Intense red emitting monoclinic LaPO₄:Eu³⁺ nanoparticles: host–dopant energy transfer dynamics and photoluminescence properties

Abstract

LaPO₄ nanoparticles were synthesized using complex polymerization method and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Varied concentration of europium ion is doped in the LaPO₄ lattice and optical properties and Judd–Ofelt analysis were investigated. It is observed that LaPO₄ nanoparticles give violet–blue emission when irradiated with UV light. On doping europium ion the band gap of LaPO₄ 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 LaPO₄ so causing an efficient energy transfer from LaPO₄ 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 C₁ for La³⁺ in LaPO₄. The critical energy-transfer distance for the Eu³⁺ ions was evaluated, based on which the quenching mechanism was verified to be an electric multipolar interaction. It is also observed that the red emission intensity of LaPO₄:Eu³⁺ (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.

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1. Introduction

Lanthanide ion doped inorganic materials have attracted significant 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 fluorescence imaging,⁵ nanothermometry⁶etc.

Inorganic compounds bearing f⁰, f⁷ or f¹⁴ lanthanide ions have a negligible chance of undergoing quenching effect through cross relaxation from Ln³⁺ ion and hence they are the preferred choice as a host material.⁷ In that context LaPO₄ 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, LaPO₄ doped with cerium and terbium ion,¹⁰ has been a commercially applied green phosphor in fluorescent 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 LaPO₄ has monazite structure; where lanthanum atoms have nine-fold coordination (LaO₉). It is reported that out of nine bonds; four of the oxygen atoms are oriented in a distorted tetrahedron (PO₄) that interpenetrates a planar pentagon formed by the other five other oxygen atoms.⁸ Europium (Eu³⁺) ions are notable among lanthanides for their characteristic red emission, which is most often excited by ultraviolet (UV) light. Eu³⁺ is also a special ion as it gives symmetry-sensitive emission because of the non-degenerate ground state ⁷F₀ and non-overlapping ^2S+1L_J multiplets and the most intense transition originates from the ⁵D₀ state which is not split into sublevels due to the crystal field. 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 ΔJ = ±2 transitions are hypersensitive to small deviation from inversion symmetry. Eu³⁺ is a preferred structural probe and is used extensively for obtaining information about local symmetry, site occupancy etc.^11–17

Substantial literature reports relate to the optical properties of europium-doped LaPO₄. Chen et al.⁸ have investigated the effect of europium doping on the morphology and optical properties of LaPO₄ crystal with varied dopant concentrations whereas Wang et al.¹⁸ has studied the influence of phosphate precursor on the PL properties of LaPO₄:Eu nanoparticles. Another group¹⁹ has studied the influences of surfactants on PL properties of LaPO₄:Eu nanoparticles. Yang et al. have studied the effects of the method of synthesis on the characteristics of the LaPO₄:Eu phosphors.²⁰ Another report on the optical properties of europium-doped LaPO₄ 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 LaPO₄:Eu. There are also further reports on the PL properties of Eu³⁺-doped LaPO₄.^23–25

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 phosphors²⁶ and in deep tissue and cell imaging studies.^27–29

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 Eu³⁺ ion in LaPO₄ and covalency of Eu–O bonds as a function of Eu³⁺ 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 LaPO₄. This citrate precursor method scores over other methods because it is a relatively simple, easy and low-cost 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_1−xEu_xPO₄. Polymeric precursor derived La_1−xEu_xPO₄ 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_1−xEu_xPO₄. We have also investigated the spectroscopic properties of La_1−xEu_xPO₄ using Judd–Ofelt theory based on photoluminescence emission spectra as a function of Eu³⁺ concentration.

2. Experimental

2.1. Synthesis

La_1−xEu_xPO₄ samples (x = 0, 0.005, 0.01, 0.02 and 0.05) were synthesized by a polymeric precursor route. The starting materials were (NH₄)₂HPO₄ (99.99%, M/s Alfa Aesar, Lancaster), La₂O₃ (99.995%, Rare-earth development division, BARC, Mumbai) and Eu₂O₃ (99.999% purity supplied by Aldrich, USA). Citric acid (99.7%, M/s Chemco fine chemicals, Mumbai) was used as the complexing agent while ethylene glycol (99.0%, M/s Thomas Baker, Mumbai) was used for polyesterification to stabilize the complex. Initially the reactants La₂O₃ and Eu₂O₃ were preheated in a highly pure argon atmosphere at 1000 °C to remove moisture. Thereafter La₂O₃ was dissolved in concentrated HNO₃ 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 (NH₄)₂HPO₄ 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 Eu₂O₃ 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.

2.2. Instrumentation

X-Ray diffraction (XRD) patterns of the powdered undoped and europium-doped LaPO₄ samples were recorded using a RIGAKU Miniflex-600 diffractometer operating in Bragg–Brentano focusing geometry. Cu-Kα radiation (λ = 1.5406 Å) 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⁻¹. 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 software provided by Edinburgh Analytical Instruments, UK. A xenon flash 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 five) were taken to minimize the fluctuations 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 reflectance spectra (UV-Vis DRS) of the samples were obtained on an UV-Vis spectrophotometer (Hitachi U-3010) using BaSO₄ as reference.

2.3. 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 exchange–correlation functional. The projector augmented wave (PAW) potentials³³ 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), Eu³⁺ (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 LaPO₄ structure, optimization was carried out with respect to plane-wave cut-off energy and k-point meshes to ensure convergence of total energy to within a precision of 0.1 meV per atom. In order to study Eu³⁺-doped systems, a 2 × 2 × 1 supercell of LaPO₄ unit cell containing 96 atoms was constructed. The Brillouin-zone (BZ) integrations were performed on an optimized Monkhorst–Pack³⁴k-point grid of 10 × 10 × 8 for LaPO₄ unit cell and 6 × 6 × 8 for 2 × 2 × 1 supercell. The total energy of LaPO₄ and Eu³⁺-doped LaPO₄ 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 eV Å⁻¹ and 0.01 GPa, respectively. The final calculation of total electronic energy and density of states (DOS) were performed using the tetrahedron method with Blöchl corrections.³⁵

3. Results and discussion

3.1. XRD study

Fig. 1 shows the XRD patterns of as-prepared samples of 0.5, 1.0, 2.0 and 5.0 mol% Eu³⁺-doped LaPO₄ samples along with undoped LaPO₄. XRD patterns of samples are in agreement with the monoclinic system of pure LaPO₄ (JCPDS no. 84-0600) which reveals that all the products are monazite LaPO₄ with monoclinic structure in P2₁/n space group. The XRD patterns shown in Fig. 1 do not show any diffraction peak related to pure oxide phase such as La₂O₃ or Eu₂O₃ phase. The fact that such phases are absent is an indication of a homogeneous solid solution of LaPO₄ and Eu³⁺, which further confirms the occupancy of Eu³⁺ ions in the lattice sites of La³⁺ in LaPO₄. Although three different polymorphs of LaPO₄ exist – hexagonal, monoclinic and tetragonal, it is the monoclinic polymorph which is mostly used in commercial purposes.

Fig. 1 XRD pattern of undoped and europium-doped LaPO₄ samples annealed at 800 °C.

3.2. Transmission electron microscopy (TEM) investigations

Fig. 2 shows a representative TEM micrograph of LaPO₄ samples. From the micrograph it can be seen that lanthanum phosphate particles formed are of nano-dimensions with irregular morphology. The average diameter of the LaPO₄ particles was found to be approximately 15–30 nm. Selected area electron diffraction (SAED) pattern shows the presence of multiple sharp rings, confirming the nano-dimension and crystalline nature of the particles (inset of Fig. 2). The rings in the patterns could be successfully indexed in terms of the monoclinic phase of LaPO₄ which is in agreement with XRD results. The formation of LaPO₄ is also confirmed using the EDAX pattern shown in the left 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. 2 Representative TEM micrographs of LaPO₄ nanoparticles synthesized using the citrate precursor route. Insets are EDS pattern (left top) and selected area electron diffraction (right top).

3.3. Luminescence study

Fig. 3 shows the excitation spectra of 0.5 mol% doped Eu³⁺-doped LaPO₄ nanoparticles by monitoring the emission at 593 nm corresponding to the ⁵D₀ → ⁷F₁ 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 filled 2p shell of oxygen to a partially filled 4f shell of Eu³⁺ ion. Fig. 3 (inset) shows a magnified region of 300–450 nm in which the peaks at 322, 364, 384 and 397 nm are observed and correspond to ⁷F_0,1 → ⁵H_3,6, ⁷F_0,1 → ⁵D₄, ⁷F_0,1 → ⁵G₃ and ⁷F₀ → ⁵L₆ transitions of Eu³⁺, respectively. These peaks are relatively of weaker intensity compared to the Eu–O charge-transfer band absorption because f → f transitions are of forbidden nature.

Fig. 3 Excitation spectrum of LaPO₄:Eu³⁺ nanoparticles at an emission wavelength of 593 nm.

Fig. 4a shows the photoluminescent spectrum of undoped LaPO₄ nanoparticles with excitation at 230 nm. It consists of a strong broad band in the range of 400–600 nm peaking at 455 nm. Such prominent emission in the case of pure LaPO₄ nanoparticles at ∼455 nm is also reported by others.^8,36

Fig. 4 (a) Emission spectrum of undoped LaPO₄ nanoparticles (λ_ex – 230 nm) and (b) room-temperature EPR spectra of undoped LaPO₄.

The presence of photoluminescence in undoped LaPO₄ 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 LaPO₄. 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 LaPO₄, is confirmed using EPR. Fig. 4b shows the room-temperature EPR spectrum of LaPO₄ 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 LaPO₄ 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 re-occupied 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. 5 Schematic model of probable mechanisms responsible for observed emission in LaPO₄. In the schematic model, the upper (red) and the lower (light blue) bands are La 5d conduction bands and O 2p valence bands, respectively. The black bar below the conduction band corresponds to the defect level created by the oxygen deficiency. White and black circles correspond to excited holes and electrons, respectively.

Fig. 6 shows the emission spectra of 0.5 mol% Eu³⁺-doped LaPO₄ 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 Eu³⁺ ions from the ground level to higher levels of the 4f-manifold.

Fig. 6 Emission spectrum of LaPO₄:Eu³⁺ (0.5 mol%) at an excitation wavelength of 256 nm.

The spectrum consists of main peaks at 593 and 614 nm corresponding to ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺, respectively. Apart from these prominent peaks, we could also observe other weak emission peaks centered at 653 and 697 nm corresponding to ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions, respectively.

The transition ⁵D₀ → ⁷F₁ is magnetic dipole (MD) allowed and is not sensitive to the local site symmetry around Eu³⁺ ion. Transition ⁵D₀ → ⁷F₁ is a hypersensitive forced electric dipole (ED) transition and it has electric dipole nature due to the mixing with 4fⁿ⁻¹5d states of the opposite parity by the crystal field components. The intensity of this transition is strongly affected by local site symmetry around the Eu³⁺ ions and by the strength of electric field in the Eu³⁺ environment. The other transition ⁵D₀–⁷F₃ has a mixed ED and MD character while transition ⁵D₀–⁷F₄ 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 ⁵D₀ → ⁷F₁ transition is the strongest and is characterized by an orange–red emission whereas the ⁵D₀ → ⁷F₂ 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 ΔJ = ±2 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 (⁵D₀ → ⁷F₂) to MDT (⁵D₀ → ⁷F₁). The fact that MDT is more intense than EDT (asymmetry ratio is less than unity), indicating that the majority of Eu³⁺ are at sites with inversion symmetry.

When the Eu³⁺ ion is inserted into any chemical/ligand environment (crystal field), the (2J + 1)-degenerate J-energy levels get split into the so-called Stark sub-levels by the ligand-field effect with the number of levels depending on the local site symmetry of the metal ion. It is reported that in LaPO₄ systems, both La³⁺ as well as phosphate ions occupy C₁ point group symmetry sites.³⁸ The substitution of La³⁺ with Eu³⁺ may not result in significant lattice distortion because of identical charges, and any distortion would arise only due to size differences (103.2 pm for La³⁺ and 94.7 for Eu³⁺). From the Stark splitting pattern shown in Fig. 7, two lines for J = 0 → J = 1 transition, two lines for J = 0 → J = 2 transition, two lines for J = 0 → J = 3 transition and four lines for J = 0 → J = 4 transition of Eu³⁺ were resolved for LaPO₄:Eu³⁺ (0.5 mol%). The fact that the ⁵D₀ → ⁷F₀ 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 Eu³⁺ in LaPO₄.

Fig. 7 Stark splitting pattern of europium emission in lanthanum phosphate nanoparticles.

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 LaPO₄ host lattice and occupies La³⁺ sites only. The fitting equation is given by eqn (1):

I = I₀ + A exp(−t/τ) (1)

where I is the emission intensity at time t, and A is the initial intensity at t = 0. Here, I₀ is a constant, t is time, and τ 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.

Fig. 8 Photoluminescence decay profile of LaPO₄:Eu³⁺ (0.5 mol%) at emission and excitation wavelength of 593 and 256 nm, respectively.

3.4. Effect of dopant ion concentration on optical properties of LaPO₄:Eu

Emission spectra of Eu³⁺ activated LaPO₄ phosphors at different Eu³⁺ 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 Eu³⁺ ion concentration. However, emission intensity increases with increase in Eu³⁺ 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 Eu³⁺ ions. To have a better understanding of the concentration quenching mechanism, the critical distance (R_c) needs to be evaluated using eqn (2):⁴¹where 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 Eu³⁺ ions, and N is the number of cations per unit cell.

Fig. 9 PL emission spectra (corrected w.r.t. source) of LaPO₄:Eu³⁺ phosphors (mol% of Eu³⁺ = 0.5, 1.0, 2.0, 5.0) excited at 256 nm. The insets show the PL emission intensities of ⁵D₀–⁷F₁ as a function of Eu³⁺ ion concentration.

For the LaPO₄ host lattice, V = 304.86 ų, N = 4 (four formula units per unit cell), and X_c = 0.02. Based on this value the critical distance is calculated to be 19.37 Å. In this case, the Eu³⁺–Eu³⁺ distance is larger than 10 Å. 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 Eu³⁺ ions in LaPO₄ 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 La³⁺ 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 19.37 Å, the probability of cross-relaxation between Eu³⁺ becomes very high and thereby reduces the lifetime. There is a close correspondence between the effect of concentration on emission intensity and PL lifetimes.

Fig. 10 PL decay curves of LaPO₄:Eu³⁺ samples (Eu³⁺ mol% = 0.5, 1.0, 2.0, 5.0) (monitored at 592 nm and excited at 256 nm).

3.5. Band gap calculations: theory and experiment (DFT and DRS measurements)

LaPO₄ has a monoclinic crystal structure (P2₁/n space group, no. 14) having four formula units per unit cell. The PAW-PBE calculated lattice parameters of this phase are a = 6.9372 Å, b = 7.1455 Å, c = 6.5411 Å (V₀ = 314.98 ų) and β = 103.73° (α = γ = 90°) (Fig. 11) which matches very well with the experimentally determined values, a = 6.8313 Å, b = 7.0705 Å, c = 6.5034 Å and β = 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 LaPO₄ unit cell are matching within 1.5% of experimentally measured room-temperature values⁴² and to within 0.2% of previous GGA calculations.⁴³

Fig. 11 (a) Unit cell of LaPO₄ (24 atoms) and (b) Eu³⁺-doped 2 × 2 × 1 supercell (96 atoms) of LaPO₄.

Table 1 Comparison of PAW-PBE calculated and experimentally determined atomic positions

	Atomic positions (this study)			Atomic positions (ref. 42)
	x	y	z	x	y	z
La (4e)	0.2851	0.1589	0.0988	0.2815	0.1603	0.1006
P (4e)	0.3042	0.1618	0.6109	0.3047	0.1639	0.6121
O1 (4e)	0.2496	0.0064	0.4414	0.2503	0.0077	0.4477
O2 (4e)	0.3811	0.3293	0.4977	0.3799	0.3315	0.4964
O3 (4e)	0.4710	0.1035	0.8042	0.4748	0.1071	0.8018
O4 (4e)	0.1287	0.2129	0.7082	0.1277	0.2168	0.7101

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The PAW-PBE calculated total and angular momentum projected electronic density of states (DOS) of LaPO₄ 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 −7.5 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.

Fig. 12 PAW-PBE calculated total and angular momentum decomposed DOS of LaPO₄.

The PAW-PBE calculated total and angular momentum projected electronic DOS of Eu³⁺-doped LaPO₄ is presented in Fig. 13. The overall DOS of Eu³⁺-doped LaPO₄ resembles LaPO₄ 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 are from solely Eu d states. Moreover, the energy range over which d and f states of Eu are distributed is exactly coincident with the valence band of LaPO₄. Therefore, our DFT study predicts host (LaPO₄) to dopant (Eu³⁺) energy transfer is preferable. Impurity energy states are contributed by Eu d and f states. An impurity state reduces the band gap of Eu³⁺-doped LaPO₄ by ∼10% with respect to pure LaPO₄.

Fig. 13 PAW-PBE calculated total and angular momentum decomposed DOS of Eu³⁺-doped LaPO₄ (doping level is 6.25% at La³⁺ position).

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 LaPO₄ 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 LaPO₄ 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 LaPO₄ crystalline potential on europium doping (which has 4f valence electrons) which also influence 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.

Fig. 14 UV-Vis diffuse reflectance spectra of LaPO₄ and La_1−xEu_xPO₄.

Fig. 15 UV-Vis absorption–energy curves of LaPO₄ and La_1−xEu_xPO₄.

Table 2 Calculated band gap for undoped and europium-doped LaPO₄

Sample	Band gap/eV
LaPO4	6.15
LaPO4:Eu (0.5 mol%)	4.13
LaPO4:Eu (1.0 mol%)	4.16
LaPO4:Eu (2.0 mol%)	4.25
LaPO4:Eu (5.0 mol%)	4.35

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Fig. 16 The energy gap dependence of LaPO₄:Eu nanoparticles on the Eu doping level.

3.6. Judd–Ofelt calculation: photophysical properties

Judd–Ofelt (JO) intensity parameters, Ω_J (J = 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 Eu³⁺ emissive properties in a matrix is nicely presented by Werts et al.,⁴⁷ and also by our group^11,12,48

Since the ⁵D₀ → ⁷F₁ transition, a magnetic dipole transitions is not affected much by environmental effects, its transition rate is constant with an approximate value of 50 s⁻¹.⁴⁹ For LaPO₄ we have adopted a value for the refractive index of 1.78 for calculation. Emission quantum efficiency of the emitting ⁵D₀ level is written as eqn (3):

where the A_R rate was obtained by summing over the radiative rates for each ⁵D₀ → ⁷F_J (J = 1–4). The JO parameter and other photophysical values are listed in Table 3.

Table 3 Photophysical properties Eu³⁺-doped LaPO₄

Eu^3+ mol%	A R/s^−1	A NR/s^−1	η (%)	Ω 2/10^−20 cm^2	Ω 4/10^−20 cm^2
0.5	179	85	87.7	0.919	3.17
1.0	210	72	88.4	0.962	3.26
2.0	269	63	91.2	1.25	3.72
5.0	234	69	89.3	1.13	3.45

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It is reported that the Ω₂ parameter is related to the symmetry and the Ω₄ parameter to long-range effects and electronic density surrounding the rare earth ion.^50–52 It was observed that at all concentrations of Eu³⁺ the Ω₄ value is greater than Ω₂ indicating the existence of a symmetric environment around europium ions in LaPO₄ which is highly probable because it occupies La³⁺ sites (no charge imbalance). This is also reflected in the intense MDT compared to EDT in our emission spectra. Also D_2d point symmetry is observed for europium ion in LaPO₄ which is relatively higher symmetry. It can also be seen from Table 3 that the Ω₂ value increases with increase in Eu³⁺ concentration up to 2.0 mol% and thereafter 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 Ω₄ is not directly related to symmetry around Eu³⁺ 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 Eu³⁺ 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 (⁵D₀ → ⁷F₂) to magnetic dipole (⁵D₀ → ⁷F₁), the asymmetry ratio (I) in Eu³⁺, is related to the degree of covalency between Eu³⁺ and O²⁻.⁵³ The larger the difference in the intensities of these two lines, the larger is the asymmetry and covalency effects. For all the samples ⁵D₀ → ⁷F₁ is markedly more intense as compared with the ⁵D₀ → ⁷F₂ line i.e. R/O (red to orange ratio) shows a marked decrease, indicating high symmetry around the Eu³⁺ ion site, as also reflected by the JO parameters.

4. Potential application of Eu³⁺-doped LaPO₄ as a luminescent material

To evaluate the material performance for phosphor applications, CIE chromaticity coordinates were evaluated for the LaPO₄: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 LaPO₄: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 LaPO₄:Eu³⁺ (2.0 mol%) and red phosphor (Y₂O₃: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 LaPO₄:Eu³⁺ (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.

Fig. 17 CIE diagram showing the co-ordinates and representing the color emitted by LaPO₄:Eu³⁺.

Fig. 18 Comparison of emission spectra of commercial red phosphor (Y₂O₃:Eu from Nichia Corporation, Japan) and LaPO₄:Eu³⁺ (2.0 mol%) under 395 nm excitation. The comparison is made on corrected spectra and integral intensity of all the peak is considered.

5. Conclusions

Monoclinic LaPO₄ 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 LaPO₄ 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 LaPO₄ 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 LaPO₄ 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 Eu³⁺ are at relatively symmetric sites. This is also reflected in the JO parameters; at all concentrations Ω₄ value is greater than Ω₂ indicating the existence of low covalency between Eu³⁺ and ligand and symmetric environment around europium ion in LaPO₄, which is highly probable because Eu³⁺ occupies La³⁺ sites (no charge imbalance). The point symmetry of Eu³⁺ in LaPO₄ is D_2d as calculated using the Stark splitting pattern. It was seen that LaPO₄: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.

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