Enhancing Sm3+ emission of LiLa(MoO4)2:Sm3+, Bi3+ phosphors by non-sensitization of Bi3+

Journal of Luminescence 214 (2019) 116590

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Enhancing Sm3+ emission of LiLa(MoO4)2:Sm3+, Bi3+ phosphors by nonsensitization of Bi3+

T

Kai Wanga, Yun Liub,*, Dinghan Liub, Guoqiang Tana, Shaojie Baia, Huijun Renc, Zubairu Siyaka Mjd a

School of Materials Science and Engineering, Xi'an, 710021, China College of Electrical and Information Engineering, Xi'an, 710021, China c School of Arts and Sciences, Shaanxi University of Science & Technology, Xi'an, 710021, China d Department of Chemistry, Ahmadu Bello University, Zaria, 810221 Nigeria b

A R T I C LE I N FO

A B S T R A C T

Keywords: Luminescent Bi3+ doping Crystal structure Non-sensitization

In order to obtain high-efficiency luminescent materials, the energy transfer between Bi3+ ions and rare earth ions co-doped compounds has been studied extensively. Herein, a novel red phosphor LiLa(MoO4)2:Sm3+, Bi3+ is synthesized by solid-state reaction. Interestingly, Bi3+ ions act as non-sensitizers to enhance the luminescent intensity of Sm3+ ions in this system. We try to explain the novel non-sensitization of Bi3+ ions. The characterization results of the samples indicate that the dopant Bi3+ ions used can generate the microstrain on the crystals. After analysis, the microstrain may be one of the important factors to enhance the luminescence of Sm3+ ions.

1. Introduction Since bismuth has multiple valences and a special ionic radius. Bi3+ doping is widely used in luminescent materials [1–3]. Bi3+ ions can be used as an activator ion to emit light of different wavelengths because the naked 6s electrons are particularly sensitive to the surrounding crystal field [4,5]. In addition, the Bi3+ ion emission band overlaps with the excitation band of rare earth (RE) ions [6,7]. Therefore, Bi3+ion is also commonly used as a sensitizer to enhance the emission of RE ions via energy transfer (ET) process. There are many RE ions that can be used for energy transfer, such as Eu3+, Tb3+ and Sm3+ [8–10]. It is well known that the host has an important influence on the luminescent properties of the phosphor. Recently, it has been reported that Bi3+ ions act as a sensitizer for RE ions in a variety of hosts, such as vanadates [11], RE oxides [12,13], tungstates [14] and molybdates [15]. Among the molybdates, alkali metal rare-earth double molybdates with the general formula MRE(MoO4)2 (M = alkali metal ions, RE = rare earth ions) are regarded as potential host materials due to their outstanding thermal and chemical stabilities [16]. LiLa(MoO4)2 (LLMO) has a scheelite structure with I41/a space group, as an important member of the alkali metal rare-earth double molybdates family. In addition, the Li+ ion can enhance the luminous efficiency and MoO42− also has a strong absorption in the NUV region to transfer energy to the

*

activator ions [17]. Therefore, LLMO is widely applied to lasers and luminescent materials [18,19]. However, we have found that Bi3+ ions do enhance the luminescent intensity of Sm3+ ions in the LLMO host. Interestingly, no ET occurred between Bi3+ ions and Sm3+ ions in this process. The recent research showed that the change caused by doping in the microstructure was also one of the reasons that enhance the emission intensity of RE ions [20]. For example, structural changes would lead to changes in the crystal refractive index, microstrain, etc. Gao [21] reported that the relative intensity of NaMgPO4:Eu3+, Al3+ phosphors was enhanced because the Al3+ doping induced crystal defects and distortion increased the microstrain. So, the reason why Bi3+ ion enhanced Sm3+ ion emission might be attributed to the effects of microstrain. In this work, a series of LLMO:Sm3+, Bi3+ phosphors were successfully prepared by solid-state reaction. The characterization methods, such as X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to analyze the crystallinity and morphology of the phosphors. The luminescent properties and possible effects of non-sensitizer Bi3+ to enhance Sm3+ emission have been investigated in detail.

Corresponding author. E-mail address: [email protected] (Y. Liu).

https://doi.org/10.1016/j.jlumin.2019.116590 Received 14 March 2019; Received in revised form 26 June 2019; Accepted 28 June 2019 Available online 29 June 2019 0022-2313/ © 2019 Published by Elsevier B.V.

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2. Experimental

respectively.), indicating a contraction and an expansion of the lattice cell due to Sm3+ ions doping and Bi3+ ions doping, respectively. However, the diffraction peaks shift slightly towards higher angle as the Bi3+ ions doping concentration is increased. This strange result could be due to the lattice distortion caused by Bi3+ ions doping. The lattice strain causes the lattices to compress and contract each other, and affects the diffraction peak shift [23]. In order to further identify the crystal structural information of the samples, the XRD patterns of LLMO, LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ are refined by the GSAS program using the Le Bail method [24,25] as shown in Fig. 2. It is very clear seen that each peak is in good agreement with the observed and calculated data. Although there are few impurities L2MoO4 appearing at low angle range, which hardly affects the luminescent properties of the phosphor. All results of the refined structural parameters are summarized in Table 1. From Table 1, the LLMO has a tetragonal structure with the space group of I41/a (88), and the lattice constants are a = b = 5.339974 Å, c = 11.78575 Å, α = β = γ = 90°, Z = 2 and V = 338.597 Å3. The refined parameters (Rp, Rwp and χ2) further suggest that the crystal structures of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ are the same as the bare host LLMO. The effective ionic radii of Sm3+ (r = 1.08 Å, CN = 8) and Bi3+ (r = 1.17 Å, CN = 8) are similar to that of La3+ (r = 1.16 Å, CN = 8), which makes it possible for the successful substitution of La3+ sites by Sm3+ and Bi3+ ions in the lattice [26]. The crystal structural model of LLMO is illustrated in Fig. 2d. Significantly, Li+ and La3+ ions occupy the same site forming [LaO8] polyhedra shared by eight oxygen atoms. Each Mo6+ is coordinated by four O2− forming [MoO4] tetrahedron and connected to [LaO8] polyhedra by their shared oxygen atoms. Fig. 3 shows the microstructural morphology of LLMO, LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ characterized by SEM. It is obvious that all the samples exhibit an irregular or diamond shaped-like morphology with similar octahedral morphology in the size range of 50–100 μm. The crystal surfaces of the host and Sm3+ singledoped (Fig. 3d and e) are smooth, but the Sm3+/Bi3+ co-doped (Fig. 3f) is rather rough because doping of Bi3+ ions changes the crystal structure and thus forms crystal defects which affect the crystal growth, which results in the increase of the crystal surface roughness. In addition, because of the presence of crystal defects, it can be inferred that there may be microstrain and subgrains in the crystal. To confirm the presence of microstrain existence on the crystal, further details morphological and structural features of the samples are further investigation by TEM, HRTEM, SAED and EDS shown in Fig. 4. Fig. 41–a3 show the irregular morphology of the particles. The HRTEM images of LLMO, LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ are presented in Fig. 4d1–d3, respectively. It is seen that the interplanar distances between the adjacent lattice fringes of the three samples are about 3.164 Å, 3.166 Å and 3.163 Å, all of which correspond to the (112) plane of the tetragonal LLMO. From the diffraction spot of the Fast Fourier Transform (FFT) patterns (see Fig. 4c1–c3), the (112) planes of all the samples of the tetragonal LLMO have been confirmed once again. From Fig. 4b1 to Fig. 4b3, the SAED patterns of the samples are changed from the disordered diffraction spots to the bright ring corresponding to (112) planes agrees well with the XRD results. In particular, in Fig. 4b3, the bright diffraction rings indicate that LLMO:0.05Sm3+, 0.07Bi3+ sample is a polycrystal. There may be a strong microstrain on the sample during the formation of polycrystals. The energy dispersive spectroscopy (EDS) was employed to estimate the profile chemical composition of LLMO:0.05Sm3+, 0.07Bi3+ phosphor as highlighted. Its result reveals the existences of La, Mo, O, Sm and Bi atomic profile as the elementary species present in the sample, indicating that Sm3+ and Bi3+ have been doped with the LLMO host. This result is consistent with the XRD data. The Li element does not appear in the EDS owing to its light relative molecular mass [27].

Li2CO3, MoO3, La2O3, Sm2O3, and Bi2O3 are all analytical graded reagent and were used as received. A series of LLMO:Sm3+, Bi3+phosphors were prepared by the traditional solid-state reaction. All raw materials were stoichiometrically weighed and then placed in an agate mortar mixed thoroughly and grounded for 30 min. The mixture was transferred into crucible with a lid placed in a muffle furnace and calcined at 900 °C for 4 h in air. The final product was grounded into powder after cooling to room temperature for the further measurement. The crystal structure information of the samples was obtained by using X-ray powder diffraction (D8 Advance, Bruker), and operating at 40 kV and 40 mA with Cu Kα radiation (λ = 1.54056 Å). The sample morphologies were measured by a scanning electron microscope (TM3000, Hitachi) with 15 kV and the elemental analysis profile was carried out by an energy dispersive spectroscopy (EDS). The transmission electron microscope (TEM), high-resolution (HRTEM) and the selected area electron diffraction (SAED) images were recorded by FEI Tecnai-G2-F20 microscope with 200 kV. Photoluminescence (PL), decay curves, and quantum yield (QY) of the samples were measured by a fluorescence spectrophotometer (Edinburgh Instruments FS5, UK) with a 150 W xenon lamp. Raman spectra were measured at room temperature by Raman spectroscopy (Renishaw) with a 785 nm laser. 3. Results and discussion 3.1. Phase and crystal structure Fig. 1a shows the XRD patterns of LLMO and LLMO:Sm3+, Bi3+ samples calcined at 900 °C. It can be seen that all the patterns are well indexed with the crystal structure of LLMO in good agreement with the diffraction patterns data base (JCPDS No. 18–0734) except for the few impurities at 2θ = 21° and 25° marked with the black diamond, indicating that the doped Sm3+ and Bi3+ do not cause the host phase change. The strong and sharp diffraction peaks in the spectra also indicate that the as-synthesized samples possess a high crystallinity from its distinct diffractograms. As shown in Fig. 1b, the XRD diffraction peaks of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.01Bi3+ are red-shifted and blue-shifted, respectively. Since the doped ions have different ionic radii (Sm3+: r = 1.08 Å, CN = 8, Bi3+: r = 1.17 Å, CN = 8 and La3+: r = 1.16 Å, CN = 8) [10,22], according to Bragg equation (2dsinθ = λ, where λ and θ are the wavelength of the X-ray and the diffraction angle,

Fig. 1. (a) XRD patterns of LLMO and LLMO:0.05Sm3+, xBi3+ (x = 0–0.09) phosphors, (b) The enlarged XRD patterns of (a) in the range of 28–28.5°. 2

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Fig. 2. Rietveld refinement XRD patterns of (a) LLMO, (b) LLMO:0.05Sm3+, (c) LLMO:0.05Sm3+, 0.07Bi3+; (d) crystal structure of the LLMO host.

states 4D3/2, 6P7/2, 4F7/2, 6P5/2, 4G9/2 and 4I13/2 + 4I9/2 within the 4f5 configuration of Sm3+, respectively [28–30]. Among these transitions, the strongest absorption peak at 405 nm is corresponding to the transition 6H5/2 → 4F7/2 of Sm3+. Under excitation at 405 nm, the emission spectrum consists of four dominating emission peaks, which are attributed to 4G5/2 → 6H5/2 (564 nm), 4G5/2 → 6H7/2 (600 nm), 4G5/2 → 6 H9/2 (647 nm) and 4G5/2 → 6H11/2 (708 nm) transitions of Sm3+, respectively [31,32]. It is seen that the value of the strongest peak (647 nm) shows a trend of increase first and then decrease with the increase of the Sm3+ doping concentration (see the inset). The emission intensity reaches a maximum when the doping concentration of Sm3+ is 5 mol%. When the concentration of Sm3+ continues to increase further, the emission intensity decreases due to the concentration quenching [33]. The excitation spectra of LLMO:0.05Bi3+, LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.05Bi3+ phosphors monitored at 647 nm are shown in Fig. 6a. It is observed that the Sm3+ single-doped and Sm3+/Bi3+ codoped excitation spectra have similar shapes and the peak positions by comparing them within the wavelength range of the investigation (350–500 nm). Meanwhile, the strongest characteristic absorption peak of Sm3+ of LLMO:0.05Sm3+, 0.05Bi3+ at 405 nm is higher than that of LLMO:0.05Sm3+, implying that the doped Bi3+ ions increase the energy absorption of Sm3+ ions in the crystal. The excitation spectra of the LLMO:0.05Bi3+ sample shows that there is no obvious absorption peak at 405 nm. Obviously, it can be inferred that the Bi3+ ions do not contribute to the luminescent intensity of LLMO:Sm3+/Bi3+. This conclusion can also be confirmed from Fig. 7. The absorption peak of Bi3+ is located at 246 nm instead of 405 nm. The emission spectrum

Table 1 Lattice constants and refinement parameters of LLMO, LLMO:0.05Sm and LLMO:0.05Sm, 0.07Bi samples. Samples

LLMO

Crystal system Space group Lattice parameters a = b (Å) ( ± Error)

tetragonal I41/a (88)

c (Å) ( ± Error) α = β = γ(deg) V (Å3) ( ± Error) Z Refinement Rp Rwp χ2

5.3399 ( ± 0.0018) 11.7857 ( ± 0.0041) 338.597 ( ± 0.352) Rietveld 6.94% 9.91% 1.969

LLMO:0.05Sm

LLMO:0.05Sm, 0.07Bi

5.3578 ( ± 0.0019) 11.7792 ( ± 0.0043) 90 338.140 ( ± 0.366) 2

5.3633 ( ± 0.0030)

6.70% 10.32% 2.317

11.7972 ( ± 0.0068) 339.350 ( ± 0.579)

8.45% 12.38% 3.432

3.2. Luminescent properties The excitation and the emission spectra of LLMO:xSm3+ (x = 0.01–0.11) phosphors are illustrated in Fig. 5. The excitation spectra (monitored at 647 nm) present a group of sharp excitation peaks at about 364, 377, 405, 421, 441 and 469 nm in the range of wavelengths between 350 nm and 500 nm, respectively. These peaks are assigned to the transitions from the ground state 6H5/2 to the excited 3

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Fig. 3. SEM images of (a) LLMO, (b) LLMO:0.05Sm3+ and (c) LLMO:0.05Sm3+, 0.07Bi3+ phosphors; partially enlarged images of (d, e, f) from (a, b, c), respectively.

shows that the LLMO:0.07Bi3+ phosphor has no emission peak under 405 nm excitation. The emission spectrum (λex = 246 nm) of the Bi3+ ion overlaps with the excitation spectrum (λem = 647 nm) of the Sm3+ ion, which indicates that there may be energy transfer between the Bi3+ and Sm3+ when the excitation wavelength is 246 nm. However, in this study the excitation wavelength (λex = 405 nm) is the characteristic absorption peak of Sm3+ ions rather than the absorption peak of Bi3+ ions. Consequently, under 405 nm excitation, the Bi3+ ions act as non-

sensitizer ions to enhance the Sm3+ ions emission intensity rather than the energy transfer. Therefore, in this case, the Bi3+ ions are different from the energy transfer process as reported in the articles as a sensitizer to enhance the emission intensity of RE ions [1,10,14]. In order to further research the subject matter, different concentrations of Bi3+ ions are doped unto LLMO: 0.05Sm3+. The emission spectra of LLMO:0.05Sm3+, xBi3+ (x = 0–0.09) phosphors under λ = 405 nm excitation are shown in Fig. 6b. It is obvious that the doped

Fig. 4. TEM, SAED, FFT and HRTEM images of (A) LLMO, (B) LLMO:0.05Sm3+ and (C) LLMO:0.05Sm3+, 0.07Bi3+ phosphors; (a1–a3) TEM images, (b1–b3) SAED patterns, (c1–c3) corresponding FFT patterns, (d1–d3) HRTEM images; (D) EDS spectrum of sample LLMO:0.05Sm3+, 0.07Bi3+ (the selected area are shown in inset). 4

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Fig. 5. Excitation and emission spectra of LLMO:xSm3+ (x = 0.01–0.11) phosphors. Fig. 7. Excitation and emission spectra of (a) LLMO:0.07Bi3+, LLMO:0.05Sm3+, and (c) LLMO:0.05Sm3+,0.07Bi3+ phosphors.

(b)

ions to enhance the emission of Sm3+ ions unto the LLMO host. As shown in Fig. 8, the quantum yields (QY) of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ samples are measured by an integrating sphere. Accordingly in Fig. 8, the QY can be calculated by the following Eq 1 [34,35]:

QY =

EB − EA SA − SB

(1)

where EB is integral intensity of the emission of the sample; EA is the integral emission intensity of the reference; SA and SB are the integral intensity of the scattered light of the reference BaSO4 and the sample, respectively. Under excitation at 405 nm, the QYs of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ are calculated to be 35.2% and 67.0%, respectively. Obviously, the QY of LLMO:0.05Sm3+, 0.07Bi3+ is almost double that of LLMO:0.05Sm3+ phosphor. The CIE chromaticity coordinates (seen in Fig. 9) of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ samples are calculated to be (0.6193, 0.3782) and (0.6241, 0.3741), respectively. It can be found that Point B and Point A are in the red region and close to each other, indicating that both are appropriate as a red phosphor for potential applications in solid-state lighting.

3.3. Analysis of the effects of Bi3+ ions From the above discussion, under the SEM, TEM and SAED analyses, all the results show that the introduction of the Bi3+ ions as dopant can generate lattice defects and form subgrains in the crystals. These subgrains produce microstrain through interactions between them. In order to further explore the formation mechanism of microstrain, the growth and evolution of LLMO:0.05Sm3+, 0.07Bi3+ prepared at different temperatures were analyzed. Fig. 10 and Fig. 11 show the XRD patterns and micromorphology of LLMO:0.05Sm3+, 0.07Bi3+ at different temperatures, respectively. At 500 °C, most of the raw materials are unreacted, and a large amount of MoO3 and La2O3 remain in the samples. This is confirmed by the XRD patterns and the dispersed raw material particles in Fig. 11a. During this time, part of the raw material undergoes a chemical reaction (2) and forms a Li2Mo2O7 phase.

Fig. 6. Excitation (a) and emission (b) spectra of LLMO:0.05Sm3+, xBi3+ (x = 0–0.09) phosphors.

Bi3+ ions almost have no effects on the shape and position of the Sm3+ emission spectra except for intensity. Fig. 6b (inset) further illustrates that the luminescent intensity enhances with the increase of Bi3+ doping concentration and reaches the maximum at x = 0.07, after which it that decreases as the doping concentration increases. This result proves that Bi3+ ions is a good candidate for used as non-sensitizer 5

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Fig. 8. Absorption and emission spectra collected by an integrating sphere to evaluate the quantum yield of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ phosphors. (Inset magnified emission spectra).

Fig. 10. XRD patterns of LLMO:0.05Sm3+, 0.07Bi3+ at different temperatures (500°C–900 °C). Fig. 9. CIE chromaticity diagram corresponding to LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ phosphors.

LiLa(MoO4)2:Sm, Bi and transform into LiLa(MoO4)2:Sm, Bi subgrains, which will produce a certain microstrain during this process. Part of the microstrain is consumed by the Li2CO3 liquid phase system, and part of it will induce LiLa(MoO4)2:Sm, Bi subgrain structure which is further distorted. The sample mainly undergoes a chemical reaction (4), and finally almost all of the LiLaMo2O8:Sm, Bi phases have been transformed into LiLa(MoO4)2:Sm, Bi phases. It can be seen from the morphology of the sample in Fig. 11c that the subgrains of LiLa(MoO4)2:Sm, Bi are present in the Li2CO3 liquid phase system and have a relatively small size (about 8–12 μm).

Li2 CO3 + 2MoO3 → x Li2 Mo2 O7 + CO2 ↑ + (1 − x )Li2 CO3 + (2 − 2x )MoO3

(2)

At 600 °C, the sample mainly undergoes a chemical reaction (3) and LiLa(MoO4)2:Sm, Bi and LiLaMo2O8:Sm, Bi are formed, of which LiLa (MoO4)2:Sm, Bi is the main phase. As shown in Fig. 11b, the sample has agglomerated into a rough block after the reaction.

LiLaMo2 O8 : Sm, Bi → LiLa(MoO4 )2 : Sm,Bi

Li2 Mo2 O7 + La2O3 + Sm2 O3 + Bi2 O3 + Li2 CO3 + MoO3 → LiLa(MoO4 )2 : Sm, Bi + LiLaMo2 O8 : Sm, Bi + CO2 ↑

(4)

At 800 °C, in the liquid phase systems of MoO3 (melting point 795 °C) and Li2CO3, LiLa(MoO4)2:Sm, Bi subgrains are further dissolved, crystallized and gradually grown. The subgrain size is about 25–30 μm (Fig. 11d), and the liquid phase is gradually reduced. The growth competition between subgrains will increase the microstrain.

(3)

When the temperature increases to 700 °C close to the melting point of Li2CO3, the LiLaMo2O8:Sm, Bi subgrains are dissolved under the action of Li2CO3 liquid phase system. They precipitate on the surface of 6

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Fig. 11. Micromorphology of LLMO:0.05Sm3+, 0.07Bi3+ at different temperatures, (a) 500 °C, (b) 600 °C, (c) 700 °C (d) 800 °C, and (e) 900 °C.

However, the reduction in the liquid phase causes the microstrain to be difficult to remove and remains in the crystal. The structure of the LiLa (MoO4)2:Sm, Bi subgrain is further distorted. Further increase the temperature to 900 °C, the liquid phase disappears, and the LiLa(MoO4)2:Sm, Bi subgrains grow to a larger polycrystal. The incomplete migration of pores between subgrains and the volatilization of Bi3+ ions form defects in LiLa(MoO4)2:Sm, Bi polycrystal (marked in blue in Fig. 11e). In summary, the phase transition of LiLaMo2O8:Sm, Bi and the growth of LiLa(MoO4)2:Sm, Bi subgrains lead to the presence of microstrain in the polycrystal. According to the previous findings, it is reported that the microstrain in the crystal may affect the emission of Sm3+ ions, because it would change the amount and the sites of activator ions in the luminescent center. To confirm this conjecture, a series of Bi3+ doping LLMO:0.05Sm phosphors are refined by the GSAS program. The strain (S) can be calculated by the following formula (5) [36]:

S=

π (Y−Y)100% i 18000

Fig. 12. Emission intensity of Sm3+ and microstrain on Bi3+ doped concentration of LLMO:0.05Sm3+, xBi3+ (x = 0–0.09) phosphors.

(5)

where (Y −Yi = LY) is the coefficient of the profile function during refinement. Fig. 12 shows the plots of dependent curves of emission intensity of Sm3+ against microstrain on Bi3+ doping concentration of LLMO:0.05Sm3+, xBi3+ (x = 0–0.09) phosphors. It is seen that as the Bi3+ doping concentration is increased from 0 to 7 mol%, the emission intensity of the Sm3+ and the strain shows the same increasing trend. However, when the Bi3+ doping concentration is greater than 7 mol%, the emission intensity of Sm3+ ion decreases, and the strain too shows an increasing trend, too. This is because as the lattice defects increases the strain on the crystal increases correspondingly with the increasing doping concentration of Bi3+ ions. At this point, the microstrain induces the quenched Sm3+ ions to be excited and emitted again. When the Bi3+ ions concentration reaches 7 mol%, the strain-inducing effect is saturated, so the emission intensity of the Sm3+ ion begins to decrease. Fig. 13 shows the Raman spectra of LLMO, LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ phosphors excited by a 785 nm laser. The spectra can be divided into three different sets of Raman modes: 100450 cm−1, 700-980 cm−1 and 1200-1850 cm−1. The relatively weak bands at 127 cm−1 and 190 cm−1 are assigned to the rotations and translation of the [MoO4] tetrahedra and the cations [37]. The bands at

317 cm−1 and 380 cm−1 correspond to the symmetric bending ν2(E) and asymmetric bending ν4(F2) of the [MoO4] tetrahedra respectively. The bands observed at 830 cm−1 and 895 cm−1 correspond to the asymmetric stretching ν3(F2) and symmetric stretching ν1(A) of the [MoO4] tetrahedra respectively [38,39]. The Raman bands in the range of 1200–1850 cm−1 should belong to internal modes of [Li/LaO8] polyhedral [40,41]. In order to study the proportion of Raman-active groups in the sample, the peak areas of different vibration modes are integrated as shown in the inset of Fig. 10. It can be seen that the integral ratios of [MoO4] and [Li/LaO8] are increased with the incorporation of Sm3+ and Bi3+. This result indicates that the microstrain generated by Bi3+ doping may induce more Sm3+ to substitute the La3+ sites. This point is consistent with the previous analyses. Therefore, the above findings suggest that microstrain enhances the emission of Sm3+ ions in the crystal to some extent. Fig. 14 as scheme describes Sm3+ ions emission enhancement, with the undoped system (see Fig. 14 left), the concentration quenching occurs between Sm3+ ions as the doping concentration of Sm3+ ions is increased. This is because the excitation energy is transferred to the 7

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Fig. 13. Raman spectra of LLMO, LLMO:0.05Sm 0.07Bi3+ phosphors.

3+

and LLMO:0.05Sm

Fig. 15. Decay curves (λex = 405 nm and λem = 647 nm) of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ phosphors.

3+

,

exponential formula (8), respectively [5,45]: lattice defect through the non-radiative transition with the energy transfer between Sm-Sm [42]. Then the energy will be lost in the form of phonons due to the lattice vibration. According to Blass's theory, the concentration quenching can be studied by the critical distance (Rc) between the Sm3+ luminescence centers and the formula is as follows (6) [43,44]:

⎜

(7)

I (t ) = I0 + A1 exp(−t / τ1) + A2 exp(−t / τ2) + A3 exp(−t / τ3)

(8)

where I(t) is the emission intensities at times t, I0 is the background intensity; A1, A2 and A3 are the fitting constants; τ1, τ2 and τ3 are the fast and slow components of the decay time. The corresponding average lifetimes (τav) can be evaluated by the following equations (9) and (10) [27,46]:

1

3V ⎞ 3 Rc ≈ 2⎛ 4π ⎝ xc N ⎠

I (t ) = I0 + A1 exp(−t / τ1) + A2 exp(−t / τ2)

⎟

(6)

τav = (A1 τ1 2 + A2 τ2 2)/(A1 τ1 + A2 τ2)

where V is the volume of the unit cell, xc is the critical concentration of Sm3+, and N is the number of cations in a unit cell. For the LLMO:xSm3+ samples, V = 338.597 Å3, xc = 0.05, and N = 2, and the Rc of Sm3+ in LLMO is calculated to be about 18.63 Å. When doped with Bi3+ ions (see Fig. 14 right), the defects are introduced into the crystal lattice to form subgrains, and the interaction between subgrains can produce microstrain. The distance (R) between quenched Sm3+ ions will be greater than the critical distance (Rc) due to the effects of microstrain. Meanwhile, the originally quenched Sm3+ ions would be excited again, so that the luminescent intensity of Sm3+ ions is enhanced as a whole. The decay curves (λex = 405 nm and λem = 647 nm) of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ phosphors were measured and represented in Fig. 15. The blue and green curves can be well fitted by the following double exponential (7) and triple

τav = (A1 τ1 2 + A2 τ2 2 + A3 τ3 2)/(A1 τ1 + A2 τ2 + A3 τ3)

(9) (10)

According to the above Eq. 9 and 10, and decay curve fitting parameters, the average lifetimes of LLMO:0.05Sm3+ and LLMO:0.05Sm3+, 0.07Bi3+ phosphors can be determined to be 0.341 and 0.347 ms, respectively. With the doping of Bi3+ ions, the lifetime of Sm3+ is not greatly increased, which can be explained as follows. From the triple exponential decay formula fitted by the latter, it can be inferred that there are three kinds of luminescence decay modes of Sm3+ ions: Sm3+ and host, Sm3+ and Sm3+, and Sm3+ and Bi3+. However, there are just two of the former (Sm3+ and host, Sm3+ and Sm3+). Therefore, the lifetime of the latter is slightly increased. Microstrain may not be the only reason to enhance the emission of Sm3+ ions in the LLMO host. The possible reasons may be guessed and

Fig. 14. Schematic diagram of non-sensitizer Bi3+ ions to enhance Sm3+ ions emission. 8

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listed below: (1) The incorporation of Bi3+ ions breaks the original distribution pattern of Sm3+ ions, resulting in the transfer of Sm3+ ions in the lattice gaps to the luminescence center positions, thus enhancing the luminescent intensity of Sm3+ ions. (2) It is well known that Li+ ions have the effects of improving the luminous efficiency of luminescent materials. After Bi3+ ions doping, Li+ ions may further improve the luminous efficiency of Sm3+ ions in the new Bi3+/Li+ group compared with Sm3+/Li+. (3) The doped Bi3+ ion may change the refractive index of the crystal, so that more Sm3+ ions can be excited by 405 nm light and the Sm3+ ions emission is enhanced.

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4. Conclusions Overall, a series of the Bi3+ co-doped LiLa(MoO4)2:Sm3+ phosphors have been synthesized by solid-state reaction. At 405 nm excitation, the emission spectra indicate that the emission intensity of Sm3+ ion reaches the maximum and the quantum yield is nearly as twice as LiLa (MoO4)2:0.05Sm3+when the doping concentration of Bi3+ is 7 mol%. However, Bi3+ ions act as non-sensitizers to enhance the luminescent intensity of Sm3+ ions. The crystal structure and the morphology of the samples are well characterized by XRD, SEM, HRTEM, and SAED. The structural analyses show that LiLa(MoO4)2:Sm3+, Bi3+ is a subgrainformed polycrystal possessing microstrain inside when doped with Bi3+ ions. We speculate that microstrain may be one of the important factors for enhancing Sm3+ luminescence and is discussed in detail. In addition, the high quantum yield and the color purity mean that the Bi3+ co-doped LiLa(MoO4)2:Sm3+ phosphors are expected to be applied to solid-state lighting.

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Acknowledgments [22]

The work was supported by the Project of the National Natural Science Foundation of China under the Grant No. 51772180 and No. 51272148.

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