KCa4(BO3)3:Ln3+ (Ln = Dy, Eu, Tb) phosphors for near UV excited white–light–emitting diodes

A series of doped KCa4(BO3)3:Ln3+ (Ln: Dy, Eu and Tb) compositions were synthesized by solid–state reaction method and their photoluminescent properties were systematically investigated to ascertain their suitability for application in white light emitting diodes. The X–ray diffraction (XRD) and nuclear magnetic resonance (MAS–NMR) data indicates that Ln3+–ions are successfully occupied the non–centrosymmetric Ca2+ sites, in the orthorhombic crystalline phase of KCa4(BO3)3 having space group Ama2, without affecting the boron chemical environment. The present phosphor systems could be efficiently excitable at the broad UV wavelength region, from 250 to 350 nm, compatible to the most commonly available UV light–emitting diode (LED) chips. Photoluminescence studies revealed optimal near white–light emission for KCa4(BO3)3 with 5 wt.% Dy3+ doping, while warm white–light (CIE; X = 0.353, Y = 0.369) is obtained at 1wt.% Dy3+ ion concentration. The principle of energy transfer between Eu3+ and Tb3+ also demonstr...


I. INTRODUCTION
Recent years have witnessed an upsurge in the research and development of white-light emitting diodes (W-LEDs) being known for general illumination sources due to their unmatchable benefits, for example: high brightness, longer lifetime, small volume, and low power consumption.][3][4][5][6] One of the most fashionable approaches to produce white light is uniting a blue InGaN based LED with a broadband yellow-emitting Y 3 Al 5 O 12 :Ce 3+ (YAG) phosphor.][9] Uniform white-light with high R a can be alternatively achieved by mixing several single color emitting phosphors with UV-LEDs. 10,11 aMgAl 10 O 17 :Eu 2+ (BAM:Eu 2+ ) and BaMgAl 10 O 17 :Mn 2+ (BAM:Mn 2+ ) are some of the well-known blue and green phosphors broadly used for the generation of white-light because of their high emission efficiency at vacuum UV excitation (VUV) and prominent R a .12,13 However, despite many advantages, the final phosphor product suffers from complexities associated with the combination of several phosphors used in the fabrication and its significant degradation during heating, which reduce the lifetime and efficiency of WLEDs.10,11,14 The pursuit in this direction has led to identify new class of phosphors having broadband near white emitting properties based on oxides, [15][16][17][18] oxyfluorides, 19,20 nitrides, 21,22 oxynitrides, [23][24][25] sulfides 26,27 and halides.28,29 Among all, the oxyfluoride (for example: Sr 2.975-x -Ba x Ce 0.025 AlO 4 F) phosphors have been seen with alacrity as the most efficient matrix for photoluminescence due to their low phonon energy.19,20 Such low phonon energies are expected to decrease the contribution of non-radiative multi-phonon relaxations during the emission process of dopant rare-earth ions, resulting into enhancement of net emission quantum efficiency.19,20 However, despite of the high quantum efficiency, fluoride based phosphor systems are very moisture sensitive in nature, thus raising concerns regarding their practical applications.20 Therefore single-host broad band emitting phosphors need to be developed for low-cost W-LEDs with improved chemical and thermal stability better reproducibility and a simpler fabrication process.10,11 With the above mentioned perspectives, the present article focuses on studying the influence of rare-earth doping/co-doping on the luminescence properties of borate based phosphors.Owing to their simple and low-cost fabrication along with high thermal and chemical stability as well as transparency over a wide spectral range (beginning from UV and extending into visible) 5,11 borate hosts are considered as potential candidates for many potential optoelectronic applications, particularly in the field of laser science and technology.11 The most known borate phosphors include M 3 Al 6 B 8 O 24 :Ce 3+ (M = Ca,Sr,Ba), 30 YCa 3 (AlO) 3 (BO 3 ) 4 :Eu 3+ , 31 Na 2 Y 2 B 2 O 7 :Eu 3+ , 31 Sr 3 Y 2 (BO 3 ) 4 :Eu 3+ , 32 Sr 2 Mg(BO 3 ) 2 :Ce 3+ , 33 NaSr 4 (BO 3 ) 3 :Ce 3+ ,Mn 2+ , 34 Na 2 Gd 2 B 2 O 7 :Ce 3+ , Tb 3+ , 35 and NaSrBO 3 :Ce 3+ .5 The aim of the present research is to develop borate (containing planar [BO 3 ] 3-groups) based phosphors having the quality of near white light emission, since borate hosts exhibited excellent thermal stability over the commercial YAG host, 5 and they are less moisture sensitive in nature than the usual oxyfluoride hosts.5,20 Very recently, we have successfully demonstrated promising near white-light emission from KCaBO 3 :Dy 3+ ,Eu 3+ phosphor upon blue excitation. 11 TheKCaBO 3 systems are found to be attractive since the enhanced luminescence properties are observed due to the occupation of large concentrations of DY 3+ /Eu 3+ ions into the Ca 2+ ions by the preferential charge-compensation mechanism of 2Ca 2+ = Dy 3+ /Eu 3+ + K + .
The present structurally attractive KCa 4 (BO 3 ) 3 host having strong absorption in the UV/near-UV (NUV) regions is expected to enhance the energy transfer process efficiency from host lattice to the activator upon NUV excitations.It is worth mentioning that the crystal structure of KCa 4 (BO 3 ) 3 has been firstly reported by Wu et al. 36 But, to the best of our knowledge, luminescence properties of rare-earth ion doped KCa 4 (BO 3 ) 3 phosphors and corresponding applications for W-LEDs are not been reported in the literature.For white-light generation, Dy 3+ ion doping is a popular choice, due to the combined emission from strong blue (B, 4 F 9/2 → 6 H 15/2 ) and yellow (Y, 4 F 9/2 → 6 H 13/2 ) emission bands.Particularly, the Dy 3+ ion emission show strong crystal field effects, therefore intensity of hypersensitive yellow color emission from the transition 4 F 9/2 → 6 H 13/2 is strongly influenced by the host environment, compared to the other non-hypersensitive emission band 4 F 9/2 → 6 H 15/2 . 11,37,38 Hce, it is interesting to observe such host matrix influence on the near-white light generation from Dy 3+ -doped phosphors directly by fine tuning the ratio of these yellow and blue prominent emission intensities. 37,38 e have also developed Eu 3+ and Tb 3+ singly and co-doped systems for red and green emission components for the generation of white-light.Nevertheless, these combinations can be interesting candidates to combine with UV/NUV light for realizing white-light with suitable color rendering index.
Eventually, Dy 3+ -singly doped and Eu 3+ /Tb 3+ -codoped KCa 4 (BO 3 ) 3 phosphors were synthesized by a solid-state reaction method and their emission spectra were obtained under NUV excitations in order to search for new compositions for white light phosphors.Intense white light was obtained in KCa 4 (BO 3 ) 3 :Dy 3+ , while Eu 3+ /Tb 3+ -codoped KCa 4 (BO 3 ) 3 phosphor showed near white light emission under the excitation of 350 nm.Based on the above referred findings, the focus has now been extended to the energy transfer mechanism, emission quenching and color rendering properties of the synthesized KCa 4 (BO 3 ) 3 phosphors.

II. EXPERIMENTAL DETAILS
The synthesis of various concentrations of Ln 3+ (Ln: Dy, Eu &amp; Tb)-doped KCa 4 (BO 3 ) 3 phosphors is accomplished from solid state reaction.Stoichiometric amounts of analytical grade K 2 CO 3 , CaCO 3 , H 3 BO 3 , Dy 2 O 3 , Tb 2 O 3 and Eu 2 O 3 were mixed and ground well in an agate mortar to obtain a fine and homogeneous powder.The mixture was then annealed in a furnace in air atmosphere at 800 o C for 7 h in alumina boats and cooled to room temperatures.
X-ray diffraction (XRD) patterns for these powders were collected by Rigaku Geigerflex D/Max, C Series, Tokyo, Japan; with Cu K α radiation (λ = 1.54 Å) in the 2θ angle range 10 o -80 o with step size 0.02 o /s.
Scanning electron microscopy (SEM; SU-70, Hitachi) attached with energy dispersive spectroscopic detection system (EDS; Bruker Quantax, Germany) was used to study the morphology and the distribution of individual atomic constituents. 11B NMR experiments were carried out on a Bruker Avance-III operating at a Larmor frequency of 128.38 MHz (B 0 = 9.4 T) and using a 4 mm double resonance probe.A solution of boric acid was used for the chemical shift reference and to set-up the radio-frequency (RF) field strength. 11B MAS NMR spectra were acquired using a pulse length of 1.0 μs and a RF field strength of 45 kHz at a spinning rate (ν R ) of 10.0 kHz with a recycling delay of 120 s.Two-dimensional (2D) t1-split Satellite Transition Magic Angle Spinning (STMAS) 39 spectrum was recorded at ν R = 10.0 kHz.The satellite transition excitation and double-quantum reconversion were obtained using hard pulses of 1.5 and 1.0 μs, respectively, with a RF of 146 kHz.Two central transition's selective soft pulses of 30.5 (180 o ) and 15.25 (90 o ) μs corresponding to a RF of 8.2 kHz were employed for the double-quantum excitation and to convert the satellite transition to observable central transition, respectively; the recycle delay was set to 1.0 s.The software package DMFIT 40 was used for the curve fitting of the 11 B MAS NMR spectra based on the number of 11 B resonances observed in the STMAS spectrum.

A. XRD and SEM studies
The observed XRD patterns of KCa 4 (BO 3 ) 3 : xLn 3+ are similar in the whole Ln 3+ doping range.As an example, Fig. 1(a) shows the XRD patterns of KCa 4 (BO 3 ) 3 , KCa 4 (BO 3 ) 3 : 10%Dy 3+ and KCa 4 (BO 3 ) 3 :5%Eu 3+ ,5%Tb 3+ .In the undoped parent sample, a mono-mineral composition was obtained with crystalline KCa 4 (BO 3 ) 3 (JCPDS:01-75-3604) phase as evidenced by the minimal mismatch between the experimental and the simulated XRD data and absence of any significant misfits in the difference plot obtained using Diffrac plus TOPAS (Bruker AXS) software and the fitting has been performed using least-square methods.The resultant fitting of XRD patterns are shown in Fig. 1(b) and the resultant structural parameters have been refined simultaneously.The figure-of-merit (R), defines the mean standard deviation, suggests the good agreement between observed and calculated data and the obtained R factor values (R exp = 8.30 R wp = 4.58, R p = 3.52 and GOF = 0.55) are well within the limits of experimental accuracy.The XRD data of KCa 4 (BO 3 ) 3 :Dy 3+ and KCa 4 (BO 3 ) 3 :5%Eu 3+ , 5%Tb 3+ phosphors is in good agreement with that obtained for KCa 4 (BO 3 ) 3 regardless of the dopant concentration as also certified by the Rietveld refinement.This implies towards the absence of any impurity phase in the as-prepared powders AIP Advances 3, 022126 ( 2013 and insignificant variations in the host crystal structure.Moreover, since the ionic radii of Dy 3+ (105 pm), Eu 3+ (109 pm) and Tb 3+ (106 pm) are closer to Ca 2+ (114 pm) instead of K + (152 pm); it has been assumed that the non-centrosymmetric Ca 2+ sites have been replaced by above mentioned ions in the KCa 4 (BO 3 ) 3 crystal lattice.
On the basis of the detailed structural investigation of the two new types of borates, cubic and orthorhombic, isolated by Wu et al., 36 it was evidenced that when the larger alkali metal cation was infixed to replace a smaller alkali metal ion, or reversely when a smaller alkaline earth metal cation was infixed to replace the larger alkali metal ions, the cubic structure became unstable and gave rise to the lower symmetric orthorhombic system.From the structural point of view, we need a flexible and stable host which will support the replacement of the internal cations with the desired metal ions without any considerable changes in the host lattice.For the non-centrosymmetric orthorhombic system, the larger cations can be easily replaced by the lanthanide ions, without changing the host crystal lattice.As the ionic radii of lanthanide ions (Dy 3+ (105 pm), Eu 3+ (109 pm), Tb 3+ (106 pm)) are quite comparable to that of Ca 2+ (114 pm), as well as their coordination geometries, the replacement of alkaline earth metal is easier and the host system becomes a perfect energy transfer system.The dopants occupy the interstitial positions symmetrically and interact with each other minimizing the resultant phonon energy within the host lattices while decreasing the luminescence quenching.It is worth to mention that, Wu and co-workers thoroughly investigated the structural correlation with energy transfer between Tm 3+ and Dy 3+ in KSr 4 (BO 3 ) 3 host.Their structures were suggested to exhibit three Sr 2+ centers and the dopants (Tm 3+ and/or Dy 3+ ) preferably occupy the closest Sr 2+ sites which are separated by a minimum distance of 3.38-3.35Å.They also suggested that for an efficient energy transfer, a closest pair up is needed, being consistent with their results.
From the structural refinement of our system we find that the closest distance among the Ca(1) and Ca(2) in KCa 4 (BO 3 ) 3 is around 3.45 Å.This finding is consistent with the pairing up of (Eu 3+ and/or Tb 3+ ) dopants, favoring an efficient energy transfer process.
The microstructural evolution of the as synthesized KCa 4 (BO 3 ) 3 phosphor material as observed in SEM-EDX is presented in Fig. 2(a).The crystalline morphology of the as synthesized phosphor powders displayed micro granularity with their particle size varying between 1-10 μm.The particles appeared to be agglomerated indicating that the structures formed by solid state reaction have not been completely destroyed during the dry milling step.The excess amount of H 3 BO 3 acting as flux during synthesis enhances the mobility of solid reactants and the formation of strong bonds within particle agglomerates.The consistent formation of KCa 4 (BO 3 ) 3 was validated by EDX element analysis and results are given in Figure 2(b) and 2(c).These EDX results support well the XRD data.However, quantifying the individual species in the present host material by EDX element analysis is very difficult because of the inability of EDX detector for accurately detecting light elements as boron.But the constant proportionality between the K and Ca elements in the present EDX graphs broadly indicates the appropriate formation of the KCa 4 (BO 3 ) 3 compound.The crystal structure shows three different 11 B sites in KCa 4 (BO 3 ) 3 in a 1:1:1 ratio.To achieve the separation of these sites we therefore acquired a STMAS spectrum on KCa 4 (BO 3 ) 3 .The 2D STMAS spectra of KCa 4 (BO 3 ) 3 (Fig. 3) show at least two well-resolved 11 B peaks (B1 and B2) on its isotropic (F1) dimension.Both sites have very similar line shapes, stating that they present similar quadrupolar parameters (see below).This reflects an almost identical chemical environment for the boron species.The isotropic chemical shift (δ iso ), quadrupolar coupling constant (C Q ) and asymmetry parameter (η Q ) were obtained for site B1 (δ iso = 1.40 ppm, C Q = 2.62 MHz and η Q = 0.15) and site B2 (δ iso = 2.75 ppm, C Q = 2.63 MHz and η Q = 0.06) through simulation of their anisotropic projections shown in the left side of Fig. 3.The relative amounts of the two boron environments was determined using the 1D 11 B single-pulse MAS employing a small flip-angle (Fig. 4(a)) and the parameters (δ iso , C Q , η Q ) extracted from the STMAS spectra as usual practice.Ratios of 64.5% and 35.5% were obtained for site B1 and B2, respectively.This result corresponds approximately to a 2:1 (B1:B2) ratio between both sites and it is most likely that two 11 B resonances corresponding to two (almost) equivalent environments are overlapped (B1 and B3) while a third boron site (B2) being more symmetric (η Q = 0.06; corresponding to a quadrupolar tensor very close to axial symmetry, i.e., η Q ∼ 0) may be well separated from the other two.The presence of three crystallographic distinct boron sites in KCa 4 (BO 3 ) 3 is therefore in agreement with the structure determined by Wu et al. 36  spectrum.The emission spectra (Fig. 5(b)) contains dominant emissions at 482 nm (blue) and 576 nm (yellow) and other low intense peaks at 670 nm (red) and 750 nm, which are attributed to the transitions from 4 F 9/2 to various energy levels 6 H 15/2 , 6 H 13/2 , 6 H 11/2 and 6 H 9/2 , respectively.In general, the yellow emission is due to magnetic dipole originated hypersensitive transition and is dominant when Dy 3+ ions are situated at low-symmetry sites with no inversion centres. 11In the present study, the yellow emission at 576 nm was found to be predominant over blue (482 nm) emission (see Fig. 5(b)), suggests that the Dy 3+ ions are situated away from inversion symmetry, and this observation is consistent with the structural study that Dy 3+ occupies the non-centrosymmetric position of Ca 2+ .In the present case, since the ionic radius of Dy 3+ (105 pm) is closer to Ca 2+ (114 pm) ionic radius in comparison to that of K + (152 pm), therefore Dy 3+ might have preference for substituting the Ca 2+ ions in the lattice.The occupation of Dy 3+ ion into Ca 2+ sites in KCa 4 (BO 3 ) 3 host eventually generates number of oxygen vacant sites, thereby lead to expand the lattice and lower the crystal density.According to Lopez et al., 41 oxygen vacancies might be useful to enhance the rare-earth ion emission by acting as sensitizers for efficient energy transfer from charge transfer states to the rare earth ions.However, the crystallinity of the host would be inevitably destroyed by these excess oxygen vacancies, which may eventually lead to quenching of rare earth ion emission.

B. 3Q-MAS-NMR structural studies
In order to investigate the emission intensity variation with respect to the content of Dy 3+ ion doping, specially to observe the effect of local surroundings on Dy 3+ , several KCa 4 (BO 3 with various dopant concentrations of Dy 3+ (1% to 10%) were prepared and their yellow emission intensities were monitored under 350 nm excitation (inset of Fig. 5(b)).As seen, the optimal value for the maximum photoluminescence intensity was found to be 5% of Dy 3+ ion concentration.The emission quenching at higher concentration is due to the inter-ion energy transfer and for this reason, the critical separation between donor and acceptors has to be estimated.The critical energy transfer distance R c between the activators could be estimated from the equation: 42 where X c is the critical concentration in mol.%,Z is the number of cations in the unit cell and V is the volume of the unit cell.By considering the experimental and analytic values of X c , Z and V [5 wt.% (0.022 mol.%), 16 and 744.637Å respectively, the critical transfer distance of Dy 3+ in the present phosphor is estimated to be about 15.93 Å.
The schematic energy levels of Dy 3+ ions are presented in Fig. 6(a) according to the calculated values of Carnall et al. 43 When the 4f higher energy levels of Dy 3+ ions are excited at 350 nm, the initial population nonradiatively relaxes to the lower energy levels by phonon-assisted processes until it arrives to the 4 F 9/2 level, then radiatively deactivates giving rise to strong blue and yellow emissions and other weak red-end emissions.With the elevated Dy 3+ content, these characteristic emissions increase upto x = 5%, and subsequently decreases drastically.
The photoluminescence decay curves of different KCa 4 (BO 3 ) 3 : x Dy 3+ phosphors for the 575 nm emission with 350 nm excitation were also recorded and the resultant decay curves could be fitted by a single exponential function: where I 0 is the initial intensity at t = 0, and τ is the decay lifetime.The corresponding decay times in KCa 4 (BO 3 ) 3 :xDy 3+ phosphor was 0.51, 0.52, 0.54, 0.49 and 0.48 ms for 1%, 3%, 5%, 7% and 10% Dy 3+ , respectively.The lifetime values increased with the increase of Dy 3+ content up to 5%, and then decreases as a result of concentration quenching.The decay curves of KCa 4 (BO 3 ) 3 :xDy 3+ phosphors were very similar to each other even for higher doping concentrations of Dy   The Y/B intensity ratio (listed in Table I) of Dy 3+ was found to assume different values, from 1.5 to 3.33 when the concentration of Dy 3+ is changed, indicating that the crystal field environment surrounding the Dy 3+ ions is changing abruptly with the incorporated amount of Dy 3+ .Hence, white light emission can be achieved from KCa 4 (BO 3 ) 3 :xDy 3+ phosphors as confirmed by CIE coordinates.The CIE chromaticity coordinates of KCa 4 (BO 3 ) 3 :xDy 3+ phosphors with different Dy 3+ concentrations were calculated from the emission spectra (Fig. 5(b)) and presented in Fig. 7(a) and the corresponding co-ordinates are listed in Table I.The values of color coordinates are nearer to pure white-light whose color coordinates are x = y = 0.3333.This fact suggests that white-light could be achieved by appropriately controlling the Dy 3+ concentration thereby adjusting the ratio of dominant blue and yellow emissions.To compare the different white emissions, the whiteness parameter (W) was calculated according to the equation. 44 where, Y is the tristimulus value, (x, y) are the chromaticity coordinates in the CIE 1931 color space and (x R , y R ) = (0.33, 0.33) are the chromaticity coordinates of the perfect white light emitter.A perfect white give W = 100%.Our results shows that the emission of KCa 4 (BO 3 ) 3 :1%Dy 3+ phosphor is the nearest of perfect white (W = 84%), which is also evident to the naked eye (inset of Fig. 5(b)).
In order to verify the potentiality as one of the effective white-light emitting phosphors for the white LEDs, the external quantum efficiency (EQE) of this phosphor was also estimated according to the process adopted by Hirosaki et al. 45 For KCa 4 (BO 3 ) 3 :1%Dy 3+ phosphor, the EQE was determined to be 52% upon excitation at 350 nm and.However, the experimental conditions and the composition of the phosphor must be optimized for further improvement in the efficiency values.It is worth to mention that the quantum efficiencies in general are influenced by the lattice defects, stoichiometric excess, nonradiative deactivation centers and self-absorption. 46Post-synthesis annealing is usually used to improve the crystallinity, surface morphology and stoichiometry to improve the phosphor's emission efficiency.Furthermore, the phosphor absorption at the long-wavelength range needs to be reduced to avoid self-absorption. 46 Photoluminescence of Eu 3+ , Tb 3+ singly and codoped KCa 4 (BO 3 ) 3 phosphors Figures 5(c) and 5(d) depicted the PLE and PL spectra of KCa 4 (BO 3 ) 3 :xEu 3+ phosphor, respectively.In the PLE spectrum of Eu 3+ (see Fig. 5(c)), monitored at the emission of 615 nm, the strong UV broad band (∼270 nm) can be assigned to the charge transfer transition between Eu 3+ and O 2− , and other sharp peaks are originating from the f-f transitions of Eu 3+ ions.The excitation peaks ranging from 350 to 500 nm can be identified as the intrinsic of Eu 3+ transitions from 7 F 0 to 5 L 6 and 5 D 2 .Upon excitation at 270 nm, the PL spectra (Fig. 5(d)) shows intense transition lines from the excited 5 D 0 level of Eu 3+ ions and their transition assignments are appropriately assigned.Two main characteristic peaks from 5 D 0 → 7 F 1 (orange, 591 nm) and 5 D 0 → 7 F 2 (red, 615 nm) are dominant.The red emission at 615 nm increases systematically with the Eu 3+ content (Fig. 5(c)) suggesting the minimum concentration quenching effects.
Interestingly, the co-doped phosphor system KCa 4 (BO 3 ) 3 :Eu 3+ − Tb 3+ showed prominent and equally intense orange-red (Eu 3+ ) and green emissions (Tb 3+ ).Fig. 8(c) shows such emission from Eu 3+ -Tb 3+ phosphor (λ ex = 350 nm), where the individual PL spectra for Eu 3+ and Tb 3+ doped phosphors are also shown for comparison.As seen in the spectrum (Fig. 8(c)), the dominant orange-red and green emissions are due to respective Eu 3+ and Tb 3+ ions, giving rise to near white-light emission.With the addition of Eu 3+ , the orange-red emissions of Eu 3+ systematically increases, at the expense of green emission (Tb 3+ : 5 D 4 → 7 F 5 ) intensity.As it is known, the tricolor combination (red, green and blue or any mixture of two of them) in different ratios can show white light emission, therefore, different mixing ratios of Eu 3+ , Tb 3+ ions are attempted in the present study.When the proportions of Eu 3+ and Tb 3+ codoping ions are 5/5 wt.% (shown in Fig. 8(c)) the near white-light emission was achieved due to the equal contribution of intensities of red, orange and green emissions, which was supported by the efficient energy transfer from Tb 3+ to Eu 3+ .Excitation spectra for KCa 4 (BO 3 ) 3 :Eu 3+ -Tb 3+ phosphor obtained by monitoring the 5 D 0 → 7 F 2 transition of Eu 3+ (615 nm) and the 5 D 4 → 7 F 5 transition of Tb 3+ (545 nm) are shown in Fig. 8(d).The overlapping excitation peaks in the region between 350-375 nm clearly indicates the close proximity of energy levels of individual ions for efficient charge transfer between Tb 3+ and Eu 3+ during near UV excitations.Fig. 6(b) shows the schematic energy level diagram to understand such energy transfer mechanism between Tb 3+ and Eu 3+ ions.During the relaxation process of Tb 3+ ions to their 7 F J states, the energy being partially transferred to ground states of Eu 3+ ions and eventually excites the electrons from the ground states to 5 D 1 level, as shown by the dashed lines (Fig. 6(b)).The following equation explains the energy transfer process: To gain further insight into the energy transfer mechanisms, the luminescent decay of Tb 3+ :541 nm and Eu 3+ :616 nm emissions were also measured.The decay curves of the Tb 3+ and Eu 3+ singly doped phosphors can be well fit by the single exponential (equation 2).The decay life times extracted from the fitted curve of 5%Tb 3+ and 5%Eu 3+ singly doped phosphors are found to have 0.451 ms and 0.463 ms respectively.The lifetime of Tb 3+ ;545 nm emission in Eu 3+ -Tb 3+ codoped sample (0.424 ms) is slightly shorter than the singly doped phosphor, while that of Eu 3+ ;615 nm (0.478 ms) was little larger than the single Eu 3+ doped phosphor.The corresponding PL decay profiles of the Tb 3+ ; 545 nm and Eu 3+ ; 615 nm emissions of the Eu 3+ -Tb 3+ codoped sample have been depicted in Fig. 9(a).In general, the PL decay of the singly doped samples resulted into a single exponential whereas the codoped phosphor was found to have a doubled exponential nature.Such double exponential decay curves can be fitted using the following equation: 47 where I(t) is the intensity of luminescence for a time, τ 1 and τ 2 are the short and long lifetimes corresponding to the intensity coefficients I 1 and I 2 , respectively.The average lifetime, hence defined as τ av = (I 1 τ 1 2 + I 2 τ 2 2 ) / (I 1 τ 1 + I 2 τ 2 ). 48Clear deviation of PL decay from single exponential behavior suggests the predominance of ion-ion interactions and energy transfer phenomena between Eu 3+ and Tb 3+ ions.This resulting into have relatively fast and slow PL decay components.
The observed color compensated white emission in the KCa 4 (BO 3 ) 3 : Eu 3+ ,Tb 3+ phosphor can be further confirmed by the CIE coordinates.As seen in the CIE diagram (Fig. 7(b)), the emission color of KCa 4 (BO 3 ) 3 : 5% Eu 3+ -5% Tb 3+ was observed to be in the near white-light region (0.36, 0.40) very close to that of pure white-light (0.33, 0.33).The PL digital image (inset of Fig. 8(c)) recorded by 350 nm excitation also confirms the near white-light.The external quantum efficiency of the PL estimates to be about 43%.The above results clearly see demonstrates near white-light due to the combined emissions of the Eu 3+ and Tb 3+ ions, supported by partial energy transfer between the ions.Hence, KCa 4 (BO 3 ) 3 :Eu 3+ ,Tb 3+ phosphor system would potentially be used as white light emitting phosphor and these findings would certainly encourage more studies on the luminescence properties for its further optimization and simplified white-light generation.

D. Thermal stability measurements
For device applications, high thermal stability of phosphor is one of the requirements to avoid any transformations in the chromaticity and brightness.Fig. 9(b) shows the temperature dependence of the luminescence intensity for selective phosphors (5%Dy 3+ , 5%Eu 3+ , 5%Tb 3+ and 5%Eu 3+ -5%Tb 3+ codoped phosphors) excited at 350 nm.At 200 o C, all the phosphors retain more than 75% of the intensity, which is favorable feature for LED applications.The activation energy (E a ) can be extracted from the expression: 5 ln(I /I 0 ) = ln A − E a /kT.(6)   Where, I 0 and I are the luminescence intensity at room and testing temperatures, respectively; A is a constant; and k is the Boltzmann constant (8.617 × 10 −5 eV K −1 ).From eq. ( 6), we determined E a to be 0.017, 0.016, 0.017 and 0.015 eV for 5%Dy 3+ , 5%Eu 3+ , 5%Tb 3+ singly doped and 5%Eu 3+ , 5%Tb 3+ codoped KCa 4 (BO 3 ) 3 phosphors, respectively.As an example, the long (I 0 /I) vs 1000/T fitting for the calculation of activation energy of KCa 4 (BO 3 ) 3 :5%Dy 3+ is also shown as inset of Fig. 9(b).

IV. CONCLUSIONS
Dy 3+ , Eu 3+ , Tb 3+ singly doped and Eu 3+ -Tb 3+ codoped KCa 4 (BO 3 ) 3 phosphors were successfully fabricated from solid-state reactions for the purpose of white-light LEDs.Dy 3+ doped KCa 4 (BO 3 ) 3 emits bright blue and yellow emissions upon excitation near UV range.As confirmed from chromatic analysis, the intensity ratios of prominent yellow and blue color emissions can be conveniently controlled by simply varying the Dy 3+ concentrations to achieve the desired near whitelight emission.More importantly, a systematic study on the PL properties of KCa 4 (BO 3 ) 3 :Ln 3+ (Ln = Eu, Tb) has been explored under near UV excitation in order to obtain multicolour-emitting phosphors.The 5 wt.%Eu 3+ and 5 wt.%Tb 3+ co-doped phosphor showed near white light emission upon near UV excitation.This was possible due to the partial energy transfer from Tb 3+ to Eu 3+ , as confirmed by the PL decay analysis.In the meantime, present phosphor systems exhibited adequate thermal stability even at 200 o C.This broadly suggests that the current phosphors can be very suitable for LED applications.Overall, the strong absorption in UV and near UV range, efficient multicolour emission including white light, moderate decay time makes it a potential candidate for efficient white light LEDs.

022126- 5 Amarnath
FIG. 2. (a) and (b) Surface morphology (SEM images) and (c) EDX analysis at four specified areas (as shown in Fig. 2(b)) for K and Ca of the KCa 4 (BO 3 ) 3 phosphor synthesized at 800 o C for 7 hours.

022126- 6 Amarnath
FIG. 3. 2D t1-split STMAS NMR spectra of KCa 4 (BO 3 ) 3 along with the F1 and F2 projections.The second-order powder patterns of the two distinguishable boron sites are displayed on the left.
FIG. 6.(a) Energy level diagrams, visible emission transition for Dy 3+ , and the resonance energy transfer among Dy 3+ ions.(b) Energy level diagrams and energy transfer behaviour of Tb 3+ and Eu 3+ ions in KCa 4 (BO 3 ) 3 .