Coprecipitation synthesis, structure and photoluminescence properties of Eu3+-doped sodium barium borate
Introduction
Borates with MNBO3 formula (M and N are mono- and divalent cations, respectively) form a large family of inorganic compounds. Its crystal structure shows a strong dependence on the relative size of M to N ions [1], [2], [3]. For example, LiMgBO3 with small M and N cations crystallizes in monoclinic form C2/c. [1]. LiCaBO3 belongs to an orthorhombic system with space group of Pbca [4]. While the corresponding borates LiNBO3 (N = Sr, Ba) with big N cation shows a typical monoclinic space group P21/n. Borates with NaNBO3 can crystallize in different crystal structures depending on the size of N in the lattices such as orthorhombic space group of Pmmn (N = Ca [1]), monoclinic P21/c (N = Sr [5], [6]) and monoclinic C12/m1 (N = Ba [3], [7]).
In MNBO3 borates, there are different possibilities for the coordination number (CN) of M and N ions. The rich structure types make it possible to tailor optical properties of rare earth ions (RE) activated MNBO3. This is due to the fact that luminescence of RE ions usually shows a strong dependence on the microstructures. In recent years, luminescence materials of RE-doped MNBO3 have been extensively investigated because of its properties such as the large band gap, moderate phonon energy, high thermal and chemical stability, and exceptional optical damage threshold. Phosphors based on MNBO3 have been widely reported as potential materials in solid state lighting [8], [9]. NaCaBO3:Ce3+ [10] and NaSrBO3:Ce3+ [11] can be efficiently excited by near ultraviolet (NUV) light, and emits bright broadband blue light. In MNBO3 host the energy transfers can take place from Ce3+ to Mn2+ ions in LiCaBO3:Ce3+/Mn2+ [12], NaCaBO3:Ce3+/Mn2+ [13], NaCaBO3:Ce3+/Tb3+/Mn2+ [14], and NaSrBO3:Ce3+/Mn2+ [15]. Orange–red luminescence is observed in LiSrBO3:Sm3+ [16], NaCaBO3:Sm3+ [17], and NaSrBO3:Sm3+ [18] phosphors. Luminescence properties of NaSrBO3:Tb3+ [19] KCaBO3:Er3+/Yb3+ [20], KCaBO3:Dy3+/Eu3+ [21] and LiMgBO3:Dy3+ [22] have been reported to have possible applications for lighting or radiation dosimetric measurement. Eu3+ ion with the 4f6 electronic configuration usually presents groups of sharp lines assigned to the transitions of 5D0 to 7FJ (J = 0, 1, 2, 3, 4) levels. The 5D0 → 7F2 transition is electric dipole-allowed hypersensitive in the local crystal field [23]. The local structures such as CNs and defects influence significantly on the luminescence of Eu3+ ions in a host [24].
As the most important emitter in the red region of the visible spectrum, Eu3+ has gained considerable interest and been widely investigated to be the efficient red-emitting phosphors [25], [26], [27], [28], [29], [30], [31]. In MNBO3 borates, the luminescence properties of Eu3+ ions doped in LiMgBO3 [32] NaSrBO3 [6], [33] LiCaBO3 [34] have been reported. There have been reported the red-emitting phosphors prepared by solid state reaction or combustion synthesis methods together with luminescence properties for possible applications in near-UV excited white-LEDs as in the references.
In this work, Eu3+ ion is selected as an activator doped in NaBaBO3 host. A modified co-precipitation is applied to prepare this phosphor. The crystalline structure was examined with X-ray powder diffraction (XRD) and refinements. The thermal stability of the green luminescence is evaluated by the luminescence decays as a function of temperature. The absolute luminescence quantum efficiency (QE) and the activation energy (ΔE) are reported.
Section snippets
Experimental
MNBO3:xEu3+ (x = 0.05–0.12) phosphors were synthesized by a modified co-precipitation method at low temperatures. The starting materials were NaNO3, Ba(NO3)2, Eu(NO3)3·6H2O and HBO3 (analytical grade, Aldrich Co., Ltd.). The synthesis procedure can be introduced as of four steps: (1) The stoichiometric amount of NaNO3, Ba(NO3)2, Eu(NO3)3·6H2O were dissolved in water forming a clear solution labeled as “A”. The solution “B” was prepared by dissolving HBO3 in ammonia under constant stirring. (2)
The crystal phase formation
Fig. 1 shows the representative experimental and calculated results from the XRD structural refinement of NaBaBO3:0.09Eu3+ carried out by using GSAS program [35]. The refined crystallographic data and the atomic coordinate parameters of 9.0 mol% Eu3+-doped NaBaBO3 are listed in Table 1, Table 2, respectively. The results indicate that the diffraction peaks of the samples are in good agreement with the ICSD standard pattern, indicating that the obtained NaBaBO3 samples are single phased. No
Conclusion
Eu3+-doped NaBaBO3 (0.5–12 mol%) were prepared by the modified coprecipitation synthesis. XRD structural refinement was completed in Eu3+-doped NaBaBO3. NaBaBO3:Eu3+ crystallizes in micro-sized particles with tabular shapes. The phosphor shows the efficient excitation around 400 nm (f–f transition of Eu3+ ions), which is much stronger than the absorption in the CT region. Under the excitation of near UV light, NaBaBO3:Eu3+ presents bright red color with CIE values of (x = 0.663, y = 0.341) and the
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A2009154). This work was also supported by Jiangsu Provincial Natural Science Foundation of China (No. BK2012635).
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