Low Temperature Synthesis of Luminescent RE 2 O 3 : Eu 3 + Nanomaterials Using Trimellitic Acid Precursors

[RE(TLA)∙(H2O)n:Eu] (RE: Y, Gd and Lu; TLA: trimellitic acid) precursor complexes were synthesized by an one step aqueous co-precipitation method. After annealing for 1 h, RE2O3:Eu nanophosphors were formed through the benzenetricarboxylate low temperature thermolysis method (500-1000 °C). The compounds were characterized by using different techniques [elemental analysis (CHN), Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG/DTG), X-ray powder diffraction (XPD) and scanning electron microscope (SEM)]. The XPD data indicated that the Y2O3:Eu materials have crystallite size range from 11 to 62 nm. The SEM and transmission electron microscopy (TEM) images show that the annealed materials keep morphological similarities with the precursor complexes. The photoluminescence properties were studied based on the excitation and emission spectra, and luminescence decay lifetimes of the D0 emitting level of the Eu ion. The experimental intensity parameters (Ωλ), lifetimes (τ), as well as radiative (Arad) and non-radiative (Anrad) decay rates were calculated and discussed. The RE2O3:Eu phosphors (RE: Y and Lu) annealed at 500 to 1000 °C have emission quantum efficiency (intrinsic quantum yield) values from 60 to 82%, indicating that this material can be potentially used for optical markers applications.


Introduction
Polycarboxylate ligands have a wide variety of structure providing large range of chemical properties when combined with metal ions.It has been drawing the attention in the areas such as metal framework systems (MOF), 1,2 selective markers for medical applications, 3 magnetic materials, 4 gas storage, 5 drug delivery, 6 precursors for materials, 7 etc.Rare earth (RE) containing materials show a versatility for application in the areas of science and technology specially in catalysis, permanent magnets in hybrid cars batteries, 8,9 electroluminescent materials, persistent phosphors, structural probes, luminescent markers, display panels, etc. [10][11][12][13][14][15] Most of those applications are consequence of their intrinsic characteristic: sharp intraconfigurational 4f N transitions, archiving high monochromatic emission colors and a wide range of emissions, from infrared to ultraviolet, 16 e.g., Nd 3+ , Eu 3+ , Gd 3+ , Tb 3+ and Tm 3+ ions which emit in the infrared, red, ultraviolet, green and blue regions, respectively.
One very important feature of the RE 3+ is their 4f-4f transitions, forbidden by the Laporte's rule.Associated to that, the shielding from the chemical environment by the filled 5s and 5p sub-shells 17 over the 4f electrons lead to a characteristic sharp lines spectra with small absorptivity and emission intensities.Taking into account the RE 3+ intraconfigurational transitions, these ions can be divided in four groups depending on their spectroscopic features: (i) Sc 3+ (3d 0 ), Y 3+ (4d 0 ), La 3+ (4f 0 ) and Lu 3+ (4f 14 ) where the 4f electrons are non-optically active due to their completely empty or fully occupied subshells; 16 (ii) Gd 3+ (4f 7 ) is a singular case due to its half-filled 4f layer, and therefore very stable.The energy difference between the lower emitting level ( 6 P 7/2 ) and the fundamental level ( 8 S 7/2 ) is approximately 32000 cm −1 opening the opportunity for its application as inorganic matrices.Due to the chemical similarity with other RE 3+ ions, it is extensively used to study the emission of the ligands in coordination complexes; (iii) Sm 3+ (4f 5 ), Eu 3+ (4f 6 ), Tb 3+ (4f 8 ) and Dy 3+ (4f 9 ): in these ions, the energy gap between the emitting and the lower levels are large enough to reduce the non-radiative decay process and accept energy from the ligands, interconfigurational transitions or charge transfer bands excited levels (Figure 1); (iv) Ce 3+ (4f 1 ), Pr 3+ (4f 2 ), Nd 3+ (4f 3 ), Ho 3+ (4f 10 ), Er 3+ (4f 11 ), Tm 3+ (4f 12 ) and Yb 3+ (4f 13 ): in these ions the energy gap between the emitting and lower levels are small, increasing the non-radiative decay process usually mediated by high energy vibrational modes in ligands (typically water molecules) or matrices (oxycarbonates, hydroxides, etc.).In these cases, the process accounts for the decreasing in the final emission efficiency.
To overcome the small absorptivity coefficients, luminescence sensitizers can be used to absorb and transfer the energy efficiently to the RE ions, keeping their desirable atomic characteristics.This phenomenon is a key feature in design of luminescent materials. 16,18,19n inorganic matrices such as vanadates, molybdates, tungstates and sesquioxides containing RE 3+ ion, generally is observed an efficient energy transfer from the ligand metal charge transfer (LMCT) band to the metal ions.In the special case, the Eu 3+ ion shows a high absorption intensity arising from the allowed LMCT transition, yielding a high intensity luminescence. 20n solid state reactions, typically, is necessary high temperatures and long reaction time periods to prepare luminescent materials.This way to synthesize materials is known as ceramic method, which promotes heterogeneous distribution of the activator ion within the matrix and generate materials with high crystallite and particle sizes.Alternative methods to obtain materials in milder reaction conditions as: sol-gel, combustion or Pechini methods, 21,22 are key to overcome the experimental limitation and improve their properties.
This report demonstrate the synthesis, characterization and optical properties of [RE(TLA):Eu 3+ (x mol%)] complexes (RE 3+ : Y, Gd and Lu; x: 0.1, 0.5, 1.0, and 5.0 mol%) and their low temperature annealing into the high luminescent RE 2 O 3 :Eu 3+ phosphors.All the precursor complexes and resulting nanophosphors were characterized by elemental analysis (CHN), Fourier transform infrared (FTIR), thermogravimetry (TG), derivative thermogravimetry (DTG), X-ray powder diffraction (XPD) and scanning electron microscopy (SEM).The photoluminescence properties of the doped materials were studied based on the excitation and emission spectra and luminescence decay curves of the Eu 3+ ion 5 D 0 excited level.
For the preparation of the [RE(TLA):Eu 3+ ] complexes, 50 mL of RECl 3(aq) (0.05 mol L −1 ) was slowly added to a 200 mL solution of Na 3 (TLA) (aq) (0.0125 mol L −1 ) at 1:1 molar ratio at ca. 100 °C.The reaction mixture was refluxed for 1 h, the precipitate was filtered and washed four times with distilled water, dried and stored at reduced pressure.
Elemental analyses were performed with a Perkin-Elmer CHN 2400 analyzer.The FTIR were acquired from 400 to 4000 cm −1 in KBr pallets form by using a Bomem MB100 FTIR.Thermogravimetry was performed from 30 to 900 °C (heating ramp of 5 °C min −1 , synthetic air dynamic atmosphere) in a TA HI-RES TGA 2850 equipment.The XPD patterns were obtained in a Miniflex Rigaku II equipment (CuK α1 ) from 5 to 70° (2θ).The SEM micrographs were recorded in a JEOL JSM 7401F field emission scanning electron microscope.The transmission electron microscope (TEM) micrographs were recorded in a JEOL USA JEM-2100 LaB 6 transmission electron microscope.
The luminescence study was based on the excitation and emission spectra recorded at room (300 K) and liquid nitrogen (77 K) temperatures.The measurements were performed in a SPEX-Fluorolog 3 instrument with double monochromators in front face mode (22.5°) using a 450 W Xenon lamp as excitation source.Luminescence decay curves were obtained by using a 150 W pulsed lamp and recorded in a SPEX 1934D phosphorimeter.

Characterization
A combination of elemental and thermogravimetric analysis (Table S1 and Figure 2) suggests an 1:1 molar ratio between the RE 3+ ion and TLA ligand ([RE(TLA)•(H 2 O) n :Eu 3+ ]; n: 4, 4 and 3 for Y 3+ , Gd 3+ and Lu 3+ , respectively). 23The TG curves of coordination compounds show a water molecules mass-loss in the temperature interval between 50 and 230 °C.Although the organic moiety decomposition of the complexes presents only one single-step between 450 and 570 °C.In this case, it was used annealing temperature of 500 °C during 1 h, in order to eliminate all the organic part leading to formation of the RE 2 O 3 :Eu 3+ luminescent material.
The infrared absorption spectra (Figure S1) present similar spectral profile for the RE 3+ complexes and Eu 3+doped matrices.The absorption bands between 1300 and 1600 cm -1 in the FTIR spectra of [RE(TLA)•(H 2 O) n :Eu 3+ ] are assigned to the carboxylate symmetric ν s (C=O) and asymmetric ν as (C=O) stretching modes, respectively. 19,24,25he narrow absorption peak around 3070 cm -1 is assigned to the C-H bond stretching of the [RE(TLA):Eu 3+ ] complexes and the broad band between 3100-3700 cm -1 correspond to the O-H stretching from the water molecules. 26he sharp absorption bands around 510 and 580 cm −1 correspond to the characteristic RE 3+ −O stretching vibration.It is worth mentioning that the broad bands from 1250 to 1600 cm −1 are assigned to stretching mode of oxycarbonate remainder from the decomposition of the organic moistly of TLA and decreases with increasing annealing temperature (Figure S1), due to oxycarbonate decomposition. 23The broad absorption band located from 2800 to 3700 cm −1 is assigned to the superficial hydroxyl groups in the nanomaterials.Therefore, the RE 2 O 3 :Eu 3+ materials originated from the [RE(TLA)] precursor complexes present similar chemical behavior compared to the sesquioxides prepared from the [RE(TMA)] complexes as reported by Silva et al. 23 The X-ray diffraction patterns of the [RE(TLA):Eu 3+ ] complexes are similar to the powder diffraction patterns (PDF) for [Gd(TLA)]:Eu 3+ and [Y(TLA)]:Eu 3+ ] (00-056-1733) and [Lu(TLA)]:Eu 3+ ] (00-058-1915), Y 3+ and Gd 3+ complexes are isomorphs.Consequently, there is no change in position or formation of new diffraction  peaks at different concentrations of the dopants.This result is consistent with the Vegard's rule 27,28 which suggests a formation of a solid solution between the Eu 3+ dopant and the RE 3+ in the host matrices due to the high similarity in the radii of these RE 3+ ions. 27he XPD patterns of the annealed materials at 500, 600, 700, 800, 900 and 1000 °C (Figure 3) reveal a formation of RE 2 O 3 :Eu 3+ in a cubic phase crystallization with the Ia -3 space group. 29The absence of 2θ shift and reflections of impurities in the patterns of the RE 2 O 3 :Eu 3+ indicates the formation of pure RE 3+ sesquioxides.The XPD data of the Y 2 O 3 , Gd 2 O 3 and Lu 2 O 3 matrices (Figure 3) are very similar.Slight differences in the (222) reflection around 28°, moving to higher 2θ values with decreasing of the ionic radius of the RE 3+ in the matrix, as predicted by Bragg's law. 30he average crystal size of the doped materials was estimated from the powder diffraction data by using the Scherrer's formula (Figure 4). 23,31The crystallite size of the RE 2 O 3 materials increases as function of the RE 3+ radius and annealing temperature.This behavior can be assigned to the higher reactivity of the Gd 2 O 3 , with lower melting point (2339 °C) compared to Y 2 O 3 (2410 °C) and Lu 2 O 3 (2427 °C). 32Therefore, the sintering process is favored for the gadolinium matrix due to the dependence of the partial melting of the nanocrystals. 23he narrowing of the diffraction peaks of RE 2 O 3 :Eu 3+ (1.0 mol%) (RE 3+ : Y, Gd and Lu) phosphors presented in the XPD patterns (Figure 3) as function of the annealing temperature, indicates that the crystallite size increases from 11, 17, 18, 37, 46 and 62 nm as the annealing temperature increases from 500, 600, 700, 800, 900 and 1000 °C (Y 2 O 3 ), respectively (Figure 4).This behavior is related to the sintering of the nanocrystallites favored at high temperatures.Although the Gd 2 O 3 :Eu 3+ annealed at 1000 °C was also included in this work, the Scherrer's formula is recommended only for crystallite sizes up to 200 nm (Figure 4).
The SEM images of the [RE(TLA):Eu 3+ (1.0 mol%)] precursors shows rods and a flower like morphologies (stacking of micrometric sheets of the material) for Y 3+ /Gd 3+ (Figures 5a and 5b) and Lu 3+ complexes, respectively (Figure 5c).After annealing up to 1000 °C, the RE 2 O 3 :Eu 3+ materials retained the original morphology of the correspond precursor complex (Figures 5d-5f).The nanosesquioxides Annealing temperature / C ° exhibit higher porosity due to the decomposition of the organic moiety.This property is important for the design of nanomaterials with controlled morphology.Since it is possible to modify the complex morphologies, the desired nanoparticle shapes can be obtained by choosing the suitable synthetic method and reaction conditions. 33,34he TEM micrographs (Figures 5g and 5h) show the cubic shape of the crystallites with high crystallinity.The particles retained the shape of the precursor agglomerates, shown in the SEM microscopy.At higher magnification no defects were observed in the crystals (except for the edges and crystallite contact points), suggesting the formation of a solid solution between the Eu 3+ ions and the host matrices, compatible with the similar RE 3+ ionic radii and chemical behavior of the Eu 3+ and RE 3+ matrices.

Photophysical properties of materials [RE(TLA):Eu 3+ ] precursor complexes
The excitation spectra of [RE(TLA):Eu 3+ (x mol%)] (RE 3+ : Y, Gd and Lu) compounds were obtained by monitoring the hypersensitive transition 5 D 0 → 7 F 2 (619 nm) at 77 K (Figure 6).For all the complexes, the absorption bands are dominated by a high intensity broad TLA ligand band centered at 295 nm assigned to the S 0 → S 1 transition, indicating an efficient energy transfer TLA → Eu 3+ .The sharp peaks are assigned to the absorption of the Eu 3+ ion originated from the ground state 7 F 0 to the 5 L 6 and 5 D 2 excited levels.The excitation spectra of the [RE(TLA):Eu 3+ (x mol%)] (RE 3+ : Y and Gd) compounds show similar profiles suggesting that this system presents equivalent chemical environments around RE 3+ ions and optical behaviors.On the other hand, [Lu(TLA):Eu 3+ (x mol%)] shows slightly different spectral profile.For all [RE(TLA):Eu 3+ ] systems, the 7 F 0 → 5 L 6 transition (25445 cm -1 for [Y(TLA):Eu 3+ (5.0 mol%)]) exhibits the highest intensity among the intraconfigurational transitions in the excitation spectra.
Using the optical data obtained from the emission spectra, it is possible to calculate the radiative rates (A 0→J ) from the 5 D 0 → 7 F J transitions using equation 1: 16,17 where σ 0→1 and σ 0→2,4 correspond to the energy barycenter of the 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2,4 transitions, respectively.The S 0→1 and S 0→J are the areas calculated under the emission of the spectral curve corresponding to the 5 D 0 → 7 F 1 and 5 D 0 → 7 F J transitions, respectively. 35ince the magnetic dipole 5 D 0 → 7 F 1 transition is almost insensitive to changes with the chemical environment around the Eu 3+ ion, the A 0→1 rate can be used as an internal standard to determine the A 0→J coefficients for Eu 3+ containing compounds. 16he lifetime (τ) of the luminescent compounds were obtained from the luminescence decay curve using a first order exponential decay, with excitation at the 7 F 0 → 5 L 6 band.The emission quantum efficiency (η, or intrinsic quantum yield, Q Ln Ln , as it has been defined by Bünzli) 36 of the 5 D 0 emitting level is determined according to equation 2: where the total decay rate, A tot = 1/τ = A rad + A nrad and the A rad = Σ J A 0→J .The A rad and A nrad quantities are the radiative and non-radiative rates, respectively.Table 1 shows the experimental values of the radiative (A rad ), non-radiative (A nrad ) rates and 5 D 0 emitting level emission quantum efficiency (η).
The [RE(TLA):Eu 3+ (x mol%)] lifetime values (Table 1 and Figure 6c) show higher values for Gd 3+ and Y 3+ containing complexes when compared to the Lu 3+ ion case.On the other hand, there are no changes in the lifetime behavior doping with an increasing concentration from 0.1, 0.5, 1.0 and 5.0 mol%, within the same system.
The 5 D 0 → 7 F 2 and 5 D 0 → 7 F 4 transitions can be used to estimate the experimental intensity parameters (Ω λ , λ = 2 and 4).The Ω 6 intensity parameter is not included in this study since the 5 D 0 → 7 F 6 transition was not observed for these systems.The coefficient of spontaneous emission, A, is given by equation 3: where, χ = n (n + 2) 2 /9 is the Lorentz local field correction and n is the refractive index of the medium (refractive index used: 1.5 for all [RE(TLA):Eu 3+ ] complexes and between 1.5 and 1.6 for RE 2 O 3 :Eu 3+ materials).The squared reduced matrix elements 〈 7 F J ||U (λ) || 5 D J 〉 2 are 0.0032 and 0.0023 calculated for J = 2 and 4, respectively. 35,37he Ω λ parameters depend mainly on the local geometry, bonding atoms and polarizabilities in the first coordination sphere of the RE 3+ metal ion, and are governed by both forced electric dipole (FED) and dynamic coupling (DC) mechanisms.Moura et al. 38 reported that the Ω 2 parameter values are very sensitive to small angular changes in the local coordination geometry (much more than the Ω 4,6 parameters).This spectroscopic behavior is associated with the hypersensitivity of certain 4f-4f transitions, to changes in the chemical environment, that are usually ruled by the Ω 2 intensity parameter.On the other hand, the Ω 4 and Ω 6 values are most sensitive the chemical bond distances to the ligating atoms around the lanthanide ion.Indeed, as concluded by Moura et al., 38 covalency in the ion-ligand bonding becomes more important with the increasing rank of the Ω λ , supporting the idea that the Ω 4 and Ω 6 parameters are better probes then Ω 2 to quantify covalency in these compounds.
The excitation spectra of RE 2 O 3 :Eu 3+ annealed phosphors (RE 3+ : Y, Gd and Lu) were recorded at 77 K in the spectral range from 200 to 590 nm, with the emission monitored at 613 nm (Figure 7 and Figure S3).They show the presence of a broad absorption band centered around (ca. 39000 cm -1 ) assigned to the O 2− (2p) → Eu 3+ (4f 6 ) LMCT transition.Besides, the narrow absorption bands arisen from 4f-4f transitions from the RE 3+ ion (ca.17000 to 34000 cm -1 ) are observed.
The excitation spectra recorded at 300 K (Figure S4) show the presence of the overlapped 7 F 0 → 5 D 1 and 7 F 1 → 5 D 1 transitions (ca.19000 cm -1 ) allowed by magnetic-dipole mechanism (∆J = 0, ±1, but 0 ↔ 0 is forbidden) for both the C 2 and S 6 symmetries.This optical results are due to the thermal population of the 7 F 1 level that are in agreement with the results previously reported for RE 2 O 3 :Eu 3+ . 42,43he absorption bands assigned to the 7 F 0 → 5 D 2 transition allowed by induced electric dipole and dynamic coupling mechanisms were observed from 21500 to 21900 cm -1 .In addition, a weak absorption band around 24100 cm -1 is assigned to the forbidden 7 F 0 → 5 D 3 transition (by ∆J selection rules) as a result of the relaxation of the selection rule due to the J-mixing effects in the 7 F J manifolds.Moreover, the other absorption bands (Figure 7) originated from 4f-4f transitions of the Eu 3+ ion were observed such as (in nm): the 7 F 0 → 5 L 6 (394), 5 G 2−6 (387), 5 L 7,8 (376), 5 D 4 (363), 5 H J' , 5 F J' , 5 I J' and 3 P 0 (between 286 and 335).
It is worth mentioning that the excitation spectra of the Gd 2 O 3 :Eu 3 present the characteristic strong absorption (nm): 8 S 7/2 → 6 P 7/2 (313), 8 S 7/2 → 6 P 5/2 (307) and 8 S 7/2 → 6 P 3/2 (302) transitions, indicating efficient energy transfer from the Gd 3+ to the Eu 3+ ion upper levels. 44The 8 S 7/2 → 6 I J (J = 7/2,9/2,17/2) (276) transitions overlap with the LMCT band.This high intensity absorption band indicates an efficient Gd 3+ to Eu 3+ energy transfer. 45he luminescent materials prepared by the benzenetricarboxylate method present comparable excitation features, indicating the reproducibility of the method even when using different benzenetricarboxylate (BTC) ligands. 23he emission spectra of the RE 2 O 3 :Eu 3+ (RE 3+ : Y, Gd and Lu) annealed at temperatures from 500 to 1000 °C were recorded at 77 K from 400 to 750 nm, under excitation in the LMCT band at 260 nm (Figure 7).All the spectra exhibit only the sharp lines arising from the 5 D 0,1,2,3 → 7 F 0-6 , transitions of the Eu 3+ ion.All materials show only one emission line assigned to 5 D 0 → 7 F 0 transition (ca.17270 cm -1 ) of the C 2 site of the cubic C-type.The 5 D 0 → 7 F 1 transition is present in both sites in the region of 16666, 16846 and 17015 cm -1 as well at 16770 and 17165 cm -1 from the C 2 and S 6 sites.
As reported by Boyer et al. 46 and Meltzer et al., 47 the refractive index (n) of the bulk RE 2 O 3 :Eu 3+ is around 1.9 and the 5 D 0 lifetime (τ) of europium ion is 1.0 ms.On the other hand, these values can be different in the case of the RE 2 O 3 nanostructured materials, with average sizes around 20-30 nm (crystallite size inferior to the wavelength of exciting radiation).Moreover, the morphology and surface/ volume ratio of the nanoparticles may play a role in the profile of the decay curves.
The radiative rate (A 01 ) of the 5 D 0 → 7 F 1 transition of Eu 3+ ion (allowed by the magnetic dipole mechanism) is formally insensitive to the ligand field environment.Therefore it can be used as a reference transition whose value is 50 s -1 assuming a refractive index equal to 1.6. 17,48,497][48] The experimental intensity parameters (Ω 2,4 ) and lifetimes (0.8-1.9 ms) values were obtained using the effective refractive index values between 1.5 and 1.6.The values of the experimental intensity parameters (Ω 2,4 ) the radiative (A rad ) and non-radiative (A nrad ) rates and emission quantum efficiencies (η) of the 5 D 0 emitting level of the RE 2 O 3 :Eu 3+ are presented in Table 2.
The values for Ω 2 (ca.12) and Ω 4 (ca.2-3) are very similar in the same matrix (Table 2) for different annealing temperatures as shown in the spectral profiles (Figure 7b). 50These results are a reflection of the observed emission intensity variations of the 5 D 0 → 7 F 2 transition of the Eu 3+ ion.This optical behavior demonstrates that the Eu 3+ ion acts as efficient luminescence probe even for the samples annealed at different temperatures.In addition, Ω 2 and Ω 4 values are also comparable changing the RE 3+ matrix, due to the similarity in the radii in the lanthanide series.
The experimental intensity parameter values for the phosphors using the TLA ligand as precursor are smaller for all the systems, as compared to those originated from the TMA ligand, especially for the of Gd 3+ matrix. 19ccording to Table 2, the RE 2 O 3 :Eu 3+ phosphors present an emission quantum efficiency values varying from 37 to 82% with the annealing temperature of 500-1000 °C.Among the materials, the Lu 2 O 3 :Eu 3+ (1.0 mol%) with annealing at 900 °C present the highest emission quantum efficiency (η = 82%).This phenomenon is probably associated to the removal of oxycarbonate from the matrices with increasing the annealing temperature.It is important to mention that the RE 2 O 3 :Eu 3+ phosphors prepared by the benzenetricarboxylate method using the TLA ligand is cheaper than compared with the TMA ligand.
The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates generated from the emission spectra of Eu 3+ doped RE 2 O 3 (Figure 8) are x: 0.650 and y: 0.335. 49The color coordinates show virtually no  (1.0mol%) nanomaterials under UV irradiation show identical strong red emission for all the phosphors annealed at temperatures from 500 to 1000 °C.

Table 1 .
Experimental values of intensity parameters (Ω λ ), radiative (A rad ) and non-radiative (A nrad ) rates, emission lifetimes and emission quantum efficiencies of the 5 D 0 emitting level determined for the [RE(TLA):Eu 3+ (x mol%)] (RE 3+ : Y, Gd and Lu) phosphors based on the emission spectra recorded at 77 K Ω λ : experimental values of intensity parameters; A rad : radiative rate; A nrad : non-radiative rate; A tot : total decay rate; τ: lifetime; η: quantum efficiency.

Table 2 .
Experimental values of intensity parameters (Ω λ ), radiative (A rad ) and non-radiative (A nrad ) rates, emission lifetimes and emission quantum efficiencies of the 5 D 0 emitting level determined for the 2 O 3 :Eu 3+ (1.0 (RE 3+ : Y, Gd and Lu) phosphors, annealed for 1 hour, based on the emission spectra recorded at 77 K λ : experimental values of intensity parameters; A rad : radiative rate; A nrad : non-radiative rate; A tot : total decay rate; τ: lifetime; η: quantum efficiency.