Sm3+

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A warm white emission of Bi3+-Eu3+ and Bi3+-Sm3+ codoping Lu2Ge2O7 phosphors by energy transfer of Bi3+-sensitized Eu3+/Sm3+ Quanfeng Li a, Shaoan Zhang b,⁎, Wenxian Lin a, Wenfeng Li a, Yuanxing Li a, Zhongfei Mu a,⁎, Fugen Wu c a b c

Experimental Teaching Department, Guangdong University of Technology, Waihuan Xi Road, No.100, Guangzhou 510006, People's Republic of China School of Optoelectronic Engineering, Guangdong Polytechnic Normal University, Guangzhou 510665, People's Republic of China School of Materials and Energy, Guangdong University of Technology, Waihuan Xi Road, No.100, Guangzhou 510006, People's Republic of China

a r t i c l e

i n f o

Article history: Received 10 May 2019 Received in revised form 22 October 2019 Accepted 2 November 2019 Available online xxxx Keywords: Bi3+-Eu3+ Bi3+-Sm3+ Energy transfer Multicolor-tunable luminescence Warm white emission

a b s t r a c t In the last few years, multicolor-tunable phosphors, especially single-composition white-light-emitting phosphors, have attracted increasing attention and interest for UV-converted white LEDs. In this paper, Lu2Ge2O7: Bi3+ phosphor presents a doublet emission ranging from UV to visible spectrum with a high QE of 76%. To obtain a warm white emission, we tried to codope Eu3+ or Sm3+ into Lu2Ge2O7 with Bi3+ ions. Multicolor-tunable emission colors of Bi3+-Eu3+ and Bi3+-Sm3+ codoped Lu2Ge2O7 samples are adjusted from cold white light, warm white light, light pink, pink, to red by changing the Eu3+ or Sm3+ doping concentration. Energy transfer process occurring from Bi3+ to Eu3+ or Sm3+ is discussed in detail and the corresponding ηETE of Bi3+-Eu3+ and Bi3+-Sm3 + can reach as high as 42% and 35%, respectively. This paper not only provides a novel UV-converted multicolortunable phosphor, but also discovery novel single-composition white-light-emitting phosphors for UVconverted white LEDs. © 2019 Published by Elsevier B.V.

1. Instruction As the next generation lighting sources, phosphor converted whitelight-emitting diodes (pc-WLEDs) have been paid more and more attention due to their outstanding merits such as energy savings, long operation time, environmentally friendliness, and so on [1–5]. However, commercial pc-WLEDs, which is manufactured by depositing the mainstream Y3Al5O12:Ce3+ (YAG:Ce) yellow phosphor onto a blue InGaN chip, exhibit a cold white luminescence with a high color temperature (CT) and poor color rendering (CR) [6,7]. Thus, great effects have been made to optimize the whole performance of commercial pc-WLEDs. Until now, several alternative methods are proposed, for example, one alternative method is to design the near ultraviolet (n-UV) excited red, green and blue phosphors deposited on the n-UV LED chip so that warm WLEDs is obtained [8–10]. However, it is unsatisfactory when pc-WLEDs work for long time because of lighting distortion, color imbalance and blue energy resorption. If single-composition white emitting phosphors, which have interesting merits such as high quantum

⁎ Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (Z. Mu).

efficiency (QE), and thermal stability, could be coated onto the n-UV LED chip, the above problems might be solved. Until now, more and more works have focused on developing novel single-composition phosphors which exhibits a warm white emission under excitation effectively with the n-UV or UV energy. According to the published works, we found that Bi3+ ions, which can be effectively pumped by near UV or UV energy, serve as sensitizers in the energy transfer process of Bi3+-Eu3+ or Bi3+-Sm3+ [11–14]. A warm white emission can be achieved by codoping transition metal (Bi3+) with red activators (Eu3+ or Sm3+) such as BaGd2O4: Bi3+, Eu3+ [11], Y2GeO5: Bi3+, Eu3+ [12] CaZnOS: Bi3+, Sm3+, Li+ [13] and La2MgGeO6:Bi3+, Sm3+ [14]. It is found that Lu2Ge2O7:Bi3+ phosphor presents a doublet emissions ranging from 300 to 600 nm with a high QE of 76%, which overlaps well with the excitation peaks of Eu3+ or Sm3+. Therefore, we planned to prepare a single-composition whitelight-emitting phosphor by codoping the sensitizers (Bi3+) into Lu2Ge2O7 host with red activators (Eu3+ or Sm3+). Fortunately, a warm white emission from Lu2Ge2O7:Bi3+, Eu3+ or Lu2Ge2O7:Bi3+, Sm3+ is obtained by energy transfer occurring between Bi3+-Eu3+ or Bi3+-Sm3+. To explain the PL properties of Lu2Ge2O7:Bi3+, Eu3+ or Lu2Ge2O7:Bi3+, Sm3+, we analyzed the crystal structure of Lu2Ge2O7 host by performing the XRD refinement. The multicolor-tunable emissions of Bi3+-Eu3+ or Bi3+-Sm3+ codoping Lu2Ge2O7 can be tuned by

https://doi.org/10.1016/j.saa.2019.117755 1386-1425/© 2019 Published by Elsevier B.V.

Please cite this article as: Q. Li, S. Zhang, W. Lin, et al., A warm white emission of Bi3+-Eu3+ and Bi3+-Sm3+ codoping Lu2Ge2O7 phosphors by energy transfer of B..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117755

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adjusting the ratio of Bi3+/Eu3+ or Bi3+/Sm3+. Fortunately, warm white emissions of Lu1.91Ge2O7:0.06Bi3+, 0.03Eu3+ or Lu1.93Ge2O7:0.06Bi3+, 0.01Sm3+ are also obtained by energy transfer of Bi3+-Eu3+ or Bi3+Sm3+. 2. Experimental section 2.1. Synthesis Lu2-xGe2O7:xBi3+ (x = 0, 0.01, 0.02, 0.04, 0.06, 0.10, and 0.14), Lu1.943+ , xEu3+ (x = 0.03, 0.06, 0.10, 0.15, 0.20 and 0.25), and xGe2O7:0.06Bi Lu1.94-yGe2O7:0.06Bi3+, ySm3+ (y = 0.005, 0.01, 0.02, 0.04, 0.06 and 0.08) samples were synthesized via the simple solid state method. High pure Lu2O3 (99.99%), Eu2O3 (99.99%), Sm2O3 (99.99%), GeO2 (99.99%), and Bi2O3 (99.99%) were used as primary materials. Taking Lu1.91Ge2O7:0.06Bi3+, 0.03Eu3+ for an example, the synthetic process was described by the following procedure: 1.876 g Lu2O3, 1.046 g GeO2, 0.070 g Bi2O3, and 0.026 g Eu2O3 were accurately weighed. Then, all the above primary materials were ground thoroughly and then put into an alumina crucible. The homogeneous mixtures are shifted into a high temperature muffle furnace and then heated at 1373 K for 10 h. After calcining process, the target materials were cooled slowly and ground again. 2.2. Characterization To characterize the composition and crystal structure of all prepared samples such as Lu2Ge2O7: Bi, Lu2Ge2O7: Bi, Eu3+, and Lu2Ge2O7: Bi, Sm3 + , the powder X-ray diffraction (XRD) were carried out by a D8 Advance diffractometer (Bruker Corporation, Germany). The photoluminescence excitation (PLE), photoluminescence (PL) spectra, high-temperature PL as well as fluorescence decay lifetimes of Lu2Ge2O7: Bi, Lu2Ge2O7: Bi, Eu3 + , and Lu2Ge2O7: Bi, Sm3+ were performed by using a ultramodern

Table 1 Rietveld refinement results of Lu2Ge2O7 host, Lu1.95Ge2O7:0.05Bi3+ samples. Items

Lu2Ge2O7

Lu2Ge2O7 host

Lu1.95Ge2O7:0.05Bi3 +

ICSD file No. Crystal system Space group Z Parameters (a, b, c)/Å Volume (V)/Å3 Rp Rwp Rexp GOF

39929 Hexagonal P 41212 (92) 4 a = b = 6.702 c = 12.175 546.86

Hexagonal P 41212 (92) 4 a = b = 6.729 c = 12.225 553.66 6.28 8.89 4.41 2.16

Hexagonal P 41212 (92) 4 a = b = 6.733 c = 12.232 554.60 7.74 10.82 4.61 3.69

spectrophotometer (FLS 980 spectrofluorometer, Edinburgh Instruments Ltd., U.K.). The internal QE were obtained by the FLS 980 spectrofluorometer with an integrating sphere. The structural refinement by the Rietveld method was performed using the Fullprof Program [15]. 2.3. Theoretical calculation The electronic structure of Lu2Ge2O7 host is calculated by the density functional theory. The calculated process in detail is the same to our published papers [16,17]. 3. Results and discussion 3.1. XRD Rietveld refinement and crystal structure To study the influence of crystal structure on the PL property of Bi3+ doped Lu2Ge2O7 host, it is indispensable to study how crystal structure of Lu2Ge2O7 host is formed. Thus, the XRD profile of Lu2Ge2O7 is refined

Fig. 1. XRD profile for Rietveld refinement results of (a) Lu2Ge2O7 host, and (b) Lu2Ge2O7: Bi3+, respectively; (c) the unit cell structure of Lu2Ge2O7 host (view along the a-axis).

Please cite this article as: Q. Li, S. Zhang, W. Lin, et al., A warm white emission of Bi3+-Eu3+ and Bi3+-Sm3+ codoping Lu2Ge2O7 phosphors by energy transfer of B..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117755

Q. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx Table 2 Main crystallographic parameters of Rietveld refinement of the Lu2Ge2O7 sample. Lu2Ge2O7 Atom

Symmetry

x

y

z

Occupation

Lu3+ Ge4+ O2− O2− O2− O2−

8b 8b 8b 8b 8b 4a

0.12620 0.60070 −0.18200 0.06100 0.70570 0.41000

0.35470 0.34950 0.36000 0.67300 0.29430 0.54000

0.11440 0.11770 0.04200 0.17600 0.25000 0.12000

1 1 1 1 1 1

Table 3 Main crystallographic parameters of Rietveld refinement of the Lu1.95Ge2O7:0.05Bi3+ sample. Lu1.95Ge2O7:0.05Bi3+ Atom

Symmetry

x

y

z

Occupation

Lu3+ Bi3+ Ge4+ O2− O2− O2− O2−

8b 8b 8b 8b 8b 8b 4a

0.12620 0.12620 0.60070 −0.18200 0.06100 0.70570 0.41000

0.35470 0.35470 0.34950 0.36000 0.67300 0.29430 0.54000

0.11440 0.11440 0.11770 0.04200 0.17600 0.25000 0.12000

0.951 0.049 1 1 1 1 1

3

information of Bi3+ occupation in Lu2Ge2O7 host, the sites, atomic coordinates, and site occupancy are also collected in Tables 2 and 3. From Tables 2 and 3, the occupation of Bi3+ is confirmed that Bi3+ ions substitutes for the sites of Lu3+ ions. According to the refinement results of Lu2Ge2O7 host in Table 2, the crystal structure of Lu2Ge2O7 host is successfully built. As shown in Fig. 1(c), slightly puckered layers are formed of heavy atoms Lu3+:Ge4+ with equal contributions of rare earth and germanium atoms per layer [17,18]. There are two different polyhedron of LuO6 and GeO4 in Lu2Ge2O7 host. The ionic radii of Lu3+ in the six-fold coordinated oxygen and Ge4+ in the four-fold coordinated oxygen are 0.861 and 0.53 Å. While, the ionic radius of Bi3+ in the hexacoordination is 1.03 Å. In other word, the ionic radius (0.861 Å) of Lu3+ in sit-fold coordinated oxygen is close to that (1.03 Å) of Bi3+ ions while the ionic radius (0.53 Å) of Ge4+ ions is far smaller than that of Bi3+ ions. Thus, the occupation of Bi3+ is the sites of Lu3+ ions not Ge4+ ions. From Fig. 1(c), slightly puckered layers are formed of heavy atoms Lu3+:Ge4+ with equal contributions of rare earth and germanium atoms per layer [18,19]. The neighboring Lu ions are shared with the oxygen and not isolated by GeO4 group. Thus, Bi3+-Sm3+ or Bi3+-Eu3+ pairs will occupy the sites of Lu3+ when they are codoped into Lu2Ge2O7 host, energy transfer possibilities of Bi3+-Sm3+ and Bi3+-Eu3+ will be largely enhanced. 3.2. PL properties of Lu2-xGe2O7: xBi3+

on the basis of the starting model (ICSD No. 39929). From Fig. 1(a) and (b), one can see that that Lu2Ge2O7 host and Bi3+ doped Lu2Ge2O7 samples are purity without a secondary phase. Meanwhile, Table 1 gives the converged weighted-profiles (Rp, Rwp and Rexp), which further confirms the credibility of Rietveld refinement results. To obtain the deeper

The PL properties of Bi3+ doped Lu2Ge2O7 not only depends on the crystal structure, but also is also affected by the electronic band structure. From Fig. 2(a), the direct band gap (Egap) of Lu2Ge2O7 is calculated to be appropriately 3.36 eV at the k-point of G. From Fig. 2(b), we can see that conduction band (CB) bottom is mainly made up of O-2p, Ge-

Fig. 2. (a) band structure results of Lu2Ge2O7 host; (b) Total and partial density of states of bulk Lu2Ge2O7; (c) Diffuse reflectance spectra of Lu2Ge2O7 host at room temperature; (d) calculated optical band gap of Lu2Ge2O7 host.

Please cite this article as: Q. Li, S. Zhang, W. Lin, et al., A warm white emission of Bi3+-Eu3+ and Bi3+-Sm3+ codoping Lu2Ge2O7 phosphors by energy transfer of B..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117755

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4s and Ge-4p states while VB top primarily consists of O-2s, Ge-4s and Ge-4p states. In Fig. 2(c), there is a platform of high reflectance in near ultraviolet (UV) and visible region (350–800 nm), and a dramatic decrease in the UV region (200–350 nm). The UV absorption edge of Lu2Ge2O7 is attributed to the excitation of electrons from valence band (VB) to the CB, in which excitation energy is theoretically same to the Egap value. Thus, Egap of Lu2Ge2O7 is also calculated based on the reflectance absorption coefficient, which is determined by the following Eqs. (1) and (2) [20,21]:
 2 ðah vÞ ¼ A hv−Eg

F ðR∞ Þ ¼

ð1−R∞ Þ2 2R∞

ð1Þ

ð2Þ

where A is a constant, hν is the photon energy, α is the absorption coefficient which is approximately substituted by F(R∞) and R∞ presents the experimentally observed reflectance [21]. The Eg value of Lu2Ge2O7 is determined by the intersection of the fitting line and horizontal axis. Thus, Eg of Lu2Ge2O7 host is calculated to be 5.14 eV, as shown in Fig. 2(d). Generally, the value of calculated Egap approximately equals to two thirds of the optical Eg value, thus the above Egap and Eg is reliable. The difference between the Eg and calculated Egap originates from the inherent shortcomings of DFT calculations [22,23]. In Fig. 3(a), PL spectra of Lu2Ge2O7: xBi3+ exhibit a UV emission peaked at 330 nm and a shoulder peak located at 485 nm under excitation at 285 nm. Thus, it is inferred that two different kinds of centers coexist in Lu2Ge2O7 host. To study the origin of these two kinds of luminescence centers, Gaussian fitting of PL spectrum of Lu1.94Ge2O7: 0.06Bi3+ sample is carried out. Obviously, PL spectrum of Lu1.94Ge2O7: 0.06Bi3+ can be well fitted into two Gaussian peaks (i) and (ii). In the above analysis on the XRD refinements, it proves only one kind of Lu3 + and Ge4+ sites in Lu2Ge2O7 host. On the basis of ionic radii and charge number of Bi3+ (1.03 Å), Lu3+ (0.861 Å) with six-fold coordinated oxygen and Ge4+ (0.53 Å) with four-fold coordinated oxygen, thus it is speculated that Bi3+ ions replace the Lu3+ sites, impossibly substitute for Ge4+ sites. Undoubtedly, the short-wave peak (i) at 335 nm is attributed to the allowed 3P1 → 1S0 transition of Bi3+. It is worth noting that ionic radius of Bi3+ is much larger than that of Lu3+ in the same coordinated environment. Therefore, the occupation of Bi3+ in Lu3+ sites results in the lattice distortion and the defects such as oxygen and germanium vacancies in Lu2Ge2O7 host. It is inferred that the longwave peak (ii) originates from donor-acceptor pair recombination with oxygen vacancies acting as donors and ionized germanium vacancies acting as acceptors. A leading example is the self-activating fluorescence of Zn2GeO4 host, whose broad emission is attributed to the donor (oxygen vacancies and zinc interstitials) and the acceptor (germanium vacancies and zinc vacancies) recombination [24–26]. To further analyze the sources of Gaussian peaks (i) and (ii), PLE spectra are also performed, respectively. PLE spectrum of Lu2Ge2O7:Bi3 + by monitoring the Gaussian peaks (i) in Fig. 3(b) presents two broad absorption band ranging from 220 to 320 nm, which is attributed to the overlap of the 1S0 → 3P1 and 1S0 → 3P0 transitions of Bi3+ ions, respectively. By monitoring the Gaussian peaks (ii), one can see that PLE spectral structure is disparate, which is assigned to the host absorption. To study the changing trend of PL intensity of Bi3+ emission (i) and (ii) dependent on the doping concentration, Fig. 3(c) gives the variation of PL integrated intensity of Gaussian peak (i) and (ii) as function of the doping Bi3+ concentration (x). It is noted that Gaussian peaks (i) and (ii) display the identical variation tendency. PL intensity of both is initially enhanced as the increasing concentration (x) of Bi3+ ions. When x is increased to be 0.06, they reach the best PL intensity. If the Bi3+ content is continuously added into Lu2Ge2O7 host, PL intensity of Gaussian peaks (i) and (ii) present a damping. Therefore, it is concluded that the optimal doping concentration (x) of Bi3+ is approximately 0.06.

Fig. 3. (a) PL spectra of Lu2Ge2O7: xBi3+ (x= 0.01, 0.02, 0.06, 0.08, 0.10 and 0.14) and inset shows the Gaussian fitting of the emission band (i) and (ii); (b) PLE spectra of Gaussian peaks (i) and (ii);(c) the variation of PL integrated intensity of Gaussian peak (i) and (ii) as function of the doping Bi3+ concentration (x).

3.3. Multicolor tunable luminescence of Lu2Ge2O7: Bi3+, Eu3+/Sm3+ To investigate energy transfer possibility of Bi3+-Eu3+ in Lu1.943+ Ge , xEu3+, Fig. 4(a) presents the PLE spectrum of 2O7:0.06Bi x Lu1.85Ge2O7: 0.15Eu3+ (λem = 620 nm, red line + square) and PL spectrum of Lu1.94Ge2O7:0.06Bi3+ (λem = 290 nm, blue line + circle). PLE spectrum of Lu1.85Ge2O7: 0.15Eu3+ is composed of two parts: a UV broad excitation band is mainly attributed to the charge transfer band of Eu3+–O2– and a series of excitation lines originate from the inter 4f4f transitions of Eu3+ such as 7F0 → 5D4 at 320 nm, 7F0 → 5L7 at 360 nm, 7F0 → 5L6 at 360 nm, 7F0 → 5D3 at 394 nm, 7F0 → 5D2 at 466 nm and 7F0 → 5D1 at 534 nm [27,28]. From Fig. 4(a), one can see

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Fig. 4. (a) PLE spectrum of Lu1.85Ge2O7: 0.15Eu3+ (λem = 620 nm, red line + square) and PL spectrum of Lu1.94Ge2O7:0.06Bi3+ (λem = 290 nm, blue line + circle ); (b) PLE spectra of Lu1.94Ge2O7:0.06Bi3+ (λem = 350 nm, black line ) , Lu1.79Ge2O7:0.06Bi3+, 0.15Eu3+ (λem = 620 nm, red line), and Lu1.85Ge2O7:0.15Eu3+ (λem = 620 nm, green line ); (c) and (d) PLE and PL spectra of Lu1.94-xGe2O7:0.06Bi3+, xEu3+ (x=0.03, 0.06, 0.10, 0.15, 0.20, and 0.25).

that Bi3+ (i) and (ii) emissions match well with the above inter 4f-4f transitions of Eu3+. Thus, energy transfer process occurring between Bi3+ and Eu3+ is confirmed. To further discuss energy transfer from

Bi3+ to Eu3+ in Lu1.94-xGe2O7:0.06Bi3+, xEu3+, Fig. 4(b) exhibits a comparison among PLE spectra of Lu1.94Ge2O7:0.06Bi3+ (λem = 335 nm, black line), Lu1.79Ge2O7: 0.06Bi3+, 0.15Eu3+ (λem = 620 nm, red line),

Fig. 5. (a) PLE spectra of Lu1.94Ge2O7:0.06Bi3+ (λem = 335 nm, green line) , Lu1.88Ge2O7:0.06Bi3+, 0.06Sm3+ (λem = 604 nm, blue line) , and Lu1.94Ge2O7:0.06Sm3+ (λem = 604 nm, red line ); (b) PLE spectra of Lu1.94-xGe2O7:0.06Bi3+, xSm3+ (λem = 604 nm, x=0.005, 0.01, 0.02, 0.04, 0.06, and 0.08);(c) and (d) PLE and PL spectra of Lu1.94-yGe2O7:0.06Bi3+, ySm3+ (y= 0.005, 0.01, 0.02, 0.04, 0.06, and 0.08).

Please cite this article as: Q. Li, S. Zhang, W. Lin, et al., A warm white emission of Bi3+-Eu3+ and Bi3+-Sm3+ codoping Lu2Ge2O7 phosphors by energy transfer of B..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117755

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Fig. 6. CIE chromaticity coordinates of Lu2Ge2O7: Bi3+, Eu3+ and Lu2Ge2O7:Bi3+, Sm3+, and the samples photos under UV light (inset).

and Lu1.85Ge2O7:0.15Eu3+ (λem = 620 nm, green line). From Fig. 4(b), one can see that PLE spectrum of Lu1.79Ge2O7: 0.06Bi3+, 0.15Eu3+ (λem = 620 nm, red line) consists of one strong broad band and a weak shoulder band in the UV range, which are ascribed to joint contribution of charge transfer band of Eu3+–O2– and the 1S0 → 3P1 transition of Bi3+. The presence of the excited transition (1S0 → 3P1) of Bi3+ in the PLE spectrum of Eu3+ does confirm the occurrence of energy transfer process from Bi3+ to Eu3+. To systemically study the effect of Eu3+ doping concentration on the PL properties of Bi3+ in Lu2Ge2O7, Fig. 4(c) and (d) presents the PLE an PL spectra of Lu1.94-xGe2O7:0.06Bi3+, xEu3+ (x = 0.03, 0.06, 0.10, 0.15, 0.20, and 0.25), respectively. From Fig. 4(c), one can see that PLE spectra of Lu1.94-xGe2O7:0.06Bi3+, xEu3+ exhibit the similar spectral structure besides the intensity when Eu3+ ions keep increasing, indicating that occupation of Eu3+ ions in Lu2Ge2O7 is the same sites, i.e., Lu sites. PL spectra in Fig. 4(d) present the similar property. From Fig. 4(d), both the broad emission of Bi3+ and the line emissions of Eu3+ are observed. The characteristic emissions of Eu3+ are assigned to the intraconfigurational f-f transitions: 5D0 → 7F0 at 579 nm, 5D0 → 7F1 at 591 nm, 5D0 → 7F2 at 617 nm, 5D0 → 7F3 at 656 nm, and 5D0 → 7F1 at 706 nm. From Fig. 4(d), one can see that PL intensity of Bi3+ continuously decreases when doping concentration of Eu3+ ions is increased. When x = 0.20, Lu1.74Ge2O7:0.06Bi3+, 0.20Eu3+ exhibits the optimal PL intensity of Eu3+ ions. However, x value exceeds 0.20, PL intensity of Eu3+ suffers a decease due to the known concentration quenching [29,30].

Table 4 The CIE chromaticity coordinates of Lu2Ge2O7:Bi3+, Eu3+/Sm3+ samples under UV light excitation. Sample (No) 1 2 3 4 5 6 7 8 9 10 11 12

Composition

CIE (x,y) coordinate 3+

Lu1.94Ge2O5: 0.06Bi Lu1.93GeO5: 0.06Bi3+, 0.01Eu3+ Lu1.91GeO5: 0.06Bi3+, 0.03Eu3+ Lu1.88GeO5: 0.06Bi3+, 0.06Eu3+ Lu1.84GeO5: 0.06Bi3+, 0.10Eu3+ Lu1.79GeO5: 0.06Bi3+, 0.15Eu3+ Lu1.74GeO5: 0.06Bi3+, 0.20Eu3+ Lu1.935GeO5: 0.06Bi3+, 0.005Sm3+ Lu1.99GeO5: 0.06Bi3+, 0.01Sm3+ Lu1.98GeO5: 0.06Bi3+, 0.02Sm3+ Lu1.98GeO5: 0.06Bi3+, 0.04Sm3+ Lu1.98GeO5: 0.06Bi3+, 0.06Sm3+

(0.213, 0.236) (0.287, 0.252) (0.347, 0.245) (0.372, 0.238) (0.457, 0.236) (0.556, 0.233) (0.558, 0.232) (0.322, 0.302) (0.356, 0.297) (0.412, 0.238) (0.432, 0.189) (0.456, 0.183)

The premise of energy transfer from Bi3+ to Sm3+ in Lu2Ge2O7 is that PL spectrum of Lu1.94Ge2O7:0.06Bi3+ should overlap with PLE spectrum of Lu1.94Ge2O7: 0.06Sm3+. Thus, PLE spectrum of Lu1.94Ge2O7: 0.06Sm3+ and PL spectrum of Lu1.94Ge2O7: 0.06Bi3+ are measured at room temperature. In Fig. 5(a), there are a series of excitation lines in PLE spectrum of Lu1.94Ge2O7:0.06Sm3+, which originate from the electric dipole forbidden transitions (4f-4f) of Sm3+, for example, 6H5/2 → 4D7/2 at 345 nm, 6H5/2 → 4F9/2 at 360 nm, 6H5/2 → 4D5/2 at 373 nm, 6 H5/2 → 4K11/2 at 403 nm, and 6H5/2 → 4F5/2 + 4I13/2, etc. [30,31]. Evidently, there is a large overlap between PLE spectrum of Lu1.94Ge2O7:0.06Sm3+ and PL spectrum of Lu1.94Ge2O7: 0.06Bi3+. Thus, there is a high possibility of energy transfer from Bi3+ to Sm3+. To further demonstrate the above possibility, a comparison among the PLE spectra of Lu1.94Ge2O7: 0.06Bi3+ (λem = 335 nm, green line), Lu1.88Ge2O7:0.06Bi3+, 0.06Sm3+ (λem = 604 nm, blue line), and Lu1.94Ge2O7:0.06Sm3+ (λem = 604 nm, red line) is given in Fig. 5(b). From Fig. 5(b), one can see that a series of excitation lines in PLE spectrum of Lu1.94Ge2O7:0.06Sm3+ (λem = 604 nm) only exist in the near UV region (350–550 nm). However, a broad absorption band in PLE spectrum of Lu1.88Ge2O7:0.06Bi3+, 0.06Sm3+ (λem = 604 nm) appears in the UV range. Compared between the PLE spectra of Bi3+ (λem = 335 nm, green line), Sm3+ (λem = 604 nm, blue line), this absorption band in UV region is attributed to the 1S0 → 3P1 transition of Bi3+. Therefore, energy transfer of Bi3+-Sm3+ occurs in Lu1.88Ge2O7:0.06Bi3+, 0.06Sm3+, indeed. Fig. 5(c) and (d) gives the PLE and PL spectra of Lu1.94-xGe2O7:0.06Bi3 + , xSm3+ (λem = 604 nm, x = 0.005, 0.01, 0.02, 0.04, 0.06, and 0.08). From Fig. 5(c), the fine structure and peak position of PLE spectra are identical besides the intensity, inferring that Sm3+ ions occupy the same position, i.e., Lu3+ site. From Fig. 5(c), under excitation at 280 nm, PL spectra of Lu1.94-xGe2O7:0.06Bi3+, xSm3+ consists of two parts: Bi3+ emission bands (i) and (ii), and Sm3+ emission peaks attributed to the 4G5/2 → 6Hj (j = 5/2, 7/2 and 9/2) transitions, respectively. To study the variation tendency of Bi3+ and Sm3+ PL intensity, Fig. 5 (d) gives the Bi3+ and Sm3+ PL dependent on the Sm3+ doping concentration. From Fig. 5(d), we can see that Bi3+ PL intensity presents a continuous decrease, while Sm3+ PL intensity initially is enhanced as Sm3+ doping concentration is increased to 0.06, and then suffers a decrease. Therefore, the effective doping concentration of Sm3+ is approximately 0.06. Multicolor-tunable inorganic materials have been developing rapidly due to their promising application in white LEDs. In this paper, we tried to achieve the precise adjustment of emission colors of the Bi3+Eu3+ and Bi3+-Sm3+ codoped Lu2Ge2O7 samples. Although Bi3+ (i) emission ranging from 300 to 380 nm can't take part in the emission color, Bi3+ (ii) emission in the region of 400–650 nm presents a bluecyan emission color. Thus, we can obtain a multicolor-tunable emission including the white emission if red component can be added by codoping the Eu3+ or Sm3+ into Lu2Ge2O7. For Lu1.94-xGe2O7: 0.06Bi3 + , xEu3+ (x = 0.01, 0.03, 0.06, 0.10, 0.15 and 0.20), the emission colors can be adjusted precisely from warm white light, light pink, pink to red by changing the Eu3+ doping concentration. While, the multicolortunable emissions of Lu1.94-yGe2O7:0.06Bi3+, ySm3+ (y = 0.005, 0.01, 0.02, 0.04, and 0.06) can be tuned from cold white light, warm white light, light pink to pink by varying the Sm3+ content. Fig. 6 gives the corresponding CIE chromaticity coordinates of Lu1.94-xGe2O7:0.06Bi3+, xEu3+ and Lu1.94-yGe2O7:0.06Bi3+, ySm3+ samples, Table 4 lists the values of CIE chromaticity coordinates, and graphs are also provided in the inset of Fig. 6. 3.4. Energy transfer Bi3+-Eu3+ and Bi3+-Sm3+of Lu2Ge2O7 To quantitatively study energy transfer process of Bi3+-Eu3+, energy transfer efficiency (ηETE) is a crucial factor, which is dependent on the lifetime decrement of the sensitizers (Bi3+ ions). Fig. 7(a) shows the representative decay curves of Lu1.94-xGe2O7:0.06Bi3+, xEu3+ by

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Fig. 7. (a) Decay curves of Lu1.94-xGe2O7:0.06Bi3+, xEu3+ (x=0.03, 0.06, 0.10, 0.15, 0.20, and 0.25, λex = 290 nm, λem = 335 nm); (b) the variation tendency of decay lifetimes of Bi3+ emission and ηETE of Bi3+-Eu3+; (c) Decay curves of Lu1.94-xGe2O7:0.06Bi3+, xSm3+ (x=0, 0.01, 0.02, 0.04, 0.06, and 0.08, λex = 290 nm, λem = 335 nm); (d) the variation tendency of decay lifetimes of Bi3+ emission and ηETE of Bi3+-Sm3+.

monitoring the Bi3+ emission at 335 nm. The average lifetime (τave) of Bi3+ is estimated by the Eq. (3) [32,33]: R∞ τave ¼ R0∞ 0

IðtÞ tdt Iðt Þ dt

ð3Þ

Thus, the τave of Bi3+ in Lu1.94-xGe2O7:0.06Bi3+, xEu3+ are calculated to be 105, 98, 91, 84, 68, and 60 ns when x = 0, 0.06, 0.10, 0.15, 0.20, and 0.25, respectively. As shown in Fig. 7(a), one can see that τave of Bi3+ in Lu1.94-xGe2O7:0.06Bi3+, xEu3+ keeps a continuous decrease, demonstrating that energy transfer possibility of Bi3+-Eu3+ becomes growing with the increasing concentration of Eu3+. Meanwhile, ηETE of Bi3+Eu3+ is estimated by the following equation [34,35]: τs ηET ¼ 1− τ0

ð4Þ

where τs0 represents the lifetime of Bi3+ doped Lu2Ge2O7, and τs are the lifetime of Bi3+ emission in Bi3+-Eu3+ codoped samples. As shown in Fig. 7(b), ηETE of Bi3+-Eu3+ keeps a continuous increase when Eu3+ doping concentration is continually added. The effective ηETE of Bi3+Eu3+ can reach as high as 42% because the optimal concentration of Eu3+ is approximately 0.20. To deeply evaluate the energy transfer probability of Bi3+-Sm3+, the decay curves of Lu1.88Ge2O7:0.06Bi3+, xSm3+ (x = 0, 0.01, 0.02, 0.04, 0.06 and 0.08) are also obtained by monitoring the emission at 335 nm under excitation with 290 nm. Fig. 7(c) exhibits a faster and faster decay rate of Bi3+ with the increasing concentration of Sm3+. These results further prove energy transfer from Bi3+ to Sm3+. By the above Eqs. (3) and (4), τave of Bi3+ and ηETE of Bi3+-Sm3+ are calculated as shown in Fig. 7(d). From Fig. 7(d), one can see that ηETE of Bi3+-Sm3+

shows the increasing tendency, however, the effective ηETE of Bi3+-Sm3 + is approximately 35%. Generally, the energy transfer mechanism of Bi3+ → Eu3+/Sm3+ corresponds to two probability: one is the exchange interaction and the other is the multipolar interaction. To know which one rules the energy transfer, the critical distance between Bi3+ and Eu3+/Sm3+ can be estimated by the following equation:  1 3V 3 Rc ≈ 2x 4πxc N

ð5Þ

where N is the number of molecules in the unit cell, V is the cell volume, and xc is the total concentration of Bi3+-Eu3+ or Bi3+-Sm3+ [31,32]. In this case, N equals to 4, the cell volume is 546.86 Å3 and xc of Bi3+-Eu3 + and Bi3+-Sm3+ is approximately 0.13 and 0.05. When Rc between sensitizers and activators is shorter than 5 Å, energy transfer mechanism is the exchange interaction. Thus, Rc of Bi3+-Eu3+ and Bi3+-Sm3 + can be calculated to be 6.3 and 8.7 Å, respectively. A conclusion can be made that the exchange interaction probably plays little role, and energy transfer from Bi3+ to Eu3+/Sm3+ is probably governed by the multipolar interactions [31,32]. The interaction types of energy transfer of sensitizers and activators is determined by the following Eq. (6) [32–34]: η0 n=3 ∞c ηs

ð6Þ

Where c is the activator's doping concentration, η and η0 correspond to the quantum yields of Bi3+ with and without the activators, i.e., Eu3+ or Sm3+. Energy transfer interactions depends on the n value (n = 6, 8, and10), corresponding to the dipole-dipole (d-d), dipole-quadrupole

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Fig. 8. Dependence of Is0/Is on Cn/3 Eu3+ ((a) n=6, (b) n=8, and (c) c=10) in Lu1.94-xGe2O7: 0.06Bi3+, xEu3+ phosphors (x = 0, 0.03, 0.06, 0.10, 0.15, 0.20, and 0.25), respectively.

(d-q), and quadrupole-quadrupole (q-q), respectively [32–34]. In this case, η0/η is approximately substituted by Is0/Is, where Is0 presents the PL integrated intensity of sensitizer (Bi3+) without the activators (Eu3+ or Sm3+) while Is is the PL integrated intensity with Eu3+ or Sm3+. Thus, Fig. 8 and 9 give the linear relation and fitting line between Is0/Is and xn/3 of Bi3+-Eu3+ and Bi3+-Sm3+ in Lu2Ge2O7 host, c respectively. By fitting the experimental data, we obtained the optimal linear relations of both Bi3+-Eu3+ (R2 = 0.99525) and Bi3+-Sm3 + (R2 = 0.99557) when n = 6, which shown in Fig. 8(a) and 9(a). Thus, it is inferred that energy transfer mechanisms of Bi3+-Eu 3+ and Bi3+-Sm3+ in Lu2Ge2O7 are the d-d interaction. 3.5. Thermal stability of white emission For practical application of single-composition white emitting emission phosphor for UV-converted white LEDs, thermal stability is an important factor which can largely affect the performance of white LED devices. Generally, optical materials suffer a thermal quenching, i.e., PL intensity of phosphors keeps a continuous reduction when the rise of working temperature continues. Furthermore, the overall performance

Fig. 9. Dependence of Is0/Is on Cn/3 Sm3+ ((a) n=6, (b) n=8, and (c) c=10) in Lu1.94-yGe2O7: 0.06Bi3+, yEu3+ phosphors (x = 0, 0.005, 0.01, 0.02, 0.04, 0.06, and 0.08), respectively.

of white LEDs devices largely depends on the thermal stability. Thus, it is indispensable to go insight into the influence of temperature on PL properties. As is known to us, the working temperature of LED chips will be increased to 150 °C. Fig. 10(a) presents the temperature-dependent PL spectra of Lu1.91Ge2O7:0.06Bi3+, 0.03Eu3+. Unsurprisingly, PL intensity of both Bi3+ and Eu3+ exhibits a continuous attenuation due to thermal quenching. Although PL intensity of Bi3+ and Eu3+ at 150 °C keeps a half of that at room temperature in Fig. 10(b), the resistance ability of thermal quenching should be further improved. For further evaluation about the influence of the temperature on the PL properties, the activation energy (ΔEa), which can reflect the resistance ability to thermal quenching, is determined by the Arrhenian equation [35–37]: IT ¼

I0 1 þ Aeð−ΔE=kB T Þ

ð7Þ

In which A is a constant; IT presents the PL intensity at a given temperature; I0 presents the initial PL intensity at room temperature; kB is the Boltzmann constant (8.629 × 10−5 eV/K). Eq. (7) can be revised as

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Fig. 10. (a) Temperature dependent PL spectra of Lu1.91Ge2O7:0.06Bi3+, 0.03Eu3+ in the range from 298 to 573 K; (b) Normalized PL intensity of Lu1.91Ge2O7:0.06Bi3+, 0.03Eu3+ at different temperature; (c) Relationship of ln[(I0/I(T))-1] versus 1/κT of Lu1.91Ge2O7:0.06Bi3+, 0.03Eu3+ sample.

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Fig. 11. (a) Temperature dependent PL spectra of Lu1.93Ge2O7:0.06Bi3+, 0.01Sm3+ in the range from 298 to 573 K; (b) Normalized PL intensity of Lu1.93Ge2O7:0.06Bi3+, 0.01Sm3 + at different temperature; (c) Relationship of ln[(I0/I(T))-1] versus 1/κT of Lu1.93Ge2O7:0.06Bi3+, 0.01Sm3+ sample.

follows:   △E I0 þ 1nA ¼ 1n − −1 KBT It

ð8Þ

Form the Eq. (8), the value of ΔEa depends on the absolute value of the fitting line's slope in Fig. 10(c). By fitting the experimental datum, ΔEa is equal to 0.136 eV. In this paper, the influence of the temperature on the PL properties of Lu1.93Ge2O5:0.06Bi3+, 0.01Sm3+ is also studied. From Fig. 11(a) and (b), PL intensity of Lu1.93Ge2O5:0.06Bi3+, 0.01Sm3+ also presents a decrease with the temperature rises. When temperature rises to 150 °C, the PL intensity of Lu1.93Ge2O5: 0.06Bi3+, 0.01Sm3+ still keeps 50% of that at room temperature. In Fig. 11(c), the corresponding value of ΔEa is also calculated to be 0.114 eV by the Eqs. (7) and (8). Given the above, the single-phase white emission phosphors Lu1.91Ge2O7:0.06Bi3+, 0.03Eu3+ and Lu1.93Ge2O5:0.06Bi3+, 0.01Sm3+ have a poor resistant ability to thermal quenching.

4. Conclusions In a word, we successfully prepared a series of Bi3+, Bi3+-Eu3+ and Bi3+-Sm3+ doped Lu2Ge2O7 phosphors. Lu2Ge2O7: Bi3+ presents a UV and visible doublet emission with a high QE of 76%, and serves as an ideal sensitizer. Energy transfer from Bi3+ to Eu3+ or Sm3+ is proved and presents a high ηETE of 42% and 35%, respectively. Furthermore, multicolor-tunable emissions of Bi3+-Eu3+ and Bi3+-Sm3+ codoped Lu2Ge2O7 phosphor is adjusted from cold white light, warm white light, light pink, pink, red by changing the Eu3+ or Sm3+ doping concentration. However, their thermal stability should be further improved. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements We deeply thanks for the financially support from the Science and Technology Program of Guangzhou, China (201804010257, 201707010324), Key Platforms and Research Projects of Department of Education of Guangdong Province (2016KTSCX031, 2017KTSCX054), National Natural Science Foundation of China (Grant Nos. 51602063) and Youth innovation talents program of Guangzhou Maritime University (F410515) of China. References [1] T. Takeda, R.J. Xie, T. Suehiro, N. Hirosaki, Nitride and oxynitride phosphors for white LEDs: synthesis, new phosphor discovery, crystal structure, Prog. Solid State Ch. 51 (2018) 41–51. [2] 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. [3] A. Arredondo, H. Desirena, I. Moreno, I.E. Orozco Hinostroza, Dual color tuning in Ce3 + -doped oxyfluoride ceramic phosphor plate for white LED application, J. Am. Ceram. Soc. 102 (2019) 1425–1434. [4] 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. [5] Y.H. Kim, P. Arunkumar, B.Y. Kim, S. Unithrattil, E. Kim, S.H. Moon, W.B. Im, A zerothermal-quenching phosphor, Nat. Mater. 16 (2017) 543. [6] Z. Xia, Q. Liu, Progress in discovery and structural design of color conversion phosphors for LEDs, Prog. Mater. Sci. 84 (2016) 59–117. [7] Z. Xia, A. Meijerink, Ce3+-doped garnet phosphors: composition modification, luminescence properties and applications, Chem. Soc.y Rev. 46 (2017) 275–299. [8] M. Zhao, H. Liao, L. Ning, Q. Zhang, Q. Liu, Z. Xia, Next-generation narrow-band green-emitting RbLi(Li3SiO4)2:Eu2+ phosphor for backlight display application, Adv. Mater. 30 (2018), 1802489. [9] J. Li, X. Zhou, J. Ding, X. Zhou, Y. Wang, Mechanism analysis of a narrow-band ultrabright green phosphor with its prospect in white light-emitting diodes and field emission displays. J. Mater. Chem. C Doi:https://doi.org/10.1039/C8TC05330H. [10] M. Shang, C. Li, J. Lin, How to produce white light in a single-phase host? Chem. Soc. Rev. 43 (2014) 1372–1386. [11] H. Wang, X. Chen, L. Teng, D. Xu, W. Chen, R. Wei, H. Guo, Adjustable emission and energy transfer process in BaGd2O4:Bi3+, Eu3+ phosphors, J. Lumin. 206 (2019) 185–191. [12] W. Zhao, S. Qi, J. Liu, B. Fan, Luminescence properties and energy transfer of a colortunable Y2GeO5: Bi3+, Eu3+ phosphor, J. Alloys and Compd. 787 (2019) 469–475. [13] Y.L. Yang, Y. Zhou, D.J. Pan, Z.J. Zhang, J.T. Zhao, The luminescence properties of CaZnOS: Bi3+, Sm3+, Li+ phosphors with tunable emissions and energy transfer for white emission, J. Lumin. 206 (2019) 578–584. [14] Q. Wang, Z. Mu, S. Zhang, Q. Du, Y. Qian, D. Zhu, F. Wu, Bi3+ and Sm3+ co-doped La2MgGeO6: a novel color-temperature indicator based on different heat quenching behavior from different luminescent centers, J. Lumin. 206 (2019) 462–468. [15] H. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr. 2 (1969) 65–71. [16] J. Li, S. Zhang, H. Luo, Z. Mu, Z. Li, Q. Du, F. Wu, Efficient near ultraviolet to near infrared downconversion photoluminescence of La2GeO5: Bi3+, Nd3+ phosphor for silicon-based solar cells, Opt. Mater. 85 (2018) 523–530. [17] Z. Li, S. Zhang, Q. Xu, H. Duan, Y. Lv, X. Lin, Y. Hu, Long persistent phosphor SrZrO3: Yb3+ with dual emission in NUV and NIR region: a combined experimental and first-principles methods, J. Alloys and Compd. 766 (2018) 663–671. [18] G.A. Bandurkin, G.V. Lysanova, V.A. Krut’ko, M.G. Komova, Cationic networks in the structures of rare earth germanates and borogermanates: typology of cationic

[19] [20] [21] [22]

[23]

[24] [25]

[26]

[27]

[28]

[29]

[30]

[31]

[32] [33]

[34]

[35]

[36]

[37]

networks in the structures of rare earth compounds, Russ. J. Inorg. Chem. 55 (2010) 238–246. U.W. Becke, J. Felsche, Phases and structural relations of the rare earth germanates RE2Ge2O7, RE=La-Lu, J. Less Common Metals. 128 (1987) 269–280. D.L. Wood, J. Tauc, Weak absorption tails in amorphous semiconductors, B. Phys. Rev. 5 (1972) 3144–3151. S.P. Tandon, J.P. Gupta, Diffuse reflectance spectra of cuprous oxide, B. Phys. Status Solidi. 38 (1970) 363–367. J.C. Chang, C.T. Chen, M. Rudysh, M.G. Brik, M. Piasecki, W.R. Liu, La6Ba4Si6O24F2:Sm3 + novel red-emitting phosphors: synthesis, photoluminescence and theoretical calculations, J. Lumin. 206 (2019) 417–425. J. Zhong, W. Zhao, Y. Zhuo, C. Yan, J. Wen, J. Brgoch, Understanding the blueemitting orthoborate phosphor NaBaBO3:Ce3+ through experiment and computation, J. Mater. Chem. C 7 (2019) 654–662. Z. Liu, X. Jing, L. Wang, Luminescence of native defects in Zn2GeO4, J. Electrochem. Soc. 154 (2007) H500–H506. L. Li, Y. Su, Y.Q. Chen, M. Gao, Q. Chen, Y. Feng, Synthesis and photoluminescence properties of hierarchical zinc germanate nanostructures, Adv. Sci. Lett. 3 (2010) 1–5. V. Bharat, R. Boppana, N.D. Hould, R.F. Lobo, Synthesis, characterization and photocatalytic properties of novel zinc germanate nano-materials, J. Solid State Chem. 184 (2011) 1054–1062. R. Guo, J. Huang, X. Chen, Q. Luo, L. Luo, Y. Xiong, S. Zhang, Pechini sol-gel synthesis of La2CaB8O16:Eu3+ red phosphor and its photoluminescence spectral properties, J. Lumin. 206 (2019) 15–20. M. Wu, X. Li, B. Milićević, M. Dramićanin, X. Jing, Q. Tang, J. Shi, Eu3+ activated Sr3ZnTa2O9 single-component white light phosphor: emission intensity enhancement and color rendering improvement, J. Mater. Chem. C Doi:https://doi.org/10. 1039/C9TC00159J. X. Li, P.L. Li, Z.J. Wang, S.M. Liu, Q. Bao, X.Y. Meng, K.L. Qiu, Y.B. Li, Z.Q. Li, Z.P. Yang, Color-tunable luminescence properties of Bi3+ in Ca5(BO3)3F via changing site occupation and energy transfer, Chem. Mater. 29 (2017) 8792–8803. M.M. Tian, P.L. Li, Z.J. Wang, Z.L. Li, J.G. Cheng, Y.S. Sun, C. Wang, X.Y. Teng, Z.P. Yang, F. Teng, Controlling multi luminescent centers via anionic polyhedron substitution to achieve single Eu2+ activated high-color-rendering white light/tunable emissions in single-phased Ca2(BO3)1-x(PO4)xCl phosphors for ultraviolet converted LEDs, Chem. Eng. J. 326 (2017) 667–679. C. Wang, P.L. Li, Z.J. Wang, Y.S. Sun, J.G. Cheng, Z.L. Li, M.M. Tian, Z.P. Yang, Crystal structure, luminescence properties, energy transfer and thermal properties of a novel color-tunable, white light-emitting phosphor Ca9-x-yCe(PO4)7:xEu2+, yMn2+, Phys. Chem. Chem. Phys. 18 (2016) 28661–28673. P.L. Li, Z.J. Wang, Z.P. Yang, Q.L. Guo, A novel, warm, white light-emitting phosphor Ca2PO4Cl: Eu2+, Mn2+ for white LEDs, J. Mater. Chem. C 2 (2014) 7823–7829. Y.S. Sun, P.L. Li, Z.J. Wang, J.G. Cheng, Z.L. Li, C. Wang, M.M. Tian, Z.P. Yang, Tunable emission phosphor Ca0.75Sr0.2Mg1.05(Si2O6): Eu2+, Mn2+: luminescence and mechanism of host, energy transfer of Eu2+→Mn2+, Eu2+→host, and host→Mn2+, J. Phys. Chem. C 120 (2016) 20254–20266. T. Li, P.L. Li, Z.J. Wang, S.C. Xu, Q. Bai, Z.P. Yang, Coexistence phenomenon of Ce3+Ce4+ and Eu2+-Eu3+ in Ce/Eu co-doped LiBaB9O15 phosphor: luminescence and energy transfer, Phys. Chem. Chem. Phys. 19 (2017) 4131–4138. J. Qiao, J. Zhao, Q. Liu, Z. Xia, Recent advances in solid-state LED phosphors with thermally stable luminescence, J. Rare Earth. Doi: https://doi.org/10.1016/j.jre. 2018.11.001. M. Xia, X. Wu, Y. Zhong, Z. Zhou, W. Y. R. Wong, A novel Na3La(PO4)2/LaPO4:Eu bluered dual-emitting phosphor with high thermal stability for plant growth lighting, J. Mater. Chem. C doi:https://doi.org/10.1039/C8TC06119J. M. Iwaki, S. Kumagai, S. Konishi, A. Koizumi, T. Hasegawa, K. Uematsu, M. Sato, Blueyellow multicolor phosphor, Eu2+-activated Li3NaSiO4: excellent thermal stability and quenching mechanism, J. Alloys Compd. 776 (2019) 1016–1024.

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