A Potential Candidate for Lamp Phosphor: Eu3+ Activated K2Y2B2O7
The novel phosphor K2Y2B2O7 doped with europium is studied for its photoluminescence properties. The studies show that the phosphor gives strong red emission (PL) at 613 nm related to 5D0-7F2 transition of Eu3+ under the 260 nm excitation (PLE) related to the charge transfer (CT) from the 2p orbital of the O2− ions to the 4f orbital of Eu3+ ions with CIE coordinates (; ). The results of PL and PLE spectra indicate the applicability of K2Y2B2O7:Eu as a red component in lamp phosphor. The phosphor is characterized through XRD pattern analysis, and morphology is explained on the basis of SEM image. Optimum concentration of Eu3+ required for the highest intensity of emission is also studied.
Borates as host for light emitting materials under UV excitation have attracted much attention. The reason behind their being in a focus of research is their variety of structure types, transparency to a wide range of wavelengths, and high optical quality [1–4]. Also borates possess excellent properties as host due to the inherent attributes of the large band gap and covalent bond energy. A variety of borate host materials doped with rare earth and other ions have been studied and reported . Moreover, B–O bond generally has no absorption in near UV region and has absorption in deep UV region because of its large covalent bond energy. Some previous works show that Eu3+ doped borates exhibit relatively strong absorption in the UV region and intense red emission with good color purity [6, 7].
Recently the alkali earth metal (M) yttrium borates (M2Y2B2O7) have been well known hosts for luminescent material with better crystallinity, lower synthesis temperature, and higher radiation efficiency, as compared to the corresponding simple alkali earth metal borates . The Na2Y2B2O7 is an excellent host for luminescent materials which supports all the possible trivalent rare earth dopants and shows intense emission from violet region to red region . In the present work, we investigate, for the first time, the photoluminescence characteristics of Eu3+ doped K2Y2B2O7. The purpose of replacing the mostly studied Na+ by K+ is to find the ion type dependence of Eu3+ luminescence in M2Y2B2O7 host. The photoluminescence properties of the synthesized materials were studied using a fluorescence spectrometer (Hitachi F-7000).
2.1. Sample Preparation
The powder samples of the phosphor K2Y2B2O7 with different concentrations of Eu3+ (0.001, 0.002, 0.005, 0.01, and 0.02) have been prepared by solution combustion technique. The method is based on the exothermic reaction between the fuel (urea) and oxidizer (ammonium nitrate). The detailed description of the method regarding the measurement of stoichiometric quantities of fuel and oxidizer is reported in our earlier work [10–13].
All the ingredients, yttrium nitrate (Y(NO3)3), boric acid (H3BO3), potassium nitrate (KNO3), urea (NH2–CO–NH2), and ammonium nitrate (NH4NO3) used, are of AR grade and the rare earth Eu2O3 (99.99% purity) used is from the Indian Rare Earths. The stoichiometric amounts of the ingredients are thoroughly mixed in an agate mortar. Later, little amount of double distilled water was added to get an aqueous homogeneous solution. The solution is then transferred into a China Basin and slowly heated at lower temperature at 90°C in order to remove the excess of water contents. The thick paste obtained after heating is then transferred into a preheated muffle furnace maintained at ()°C. The paste boils foams and ignites to burn with flame; lastly, a voluminous, foamy powder is obtained. This entire combustion process is over in about 5 min. Following the combustion, the resulting fine powders were annealed in slightly reducing atmosphere (produced by burning charcoal) at a temperature of 750°C for about 60 min and suddenly cooled to room temperature . The same process of synthesis is repeated for different concentrations of activator.
2.2. Characterization Techniques
K2Y2B2O7:Eu3+ materials were characterized by using Rigaku Miniflex II X-ray diffractometer with scan speed of 2.000°/min and with Cu Kα radiation. SEM images are produced on Philips XL 30 SEM system having a resolution of 2.0 nm at 30 kV and 0.5 nm at 1 kV. The PL and PLE spectra are recorded on Hitachi F-7000 fluorescence spectrometer by keeping slit window at 1.0 nm for excitation and emission and PMT voltage at 400 V.
3. Results and Discussion
3.1. XRD Pattern for K2Y2B2O7
The studies on novel yttrium based borate phosphor become a hot issue in exploring new phosphor materials, which have also been proved to be efficient in the application of light conversion. Na2Y2B2O7 phosphor was first reported by Zhang and Li  as a near-UV white LEDs phosphor prepared by high temperature solid state synthesis method. The material was described in the monoclinic crystal system. In this paper it has been decided to replace sodium by potassium (having the same valence) to get K2Y2B2O7.
The X-ray diffraction patterns of K2Y2B2O7 with different concentrations of activator Eu3+ are shown in Figure 1 to verify the phase purity and crystal similarity. All the peaks in different patterns are in good agreement with each other, which indicates that all the prepared samples are in a similar phase, and the increasing concentration of dopants does not affect the crystal structure. However, the obtained diffraction peaks of compound are in some agreement with the two different phases in the ICDD database, namely, Y2B2O6(~80%) and K2O (~20%). Having in mind that the quantities of the starting materials are chosen according to the given chemical composition of K2Y2B2O7:Eu3+ matrix, therefore (even if in a mixed phase) this was named as K2Y2B2O7:Eu3+ phosphor in this paper .
The results also imply that the prepared samples are not the simple physical mixtures of precursor used but may be a new single-host K2Y2B2O7 material. The detailed studies on structures of these materials are still under investigation. Therefore, only XRD data of K2Y2B2O7 is reported in this paper.
3.2. Surface Morphology of K2Y2B2O7:Eu3+
The SEM photographs of K2Y2B2O7:Eu3+ phosphor clearly show that the grains are irregular in shape and have a size less than 1 μm. The typical morphological images are represented in Figure 2. The particles possess foamy like morphology formed from highly agglomerated crystallites. An average crystallite size is in submicrometer range of K2Y2B2O7:Eu3+ phosphors.
3.3. Luminescence Properties of Eu3+ Activated K2Y2B2O7
Figure 3 represents excitation and emission behaviors of Eu3+ activated K2Y2B2O7 prepared at 750°C. The excitation spectra exhibit a broad band centered at 260 nm related to the charge transfer (CT) from the 2p orbital of the O2− ions to the 4f orbital of Eu3+ ions . Other minor peaks, 300–400 nm, are associated with the direct excitation of electrons for the f-f shell transitions in Eu3+ ions.
The emission spectra of Eu doped K2Y2B2O7 under the charge transfer (CT) band excitation at 260 nm essentially contain groups of lines between 500 and 650 nm. The lines in the 500–550 nm range (shown in Figure 3 (inset)) are attributed to 5D1 to 7 transition of Eu3+ and are found to be very weak. The lines in the 550–650 nm range correspond to transitions from the first excited state 5D0 to the 7 levels (). It is well known that the relative intensity of the 5D0-7F1 and 5D0-7F2 transitions strongly depends on the local symmetry of the Eu3+ ions. When the Eu3+ ions occupy sites with inversion centers, the 5D0-7F1 (the magnetic dipole) transition should be relatively strong, while the 5D0-7F2 (the electric dipole) transition is parity-forbidden and should be very weak. The emission spectra of the compound K2Y2B2O7 exhibit strong red luminescence of 5D0-7F2 at 613 nm indicating that the Eu3+ ion is located in a noncentrosymmetric position in the matrix. For Eu3+ doped Na2Y2B2O7, after the Eu3+ ions have entered the crystal lattice, they will occupy Y sites and Y ions indeed occupy very low symmetry sites, C1. According to group theory selection rules, the magnetic dipole and the electric dipole are permitted and the electric dipole transition is the stronger one . Taking the same reference in account it may be suggested that in Eu3+ doped K2Y2B2O7 phosphor Eu3+ also takes the same low symmetry site.
3.4. Concentration Quenching Effect
Generally speaking, the doping concentration has a significant effect on the phosphor performance. The effect of Eu3+ doping concentration () in the K2Y2B2O7:Eu3+ (, 0.002, 0.005, 0.01, and 0.02) phosphors on the relative intensity of the electronic dipole transition (5D0-7F2) is shown in Figure 4. The result shows that, as the concentration of Eu3+ increases, the luminescent intensity also increases and reaches the highest intensity when the doping concentration of Eu3+ increases to 0.01. However, the luminescent intensity slightly decreases as long as the concentration is over 0.01 due to concentration quenching. The energy transfer within the same rare earth ions results in the concentration quenching associated with the exchange interaction .
The concentration quenching phenomenon can be theoretically supported by the relationship between luminescent intensity and doping concentration mathematically expressed as where is the intrinsic transition probability of the sensitizer and is the index of electric multipole, for electric dipole-electric dipole, electric dipole-electric quadrapole, and electric quadrapole-electric quadrapole corresponding to 6, 8, and 10, respectively. denotes a dimension of the sample, and here owing to the energy transfer between Eu3+ inside the grains. and are constants, and is a function. From (1), it can be obtained that where is independent of the doping concentration. Figure 5 presents the plots for the 5D0-7F2 transition of Eu3+ in the K2Y2B2O7 host. According to (2), the value of the slope parameter is calculated to be −0.89 (close to −1) corresponding to by linear approximation idea . This suggests that the exchange interaction mechanism plays a central role in the energy transfer among Eu3+ ions in the phosphors K2Y2B2O7.
3.5. CIE Diagram
The CIE 1931 color space chromaticity diagram to illustrate the chromaticity of K2Y2B2O7:Eu3+ phosphors. Figure 6 represents that the CIE coordinates of K2Y2B2O7:Eu3+ were measured as (; ). The location of coordinate has been marked in Figure 3 with a green circle. The CIE coordinates of K2Y2B2O7:Eu3+ are in the bright red area.
For K2Y2B2O7 doped with europium, the XRD pattern shows that the material was highly crystalline, having multiple peaks related to precursor or dopants, and the SEM image indicates the crystalline nature of this phosphor, with agglomerated regular morphology. Photoluminescence properties show that the phosphor gives strong red emission (PL) at 613 nm related to 5D0-7F2 transition of Eu3+ under the 260 nm excitation (PLE) related to the charge transfer (CT) from the 2p orbital of the O2− ions to the 4f orbital of Eu3+ ions with CIE coordinates (; ). The results of PL and PLE spectra indicate the applicability of K2Y2B2O7:Eu3+ as a red component in lamp phosphor.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
One of the authors, K. A. Koparkar, is thankful to the head of Department of Physics for providing XRD facility implemented under FIST Program-2010.
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