Synthesis , Structural and Photophysical Properties of Gd 2 O 3 : Eu 3 + Nanostructures Prepared by a Microwave Sintering Process

In this paper, we report the obtention of gadolinium oxide doped with europium (Gd2O3:Eu+3) by thermal decomposition of the Gd(OH)3:Eu3+ precursor prepared by the microwave assisted hydrothermal method. These systems were analyzed by thermalgravimetric analyses (TGA/DTA), X-ray diffraction (XRD), structural Rietveld refinement method, fourrier transmission infrared absorbance spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) measurement. XRD patterns, Rietveld refinement analysis and FT-IR confirmed that the Gd(OH)3:Eu3+ precursor crystallize in a hexagonal structure and space group P6/m, while the Gd2O3:Eu3+ powders annealed in range of 500 ̊C and 700 ̊C crystallized in a cubic structure with space group Ia-3. FE-SEM images showed that Gd(OH)3:Eu3+ precursor and Gd2O3:Eu3+ are composed by aggregated and polydispersed particles structured as nanorods-like morphology. The excitation spectra consisted of an intense broad band with a maximum at 263 nm and the Eu3+ ions can be excitated via matrix. The emission spectra presented the characteristics 5 7 0 0,1,2,3 and 4 D F → transitions of the Eu3+ ion, whose main emission, 5 7 0 2 D F → , is observed at 612 nm. The photophysical properties indicated that the microwave sintering treatment favored the Eu3+ ions connected to the O-Gd linkages in the Gd2O3 matrix. Also, the emission in the Gd2O3:Eu3+ comes from the energy transfered from the Gd-O linkages to the [ ] 8 EuO clusters in the crystalline structure. Corresponding author. A. P. de Moura et al.


Introduction
One-dimensional nanomaterials, such as nanotubes, nanowires, nanobelts or nanoribbons have attracted much interest in the past decade due to their physical properties and potential applications in nanotechnology fields [1]- [8].Moreover, these materials can be applied as displays, catalysts, biological sensing, and other optoelectronic devices [9]- [11].
The demand for efficiency and high resolution waveguides, lamps and other optical devices has also stimulated the discovery of new luminescent materials with superior properties.Thus, there has been a tremendous interest in the subject of materials science for the development of new luminescent materials.The improved performance of display requires high-quality phosphors for sufficient brightness and long-term stability.To enhance the luminescent characteristics of phosphors, extensive research has been carried out on rare-earth activated oxide phosphors due to their superiority in color purity, chemical and thermal stabilities [12]- [14].In this context, lanthanide hydroxides and oxides have actively been investigated for its application in multilayered capacitors, luminescent lamps and displays, solid-laser devices, optoelectronic data storages, waveguides, and heterogeneous catalysts.Their composition, structure and particle size depend on the synthesis method.Moreover, the chemical homogeneity and morphology of the synthesized products determine the effectiveness of their properties [11] [15] [16].When they are applied for a fluorescent labeling, for instance, there are several advantages such as sharp emission spectra, long lifetimes, and high resistance against photobleaching in comparison with conventional organic fluorophores and quantum dots [17]- [19].
Europium ion in a trivalent state is one of the most studied rare earth element because of the simplicity of its emission spectra and due to the wide application as red phosphor in color TV screens.Eu 3+ f-f transitions are sensitive to its local environment.The monitoring of different concentrations of the Eu 3+ content into a ceramic material is very interesting in understanding the nature of the lattice modifiers as well as the degree of order-disorder into its crystalline structure.The most intense f-f transition is the 5 7 0 2 D F → transition at 616 nm.When this ion is presented in a non-centrosymmetric site, it can be used as an activator ion with red emission which has been used in the most commercial red phosphor.Moreover, the intensity of Eu 3+ excitations at around 394 and 465 nm is improved in these materials as compared with most other Eu 3+ doped phosphors [24] [25].Because of it, this ion is able to be applied as biological sensors, phosphors, electroluminescent devices, optical amplifiers or lasers when it is used as a dopant in a variety of ceramic materials [26]- [28].
A variety of preparation methods have been developed to reduce the reaction temperature and achieve a small particle size of high quality Gd 2 O 3 :Eu 3+ phosphors [11] [29]- [32].
Microwave heat processing has been successfully applied for the preparation of micro or nanosized inorganic materials [33]- [38].The microwave-assisted heating is a greener approach to synthesize materials in a shorter time (from several minutes to a few hours) and with lower power consumption (hundreds of Watts) compared to the conventional heating at the same temperatures [39]- [43].This is a consequence of directly and uniformly heating of the components, and exchange in the reaction selectivity, which can increase the reactional rates (microwave catalysis).Consequently, microwave synthesis is becoming quite common in several material sciences areas, nanotechnology, inorganic, organic, biochemical, or pharmaceutical laboratories [44]- [50].
In the present work, we investigated the photo-physical properties of Gd 2 O 3 : Eu 3+ phosphors obtained by the thermal decomposition in range of 500˚C and 700˚C of the Gd(OH) 3 :Eu 3+ precursor prepared by the microwave assisted hydrothermal method.These materials were structured and microstructurally analyzed by means of X-ray diffraction (XRD), Rietveld refinement method, fourrier transmission infrared absorbance spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM).The photo-physical properties were investigated through the excitation and emission spectra of the Eu 3+ ion as well as lifetime measurements.

Synthesis of the Precursors
The synthesis of the precursors was performed using the following procedure: In a typical synthesis, 1.8 g of Gd 2 O 3 and 0.018 g of Eu 2 O 3 were dissolved in 3.0 mL of the HNO 3 solution.After the formation of a clear solution, this solution was kept under constant heating until complete evaporation of the acid.Then 80 mL of distilled water were added to the solution and stirred for 30 min at room temperature.After that, an aqueous KOH (2.0 M) solution was added until the pH of solution was adjusted to be in the range of 12 giving rise to a colloidal precipitates.After stirring for about 30 min, the resultant solution was transferred to a Teflon lined stainless autoclave.This autoclave was then sealed and placed into a microwave system (MH) using 2.45 GHz microwave radiation with maximum power of 800 W. The MH conditions were kept at 140˚C for 1 minute.The white powders obtained (Gd(OH) 3 :Eu 3+ ) were collected, washed with water and ethanol, and then dried at 60˚C for 8 h under atmospheric air in a conventional furnace.

Synthesis of Gd2O3:Eu 3+ Powders
The Gd 2 O 3 :Eu 3+ powders were obtained from thermal decomposition of the Gd(OH) 3 :Eu 3+ precursors.These precursor powders were placed in ceramic crucibles and heated in a microwave sintering furnace at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C for 5 min under an ambient atmosphere using a heating rate of 5˚C/min producing white powders denoted as Gd 2 O 3 :Eu 3+ .

Characterization
The Gd(OH) 3 :Eu 3+ and Gd 2 O 3 :Eu 3+ powders were structurally characterized by X-ray diffraction (XRD) in normal routine and Rietveld routine using a Rigaku-DMax/2500PC (Japan) with Cu-Kα radiation (λ = 1.5406Å) and in the 2θ range from 10˚ to 130˚ with a scanning rate of 0.02˚/min.Fourier Transmission Infrared absorbance spectroscopy (FT-IR) analysis were taken in a FT-IR Bruker model EQUINOX spectrophotometer in range of 500 and 4000 cm −1 .Crystals morphologies were verified using a Scanning Electron Microscope (Jeol JSM-6460LV microscope).Photoluminescence (PL) was measured with a Thermal Jarrel-Ash Monospec 27 monochromator and a Hamamatsu R446 photomultiplier.The 350.7 nm exciting wavelength of a krypton ion laser (Coherent Innova) was used, with the nominal output power of the laser power kept at 200 mW.All the measurements were taken at room temperature.The excitation and emission spectra of the Gd 2 O 3 :Eu 3+ powders were measured in a Jobin Yvon-Fluorolog 3 spectrofluorometer at room temperature using a 450 W xenon lamp as excitation energy source.Lifetime data of the Eu 3+ 5 7 0 2 D F → (λ exc = 394 nm, λ em = 612 nm) transition in the Gd 2 O 3 :Eu 3+ samples were evaluated from the decay curves using the emission wavelength set at 612 nm and excitation wavelength set at 393 nm.

Thermogravimetric Analyses (TGA/DTA)
Figure 1 presents the TGA curve of the as-prepared Gd(OH) 3 :Eu 3+ powder.It can be seen in this figure, during the MH process, the thermal degradation of the Gd(OH) 3 :Eu 3+ powders occurs in a two-step process in accordance to observed by Chang et al. [51].The first-step occurs at 436˚C, where a weight loss of 8.3% is reported, while the second-step, occurs from 436˚C to 700˚C, a weight loss of 3.3% is revealed.From these results, it was confirmed that the MH treatments of the hydroxide precursor give rise to a stable Gd 2 O 3 :Eu 3+ matrix at temperatures up to 400˚C.Moreover, the two-step dehydration process indicates the presence of an intermediate phase in addition to the starting hexagonal Gd(OH) 3 :Eu 3+ and the final cubic Gd 2 O 3 :Eu 3+ .The theoretical weight loss of each process in this work is in a good agreement with shown by the literature.

X-Ray Diffraction (XRD) and Rietveld Refinement Analyses
XRD patterns of the as-prepared Gd(OH) 3 :Eu 3+ are presented in Figure 2(A), where the reflectance peaks of a pure hexagonal Gd(OH) 3 :Eu 3+ phase with space group P63/m can be perfectly indexed in agreement with the respective Inorganic Crystal Structure Database (ICSD) card number 200,093.After the thermal decomposition of the Gd(OH) 3 :Eu 3+ in range of 500˚C to 700˚C for 5 min (Figure 2(B)), it was observed that all the samples can be perfectly indexed to the cubic structure of crystalline Gd 2 O 3 and space group Ia-3 (ICSD # 94892).None secondary phases were detected in these samples, indicating that all the Gd(OH) 3 :Eu 3+ precursor heated from 500˚C to 700˚C giving rise to Gd 2 O 3 :Eu 3+ powders.In addition, in this range of temperature, any significant change in the XRD peak profiles of the Gd 2 O 3 :Eu 3+ samples were detected.
To better analyze the influence of the precursor thermal decomposition, the Gd(OH) 3 :Eu 3+ and Gd 2 O 3 :Eu 3+ powders were submitted to the Rietveld refinement analysis.
The Rietveld refinement is a method in which the profile intensities obtained from step-scanning measurements of the powders allow to estimate an approximate structural model for the real structure [52].In our work, the Rietveld refinement was performed using the General Structure Analysis (GSAS) program [53].In these analyses, the parameters like scale factor, background, shift lattice constants, profile half-width parameters (u, v, w), isotropic thermal parameters, strain anisotropy factor, occupancy and atomic functional positions were refined.The background was corrected using a Chebyschev polynomial of the first kind.The peak profile function was modeled using a convolution of the Thompson-Cox-Hastings pseudo-Voigt (pV-TCH) [54] with asymmetry function described by Finger et al. [55].To account for the anisotropy in the half width of the reflections, it was used the model proposed by Stephens [56].
The obtained results from the Rietveld refinement analyses of the crystalline Gd(OH) 3 :Eu 3+ and Gd 2 O 3 :Eu 3+ are shown in Figure 3.The results showed that the Gd(OH) 3 :Eu 3+ powder crystallize in a hexagonal structure, space group P63/m and two clusters per unit cell (Z = 2), while the Gd 2 O 3 :Eu 3+ powders crystallize in a cubic structure, space group Ia-3 and sixteen formula units per cell (Z = 16).These results showed a good relation between the observed XRD patterns and the theoretical ones, as shown by a line (Y obs − Y cal ) at Figure 3.Moreover, it was not verified the presence of secondary phases related to the Eu 3+ ions, probably indicating that these ions were incorporated to the hydroxide and oxide matrixes.The lattice parameters, unit cell volume, unit cell angles and correlation parameters (R Bragg , χ 2 and R wp ) obtained by means of the structural refinement data for analyzed powders are listed in Table 1 and Table 2.
From the lattice parameters, unit cell volume and atomic positions obtained from the Rietveld refinement data, it was possible to model a schematic representation of the hexagonal Gd(OH) 3 :Eu 3+ unit cell and space group P63/m Figure 4(A), using the Visualization for Electronic and Structural Analysis (VESTA) program version 2.1.6for Windows [57].
In the literature, the crystalline structure of the Gd(OH) 3 has been studied by Chang et al. [51].From this schematic representation is was possible to observed that the hexagonal Gd(OH) 3 is composed by [Gd(OH) 3 ] −   clusters connected to each other.Moreover, the addition of Eu 3+ in its crystalline structure promotes the expansion of the Gd(OH) 3 unit cell as a consequence of the substitution of some Gd sites by the Eu ion ones.
The unit cell of the Gd 2 O 3 :Eu 3+ powder was also simulated through the VESTA software and its schematic representation is illustrated at GdO clusters which are connected to each other and all dispersed to the cubic crystalline structure, which are in good agreement with References [58] and [59].
From Table 2, it can be seen any significant changes in lattice parameters (a, b and c) of Gd 2 O 3 system as function of the heat treatment.When the Gd(OH) 3 :Eu 3+ material is subjected to a thermal treatment in a microwave oven sintering, there is an interaction of the Gd, O and H atoms with the microwave radiation in the crystalline structure.Consequently, the heating of these systems occurs.This process is able to promote the dehydration of the Gd(OH) 3 :Eu 3+ material giving rise to Gd 2 O 3 crystalline structure.Due to low heating rate used (5˚C /min), this interaction occurs slowly and the quantity of surface defects is reduced as well as particle growth is promoted [60].

Fourier Transmission Infrared Absorption Spectroscopy (FT-IR)
Figure 5 shows the FT-IR transmission spectra of the Gd(OH) 3 :Eu 3+ precursor and of the Gd 2 O 3 :Eu 3+ nanocrystals obtained heated at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C for 5 min.
The FT-IR of the Gd(OH) 3 :Eu 3+ precursor presents a sharp absorption band at 3610 cm −1 which is characteristic of the Gd-OH matrix.The bands at around 3460 and 1627 cm −1 are due to the OH stretching (υ) and OH deformation vibrations (δ), respectively.The broad at absorptions around 2333 cm −1 is assigned to the existence of CO 2 .The absorption bands at around 1480 and 400 cm −1 are ascribed to the CO asymmetric vibration (υ as ).
The sharp peak observed for the Gd(OH) 3 :Eu 3+ precursors is ascribed to the OH stretching vibration, and indicates the absence of hydrogen bonds between the hydroxyls group [61].Thus, it can be supposed that the [Gd(OH) 3 ] − clusters in the Gd(OH) 3 are connected to each other, in accordance to the Rietveld refinement analysis.In the IR absorption spectra of the Gd 2 O 3 :Eu 3+ powders annealed from 500˚C to 700˚C, it was observed the appearance of some absorption at around 525 and 830 cm −1 characteristic of Gd-O vibrations, confirming the formation of an oxide matrix after the heat treatment process [62] [63].Also, the broad absorption bands observed at 3460 cm −1 in these samples are only corresponded to the water absorbed in its superficies.All these results are also in agreement with the Rietveld refinement analysis.
The intensity of the absorption bands corresponding to the OH and CO groups are strong dependent of the annealing temperature, indicating that the powder prepared in air atmosphere have strong absorption to water and CO 2 and it can be potentially applied as gas sensors in monitoring gases such as water and CO 2 .

Field Emission Scanning Electron Microscopy (FE-SEM)
Figure 6 shows the FE-SEM images of the Gd(OH) 3 :Eu 3+ precursor (A) and Gd 2 O 3 :Eu 3+ samples heated at 500˚C (B), 550˚C (C), 600˚C (D), 650˚C (E) and 700˚C (F), respectively.The FE-SEM images showed that the Gd 2 O 3 :Eu 3+ and Gd(OH) 3 :Eu 3+ powders are composed by aggregated and polydispersed particles structured as nanorods-like morphology.Moreover, the size and thickness of the Gd 2 O 3 :Eu 3+ powders (Figures 6(B)-(F)) vary as function of the annealing temperature.It was also noticed that as the temperature increases the particles have a tendency to agglomerate.
FE-SEM images were also employed to evaluate the average particle size distribution (width) of the Gd(OH) 3 :Eu 3+ and Gd 2 O 3 :Eu 3+ nanostructures.During this measurement was considered around 100 nanostructures and the best fit for this system was adjusted as a lognormal function, which is described by the following equation: where y 0 is the first value in y-axis, A is the amplitude, w is the width, π is a constant, x c is the center value of the distribution curve in x-axis.
The obtained results shown in Figure 7 presented an assymmetrical distribution on the logarithmic scale of average particle size.In this case, it was noted that almost all particles presented an average width between 8 and 20 nm.

Photoluminescence (PL) Emission Measurements
Figure 8(A) illustrates the PL spectra of the Gd(OH) 3 :Eu 3+ precursor and Gd 2 O 3 :Eu 3+ samples heat treated at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C, respectively and (B) the PL spectrum of the non-doped Gd 2 O 3 prepared by the microwave assisted hydrothermal method.All the measurements were recorded at room temperature.
To a better understanding of the PL properties and its dependence on the structural order-disorder in the Gd 2 O 3 lattice, the PL emission spectra of the Gd(OH) 3 :Eu 3+ and Gd 2 O 3 :Eu 3+ powders were performed at room temperature, using an excitation of a krypton laser source at 350.7 nm. Figure 8(A) also shows a broad emission band from 400 to 600 nm.This band can be ascribed to the emission of the Gd 2 O 3 matrix as it was confirmed by the PL emission spectrum of Gd 2 O 3 powder where a broad band with maximum situated at 449 nm were observed (Figure 8(B)).Moreover, in range of 600 and 700 nm, it was possible to noticed the intra-configurational Figure 9 shows the excitation spectra of Gd 2 O 3 :Eu 3+ powders annealed at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C.The excitation spectra were recorded monitoring the emission wavelength at 612 nm.It can be clearly seen that the excitation spectra consist of a main intense broad band with a maximum at 263 nm attributed to the the transitions situated at 274 and 311 nm, respectively.These transitions are possible to be detected as a consequence of the 3 3 Gd Eu + + → energy transfer [65] [66] in the Gd 2 O 3 matrix (CTB).Above 330 nm, it was noted the Eu 3+ 4 f 6 intra-configurational transitions from the ground state 7 F 0 to the excited states 5 G 6 at 362 nm, 5 H 4 at 380 nm, 5 L 6 at 393 nm, 5 D 2 at 464 nm and 5 D 1 at 532 nm, respectively (Figure 9).In this case, we observed that the Eu 3+ ion can be excited via matrix in a wide range of wavelength and the most intense absorption band is correspondent to the 8   6   S I → transition of the Gd 2+ ions at 263 nm.Moreover, as the temperature increases the relative intensity of the bands corresponding to its transitions also increases [68]., a magnetic dipole transition is almost independent of the Eu 3+ surroundings.Thus, the ratio 5 emission intensity gave us valuable information about the environmental changes around the rare earth ions and can be used as a dimension of the degree of distortion from the inversion symmetry of the Eu 3+ site in the lattice [69].
The ratio 5 values as the temperature increases from 500˚C to 650˚C is a indicative that the Eu 3+ ions is occupying higher symmetry sites.In this case, we believe that some Eu 3+ is already connected to the -OH linkages from the Gd(OH) 3 precursor.However, there is an increase of the ratio 5 D 0 → 7 F 2 / 5 D 0 → 7 F 1 value at 700˚C, indicating that now the Eu 3+ ions is occupying lower symmetric sites.The temperature favored the higher symmetric coordination sites, since there is Eu 3+ ions linked to the O-Gd linkages in the Gd 2 O 3 matrix.Some of these oxygen atoms are in the first coordination sphere of Eu 3+ ions giving rise to [ ] According to these data, it was observed that the lifetime of Eu 3+ increases as the annealing temperature increases.This behavior is probably due to the increase of the energy transfer from the Gd-O linkages to the [ ] 8 EuO • clusters.All these results are in accordance to the emission and excitation measurements.

Conclusion
In summary, the obtained results showed that the Gd(OH) 3 :Eu 3+ (precursor) was synthesized by the microwave assisted hydrothermal method in a short period of time (30 minutes).After heated treated from 500˚C to 700˚C,  • clusters in the crystalline structure.Moreover, these are in accordance to the lifetime values, which presented an increase as the temperature increases.This method is very simple and effective, and can be extended to synthesize some other rare earth and metal oxide nanorods.

8 EuO
that the microwave sintering treatment favored the Eu 3+ ions connected to the O-Gd linkages in the Gd2O3 matrix.Also, the emission in the Gd2O3:Eu 3+ comes from the energy transfered from the Gd-O linkages to the[  ]    • clusters in the crystalline structure.

Figure 4 (
B). From this schematic representation the Gd 2 O 3 :Eu 3 are composed by the Gd atoms coordinated to ten oxygen atoms forming [ ] 10

Figure 8 .
Figure 8. PL emission spectra of the Gd(OH) 3 :Eu 3+ and Gd 2 O 3 :Eu 3+ powders heated in different temperatures (A) and pure Gd 2 O 3 heat treated at 600˚C.Insert: FE-SEM image of pure Gd 2 O 3 powder.4f 6 transitions of the Eu 3+ ions specifically the 7 5 0 F D J → (J = 1, 2, 3 and 4) transitions at 590, 615, 633 and 710 nm, respectively, for all analyzed samples.Figure9shows the excitation spectra of Gd 2 O 3 :Eu 3+ powders annealed at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C.The excitation spectra were recorded monitoring the emission wavelength at 612 nm.It can be clearly seen that the excitation spectra consist of a main intense broad band with a maximum at 263 nm attributed to the the2

Figure 9 .
Figure 9. Excitation spectra of Gd 2 O 3 :Eu 3+ samples heated at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C, λ em = 612 nm.The emission spectra of Gd 2 O 3 :Eu 3+ for the samples annealed at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C are shown in Figure 10.These spectra were obtained setting the excitation wavelength into the energy transfer band CTB of Eu 3+ at 263 nm.The emission spectra of the Eu 3+ ion present narrow bans ascribed to the 5 7 0 0,1,2,3 and 4 D F → transitions at around 578, 589, 614, 652 and 699 nm, respectively.The most intense band is related to 5 7 0 2 D F → transition with maximum situate at 612 nm.The hypersensitive transition 5 7 0 2 D F → is dependent on the local Eu 3+ environment due to its electric dipole character, while the intensity of 5 7 0 2 D F →, a magnetic dipole transition is almost independent of the Eu 3+ surroundings.Thus, the ratio5 the Gd 2 O 3 :Eu 3+ powders annealed at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C is of 18.0, 17.1, 15.8, 11.95 and 18.3, respectively.The decrease in the ratio

8 EuO• 8 EuO
clusters in this powder.All these results indicates that probably the emission in the Gd 2 O 3 :Eu 3+ powders annealed from 500˚C to 700˚C comes from the energy transfer from the Gd-O linkages to the [ ] • clusters in the crystalline structure.Moreover, the increase of the relative intensity of the Eu 3+ formation of these complex clusters in the Gd 2 O 3 matrix.The photoluminescence decay curves and lifetime of the Eu 3+ and excitation wavelengths set at 612 nm and 263 nm, respectively.In this work, the lifetime value of the Gd 2 O 3 :Eu 3+ samples as a function of the annealing temperatures are shown in Figure11.All these curves can be fitted into a single exponential function as is the lifetime of the rare earth ion).

Figure 11 .
Figure 11.Decay curves and lifetime of the 5 D 0 → 7 F 2 transition characteristic of the Eu 3+ of the Gd 2 O 3 :Eu 3+ nanorods heat treated at 500˚C, 550˚C, 600˚C, 650˚C and 700˚C (λ ex = 612 nm and λ em = 612 nm). the XRD patterns and Rietveld refinement and FT-IR analyses indicated the formation of Gd 2 O 3 :Eu 3+ powders which crystallizes in a cubic structure of crystalline Gd 2 O 3 and space group Ia-3.No secondary phases related to the Eu 3+ ions were detected indicating that these ions were incorporated to the hydroxide and oxide matrixes in the analyzed powders.FE-SEM images indicated that the Gd(OH) 3 3+ precursor and Gd 2 O 3 :Eu 3+ powders are composed by several aggregated particles with nanorods-like morphology, which sizes are in the range of 8 and 20 nm.Eu 3+ emission and excitation spectra pointed out that the emission in the Gd 2 O 3 :Eu 3+ powders comes from the energy transfer from the Gd-O and [ ] 8 EuO • clusters in the crystalline structure.Moreover, these are

Table 1 .
Lattice parameters, unit cell volume of Gd(OH) 3 :Eu 3+ obtained by MH processing at 140˚C for 1 min.

Table 2 .
Lattice parameters, unit cell volume of Gd 2 O 3 :Eu 3+ obtained by thermal decomposition of the Gd(OH) 3 :Eu 3+ precursor at 700˚C.