Crystal structure and Temperature-Dependent Luminescence Characteristics of KMg4(PO4)3:Eu2+ phosphor for White Light-emitting diodes

The KMg4(PO4)3:Eu2+ phosphor was prepared by the conventional high temperature solid-state reaction. The crystal structure, luminescence and reflectance spectra, thermal stability, quantum efficiency and the application for N-UV LED were studied respectively. The phase formation and crystal structure of KMg4(PO4)3:Eu2+ were confirmed from the powder X-ray diffraction and the Rietveld refinement. The concentration quenching of Eu2+ in the KMg4(PO4)3 host was determined to be 1mol% and the quenching mechanism was certified to be the dipole–dipole interaction. The energy transfer critical distance of as-prepared phosphor was calculated to be about 35.84Å. Furthermore, the phosphor exhibited good thermal stability and the corresponding activation energy ΔE was reckoned to be 0.24eV. Upon excitation at 365nm, the internal quantum efficiency of the optimized KMg4(PO4)3:Eu2+ was estimated to be 50.44%. The white N-UV LEDs was fabricated via KMg4(PO4)3:Eu2+, green-emitting (Ba,Sr)2SiO4:Eu2+, and red-emitting CaAlSiN3:Eu2+ phosphors with a near-UV chip. The excellent color rendering index (Ra = 96) at a correlated color temperature (5227.08K) with CIE coordinates of x = 0.34, y = 0.35 of the WLED device indicates that KMg4(PO4)3:Eu2+ is a promising blue-emitting phosphor for white N-UV light emitting diodes (LEDs).

the temperature-dependent luminescence characteristics as well as the application of KMg 4 (PO 4 ) 3 :Eu 21 pumped for n-UV LEDs have not been investigated. In this paper, the KMg 4 (PO 4 ) 3 :Eu 21 phosphor was firstly prepared by the conventional high temperature solid-state reaction method. The crystal structure, reflectance spectra, thermal stability, quantum efficiency and applications in white NUV LED are studied respectively. White LEDs was fabricated by combing an N-UV LED chip (l max 5 385 nm) with the KMg 4 (PO 4 ) 3 :Eu 21 , along with green and red phosphors, and its optical properties have also been investigated. The results demonstrate that the blue-emitting KMg 4 (PO 4 ) 3 :Eu 21 is a promising blue-emitting phosphor for white N-UV LEDs.

Results
XRD Refinement and Crystal Structure. Figure 1 depicts the XRD patterns of the series of as-synthesized KMg 4 (PO 4 ) 3 :xEu 21 (x 5 0, 0.02, 0.06, and 0.1), and the standard pattern (JCPDS 15-4111) of KMg 4 (PO 4 ) 3 is shown as a reference. It can be found from the Figure 1 that all XRD patterns agree well with the standard pattern and no other phase is observed, which demonstrates that the single phase of KMg 4 (PO 4 ) 3 :xEu 21 was obtained and the doping of Eu 21 ions did not cause any notable impurities or any structural variation. Besides, the main diffraction peaks shift slightly to the higher angle side with increasing Eu 21 concentrate, as shown in Figure 1(b). This observation means that the lattice was distorted by substitution the ions which are comparatively big radius in KMg 4 (PO 4 ) 3 host lattice with Eu 21 ions 14 . Thus, it is reasonable to assume that Eu 21 (r 5 1.25 Å for coordinate number (CN) 5 8 and r 5 1.17 Å for CN 5 6) ions substituted the position of K 1 sites (r 5 1.51 Å for CN 5 8) because both the Mg 21 (r 5 0.72 Å for CN 5 6 and r 5 0.66 Å when CN 5 5) and P 51 (r 5 0.17 Å for CN 5 4) sites are smaller than the Eu 21 ions 15,16 . For further understanding the phase purity and the occupancy of Eu 21 ions on K 1 sites in KMg 4 (PO 4 ) 3 :Eu 21 , the Rietveld refinement of KMg 4 (PO 4 ) 3 and KMg 4 (PO 4 ) 3 :0.06Eu 21 phosphors were analyzed via the GSAS program as shown in Figure 2. The KMg 4 (PO 4 ) 3 was served as an initial structural model. The results of Rietveld refinement further demonstrate that neither the host nor the doped 0.06 mol Eu 21 ions generated any impurity or secondary phases in KMg 4 (PO 4 ) 3 . The KMg 4 (PO 4 ) 3 :xEu 21 belongs to an orthorhombic structure with the space group Pnnm(58). For the crystal of KMg 4 (PO 4 ) 3 host, the lattice parameters were fitted to be a 5 16.3707(7) Å , b 5 9.5627(4) Å , c 5 6.1667(5) Å , cell volume (V) 5 965.361(23) Å 3 and the weighted profile R-factor (R wp ), the expected R factor (R p ) are 8.86% and 6.86%, respectively. As doped with Eu 21 , the lattice parameters of KMg 4 (PO 4 ) 3 :0.06Eu 21 became a 5 16.3563(4) Å , b 5 9.5570(2) Å , c 5 6.1663(1) Å and V 5 963.904 (67) Å 3 . The refinement data converged to R wp 5 10.54% and R p 5 7.97%, as summarized in Table 1. The volumetric constriction with increasing Eu 21 doping concentration also indicates that the Eu 21 occupied the K 1 ions sites. Figure 3 illustrates the crystal structure of KMg 4 (PO 4 ) 3 and the coordination environment of the K 1 ions. The compound of KMg 4 (PO 4 ) 3 is consist of PO 4 tetrahedra, MgO 6 octahedra and MgO 5 polyhedra which are linked by P-O-Mg bridges. The K 1 ions are located in the tunnels along b-axis and surrounded by the threedimensional framework forming via interconnected polyhedra. As presented in Figure 3c, the K 1 ions in KMg 4 (PO 4 ) 3 are eight-fold coordinated by oxygen ions with 4g position and m site symmetry.
Reflectance and Photoluminescence properties of the KMg 4 (PO 4 ) 3 : xEu phosphor at RT. The reflectance spectra of KMg 4 (PO 4 ) 3 host and KMg 4 (PO 4 ) 3 :0.05Eu 21 are presented in Figure 4a. The KMg 4 (PO 4 ) 3 host shows an energy absorption band ranging from 200 to 300 nm, and a high reflection ranging from 300 to 700 nm. The band gap of the virgin KMg 4 (PO 4 ) 3 is calculated by using the following formula: 17 in which hn means the energy per photon, C is a proportional constant and E g represents the value of the band gap, n 5 1/2 stands for an indirect allowed transition, 2 means a direct allowed transition, 3/2 represents a direct forbidden transition, or 3 indicates an indirect forbidden transion, R ' 5 R sample /R standard . The F(R') means the Kubelka2Munk function which can be formulated to the following equation: where K, S, and R represent the absorption, scattering, and reflectance parameter, respectively. As illustrated in Figure 4b, the band gap energy of KMg 4 (PO 4 ) 3 host is estimated to be about 5.74 eV from the extrapolation of the line for [F(R')hn] 2 5 0. As Eu 21 ions were introduced into the host, strong broad absorption appeared in the 250-400 nm N-UV range, which is matched well with the excitation spectrum.
The photoluminescence emission (PL, l ex 5 300 and 365 nm) and excitation (PLE, l em 5 450 nm) spectra of KMg 4 (PO 4 ) 3 :0.06Eu 21 are also depicted in Figure 4a. The PLE spectrum of KMg 4 (PO 4 ) 3 :  0.06Eu 21 presents a broad hump ranging at 250-400 nm, which originates from the 4f 7 -4f 6 5d transition of Eu 21 ions. It indicates that the broad excitation spectrum of KMg 4 (PO 4 ) 3 :Eu 21 matches well with the emission of the commercial N-UV chip (365-420 nm). The PL spectra of the KMg 4 (PO 4 ) 3 :0.06Eu 21 phosphor present a 52.2 nm full width at half-maximum (FWHM) broad blue emission band extending from 400 to 525 nm peaking at 450 nm, which is assigned to the 4f 6 5d-4f 7 transition of the Eu 21 ions. Moreover, The PL of KMg 4 (PO 4 ) 3 :0.06Eu 21 detected under 365 nm is similar to that under 300 nm in addition to the difference of the relative intensity, which verifies that the Eu 21 ions occupy the same lattice site (K 1 sites) in KMg 4 (PO 4 ) 3 host 18 . Above result is in accord with the conclusion from Rietveld refinement. The CIE chromaticity coordinates of KMg 4 (PO 4 ) 3 :0.01Eu 21 and commercial BAM phosphors under 365 nm UV excitation are illustrated in Figure 5. The color coordinates of KMg 4 (PO 4 ) 3 :0.01Eu 21 and BAM are calculated to be (x 5 0.1507, y 5 0.0645) and (x 5 0.1471, y 5 0.0628), respectively. The inset shows the digital photograph of KMg 4 (PO 4 ) 3 :0.01Eu 21 phosphor under a 365 nm UV lamp, which indicates KMg 4 (PO 4 ) 3 :Eu 21 phosphor can be used as a blue-emitting phosphor for w-LEDs application.
As Figure 4a shows, the PL and PLE spectra of KMg 4 (PO 4 ) 3 : 0.06Eu 21 overlap partially, which demonstrates the existence of energy transfer between Eu 21 -Eu 21 19 . As we know, two main mechanisms can be explanatory for the resonant energy-transfer: exchange interaction or electric multipolar interaction 20 . In order to further investigate the process of energy transfer between activators or between sensitizer and activator, the Eu 21 concentrationdependent PL spectra of KMg 4 (PO 4 ) 3 :xEu 21 (x 5 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) phosphors under 365 nm light excitation are shown in Figure 6. The optimal doping concentration of Eu 21 for the PL intensity in KMg 4 (PO 4 ) 3 :xEu 21 is 1% mol. When the doping content of Eu 21 exceeded 0.01 mol%, the PL intensity began to decrease because of the concentration quenching effect which is due to the energy consumed via energy transfer from one activator to another 21 . Thus, the critical distance (Rc) for energy transfer among Eu 21 is necessary to obtain for further understanding the concentration quenching interaction mechanism. The value of the critical distance (Rc) can be reckoned via the following equation: 22 where V means the unit cell volume, x c represents the concentration of activator ion where the quenching occurs and N is the number of the K 1 ion in per unit cell. For the KMg 4 (PO 4 ) 3 host, x c 5 0.01, N 5 4, and V 5 965.361 Å 3 , hence, the value of R c is calculated to about 35.84 Å . Owing to the typical critical distance of the exchange interaction is about 5 Å and the exchange interaction only fits the energy transfer of forbidden transitions 5,23 . Therefore, the electric multipolar in which x is the concentration of activation, which is not less than the critical concentration, I/x is the emission intensity (I) per activator concentration (x); K and b are constants under the same excitation condition of host lattice; and h is a function of electric multipolar character. h 5 6, 8, 10 for dipole-dipole (d-d), dipolequadrupole (d-q), quadrupole-quadrupole (q-q) interactions, respectively. In order to estimate the h value, the dependence of lg(I/x) on lg(x) is illustrated in the inset of Figure 6. A relatively linear relation can be observed and the slope of the straight line is fitting to   To further explore the energy transfer process, the room temperature luminescence decay curves of Eu 21 ions in KMg 4 (PO 4 ) 3 :xEu 21 (x 5 0.005, 0.01, 0.02, 0.04, 0.06, 0.08; l ex 5 365 nm, l em 5 450 nm) were measured, as shown in Figure 7. The decay curves can be fitted with an approximate single-exponential decay model as: where I 0 represents the initial emission intensity when t is 0 and t means the lifetime. Influence of Temperature on emission intensity and FWHM. The thermal quenching of luminescence is one of important technological parameters to be considered for phosphor materials applied in high power LEDs, because it significantly affects the light output and service life. The temperature-dependent PL spectra of the KMg 4 (PO 4 ) 3 :0.01Eu 21 exited by 365 nm N-UV light is depicted in Figure 8. With increasing temperature (30uC-300uC), the emission intensity decreases from 100% to 74.26% of that at 30uC, and the FWHM of the emission band increases from 50.34 to 60.42 nm. The inset of Figure 8 illustrates the comparison of the thermal luminescence quenching of KMg 4 (PO 4 ) 3 :0.01Eu 21 with that of commercial BAM:Eu 21 and the FWHM of KMg 4 (PO 4 ) 3 :0.01Eu 21 emission as a function of the temperature. As shown in Figure 8, it can be found only 9% decay at 150uC for KMg 4 (PO 4 ) 3 :0.01Eu 21 , which indicates that the thermal stability of KMg 4 (PO 4 ) 3 :Eu 21 is superior to that of commercial BAM:Eu 21 below 200uC and this phosphor could be used as a promising phosphor for high-power LED application. To better understand the thermal quenching process, the configurational coordinate diagram can be used to in which I 0 and I are the luminescence intensity of KMg 4 (PO 4 ) 3 :Eu 21 at room temperature and a given temperature, respectively; A is a constant; k is the Boltzmann constant (8.617 3 10 25 eV K 21 ). From above equation, the DE is calculated to be about 0.24 eV (Figure 9). The temperature-dependent of emission FWHM is related to the configuration coordinate model and the Boltzmann distribution, and can be expressed by: 18,29 where W 0 is the FWHM at 0uC, hv represents the vibrational phonon energy, S means the Huang2Rhys parameter, and k is the Boltzmann constant. With the temperature increase, the excited electrons spread to higher vibration levels and the radiative transitions from these different levels cause the emission band broadening.
Quantum efficiency and Electroluminescence properties of White-Light LED Lamp. Quantum efficiency of phosphors is another important technological parameter for practical application. The internal quantum efficiency (QE) of KMg 4 (PO 4 ) 3 :0.01Eu 21 were measured and calculated by the following equations: 30 in which L s represents the luminescence emission spectrum of the sample; E R is the spectrum of the excitation light from the empty integrated sphere (without the sample); E S means the excitation   spectrum for exciting the sample. As given in Figure 10, the internal QE of the KMg 4 (PO 4 ) 3 :0.01Eu 21 phosphor is estimated to be about 50.44% under 365 nm excitation. As a comparison, the internal QE of commercial BAM:Eu 21 phosphor is detected at the same condition and calculated to about 88.99%. The QE of KMg 4 (PO 4 ) 3 :0.01Eu 21 can be further improved by optimization of the preparation conditions, because the QE depends closely on the prepared conditions, crystalline defects, particle size and morphology of the phosphor 31,32 .
To demonstrate the potential application of KMg 4 (PO 4 ) 3 :Eu 21 phosphor, the electroluminescent spectrum of white LED lamp which was fabricated via using N-UV LED chips (l max 5 385 nm) combing with blue-emitting KMg 4 (PO 4 ) 3 :0.01Eu 21 phosphor, green-emitting (Ba,Sr) 2 SiO 4 :Eu 21 phosphor, and red-emitting CaAlSiN 3 :Eu 21 phosphor was measured as given in Figure 11 with forward bias current of 2 mA. The CIE color coordinates, correlated color temperature (CCT) and color rendering index (Ra) of this fabricated WLED lamp are determined to be (0.34, 0.35), 5227.08 and 96, respectively. The Ra was decided from the full set of the first eight CRIs shown in Table 2. The appropriate CCT value (5227.08) and high Ra value (96) demonstrate that the KMg 4 (PO 4 ) 3 :Eu 21 can be a promising candidate for a blue-emitting phosphor for application of WLEDs.

Discussion
In conclusion, we report a systematic study on the preparation and crystal structure analysis of blue-emitting KMg 4 (PO 4 ) 3 :Eu 21 phosphor and investigate their reflectance spectra, thermal stability, quantum efficiency and applications in N-UV LED. The phase composition and crystal structure of KMg 4 (PO 4 ) 3 :Eu 21 were determined via the powder X-ray diffraction patterns and Rietveld refinement analysis. The optimal Eu 21 doping concentration in the KMg 4 (PO 4 ) 3 host is 1 mol%. The critical energy transfer distance of this phosphor was calculated to be about 35.84 Å and the concentration quenching mechanism is proved to be the dipole-dipole interaction. The investigation results also reveal that the as-prepared phosphor shows good thermal stability, and the internal quantum efficiency is 50.44%. The white N-UV LEDs packaged by an N-UV chip with blue-emitting KMg 4 (PO 4 ) 3 :Eu 21 , green and red-emitting phosphors generate white light with high color rendering index (Ra 5 96) and an appropriate correlated color temperature (5227.08 K). These results demonstrate that KMg 4 (PO 4 ) 3 :Eu 21 is a promising blue-emitting phosphor for N-UV LEDs. were weighed in stoichiometric proportions and ground homogeneously in agate mortar. Firstly, the mixtures were preheated at 600uC for 2 h in a muffle furnace in air to release NH 3 , CO 2 , and H 2 O. Then, the precursor was reground and heated at 1000uC for 6 h in the thermal carbon reducing atmosphere (TCRA). Finally the furnace cooled to room temperature and the mixtures were ground in an agate mortar.
Materials Characterization. X-ray powder diffraction (XRD) patterns of the final products were identified on a D8 Advance diffractometer (Bruker Corporation, Germany) with Cu Ka radiation (l 5 0.15406 nm) radiation. High quality XRD data for Rietveld refinement were collected by step scanning rate (8 s per step with a step size of 0.02u) over a 2h range from 5u to 100u. The photoluminescent excitation/ emission (PLE/PL) spectra were detected by a Hitachi F-4600 fluorescence spectrophotometer (Japan) equipped using a150 W Xe lamp as the excitation source. The temperature-dependent luminescence properties were measured on the same spectrophotometer which was assembled with a computer-controlled electric furnace and a self-made heating attachment. The diffuse reflectance spectra were obtained by a Varian Cary-5000 UV2vis2NIR spectrophotometer attached with an integral sphere. The room-temperature luminescence decay curves were obtained from a spectrofluorometer (Horiba, Jobin Yvon TBXPS) using a tunable pulse laser radiation (nano-LED) as the excitation. Quantum efficiency was measured by a fluoromax-4 spectrofluorometer (Horiba, Jobin Yvon) with an integral sphere at room temperature.