better thermal stability which can be reasonably ascribed to its much favored crystal structure and photoluminescence properties. These results suggest that both BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ possess excellent thermal stability which can be used as good red-emitting components for their potential application in WLED.

Table 2 Chromaticity parameters for fabricated WLED device under different driven currents. Type

BMGO: 0.30 Eu3+

BZGO: 0.25 Eu3+

8

Current

CIE coordinates

(mA)

x

y

20 40 60 80 100 120 20 40 60 80 100 120

0.3453 0.3488 0.3474 0.3478 0.3456 0.3453 0.3213 0.3165 0.3062 0.3235 0.3195 0.3155

0.3488 0.3538 0.3531 0.3539 0.3551 0.3540 0.3308 0.3289 0.3183 0.3393 0.3381 0.3337

CCT (K)

Ra

R9

Luminance efficiency (lm/W)

4982 4871 4919 4906 4996 5001 6046 6304 6989 5911 6106 6329

83.3 83 84.3 84.3 84.5 83.9 75.3 76.7 75.4 79.3 79.4 78.3

89.6 91.2 91.4 90.8 88.9 89.6 82.3 87 86.9 85.7 87.5 85.4

14.26 14.09 13.73 13.38 13.09 12.77 10.01 10.25 10.31 9.61 9.53 9.47

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

Fig. 12. The CIE chromaticity diagram of the fabricated WLEDs devices based on BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ as red phosphors driven by 20 mA. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 13. The EL spectra of the WLEDs devices under different driven currents based on BMGO: 0.30 Eu3+ (a) and BZGO: 0.25 Eu3+ (b) as red phosphor. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 12 gives the CIE chromaticity diagram of the fabricated WLEDs devices based on BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ as red phosphors driven by 20 mA. Unsurprisingly, the color point of BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ based WLEDs devices lied on the back body locus. It is interesting to find that the WLEDs device based on BMGO: 0.30 Eu3+ as red emitting components exhibited excellent luminescence properties with the corresponding CIE chromaticity coordinates of (0.3453, 0.3488), the CCT of 4982 K, the Ra of 83.3, R9 of 89.6 and the luminance efficiency of 14.26 lm/W when driven by 20 mA current and 3 V voltage, respectively. In addition, the WLEDs device based on BZGO: 0.25 Eu3+ as red emitting components presents the CIE chromaticity coordinates of (0.3213, 0.3308), the CCT of 6346 K, the Ra of 75.3, R9 of 82.3 and the luminance efficiency of 10.01 lm/ W. In contrast, the relatively higher luminescence performance for BMGO: 0.30 Eu3+ based WLED device maybe reasonably attributed to its more favorable crystal structure and much better photoluminescence properties. Fig. 13 presents that the EL emission intensity of the WLEDs device based on BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ gradually rise with the increase of driven current from 20 mA to 120 mA. It can be seen that from Table 2, there are only slightly variations in the values of CIE chromaticity coordinates, CCT, Ra, R9 and luminance efficiency for the WLEDs devices based on BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ with the changes of the driven current increased. It is known that the corresponding color temperature R9 is a critical parameter for high-quality color renditions of the WELDs. Significantly, the BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ based WLEDs devices both possess greater R9 values, indicating that both of these WLEDs device denotes the color reproduction in the strong red region. These results indicate that both the red-emitting phosphors BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ can act as excellent candidates for their application used in near ultraviolet excited WLEDs devices. In comparison with BZGO: 0.25 Eu3+, the BMGO: 0.30 Eu3+ based WLEDs device shows much better EL performance, which is consistent with its outstanding PL and thermal stability

properties. 4. Conclusions In summary, Eu3+ ions activated high performance red-emitting phosphors Ba2(1-x)AGe2O7: 2xEu3+ (A = Mg, Zn, 0 ≤ x ≤ 0.2) were synthesized by high-temperature solid-state procedure. The Among them, the phosphors BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+exhibited the best PL properties compared to their counterparts in the same series. In contrast, host lattice Ba2MgGe2O7 not only presents larger band gap but also possess better crystallinity, thus improving the PL properties of BMGO: 2xEu3+ (0 ≤ x ≤ 0.2). In addition, the phosphors BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ also exhibited high color purity of 90% and 90.3%, and excellent thermal stability which remains about 88.74% and 81.8% of the PL intensity at 423 K in comparison than that at room temperature, respectively. Accordingly, BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ based WLEDs devices exhibit high EL performance with the CIE chromaticity coordinates of (0.3453, 0.3488) and (0.3213, 0.3308), the CCT of 4982 K and6346 K, the luminance efficiency of 14.26 lm/W and 10.01 lm/W, the Ra of 83.3 and 75.3, and the significantly R9 of 89.6 and 82.3 when driven by 20 mA current and 3 V voltage, respectively. The relatively better WLEDs performance of BMGO: 0.30 Eu3+ is primarily attributed to its much favorable crystal structure and thermal stability. As a conclusion, BMGO: 0.30 Eu3+ and BZGO: 0.25 Eu3+ can act as efficient red-emitting phosphors for promising high performance WLEDs pumped by near ultraviolet LED chips. Acknowledgements The work is supported by the Scientific Research Fund of Hunan Provincial Education Department (No. 18B450, 17B138 and 18C0886); the Open Foundation of Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials (No. TC201704); the double first-class discipline construction program of Hunan province (NO. Xiang Jiao 9

Journal of Luminescence 216 (2019) 116734

C. Ji, et al.

Tong [2018]469); the Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials (No. 2016TP1028); the Natural Science Foundation of Hunan Province (No. 2018JJ3251); In addition, high performance computing center of science & engineering of Sichuan University of Science & Engineering is gratefully acknowledged for DFT studies.

[18] C. Ji, Z. Huang, X. Tian, W. Xie, J. Wen, H. He, C. Zhou, T. Zeng, Synthesis and photoluminescence properties of a novel BaGe4O9:Eu3+ red emitting phosphor for warm white LEDs, Dyes Pigments 160 (2019) 772–778. [19] S.H. Lee, P. Du, L.K. Bharat, J.S. Yu, Ultraviolet radiation excited strong red-emitting LaAlO3 :Eu3+ nanophosphors: synthesis and luminescent properties, Ceram. Int. 43 (2017) 4599–4605. [20] C.R. Garcia, J. Oliva, M.A. Garcia-Lobato, A.I. Martínez, R. Ochoa-Valiente, G.A. Hirata, Red-emitting SrGe4O9:Eu3+ phosphors obtained by combustion synthesis, Ceram. Int. 43 (2017) 12876–12881. [21] W. Xie, Y. Mo, C. Zou, F. Kang, G. Sun, Broad color tuning and Eu3+-related photoemission enhancement via controllable energy transfer in the La2MgGeO6:Eu3+,Bi3+ phosphor, Inorg. Chem. Front. 5 (2018) 1076–1084. [22] J. Grigorjevaite, A. Katelnikovas, Luminescence and luminescence quenching of K2Bi(PO4)(MoO4):Eu3+ phosphors with efficiencies close to unity, ACS Appl. Mater. Interfaces 8 (2016) 31772–31782. [23] B. Li, S. Wang, Q. Sun, C. Lu, H. Guo, X. Huang, Novel high-brightness and thermalstable Ca3Gd(AlO)3(BO3)4 :Eu3+ red phosphors with high colour purity for NUVpumped white LEDs, Dyes Pigments 154 (2018) 252–256. [24] A.A. Kaminskii, L. Bohatý, P. Becker, J. Liebertz, P. Held, H.J. Eichler, H. Rhee, J. Hanuza, Tetragonal Ba2MgGe2O7- a novel multifunctional optical crystal with numerous manifestations of nonlinear-laser effects: almost sesqui-octave Stokes and anti-Stokes combs and cascadedχ(3)↔χ(2)lasing with involved second and third harmonic generation, Laser Phys. Lett. 5 (2008) 845–868. [25] S.J. Clark, M.D. Segall, C.J. Pickard, P.J. Hasnip, M.I.J. Probert, K. RefsonV, M.C. Payne, First principles methods using CASTEP, Z. Kristallogr. 220 (2005) 567–570. [26] B.H. Toby, Expgui, a graphical user interface for GSAS, J. Appl. Crystallogr. 34 (2001) 210–213. [27] S. Dey, R. Mondal, S.K. Dey, S. Majumder, P. Dasgupta, A. Poddar, V.R. Reddy, S. Kumar, Tuning magnetization, blocking temperature, cation distribution of nanosized Co0.2Zn0.8Fe2O4 by mechanical activation, J. Appl. Phys. 118 (2015) 103905. [28] L. Mukhopadhyay, V.K. Rai, Investigation of photoluminescence properties, Judd–Ofelt analysis, luminescence nanothermometry and optical heating behaviour of Er3+/Eu3+/Yb3+:NaZnPO4 nanophosphors, New J. Chem. 42 (2018) 13122–13134. [29] M.L. Balmer, Y. Su, H. Xu, E. Bitten, D. Mccready, A. Navrotsky, Synthesis, structure determination, and aqueous durability of Cs2ZrSi3O9, J. Am. Ceram. Soc. 84 (2001) 153–160. [30] C. Ji, Z. Huang, J. Wen, J. Zhang, X. Tian, H. He, L. Zhang, T.-H. Huang, W. Xie, Y. Peng, Blue-emitting Bi-doped double perovskite Gd2ZnTiO6 phosphor with nearultraviolet excitation for warm white light-emitting diodes, J. Alloy. Comp. 788 (2019) 1127–1136. [31] P. Du, X. Huang, J.S. Yu, Facile synthesis of bifunctional Eu3+-activated NaBiF4 redemitting nanoparticles for simultaneous white light-emitting diodes and field emission displays, Chem. Eng. J. 337 (2018) 91–100. [32] D.L. Dexter, J.H. Schulman, Theory of concentration quenching in inorganic phosphors, J. Chem. Phys. 22 (1954) 1063–1070. [33] J. Qiao, L. Ning, M.S. Molokeev, Y.C. Chuang, Q. Liu, Z. Xia, Eu2+ Site preferences in the mixed cation K2BaCa(PO4)2 and thermally stable luminescence, J. Am. Chem. Soc. 140 (2018) 9730–9736. [34] T.S. Sreena, P. Prabhakar Rao, A.K.V. Raj, T.R. Aju Thara, Exploitation of Eu3+ red luminescence through order–disorder structural transitions in lanthanide stannate pyrochlores for warm white LED applications, Phys. Chem. Chem. Phys. 20 (2018) 24287–24299. [35] L. Li, X. Tang, Z. Wu, Y. Zheng, S. Jiang, X. Tang, G. Xiang, X. Zhou, Simultaneously tuning emission color and realizing optical thermometry via efficient Tb3+→Eu3+ energy transfer in whitlockite-type phosphate multifunctional phosphors, J. Alloy. Comp. 780 (2019) 266–275. [36] G.S. Ofelt, Intensities of crystal spectra of rare-earth ions, J. Chem. Phys. 37 (1962) 511–520. [37] B.R. Judd, Optical absorption intensities of rare-earth ions, Phys. Rev. 127 (1962) 750–761. [38] L. Li, X. Tang, Y. Zheng, Z. Wu, W. Chang, S. Jiang, G. Xiang, X. Zhou, A novel dazzling Eu3+-doped whitlockite-type phosphate red-emitting phosphor for white light-emitting diodes, J. Am. Ceram. Soc. 101 (2018) 4095–4107.

References [1] J. Xiang, J. Zheng, Z. Zhou, H. Suo, X. Zhao, X. Zhou, N. Zhang, M.S. Molokeev, C. Guo, Enhancement of red emission and site analysis in Eu2+ doped new-type structure Ba3CaK(PO4)3 for plant growth white LEDs, Chem. Eng. J. 356 (2019) 236–244. [2] X. Zhang, Z. Zhu, Z. Guo, Z. Sun, Y. Chen, A ratiometric optical thermometer with high sensitivity and superior signal discriminability based on Na3Sc2P3O12: Eu2+, Mn2+ thermochromic phosphor, Chem. Eng. J. 356 (2019) 413–422. [3] S. Liu, P. Sun, Y. Liu, T. Zhou, S. Li, R.J. Xie, X. Xu, R. Dong, J. Jiang, H. Jiang, Warm white light with a high color rendering index from a single Gd3Al4GaO12:Ce3+ transparent ceramic for high-power LEDs and LDs, ACS Appl. Mater. Interfaces 11 (2019) 2130–2139. [4] J. Lee, K. Min, Y. Park, K.S. Cho, H. Jeon, Photonic crystal phosphors integrated on a blue LED chip for efficient white light generation, Adv. Mater. 30 (2018) 1703506. [5] O. Sidletskiy, K. Lebbou, D. Kofanov, V. Kononets, I. Gerasymov, R. Bouaita, V. Jary, R. Kucerkova, M. Nikl, A. Polesel, et al., Progress in Fabrication of Long Transparent YAG:Ce and YAG:Ce,Mg Single Crystalline Fibers for HEP Applications, CrystEngComm, 2019. [6] S. Liu, M. He, X. Di, P. Li, W. Xiang, X. Liang, CsPbX3 nanocrystals films coated on YAG:Ce3+ PiG for warm white lighting source, Chem. Eng. J. 330 (2017) 823–830. [7] Y. Tang, S. Zhou, X. Yi, D. Hao, X. Shao, J. Chen, The characterization of Ce/Prdoped YAG phosphor ceramic for the white LEDs, J. Alloy. Comp. 745 (2018) 84–89. [8] X. Ding, Y. Wang, Commendable Eu2+-doped oxide-matrix-based LiBa12(BO3)7F4 red broad emission phosphor excited by NUV light: electronic and crystal structures, luminescence properties, ACS Appl. Mater. Interfaces 9 (2017) 23983–23994. [9] H. Ming, S. Liu, L. Liu, J. Peng, J. Fu, F. Du, X. Ye, Highly regular, uniform K3ScF6:Mn4+ phosphors: facile synthesis, microstructures, photoluminescence properties, and application in light-emitting diode devices, ACS Appl. Mater. Interfaces 10 (2018) 19783–19795. [10] Y.Y. Zhou, E.H. Song, T.T. Deng, Q.Y. Zhang, Waterproof narrow-band fluoride red phosphor K2TiF6:Mn4+ via facile superhydrophobic surface modification, ACS Appl. Mater. Interfaces 10 (2018) 880–889. [11] T. Suehiro, R.-J. Xie, N. Hirosaki, Facile synthesis of (Sr,Ca)2Si5N8:Eu2+-based redemitting phosphor for solid-state lighting, Ind. Eng. Chem. Res. 52 (2013) 7453–7456. [12] P. Du, J.S. Yu, sear-infrared light-triggered visible upconversion emissions in Er3+/ Yb3+-codoped Y2Mo4O15 microparticles for simultaneous noncontact optical thermometry and solid-state lighting, Ind. Eng. Chem. Res. 57 (2018) 13077–13086. [13] X. Zhang, J. Zhang, Y. Chen, Broadband-excited and efficient blue/green/redemitting Ba2Y5B5O17 : Ce3+, Tb3+, Eu3+ phosphors using Tb3+ -bridged Ce3+Eu3+ energy transfer, Dyes Pigments 149 (2018) 696–706. [14] R. Vijayakumar, H. Guo, X. Huang, Energy transfer and color-tunable luminescence properties of Dy3+ and Eu3+ co-doped Na3Sc2(PO4)3 phosphors for near-UV LEDbased warm white LEDs, Dyes Pigments 156 (2018) 8–16. [15] L.K. Bharat, P. Du, J.S. Yu, Long-wave UVA radiation excited warm white-light emitting NaGdTiO4 : Tm3+/Dy3+/Eu3+ ions tri-doped phosphors: synthesis, energy transfer and color tunable properties, J. Alloy. Comp. 666 (2016) 270–278. [16] X. Li, B. Milićević, M.D. Dramićanin, X. Jing, Q. Tang, J. Shi, M. Wu, Eu3+Activated Sr3ZnTa2O9 single-component white light phosphors: emission intensity enhancement and color rendering improvement, J. Mater. Chem. C 7 (2019) 2596–2603. [17] P. Dang, G. Li, S. Liang, H. Lian, J. Lin, Multichannel photoluminescence tuning in Eu-doped apatite phosphors via coexisting cation substitution, energy transfer and valence mixing, J. Mater. Chem. C (2019), https://doi.org/10.1039/C9TC01051C.

10