Photoluminescence and Raman Spectra of Double-Perovskite Sr2Ca ...

Journal of The Electrochemical Society, 155 共6兲 J148-J151 共2008兲

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Photoluminescence and Raman Spectra of Double-Perovskite Sr2Ca„MoÕW…O6 with A- and B-Site Substitutions of Eu3+ Shi Ye, Chun-Hai Wang, and Xi-Ping Jingz Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry & Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China A- and B-site substituted double-perovskite Sr2CaMoO6 by Eu3+ have been synthesized using solid-state reactions and characterized by X-ray diffraction, Raman spectroscopy, and photoluminescence measurement. Raman spectra are used to identify the Aand B-site substitutions, because specific Raman peaks corresponding to different ions motion are sensitive to each situation. Raman data reveal that both the A- and B-site substituted solid solutions are formed. The photoluminescence intensity of the B-site substituted Sr2CaMoO6 is evidently higher than that of the A-site substituted phosphor. The WO6 group introduced into Sr2CaMoO6:Eu3+,Li+ acts as an energy obstacle to prevent energy transfer among MoO6 groups, leading to more energy being trapped by Eu3+, and a higher photoluminescence intensity is obtained. The phosphor with optimized composition Sr2Ca0.80Li0.10Eu0.10Mo0.10W0.90O6 shows a better luminescence intensity than the commercial phosphor Y2O2S:Eu under 395 nm excitations. © 2008 The Electrochemical Society. 关DOI: 10.1149/1.2898897兴 All rights reserved. Manuscript submitted January 10, 2008; revised manuscript received February 20, 2008. Available electronically April 8, 2008.

Phosphors for the applications in white light ultraviolet-lightemitting-diodes 共w-UVLEDs兲 have attracted much attention recently.1-6 Molybdates doped with Eu3+ are promising candidates as red phosphors4-6 for these applications. In many molybdates, such as CaMoO4:Eu,Li,4 Mo is 4-oxygen coordinated and the charge transfer state 共CTS兲 of MoO4 is located around 300 nm. For these phosphors, 5D0 → 7F1,2 orange/red emissions of Eu3+ have a relatively strong line excitation at ⬃395 nm in the near-UV region, which originates from the 7F0 − 5L6 transition of Eu3+. This is suitable for the excitation by UVLEDs, because they normally give efficient emissions in the range from 370 to 400 nm. Sivakumar and Varadaraju5,6 reported a Eu3+-doped perovskite-type molybdate Sr2CaMoO6, in which Mo is 6-oxygen coordinated and the CTS of MoO6 has a broad peak around 400 nm, suitable for the excitation of UVLED for Eu3+ red/orange emissions. The structure of Sr2CaMoO6 is called double perovskite,7 because both Ca and Mo occupy the B sites, in which the cations Ca and Mo are arranged like the rock salt structure. Two cation sites 共Sr at the A site and Ca at the B site兲 in this double-perovskite phase can be substituted by Eu3+, but previous research did not supply strong proofs to support whether A- or B-site substitution was applied in this phosphor. This paper is mainly focused on the photoluminescence 共PL兲 difference of A- and B-site substituted Sr2CaMoO6. Characterizations of X-ray diffraction 共XRD兲 and Raman spectroscopy were also conducted. Raman active peaks are sensitive to the change of local structure of materials;8 therefore, it was used to detect the Eu3+ sites 共at the A- or B-site兲 combined with XRD data. Results indicate that Eu3+ prefers to enter the B-site, and the samples with B-site substitution perform better luminescence than those with A-site substitution. Additionally, studies on the substitution of Mo by W were carried out, which shows that the introduction of WO6 into the lattice enhances the PL intensity significantly under near-UV excitations. The mechanism of this enhancement is discussed.

ders were first prefired at 600°C for 12 h, then at 900°C for another 12 h, and finally heated at 1100°C–1200°C for 24 h with several intermediate grindings. Phase purity was examined by XRD using a Rigaku Dmax 2000 X-ray diffractometer. PL measurements were carried out on a Hitachi F4500 fluorescence spectrophotometer. Raman spectra were recorded using a LabRAM HR800 high-resolution Raman spectrometer with a 632.8 nm laser beam as light source. Results and Discussion XRD and Raman characterizations of A- and B-site substitutions of Eu3⫹.— XRD patterns of the selected samples with A- and B-site substitution are illustrated in Fig. 1. For the B-site substituted series Sr2Ca1−2xEuxLixMoO6, in which Li acts as a charge compensator, the sample with x = 0.10 is pure phase.7 Actually, in this series, no obvious impurity phase appears even when x = 0.20, which means the solid solution range in this series can extend at least to this content. For the A-site substituted series Sr2−2xEuxLixCaMoO6, an impurity phase 共 ⴱ marked at 2␪ = 27.7°, SrMoO4, JCPDS 85–809兲 can be detected clearly at x = 0.10. The solid solution range of the A-site substituted series is quite narrow and the XRD data cannot give strong evidence whether Eu3+

Experimental All samples were synthesized by high-temperature solid-state reactions. The raw materials were BaCO3 共analytical reagent, AR兲, SrCO3 共AR兲, CaCO3 共luminescence pure兲, MoO3 共AR兲, WO3 共AR兲, Li2CO3 共AR兲, and Eu2O3 共99.99%兲. Proper amounts of the raw materials were weighed and mixed in an agate mortar. The mixed pow-

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Figure 1. XRD patterns of some selected samples.

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Figure 3. Variations of wavenumber and full width at half maximum of Raman peak at ⬃790 cm−1 with Eu3+ contents in A-site substituted series Sr2−2xLixEuxCaMoO6 and B-site substituted series Sr2Ca1−2xLixEuxMoO6.

Figure 2. Raman spectra of 共a兲 Sr2CaMoO6 and 共b兲 Ba2CaMoO6.

really enters the lattice following the nominal formula Sr2−2xEuxLixCaMoO6. To identify the A- and B-site substitution, Raman spectra for both series samples were collected. Both Ba2CaMoO6 and Sr2CaMoO6 have a double-perovskite structure with 1:1 B-site cation ordering, but the former has a higher symmetry 共cubic unit cell with space group Fm3m兲9 than the latter 共monoclinic unit cell with space group P21 /n兲.7 So, the Raman spectrum of Ba2CaMoO6, which has been well understood,6,8 is simpler than that of Sr2CaMoO6. Understanding the Raman spectrum of Ba2CaMoO6 first would help us to well understand that of Sr2CaMoO6. For Ba2CaMoO6, the perovskite framework is almost ideal, but for Sr2CaMoO6, the ordered double-perovskite framework is constructed by the tilting connection of 共Ca/Mo兲O6 octahedra, which is derived from the small size of Sr2+ compared to that of Ba2+. Previous work6,10 showed that Ba2CaMoO6 and Sr2CaMoO6 could form a complete solid solution, which also reflects the structural correlation between these two phases. Prosandeev et al.8 have carefully studied the Raman spectra of the two kinds of perovskite structures with and without octahedral tilting. Figure 2 shows the Raman spectra of these two phases. The profiles of the two spectra are consistent with those depicted in Ref. 8. Ba2CaMoO6 has four Raman active modes: T2g共1兲, T2g共2兲, Eg, and A1g, located at 103, 407,

647, and 804 cm−1, respectively.8,10,11 For Sr2CaMoO6, due to the tilting of the 共Ca/Mo兲O6 octahedra, the Raman peaks correlated with modes T2g共1兲 and T2g共2兲 of Ba2CaMoO6 are split into several peaks covering the range 75–200 cm−1 and 300–500 cm−1, respectively. Especially, the peaks correlated with mode T2g共1兲 of the Ba phase change dramatically,8,10 while the peak correlated with mode A1g of the Ba phase only has a slight redshift.8,10 According to Ref. 8, mode A1g is a fully symmetric breathing vibration of oxygen octahedra, and mode T2g共1兲 is an A-site cation related vibration contributed by oxygen atoms and an A-site cation. Furthermore, mode A1g has a relatively small correlation radius, which means that its vibration is confined in small regions with short-range correlations of atoms. Mode T2g共1兲 has a larger correlation radius, which is very sensitive to the order/disorder transition of the B-site cations. Based on their vibration manner, it can be deduced that mode A1g is sensitive to the B-site substitution, whereas mode T2g共1兲 is sensitive to the A-site substitution. In this work, the Raman peaks of Sr2CaMoO6 evolved from modes A1g and T2g共1兲 of the Ba phase are used to identify the site substitutions of Eu3+. Fortunately, the variations of the spectrum profiles were observed, as illustrated in Fig. 3 and 4. The peak at ⬃790 cm−1 共correlated with mode A1g of Ba2CaMoO6兲 broadens for both the A- and the B-site substitution, but the peak shows a blueshift clearly for the B-site substitution, while for the A-site substitution, the peak wavenumber changes slightly. The peaks in the range 100–180 cm−1 关correlated with mode T2g共1兲 of Ba2CaMoO6兴 broaden for both site substitutions, but only the A-site substitution gives the blueshift of the peaks clearly, although the substituted content of Eu3+ is small 共x ⱕ 0.10兲. No clear peak shift is observed for the B-site substitu-

Figure 4. Profiles of Raman peaks in the range 100–180 cm−1 for 共a兲 the B-site substituted series Sr2Ca1−2xLixEuxMoO6 and 共b兲 the A-site substituted series Sr2−2xLixEuxCaMoO6.


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Journal of The Electrochemical Society, 155 共6兲 J148-J151 共2008兲 theory,13 in order to have a high energy transfer efficiency in the MOx–Eu3+ 共where M is a high-valence d0 ion兲 system, the angle M–O–Eu3+ should be kept near 180° so that the wave functions of Eu3+ and Mo6+ can be well overlapped through O2−. When Eu3+ substitutes for Ca2+ 共B-site substitution兲, the above condition is met. Thus, the overlap between Eu3+ and Mo6+ is supposed to be large, analogically through ␴ bonding. For the samples where Eu3+ is located at the A-site, the overlap is not supposed to be enough, analogically through ␲ bonding.13 Therefore, the B-site substituted samples have a stronger PL intensity than the A-site substituted samples.

Figure 5. PL spectra of 共a兲 Sr1.96Li0.02Eu0.02CaMoO6, Sr1.90Li0.05Eu0.05CaMoO6, and 共c兲 Sr2Ca0.90Li0.05Eu0.05MoO6.

Photoluminescence of Sr2Ca0.80Li0.10Eu0.10Mo1⫺yWyO6 solid solutions.— With the introduction of W into the phosphor Sr2Ca0.80Li0.10Eu0.10MoO6, based on the XRD data, the full-range solid solutions Sr2Ca0.80Li0.10Eu0.10Mo1−yWyO6 are formed. The PL measurements indicate that the intensity of this series continuously increases when y ⬎ 0.20 under 400 nm excitation, as shown in Fig. 6a. The result is similar to our previous work10 on Ba2Ca共Mo/W兲O6:Eu3+,Li+ phosphors. The excitation spectra are composed of some broad bands 共see Fig. 6b兲, which originate from the CTS transitions of MoO6, WO6, and EuO6 共or Eu3+–O2−兲. The assignments are given so that the CTS band of the MoO6 group covers the range 350–450 nm and that of the WO6 group is located around 250–350 nm. Referring to the data reported for other Eu3+-doped phosphors,14 the CTS band of Eu3+–O2− should be located around 250–320 nm. However, as the Eu3+ content is relatively low, its CTS band is weak and immersed in the CTS bands of MoO6 and WO6. The excitation band appearing below 240 nm is assigned to interband transitions 共conduction band兲. With the increase of W content y, the CTS band of MoO6 gradually shifts to a short wavelength, which implies that the correlations among MoO6 groups exist, although they are separated by CaO6 and WO6 groups. With the replacement of W for Mo, the average distance between the MoO6 groups increases and the electron delocalization among the MoO6 groups decreases; thus, the blueshift of the MoO6 CTS band is observed. The decline of PL intensity with the W content increase in the range y ⱕ 0.20 may originate from the decrease of the MoO6 content. When y is beyond 0.20, the intensity clearly increases with the W content, which suggests that more energy is trapped by Eu3+. When the absorbed energy of MoO6 transfers along the 共Mo/Ca兲O6 framework, it is mostly quenched and only little is transferred to Eu3+. Figure 6 indicates that the CTS level of WO6 is higher than that of MoO6; MoO6 cannot transfer its absorbed energy to WO6.10 With the replacement of W for Mo, MoO6 is segregated by WO6

共b兲

tion; even the substituted content of Eu3+ is up to x = 0.15. Based on the results above, it is believed that both the A-site substituted solid solutions Sr2−2xLixEuxCaMoO6 and the B-site substituted solid solutions Sr2Ca1−2xLixEuxMoO6 are formed. However, Eu3+ prefers to enter the B site, and the B-site substituted series has a wide solid solution range. Photoluminescence of the A- and the B-site substituted series.— PL spectra of the A- and the B-site substituted samples are depicted in Fig. 5. For the B-site substituted samples, e.g., Sr2Ca0.90Li0.05Eu0.05MoO6, the emission spectra are dominated by the magnetic dipole transition of 5D0 → 7F1. This suggests that the B-site is a center-symmetric site,12 which is coincident with the structural data.7 In the A-site substituted series, the PL spectrum of the sample with the low Eu3+ content x = 0.02 shows an analogical profile to that of B-site substituted samples. At such low content, we are not sure whether Eu3+ really enters the A-site following the nominal formula Sr2−2xLixEuxCaMoO6 or if it enters the B-site as it prefers. When x increases to 0.05, the spectrum shows a dramatic change where the electric dipole transition 5D0 → 7F2 inversely becomes stronger than the magnetic dipole transition 5D0 → 7F1, which make us believe that Eu3+ surely enters the A-site, a structurally noncentrosymmetric site.7,12 Interestingly, the PL intensity of the B-site substituted sample Sr2Ca0.90Li0.05Eu0.05MoO6 is much stronger than that of the A-site substituted sample Sr1.90Li0.05Eu0.05CaMoO6. According to Blasse’s

Figure 6. Variations of 共a兲 the PL intensity of the dominant transition Sr2Ca0.80Li0.10Eu0.10Mo1−yWyO6.

5

D0 − 7F1 and 共b兲 the excitation spectra with W content y in the series



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to the breathing vibration of oxygen in 共Ca/Mo兲O6 octahedra, which is sensitive to the B-site substitution. Raman spectra can clearly identify the B- and the A-site substitutions, which proves that both the A- and B-site substituted solid solutions are formed. The solid solution range of the former is narrower than that of the latter, and also the B-site substituted Sr2CaMoO6 shows a much better PL performance than the A-site substituted phosphor. The introduction of W into the Sr2CaMoO6:Eu3+,Li+ phosphor benefits the energy transfer from MoO6 to Eu3+, because the WO6 group acts as an energy obstacle to block energy transfer among MoO6 groups, which results in more energy being trapped by Eu3+. The phosphor with the optimized composition shows a higher PL intensity than the commercial phosphor Y2O2S:Eu under 395 nm excitation, although its color purity needs to be improved. Figure 7. PL spectra of Sr2Ca0.80Li0.10Eu0.10Mo0.10W0.90O6 and the commercial phosphor Y2O2S:Eu.

Acknowledgments We are thankful for the financial support from the National Natural Science Foundation of China 共20221101, 20423005兲. Peking University assisted in meeting the publication costs of this article.

even further and the energy transfer among MoO6 is blocked by WO6. In this case, more energy is permitted to be transferred to Eu3+, leading to the increase of Eu3+ emission. The composition optimized phosphor Sr2Ca0.80Li0.10Eu0.10Mo0.10W0.90O6 has a higher PL intensity than that of the commercial phosphor Y2O2S:Eu under 395 nm excitation, as demonstrated in Fig. 7, although its color purity is not very good. Conclusions A- and B-site substituted Sr2CaMoO6 by Eu3+ are prepared and characterized by XRD, Raman spectroscopy, and PL measurement. The Raman active modes located around 100–180 cm−1 are related to the vibration of oxygen and an A-site cation, which is sensitive to the A-site substitution. The mode appearing at ⬃790 cm−1 is related

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