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Abstract
A series of Eu3+-doped REMO3 (RE = Lu/Y/La; M = B/Al) phosphors have been synthesized via a high-temperature solid-state method. With the increase of the cation radius from Lu3+ ion to Y3+ ion to La3+ ion in Eu3+-doped REMO3 matrices, the Eu-O charge transfer (CT) energy decreases and the full width at half maximum (FWHM) increases. With the change of cation in the borate radical and aluminate radical from B3+ ion (0.27 Å) to Al3+ ion (0.535 Å), the red-shift of the Eu-O CT band occurs, and their FWHMs increase. Our work provides a reference for Eu3+-doped other complex oxides.
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1. Introduction
Rare earth materials have been widely used in various fields such as physical optics, surface chemistry, biomedicine and magnetism because of their good chemical and physical characters [1–4]. Especially recently, there is a great of interest in the rare earth aluminate and orthoborate when they act as potential hosts [5,6]. Orthoborate is very suitable host lattices for luminescence ions due to its large band gap. The REBO3 (RE = Lu/Y/La) samples have attracted considerable attention due to the high UV transparency, non-linear optical property, high conversion efficiency, wide color gamut, good thermal and chemical stabilities [7–9]. They were well applied to bioimaging materials, the Hg-free fluorescence lamps, and the plasma display panels [10–12]. Rare earth doped REAlO3 (RE = Lu/Y/La) microcrystals have been widely studied owing to their favorable physical, optical, thermal, and mechanical properties [4,13]. These properties make the REAlO3 (RE = Lu/Y/La) microcrystals particularly attractive for applications in solid state laser hosts, plasma display panels and cathode-ray tubes [14–16].
Generally speaking, Eu3+ ion is widely used as activator in many phosphors owing to the transitions of 5D0 → 7FJ (J = 0, 1, 2, 3, 4, 5, 6, 7) [17–22]. It is well known that the transition of 5D0 → 7F2 is extremely sensitive to the local environment [23]. When the local environment changes slightly, the luminescence properties of Eu3+ ion are apparently different. Therefore, Eu3+ ion is often used as a structure probe to estimate the local symmetry of the doped cation sites in phosphors [24,25]. Eu-O charge transfer (CT) band in Eu3+ doped REMO3 (RE = Lu/Y/La; M = B/Al) phosphors occurs from ligands O2− ion to the central ion Eu3+ ion. When Eu3+ ion was doped in different rare earth aluminate and orthoborate, the Eu-O CT band peak positions are misalign [26]. This is caused by Eu-O bond length and crystal structure [27]. Therefore, it is meaningful and interesting to investigate shifting trend of the Eu-O CT band in Eu3+ doped REMO3 (RE = Lu/Y/La; M = B/Al) phosphors and predict appropriate rare earth aluminate and orthoborate matrices.
Up to now, many researchers have been working on the issues of Eu-O CT band in complex oxides for many years [28]. Strzep et al. reported that the Eu-O CT band and O-As CT band intensities vary with the annealing temperature in Eu3+ doped YAsO4 phosphors [29]. Wang et al. reported a novel optical temperature sensing strategy based on the energy transfer from CT bands of W-O and Eu-O to Eu3+-Dy3+ ions in SrWO4 phosphors [30]. However, the shift of position of Eu-O CT band in a series of Eu3+ doped REMO3 (RE = Lu/Y/La; M = B/Al) phosphors has not yet been demonstrated.
In this paper, we prepared a series of Eu3+ doped REMO3 (RE = Lu/Y/La; M = B/Al) phosphors by a high-temperature solid-state method. The peak position of Eu-O CT band is red-shifted due to the change of the rare earth cation from Lu3+ ion to La3+ ion in REMO3 matrices, and the full width at half maximum (FWHM) increases. With the change of cation from B3+ ion (0.27 Å) to Al3+ ion (0.535 Å) in borate radical and aluminate radical, the peak position of Eu-O CT band is red-shifted and the FWHM increases.
2. Experimental details
2.1 Materials
All chemicals are of analytical grade and used without further purification. H3BO3 (99.99%), Al (NO3)3·9H2O (99.99%), Y (NO3)3·6H2O (99.99%), LaCl3·7H2O (99.99%), EuCl3·6H2O (99.99%), LuCl3·6H2O (99.99%) are supplied by Yutai Qingda Chemical Technology Co., Ltd. China. Ethanol is supplied by Beijing Fine Chemical Company.
2.2 Synthesis of Eu3+-doped REBO3 (RE = Lu/Y/La)
The powder samples were prepared by a high-temperature solid-state method. Raw materials 0.95 mmol LuCl3·6H2O, 0.05 mmol EuCl3·6H2O and 2 mmol H3BO3 were thoroughly mixed by using an agate mortar for 30 min. And the precursors were loaded into alumina crucibles, and calcined in the muffle at 900 °C for 2 h to obtain the LuBO3: 5 mol% Eu3+ powder. 1 mmol excess amount of H3BO3 was used to compensate an evaporation loss during heating [31]. According to the same method, YBO3: 5 mol% Eu3+ and LaBO3: 5 mol% Eu3+ powders were also obtained. The as-synthesized phosphors were grounded and subjected to phase characterization and luminescence properties study.
2.3 Synthesis of Eu3+-doped REAlO3 (RE = Lu/Y/La)
A series of Eu3+-doped REAlO3 (RE = Lu/Y/La) samples were prepared by a high-temperature solid state method. In the study, 0.95 mmol LuCl3·6H2O, 0.05 mmol EuCl3·6H2O and 2 mmol Al (NO3)3·9H2O were grounded thoroughly by an agate mortar for 30 min. 1 mmol excess amount of Al (NO3)3·9H2O was used to compensate an evaporation loss during heating. Subsequently, the mixture was put into the crucible which was placed in a muffle for 2 h at 900 °C. After it was cooled to a room temperature, LuAlO3: 5 mol% Eu3+ powder was synthesized. Using the same method, we prepared YAlO3: 5 mol% Eu3+ and LaAlO3: 5 mol% Eu3+ powders.
2.4 Characterization
The phase purity and crystal structure of the powder samples were examined by X-ray diffraction (XRD) analysis with a powder diffractometer (Model Rig-aku RU-200b), using Ni-filtered Cu Kα radiation (λ = 1.5406 Å). The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were recorded with a Hitachi fluorescence spectrometer F-7000. All tests were measured at a room temperature.
3. Results and discussion
The phase purity and crystal structure of as-prepared samples characterized by the XRD are clearly shown in Fig. 1. All the diffraction peaks of the samples can be indexed to the hexagonal LuBO3 (JCPDS NO.13-481), hexagonal YBO3 (JCPDS NO.13-531), orthorhombic LaBO3 (JCPDS NO.13-113), cubic LuAlO3 (JCPDS NO.18-761), cubic YAlO3 (JCPDS NO.33-40) and rhombohedral LaAlO3 (JCPDS NO.31-0022), respectively. No other phase peaks or traces of impurities were detected in any of the diffraction patterns. This indicates that the samples were completely converted into pure REMO3 (RE = Lu/Y/La; M = B/Al) samples and the Eu3+ ions were successfully doped into the hosts without significantly changing the lattice structure.
Fig. 1. XRD patterns of a series of Eu3+-doped REMO3 phosphors: (a) RE = Lu/Y/La, M = B; (b) RE = Lu/Y/La, M = Al.
In order to investigate their luminescence properties, we measured the PLE and PL spectra of REBO3: 5 mol% Eu3+ (RE = Lu/Y/La) phosphors. In Fig. 2 (a-c), the PLE spectra monitored with 593 (616) nm show broad excitation bands and the characteristic f-f transition lines of Eu3+ ion. The sharp lines in the wavelength range 300 nm to 500 nm centered at 320 nm, 361 nm, 379 nm, 393 nm, 416 nm, and 465 nm are attributed to 7F0 → 5H3, 7F0 → 5D0, 7F0 → 5L7, 7F0 → 5L6, 7F0 → 5D4 and 7F0 →5D2 transitions of Eu3+ ion, respectively [32]. Their broad and strong PLE bands in the range of 200 nm to 350 nm were ascribed to the Eu-O CT band which occurs in an electron transfer from an O2- 2p orbital to an Eu3+ ion empty orbital [33].
As is known from the archival literature, the energy position of the Eu-O CT band is related to the stabilization of the O2- ions by the potential field created by the nearby cations: the bigger the radius and the lower the electronegativity of these ions; the weakening of the bond strength between the ligand O2- ion and the central ion Eu3+, the more the Eu-O CT moves to low energy [28,29]. The radius is Lu3+ ion (0.861 Å) < Y3+ ion (0.9 Å) < La3+ ion (1.1 Å); the electronegativity is La3+ ion (1.1) < Y3+ ion (1.22) < Lu3+ ion (1.27). Therefore, for the La3+ ion, with the greatly decreasing of electronegativity and increasing of the radius, larger CT red shift is found than that for Lu and Y ion in REAlO3 (RE = Lu/Y/La) matrices from 239 nm to 321 nm. In the same way, with the increase of the rare earth cation radius in REBO3 (RE = Lu/Y/La) matrices from Lu3+ ion, Y3+ ion then to La3+ ion, the red-shift of their Eu-O CT bands occurred from 238 nm to 280 nm. Their Eu-O CT band peak positions and their FWHMs are listed in Table 1. The more the Eu-O CT band red shifts, the more the FWHM increases.
Table 1. The positions of Eu-O CT band and the FWHMs of REMO3: 5 mol% Eu3+ (RE = Lu/Y/La; M = B/Al)
Their PL spectra were obtained under the Eu-O CT band excitation which are composed of 5D0 → 7FJ (J = 1, 2, 3, and 4) emission lines of the 4f6 configuration of Eu3+ ion [34]. When the Eu3+ ion occupies a site with inversion symmetry, the 5D0 → 7F1 magnetic dipole transition is prominent, while when the Eu3+ ion occupies a site without inversion symmetry, the 5D0 → 7F2 electronic dipole transition is prominent [35]. In LuBO3: 5 mol% Eu3+ and YBO3: 5 mol% Eu3+ powders, the magnetic dipole transition at about 593 nm (5D0→7F1) is dominant over the electric dipole transition 616 nm (5D0→7F2), suggesting a higher occupancy of Eu3+ ion occupy in inversion symmetry in LuBO3 and YBO3 matrices [36]. In LaBO3: 5 mol% Eu3+ powders, the electric dipole transition 616 nm (5D0→7F2) is dominant compared to the magnetic dipole transition at about 593 nm (5D0→7F1), showing that Eu3+ ion is in a symmetric environment. Therefore, the (5D0→7F2)/(5D0→7F1) intensity ratio of Eu3+ ion can be used as a measure of the site symmetry.
As shown in Fig. 3 (a-c), the PLE spectra of REAlO3: 5 mol% Eu3+ samples (RE = Lu/Y/La) monitored with 593 nm show broad excitation bands and the characteristic f-f transition lines of Eu3+ ion. The broad excitation bands peaked at 239 nm (LuAlO3), 240 nm (YAlO3) and 321 nm (LaAlO3) are assigned to CT between O2- ion and Eu3+ ion. Their Eu-O CT band peak positions are listed in Table 1. As well known, with the increase of rare earth cation radius, the energy of Eu-O CT band decreases.
Therefore, in the REAlO3: 5 mol% Eu3+ (RE = Lu/Y/La) powders, with the change of the rare earth cation from Lu3+ ion to Y3+ ion then to La3+ ion, the red-shift of Eu-O CT band from 239 nm to 321 nm occurred. Their FWHMs are also listed in Table 1. The corresponding FWHMs of LuAlO3: 5 mol% Eu3+, YAlO3: 5 mol% Eu3+ and LaAlO3: 5 mol% Eu3+ increase from 25.91 nm to 27.90 nm and to 52.03 nm. It also can be seen that along with the red-shift of Eu-O CT band, the FWHM increases, as shown in Fig. 4. Other lines between 300 nm and 500 nm are associated with the characteristic 4f-4f transitions of Eu3+ ion, namely the 7F0 → 5H3, 7F0 → 5D4, 7F0 → 5L7, 7F0 → 5L6, 7F0 → 5D4 and 7F0 → 5D2 transitions at wavelengths 320 nm, 361 nm, 379 nm, 393 nm, 416 nm and 465 nm, respectively.
Fig. 4. The relationship between the positions of the Eu-O CT band and the FWHMs in the crystal systems REMO3: 5 mol% Eu3+ (RE = Lu/Y/La; M = B/Al).
Their PL spectra of REAlO3: 5 mol% Eu3+ samples were obtained under the excitations at 239 nm (LuAlO3), 240 nm (YAlO3) and 321 nm (LaAlO3) (the peak position of Eu-O CT band). The PL spectra showed strong characteristic emissions of Eu3+ ion, such as 5D0 →7FJ (J = 1, 2, 3, and 4). The magnetic dipole transition 593 nm (5D0 → 7F1) is prominent over the electric dipole transition 616 nm (5D0→7F2), showing that the Eu3+ ion occupies a site with inversion symmetry in REAlO3 (RE = Lu/Y/La) matrices.
In LaBO3: 5 mol% Eu3+ and LaAlO3: 5 mol% Eu3+ powders, with the change of cation from the B3+ ion (0.27 Å) to Al3+ ion (0.535 Å) in the borate radical and aluminate radical, the Eu-O CT band position shifting to the low energy zone (red-shift) can be attributed to two aspects. One is the effect of the coupling strength of the B-O bond and Al-O bond. With the change of cation from the B3+ (0.27 Å) ion to Al3+ (0.535 Å) ion in the borate radical and aluminate radical, the coupling strength of the Al-O bond is weaker compared with that of the B-O bond. Therefore, an electron transferring from an O2- ion 2p orbital to an Eu3+ ion empty orbital needs less energy in the LaAlO3: 5 mol% Eu3+ powder than that in LaBO3: 5 mol% Eu3+ powder. The other aspect is electronegativity. With the electronegativity decrease of the Al3+ ion (Al3+ ion (1.714) < B3+ ion (2.275)), the potential field generated at the O2- ion lattice position decreases, the transfer electrons from O2- to the Eu3+ will need less energy. The Eu-O CT band in the LaAlO3: 5 mol% Eu3+ powder is red-shifted from 280 nm to 321 nm. [37–39]. It can also be found that the more the red-shift (or with decreasing CT energy) of the Eu-O CT band, the more the FWHM increases, as can be seen in Fig. 4.
4. Conclusions
In this work, we synthesized a series of REMO3: 5 mol% Eu3+ (RE = Lu/Y/La; M = B/Al) samples using a high-temperature solid-state method. In REMO3: 5 mol% Eu3+ (RE = Lu/Y/La; M = B/Al) phosphors, the relationship between cation radius and Eu-O CT band red-shift has been discussed. In ReBO3: 5 mol% Eu3+ (RE = Lu/Y/La) phosphors, with the increase of RE3+ ion radius (from Lu3+ ion, Y3+ ion, then to La3+ ion), the Eu-O CT band peak positions are red-shifted from 238 nm to 280 nm and their FWHMs increase from 25.71 nm to 38.39 nm. In Eu3+ doped REAlO3 (RE = Lu/Y/La) phosphors, with the increase of RE3+ ion radius (from Lu3+ ion, Y3+ ion, then to La3+ ion), the red-shift of Eu-O CT band peak positions occurs from 239 nm to 321 nm, and the FWHMs increase from 25.91 nm to 52.03 nm. In addition, we found that the FWHM increases with red-shift of the Eu-O CT band. Because of their fine luminescence properties, the synthesized Eu3+-doped REMO3: (RE = Lu/Y/La; M = B/Al) samples are promising candidates for applications in display and solid-state lighting fields.
Funding
Jilin Province Education (JJKH20220665KJ); Scientific and Technological Developing Project of Jilin Province (20200201256JC).
Acknowledgements
This work was supported by the Scientific and Technological Developing Project of Jilin Province (20200201256JC) and Jilin Province Education (JJKH20220665KJ).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data Availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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