Intense red emitting monoclinic LaPO 4 :Eu 3+ nanoparticles: host – dopant energy transfer dynamics and photoluminescence properties

LaPO 4 nanoparticles were synthesized using complex polymerization method and characterized systematically using X-ray di ﬀ raction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Varied concentration of europium ion is doped in the LaPO 4 lattice and optical properties and Judd – Ofelt analysis were investigated. It is observed that LaPO 4 nanoparticles give violet – blue emission when irradiated with UV light. On doping europium ion the band gap of LaPO 4 decreases. Based on DFT calculations it is proposed that energy range over which d and f states of Eu are distributed is coincident with the valence band of LaPO 4 so causing an e ﬃ cient energy transfer from LaPO 4 to europium ion. The actual site symmetry for europium ion in lanthanum orthophosphate was also evaluated as D 2d based on the Stark splitting pattern although it is C 1 for La 3+ in LaPO 4 . The critical energy-transfer distance for the Eu 3+ ions was evaluated, based on which the quenching mechanism was veri ﬁ ed to be an electric multipolar interaction. It is also observed that the red emission intensity of LaPO 4 :Eu 3+ (2.0 mol%) is almost 85% of a commercial red phosphor, which clearly demonstrates that the as-prepared samples are promising red phosphors under near-UV for use in white-light emitting diodes.


Introduction
Lanthanide ion doped inorganic materials have attracted signicant amount of attention in all areas of science and technology because of their interesting physico-chemical properties and potential applications in many new areas such as drug carriers, 1 telecommunication, 2 photovoltaics and photocatalysis, 3 white light application, 4 contrast agents for MRI and uorescence imaging, 5 nanothermometry 6 etc.
Inorganic compounds bearing f 0 , f 7 or f 14 lanthanide ions have a negligible chance of undergoing quenching effect through cross relaxation from Ln 3+ ion and hence they are the preferred choice as a host material. 7 In that context LaPO 4 is good luminescence host. Also because of its other desirable properties such as low toxicity, environmental stability, ability to accommodate large lanthanide ions, high refractive index and high thermal and chemical stability, it is extensively used as a host matrix. 8,9 In fact, LaPO 4 doped with cerium and terbium ion, 10 has been a commercially applied green phosphor in uorescent lamps. Lanthanum orthophosphate has three different polymorphs; hexagonal, monoclinic and tetragonal. Because the tetragonal phase is stable at high temperature and the hexagonal phase is stable at low temperature, they are not preferred choices for a luminescence host matrix, and it is the monoclinic form which is mainly explored for lanthanide-based photoluminescence (PL). Monoclinic LaPO 4 has monazite structure; where lanthanum atoms have nine-fold coordination (LaO 9 ). It is reported that out of nine bonds; four of the oxygen atoms are oriented in a distorted tetrahedron (PO 4 ) that interpenetrates a planar pentagon formed by the other ve other oxygen atoms. 8 Europium (Eu 3+ ) ions are notable among lanthanides for their characteristic red emission, which is most oen excited by ultraviolet (UV) light. Eu 3+ is also a special ion as it gives symmetry-sensitive emission because of the nondegenerate ground state 7 F 0 and non-overlapping 2S+1 L J multiplets and the most intense transition originates from the 5 D 0 state which is not split into sublevels due to the crystal eld. Furthermore, in a crystal site with inversion symmetry the electric dipole transitions (EDT) are strictly forbidden and the magnetic dipole transitions (MDT) are usually the most intense emission peaks, while for a site without inversion symmetry EDT is usually the strongest emission line, because DJ ¼ AE2 transitions are hypersensitive to small deviation from inversion symmetry. Eu 3+ is a preferred structural probe and is used extensively for obtaining information about local symmetry, site occupancy etc. [11][12][13][14][15][16][17] Substantial literature reports relate to the optical properties of europium-doped LaPO 4 . Chen et al. 8 20 Another report on the optical properties of europium-doped LaPO 4 nanorods and nanoparticles also exists in the literature. 21 Lei et al. 22 have explored the effect of lithium ion co-doping on the PL properties of LaPO 4 :Eu. There are also further reports on the PL properties of Eu 3+ -doped LaPO 4 . [23][24][25] The most general method of synthesizing a phosphor material is doping different lanthanide ion into suitable inorganic hosts depending on the requirements. But in some cases even without doping any activator ion; hosts show intense self activated emission and subsequently can transfer their excitation energy to activator ions when doped. Therefore, energy transfer from dopant to host has become an effective route to enhance the emission intensity of activators. Some of the recent reports on application of host-dopant energy transfer, where defects play a prominent role, are in NIR phosphorescent phosphors 26 and in deep tissue and cell imaging studies. [27][28][29] None of these previous studies discuss about the origin of the violet-blue emission in lanthanum phosphate nanoparticles or explained the energy transfer mechanism on europium doping. Moreover none of these studies discuss about the local site symmetry around Eu 3+ ion in LaPO 4 and covalency of Eu-O bonds as a function of Eu 3+ concentration.
Doping, which forms the basis of most photonic applications, is best achieved through a wet chemical method such as sol-gel, combustion, use of citrate precursor, microemulsion method. We have employed a citrate precursor route for the synthesis of LaPO 4 . This citrate precursor method scores over other methods because it is a relatively simple, easy and lowcost technique employing common chemicals with good stoichiometric and particle size control.
In this work we have presented a combined experimental and theoretical study on the optical properties of La 1Àx Eu x PO 4 . Polymeric precursor derived La 1Àx Eu x PO 4 are characterized by X-ray diffraction (XRD), photoluminescence spectroscopy (PL) and ultraviolet-visible absorption analysis (UV-Vis). An effort was made to correlate the experimental and theoretical results for host-dopant energy transfer. We have studied the effect of europium ion concentration on photoluminescence properties and optical absorption spectroscopy of La 1Àx Eu x PO 4 . We have also investigated the spectroscopic properties of La 1Àx Eu x PO 4 using Judd-Ofelt theory based on photoluminescence emission spectra as a function of Eu 3+ concentration.

Synthesis
La 1Àx Eu x PO 4 samples (x ¼ 0, 0.005, 0.01, 0.02 and 0.05) were synthesized by a polymeric precursor route. The starting materials were (NH 4 ) 2 HPO 4 (99.99%, M/s Alfa Aesar, Lancaster), La 2 O 3 (99.995%, Rare-earth development division, BARC, Mumbai) and Eu 2 O 3 (99.999% purity supplied by Aldrich, USA). Citric acid (99.7%, M/s Chemco ne chemicals, Mumbai) was used as the complexing agent while ethylene glycol (99.0%, M/s Thomas Baker, Mumbai) was used for polyesterication to stabilize the complex. Initially the reactants La 2 O 3 and Eu 2 O 3 were preheated in a highly pure argon atmosphere at 1000 C to remove moisture. Thereaer La 2 O 3 was dissolved in concentrated HNO 3 followed by addition of citric acid. Citric acid was added to maintain the fuel to oxidant ratio as unity. The solution was kept on the hot plate at 80 C and the required amount of (NH 4 ) 2 HPO 4 was added. Ethylene glycol was then added and the mixture stirred. The citric acid and ethylene glycol were added in the mass ratio of 6 : 4. The solution was kept overnight for stable polyester formation. For the Eu-doped samples, the required amount of Eu 2 O 3 was also added in the nitric acid dissolution step. Upon evaporation of the reaction solution a transparent polymeric gel was formed. The gel was then burned at 500 C in a preheated furnace for 1 h. The obtained foamy carbonaceous precursor was then ground, pelletised and kept at 800 C for 6 h.

Instrumentation
X-Ray diffraction (XRD) patterns of the powdered undoped and europium-doped LaPO 4 samples were recorded using a RIGAKU Miniex-600 diffractometer operating in Bragg-Brentano focusing geometry. Cu-Ka radiation (l ¼ 1.5406Å) was used as the X-ray source and the operating voltage and current of the instrument was kept at 40 kV and 30 mA, respectively. The XRD patterns were collected with scan rate of 1 min À1 . PL data were recorded on an Edinburgh CD-920 unit equipped with M 300 monochromators. The data acquisition and analysis were done by F-900 soware provided by Edinburgh Analytical Instruments, UK. A xenon ash lamp with frequency range of 10-100 Hz was used as the excitation source. Emission spectra for samples were recorded with a lamp frequency of 100 Hz. Multiple scans (at least ve) were taken to minimize the uctuations in peak intensity and to maximize S/N ratio. Fluorescence lifetime measurements were based on well established time-correlated single-photon counting (TCSPC) technique. Samples for TEM were prepared by suspending particles in a non-aqueous medium. To ensure uniform distribution samples were subjected to ultrasonication for 20 min and a few drops from the liquid were allowed to settle on 200 micron carbon coated copper grids. The grids were allowed to dry under vacuum for a few hours prior to loading in a transmission electron microscope (TEM). A Technai T-20 instrument of FEI was used for all TEM investigation. EPR spectra were recorded on a Bruker ESP-300 spectrometer operating at X-band microwave frequency (9.4186 GHz). Diphenyl picrylhydrazyl (DPPH) was used for the calibration of g-values of paramagnetic species. UV-Vis diffuse reectance spectra (UV-Vis DRS) of the samples were obtained on an UV-Vis spectrophotometer (Hitachi U-3010) using BaSO 4 as reference.

Computational methodology
All calculations in this study are based on density functional theory (DFT) in conjunction with projector augmented wave (PAW) potentials, which is implemented in the plane wave based Vienna Ab initio Simulation Package (VASP). 30,31 The generalized gradient approximation (GGA) parameterized by Perdew-Burke-Ernzerhof (PBE) 32 was used as the exchangecorrelation functional. The projector augmented wave (PAW) potentials 33 were used for the ion-electron interactions including the valence states of La (5s, 5p, 5f -11 valence electrons), P (3s, 3p -5 valence electrons), Eu 3+ (5p, 6s, 5d -9 valence electrons) and O (2s, 2p -6 valence electrons). In our calculations, the Kohn-Sham single-particle wave functions were expanded in a plane-wave basis with kinetic energy cutoff of 500 eV and it was shown that the results were well converged at this cut off. For monoclinic LaPO 4 structure, optimization was carried out with respect to plane-wave cut-off energy and kpoint meshes to ensure convergence of total energy to within a precision of 0.1 meV per atom. In order to study Eu 3+ -doped systems, a 2 Â 2 Â 1 supercell of LaPO 4 unit cell containing 96 atoms was constructed. The Brillouin-zone (BZ) integrations were performed on an optimized Monkhorst-Pack 34 k-point grid of 10 Â 10 Â 8 for LaPO 4 unit cell and 6 Â 6 Â 8 for 2 Â 2 Â 1 supercell. The total energy of LaPO 4 and Eu 3+ -doped LaPO 4 were optimized with respect to volume (or lattice parameter, b/a, c/a ratio), angle and atomic positions. The structural relaxations (b/a, c/a ratio and atomic positions) were performed for each structure using the conjugate gradient algorithm until the residual forces and stress in the equilibrium geometry were of the order of 0.005 eVÅ À1 and 0.01 GPa, respectively. The nal calculation of total electronic energy and density of states (DOS) were performed using the tetrahedron method with Blöchl corrections. 35  the monoclinic system of pure LaPO 4 (JCPDS no. 84-0600) which reveals that all the products are monazite LaPO 4 with monoclinic structure in P2 1 /n space group. The XRD patterns shown in Fig. 1 do not show any diffraction peak related to pure oxide phase such as La 2 O 3 or Eu 2 O 3 phase. The fact that such phases are absent is an indication of a homogeneous solid solution of LaPO 4 and Eu 3+ , which further conrms the occupancy of Eu 3+ ions in the lattice sites of La 3+ in LaPO 4 . Although three different polymorphs of LaPO 4 existhexagonal, monoclinic and tetragonal, it is the monoclinic polymorph which is mostly used in commercial purposes.  Fig. 2). The rings in the patterns could be successfully indexed in terms of the monoclinic phase of LaPO 4 which is in agreement with XRD results. The formation of LaPO 4 is also conrmed using the EDAX pattern shown in the le inset. The intense copper line arises from the copper-grid used in sample preparation while L and M lines of La and K lines of P and O are also present. Fig. 3 shows the excitation spectra of 0.5 mol% doped Eu 3+doped LaPO 4 nanoparticles by monitoring the emission at 593 nm corresponding to the 5 D 0 / 7 F 1 transition of europium ion. In the spectra a broad band centred at 256 nm is observed which is attributed to a charge transition state (CTS) arising due to transfer of an electron from the lled 2p shell of oxygen to a partially lled 4f shell of Eu 3+ ion. Fig. 3 (inset) shows a magnied region of 300-450 nm in which the peaks at 322, 364, 384 and 397 nm are observed and correspond to 7 F 0,1 / 5 H 3,6 ,   The presence of photoluminescence in undoped LaPO 4 reveals the presence of defects such as oxygen vacancies, lanthanum interstitials or dangling bonds which can be created during the synthesis procedure. Out of all these, oxygen vacancies are most likely candidate responsible for visible emission in LaPO 4 . Different types of oxygen vacancies (neutral, singly ionized or doubly ionized) could be responsible for such an intense emission without any activator ion. The presence of oxygen defects, which is responsible for visible emission in LaPO 4 , is conrmed using EPR. Fig. 4b shows the roomtemperature EPR spectrum of LaPO 4 which shows an intense peak at g $ 1.9684 which is typical of a singly ionized oxygen vacancy. 37 Such intense violet-blue emission can arise in undoped LaPO 4 from transitions of an electron which is formed during photon irradiation near to the conduction band (CB) to single ionized oxygen vacancy centres (V O + ). Emission in such a case arises from a recombination process wherein an excited electron of the conduction band (CB) loses its energy and reoccupied the energy levels of an electron hole in the valence band (VB) through the localized defect levels. The schematic of defect induced emission is shown in Fig. 5. Fig. 6 shows the emission spectra of 0.5 mol% Eu 3+ -doped LaPO 4 nanoparticles when excited with 256 nm radiation corresponding to the charge-transfer state. As can be seen, it consists of various sharp lines due to the direct excitation of the Eu 3+ ions from the ground level to higher levels of the 4f-manifold.

Luminescence study
The spectrum consists of main peaks at 593 and 614 nm corresponding to 5 D 0 / 7 F 1 and 5 D 0 / 7 F 2 transitions of Eu 3+ , respectively. Apart from these prominent peaks, we could also observe other weak emission peaks centered at 653 and 697 nm corresponding to 5 D 0 / 7 F 3 and 5 D 0 / 7 F 4 transitions, respectively.
The transition 5 D 0 / 7 F 1 is magnetic dipole (MD) allowed and is not sensitive to the local site symmetry around Eu 3+ ion.   This journal is © The Royal Society of Chemistry 2015 Transition 5 D 0 / 7 F 1 is a hypersensitive forced electric dipole (ED) transition and it has electric dipole nature due to the mixing with 4f nÀ1 5d states of the opposite parity by the crystal eld components. The intensity of this transition is strongly affected by local site symmetry around the Eu 3+ ions and by the strength of electric eld in the Eu 3+ environment. The other transition 5 D 0 -7 F 3 has a mixed ED and MD character while transition 5 D 0 -7 F 4 is an ED transition.
The absence of broad host emission due to surface defects in emission spectra of the doped sample indicate incomplete energy transfer at 0.5 mol% of dopant ion concentration.
Among all the transitions observed, the 5 D 0 / 7 F 1 transition is the strongest and is characterized by an orange-red emission whereas the 5 D 0 / 7 F 2 transition is characterized by red emission. Generally in a crystal site with inversion symmetry the EDT are strictly forbidden and the MDT are usually the most intense emission peak whereas in a site without inversion symmetry EDT is usually the strongest emission line, because DJ ¼ AE2 transitions are hypersensitive to small deviations from inversion symmetry. A sensitive parameter for understanding symmetry is the asymmetry ratio (I) which is described as ratio of integral intensities of EDT ( 5 D 0 / 7 F 2 ) to MDT ( 5 D 0 / 7 F 1 ). The fact that MDT is more intense than EDT (asymmetry ratio is less than unity), indicating that the majority of Eu 3+ are at sites with inversion symmetry.
When the Eu 3+ ion is inserted into any chemical/ligand environment (crystal eld), the (2J + 1)-degenerate J-energy levels get split into the so-called Stark sub-levels by the ligand-eld effect with the number of levels depending on the local site symmetry of the metal ion. It is reported that in LaPO 4 systems, both La 3+ as well as phosphate ions occupy C 1 point group symmetry sites. 38 The substitution of La 3+ with Eu 3+ may not result in signicant lattice distortion because of identical charges, and any distortion would arise only due to size differences (103.2 pm for La 3+ and 94.7 for Eu 3+ ). From the Stark splitting pattern shown in Fig. 7, two lines for J ¼ 0 / J ¼ 1 transition, two lines for J ¼ 0 / J ¼ 2 transition, two lines for J ¼ 0 / J ¼ 3 transition and four lines for J ¼ 0 / J ¼ 4 transition of Eu 3+ were resolved for LaPO 4 :Eu 3+ (0.5 mol%). The fact that the 5 D 0 / 7 F 0 transition is absent rules out the  possibility of the presence of C 4v symmetry. 39,40 Therefore the other possibility is D 2d site symmetry of Eu 3+ in LaPO 4 . As far as PL decay behavior is concerned, the 0.5 mol% doped sample shows monoexponential behavior (Fig. 8) which indicates that europium ion is uniformly distributed in the LaPO 4 host lattice and occupies La 3+ sites only. The tting equation is given by eqn (1): where I is the emission intensity at time t, and A is the initial intensity at t ¼ 0. Here, I 0 is a constant, t is time, and s is the lifetime. The lifetime value obtained in this case is 4.89 ms, which are typically larger than that for most europium-doped phosphor materials, and thus could be a potential material for optical applications.

Effect of dopant ion concentration on optical properties of LaPO 4 :Eu
Emission spectra of Eu 3+ activated LaPO 4 phosphors at different Eu 3+ concentrations with excitation wavelength of 256 nm are shown in Fig. 9. It can be seen that there is not much change in the emission spectral features as a function of Eu 3+ ion concentration. However, emission intensity increases with increase in Eu 3+ ion concentration reaching a maximum at 2.0 mol%. Beyond 2.0% the intensity decreases due to concentration quenching, this resulting from resonant energy transfer between Eu 3+ ions. To have a better understanding of the concentration quenching mechanism, the critical distance (R c ) needs to be evaluated using eqn (2): 41 where R c is the critical distance between the dopant ion and quenching site, V is the volume of the unit cell, X c is the critical concentration of Eu 3+ ions, and N is the number of cations per unit cell. For the LaPO 4 host lattice, V ¼ 304.86Å 3 , N ¼ 4 (four formula units per unit cell), and X c ¼ 0.02. Based on this value the critical distance is calculated to be 19.37Å. In this case, the Eu 3+ -Eu 3+ distance is larger than 10Å. Thus exchange interactions can be ruled out. Therefore, the electric multipolar interaction is believed to be the only mode for the energy transfer among the Eu 3+ ions in LaPO 4 phosphor.
The effect of dopant ion concentration on lifetime value was also investigated and is shown in Fig. 10. It can be seen that all the samples exhibit monoexponential behavior indicating uniform distribution of europium at La 3+ sites. The lifetime values increase up to 2.0 mol% and beyond that decreases due to concentration quenching. The lifetime values are 4.89, 5.77, 6.23, 6.08 ms for 0.5, 1.0, 2.0, 5.0 mol% europium, respectively. At higher concentration, when the critical distance reaches the value of 19.37Å, the probability of cross-relaxation between Eu 3+ becomes very high and thereby reduces the lifetime. There is a close correspondence between the effect of concentration on emission intensity and PL lifetimes.

Band gap calculations: theory and experiment (DFT and DRS measurements)
LaPO 4 has a monoclinic crystal structure (P2 1 /n space group, no. 14) having four formula units per unit cell. The PAW-PBE calculated lattice parameters of this phase are a ¼ 6.9372Å, b ¼ 7.1455Å, c ¼ 6.5411Å (V 0 ¼ 314.98Å 3 ) and b ¼ 103.73 (a ¼ g ¼ 90 ) (Fig. 11) which matches very well with the experimentally determined values, a ¼ 6.8313Å, b ¼ 7.0705Å, c ¼ 6.5034Å and b ¼ 103.27 . 42 The calculated relaxed atomic positions are compared with experimentally determined atomic positions in Table 1. All the calculated structural parameters of the LaPO 4 unit cell are matching within 1.5% of experimentally measured room-temperature values 42 and to within 0.2% of previous GGA calculations. 43 The PAW-PBE calculated total and angular momentum projected electronic density of states (DOS) of LaPO 4 is presented in Fig. 12. From Fig. 12 it can be noted that the conduction band from 4.5 to 4.8 eV is formed primarily by the La 5d and f states. The valence band is located from À7.5 eV to 0. Besides the small contribution from La s, d and f states, the main contribution of the valence band arises from the O 2p states and P 3s, 3p states due to the strong hybridization effects. It is evident from Fig. 12 that the top of the valence band is formed predominantly by p states of oxygen with admixture of small amounts of La d and f states. The lower part of the valence band is mainly s and p state of P, p states of O along with very small contribution of La d and f states.
The PAW-PBE calculated total and angular momentum projected electronic DOS of Eu 3+ -doped LaPO 4 is presented in Fig. 13. The overall DOS of Eu 3+ -doped LaPO 4 resembles LaPO 4 DOS except for the impurity states very near to the Fermi level. The major contribution of Eu atom to the valence band is from d as well as f states and contributions to the conduction band   Fig. 14 and 15 illustrate the UV-Vis absorption spectra and absorption-energy curves (according to the Kubelka-Munk function), respectively, of undoped and europium-doped LaPO 4 nanoparticles. It can be seen from the spectra that there is an absorption edge at around 252 nm which appears only on europium doping. Band gap values at different europium concentrations according to the KM spectra are listed in Table 2. It was observed that the band gap value of LaPO 4 becomes narrower on europium doping and this phenomenon is known as band gap narrowing (BGN), which is in accordance with DFT calculations, and can be easily seen from Fig. 16. BGN is related to the change in the nature and strength of LaPO 4 crystalline potential on europium doping (which has 4f valence electrons) which also inuence its electronic level. 44 Upon doping even at a low level of 0.5 mol% the impurity band broadens and merges with the bottom of the conduction band and thereby decreases the band gap of the material. With increase in europium concentration beyond 0.5% there is essentially no further change in band gap value.

Judd-Ofelt calculation: photophysical properties
Judd-Ofelt (JO) intensity parameters, U J (J ¼ 2, 4) calculated using Judd-Ofelt theory 45,46 provide information about the local environment in the proximity of lanthanide ion. The application of JO theory to the quantitative analysis of Eu 3+ emissive properties in a matrix is nicely presented by Werts et al., 47 and also by our group 11,12,48 Since the 5 D 0 / 7 F 1 transition, a magnetic dipole transitions is not affected much by environmental effects, its transition rate is constant with an approximate value of 50 s À1 . 49 For LaPO 4 we have adopted a value for the refractive index of 1.78 for calculation. Emission quantum efficiency of the emitting 5 D 0 level is written as eqn (3): where the A R rate was obtained by summing over the radiative rates for each 5 D 0 / 7 F J (J ¼ 1-4). The JO parameter and other photophysical values are listed in Table 3. It is reported that the U 2 parameter is related to the symmetry and the U 4 parameter to long-range effects and electronic density surrounding the rare earth ion. [50][51][52] It was observed that at all concentrations of Eu 3+ the U 4 value is greater than U 2 indicating the existence of a symmetric environment around europium ions in LaPO 4 which is highly probable because it occupies La 3+ sites (no charge imbalance). This is also reected in the intense MDT compared to EDT in our emission spectra. Also D 2d point symmetry is observed for europium ion in LaPO 4 which is relatively higher symmetry. It can also be seen from Table 3 that the U 2 value increases with increase in Eu 3+ concentration up to 2.0 mol% and thereaer decreases. This reveals that with increase in europium ion concentration the covalency of the M-L bond increases as does the distortion.
The value of U 4 is not directly related to symmetry around Eu 3+ but it does depend on electron density around the ligands. The fact that its value also increases with increase in dopant ion concentration indicates that the electron density on the ligand decreases.  Form Table 3 it can also be seen that, with increase in Eu 3+ ion concentration up to 2.0 mol% that the radiative transition rate (A R ) increases and non-radiative rate (A NR ) decreases. This is matching with our emission spectral data where concentration quenching was observed beyond 2.0 mol% due to cross relaxation between europium ions. The ratio of intensity of electric dipole ( 5 D 0 / 7 F 2 ) to magnetic dipole ( 5 D 0 / 7 F 1 ), the asymmetry ratio (I) in Eu 3+ , is related to the degree of covalency between Eu 3+ and O 2À . 53 The larger the difference in the intensities of these two lines, the larger is the asymmetry and covalency effects. For all the samples 5 D 0 / 7 F 1 is markedly more intense as compared with the 5 D 0 / 7 F 2 line i.e. R/O (red to orange ratio) shows a marked decrease, indicating high symmetry around the Eu 3+ ion site, as also reected by the JO parameters.

Potential application of Eu 3+doped LaPO 4 as a luminescent material
To evaluate the material performance for phosphor applications, CIE chromaticity coordinates were evaluated for the LaPO 4 :Eu (2.0 mol%) sample adopting standard procedures. This is represented as the point '*' in the CIE diagram shown in Fig. 17. It is clear from the values that LaPO 4 :Eu shows red emission with very high quantum efficiency of 91.2%. This is because of the fact that non-radiative losses are very low in this particular system. For the application of red phosphors in white LEDs with near UV light as the excitation source, the emission spectra of LaPO 4 :Eu 3+ (2.0 mol%) and red phosphor (Y 2 O 3 :Eu from Nichia Corporation, Japan) were compared using the same excitation wavelength of 395 nm (near UV light) (Fig. 18). It is observed that the red emission intensity of LaPO 4 :Eu 3+ (2.0 mol%) is almost 85% of the commercial red phosphor, which clearly demonstrates that the as-prepared samples are a promising red phosphor under near-UV radiation for white light emitting diodes.

Conclusions
Monoclinic LaPO 4 nanoparticles of 20-30 nm size were synthesized using complex polymerization method and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. XRD shows the formation of the pure phase    of LaPO 4 and europium doping does not distort the crystal structure. The crystallinity and purity of the sample is further studied using EDAX and SAED. Based on PL and EPR it was inferred that intense violet-blue emission in the undoped sample is due to transitions of electrons which are formed during photon irradiation near to the conduction band (CB) to generate single ionized oxygen vacancy centres in LaPO 4 nanoparticles. On europium doping, energy is completely transferred from the host to dopant ion, as also predicted using DFT calculations. Europium doping reduces the band gap of LaPO 4 by almost 10%. Effects of europium ion on PL intensity, lifetime and photophysical properties were studied extensively.
MDT is more intense than EDT in all samples indicating that the majority of Eu 3+ are at relatively symmetric sites. This is also reected in the JO parameters; at all concentrations U 4 value is greater than U 2 indicating the existence of low covalency between Eu 3+ and ligand and symmetric environment around europium ion in LaPO 4 , which is highly probable because Eu 3+ occupies La 3+ sites (no charge imbalance). The point symmetry of Eu 3+ in LaPO 4 is D 2d as calculated using the Stark splitting pattern. It was seen that LaPO 4 :Eu gives red emission with very high quantum efficiency of 91.2% for a 2.0 mol% doped sample. It is almost comparable to commercial red phosphor in terms of its efficiency indicating its potential for use as a red emitter in future LEDs.