Optical Properties of LiIn_(1−𝑥)Tm_𝑥(WO₄)₂ Blue Phosphor

Abstract

LiIn_1−𝑥Tm_𝑥(WO₄)₂ (𝑥=0, 0.5, 1, 5, and 10 at.%) polycrystalline powders blue phosphors were prepared via the classical solid-state reaction method. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation, and emission spectra were used to characterize LiIn_1−𝑥Tm_𝑥(WO₄)₂ phosphors. By analyzing the excitation and emission spectra of LiIn_1−𝑥Tm_𝑥(WO₄)₂ samples, the result indicates that there exists the energy transfer only from the WO₄²⁻ group to the ¹G₄ energy level of Tm³⁺ ion. On the other hand, the influence of the thulium concentration on the blue emission transition ¹D₂→³F₄ and ¹G₄→³H₆ and the emission of WO₄²⁻ group are investigated.

1. Introduction

Rare-earth ions-doped double tungstates have attracted a great deal of interest in recent years, due to their efficient radiative emissions in the visible and mid-infrared spectral regions [1–5]. These emissions are suitable for developing potential applications such as active media in solid-state lasers [6–10] and inorganic scintillation materials thanks to their high density (7.46 (g/cm³) for LiIn(W₄)₂) and to the fast emission of the (WO₄)⁻² tungstate group [11, 12].

Trivalent thulium could be an attractive activator ion exhibiting the suitable strong absorption band for the commercial GaAlAs laser diodes pumping around 800 nm, corresponding to the ³H₆→³H₄ transition. Especially, the 2 𝜇m region meets a great interest in many fields and applications like optical communications, medical equipment, and remote sensing [13–15].

Furthermore, Tm-doped materials, such as YLF, YAG, and NYW, generate blue laser radiation through the up-conversion of infrared radiation into visible radiation [16].

Especially, solid-state blue light sources are desirable for several applications, including high-density optical storage and colour displays, but, it has been difficult to find a suitable blue phosphor. Tm³⁺ ion is among activators which can be used for obtaining blue emission corresponding to its ¹D₂→³F₄ and ¹G₄→³H₆ transitions [17].

LiIn(WO₄)₂ belongs to the large family of inorganic compounds with the general formula 𝑀^I𝑀^III(𝑀^VIO₄)₂, where 𝑀^𝐼=alkalimetal, or monovalent transition metal ions; 𝑀^III=Al, In, Sc, Cr, Bi, Fe, rare earth ion, and 𝑀^VI=Mo, W. These materials may crystallize in a wide number of structures, which may be related as originating from either the structure of scheelite (CaWO₄), with isolated 𝑀^VIO₄ tetrahedra, or the structure of wolframite (FeWO₄), with octahedral coordination of 𝑀^VI Atoms [18, 19].

Due to the useful laser transitions of Tm³⁺ at both the visible and infrared frequencies, much attention has been paid in identifying optimal host crystals for these ions. In fact, the Tm³⁺ ions have demonstrated laser action in different spectral ranges in a wide variety of host crystals pumped with flash lamp, ion laser, or diode laser. However, as far as we know, there are no reports concerning thulium-doped monoclinic LiIn(WO₄)₂ host.

In paper, polycrystalline powders of Tm³⁺∶LiIn(WO₄)₂ were prepared using the classical solid-phase chemical reaction. First results of the structural and spectroscopic investigations of this material are presented. FTIR and photoluminescence spectra, especially charge transfer bands observed in the UV region, are analyzed. We also investigate the dependence of the blue emission intensity versus Tm³⁺ ions concentration in the material, under resonant ¹D₂ level excitation (𝜆_ex=360 nm) and W-O ligand-to-metal charge transfer band energy.

2. Experimental Procedures

2.1. Powder Preparation

Polycrystalline powders of LiIn_1−𝑥Tm_𝑥(WO₄)₂, where x is the Tm³⁺ ions concentration (𝑥=0, 0.5, 1, 5, and 10 at.%), are synthesized by means of classical solid-state reaction according to the following reaction: 12Li₂CO₃+(1−𝑥)2In₂O₃+12𝑥Tm₂O₃+2WO₃→LiIn_1−𝑥Tm_𝑥(WO₄)₂+12CO₂↑. (1)

Stoichiometric amounts of the starting materials: Li₂CO₃ (Alfa Aesar, 99.998%), In₂O₃ (CERAC, 99.99%), WO₃ (Ventron, 99.7%), and Tm₂O₃ (Aldrich Chemical Cie, 99.99%), of analytical grade purity, weighed in suitable molar proportions (Mettler Toledo PR 2003 balance) were ground and mixed manually in an agate mortar. The mixture was placed in a Pt crucible, and then heated in an electrical furnace. The mixtures were carefully ground, mixed, and then calcined in air at 700^∘C for 15 hours. After regrinding, the samples were finally calcined in air at 950^∘C fot 15 hours.

The samples were gradually cooled down to room temperature, weighed, ground, and analyzed using XRD method. After the final heating cycle, the samples were examined by Differential Thermal Analysis (DTA).

2.2. Characterization Techniques

Phase purity and crystal structure of the synthesized compounds were determined by X-ray diffraction analysis (XRD). Powder XRD patterns were collected at ambient temperature with a Philips PW1820 diffractometer (Cu K𝛼 radiation, 𝜆=1.54056Å, 2𝜃=10-50^∘). Lattice parameters were refined from XRD data recorded on powder samples, using the WinPLOTR-2006 program (LLB Saclay- LCSIM Rennes).

The morphology and microstructure were characterized by scanning electron microscope (SEM, Philips XL-30). IR spectra at 300 K were recorded with Thermo-Nicolet “Nexus”-670 FT-IR spectrophotometer, yielding a spectral resolution of 4 cm⁻¹. The samples were mixed with KBr with a sample/KBr weight ratio of ∼1/100 and compressed to give self-supporting pellets. For each sample 128 scans were recorded.

Room temperature diffuse absorption measurements were performed using a CARY 500 UV-VIS-NIR spectrophotometer in the range 250–1350 nm, with a resolution of 0.3 nm in the UV-VIS wavelength region and 1 nm in the NIR. This spectrometer is equipped with an integrating sphere accessory coated with PTFE (Poly Tetra Fluoro Ethylene) white material. The diffuse reflectance measurements were converted into absorption (arbitrary units) using the Kubelka-Munk function (f(𝑅_∞) = K/ 𝑆_sca = (1−𝑅_∞)²/2𝑅_∞) [20],where K is the absorption, 𝑅_∞ is the diffuse reflectance rate and S_sca is the diffusion factor.

Perkin-Elmer LS-50B spectrofluorimeter was used for room temperature steady-state luminescence measurements. For recording the excitation and the emission spectra, unpolarized light from the Xenon lamp (150 W) was used. The spectra were collected at right angle using a special sample holder. The values of the slit widths for the excitation and emission monochromators were set at spectral resolutions of 2.5 nm and 2.5 nm, respectively. The limit range is between 200 to 900 nm.

3. Results and Discussion

3.1. Crystal Structure of the Host

Monoclinic LiIn(WO₄)₂ crystallizes in the wolframite structure and is isostructural to LiFe(WO₄)₂ with the space group 𝐶2/𝑐=𝐶⁶_2ℎ and 𝑍=4.Accordingly to the C2/c space group, the Li and In ions occupy the nonequivalent positions on the 2 and 2₁ axes [18, 19, 21, 22]. The unit-cell dimensions at room temperature are, 𝑎=9.58Å, 𝑏=11.59Å, 𝑐=4.956Å, and 𝛽=91.078^∘. The crystal is built up of (WO₆)⁶⁻ octahedron connected to each other by means of common edges, forming (WO₄)²⁻ zigzag chains along the 𝑐-direction. Lithium and indium atoms occupy both octahedral sites and constitute (LiO₄)⁷⁻ and (InO₄)⁵⁻ chains ordered in a and b directions. Consequently, In³⁺ octahedrons share common edges in the chain. This type of structure is encountered in compounds where difference of ionic radii of M⁺ and M³⁺ is low (r_Li⁺=0.76Å, r_In³⁺=0.80Å [23]).

The XRD patterns of LiIn_1−𝑥Tm_𝑥(WO₄)₂ (x = 0, 0.5, 5, and 10 at.%), are shown on Figure 1, from which we can see that for x = 0, 0.5, and 5 at.% all the diffraction peaks of each sample can be indexed to the phase of monoclinic LiIn(WO₄)₂ (pdf code 00-049-0963). For the sample which 𝑥=10 at.%, there are two ones additional peaks with weaker intensity at the diffraction angles: 2𝜃=32^∘, 20.4^∘ of non identified phase.

Figure 1 

XRD patterns of LiIn1-xTmx(WO4)2 (𝑥=0, 0.5, 1, 5, and 10 at.%).

The knowledge of the unit cell variation with Tm³⁺ doping is important, for example, for epitaxial growth of doped layers on undoped substrates. In Table 1, we present the optimized unit-cell parameters deduced from the previous XRD analysis for all concentration except 10%. The unit-cell parameters a, c, b, and 𝛽 remain basically constant when the doping concentration of Tm³⁺ increases. For 𝑥=10 at %, at that time no interpretation can be done. Growth of single crystals will be accomplished in order to resolve this problem.

Table 1 

Calculated unit cell parameters for various thulium concentrations.

Figures 2(a) and 2(b) show the DTA curves of pure LiIn(WO₄)₂ sample corresponding to the melting and crystallization process, respectively. A single sharp endothermic peak is observed at around 1093^∘C. This peak exhibits the characteristics of a first-order phase transition which corresponds to a solid-liquid phase change, identified as the melting point. DTA analysis indicates that the fusion of the product is clearly congruent, and then one can attempt to pull a single crystal by the Czochralski technique.

(a)

(b)

Figure 2 

DTA curves of pure LiIn(WO4)2 during the (a) heating cycle and (b) cooling cycle.

3.2. Surface Morphological Analysis

In order to observe the grain morphology of the prepared powders with varying the Tm³⁺ concentration, we have taken for each sample some scanning electron microscope (SEM) images. For all studied Tm³⁺ concentrations, the SEM picture (Figure 3) exhibits particles with the same and definite shapes and sizes. The distribution of these particles is random and overlaps each other. The typical crystalline grain size is estimated to be in the range 0.5–2.0 micrometers.

Figure 3 

SEM micrograph of LiIn1-xTmx(WO4)2.

3.3. Infrared Spectroscopy

Figure 4 shows the room temperature Infrared spectra of LiIn_1−𝑥Tm_𝑥(WO₄)₂ samples (𝑥 = 0, 0.5, 1, 5, and 10 at.%). The spectra exhibits a number of broad absorption bands, particularly in the range 400–1100 cm⁻¹. These bands are assigned to the vibration modes of WO₆ groups. These results showed that the bands in the 940–750 cm⁻¹ region can be attributed to symmetric and asymmetric stretching vibrations of the terminal (short) W-O bonds [22]. Stretching vibrations of the longer W-O bonds, which take part in the formation of the (W^O_OW) bridges, were observed in the 460–725 cm⁻¹ region. In the 407–445 cm⁻¹ region lie the bending vibrations of the W-O bonds. These attributions are in good agreement with the previous works of Maczka [22] and Fomichev [24]. However we note that a change appears in the 785–560 cm⁻¹ region for 𝑥=10%. This is probably due to the phase change as mentioned above.

Figure 4 

Room temperature FTIR spectra of LiIn1-xTmx(WO4)2 (𝑥=0, 0.5, 1, 5, and 10 at.%).

3.4. Optical Absorption

The diffuse absoption spectra of LiIn(WO₄)₂ doped with different concentrations of Tm³⁺ are shown in Figure 5(a). They are recorded at room temperature, in the visible and near infrared regions. One can clearly distinguish four groups of peaks which can be assigned to the intraconfigurationnal 4f¹²-4f¹² absorption transitions of Tm³⁺ from the ground state ³H₆, to the ³H₅ (∼1200 nm), ³H₄ (∼800 nm), ³F₃-³F₂ (∼700 nm), and ¹G₄ (∼470 nm) excited states. The intensity of these peaks increases with increasing concentration of Tm³⁺ ions. In the UV region (Figure 5(b)), the spectra present a large band located between 280 and 345 nm and attributed to the W-O charge transfer. This band is very intense compared with the peaks observed in the visible and near infrared regions.

(a)

(b)

Figure 5 

Diffuse absorption spectra LiIn1-xTmx(WO4)2 (𝑥=0, 0.5, 1, 5, and 10 at.%): (a) visible and (b) UV regions.

3.5. Photoluminescence

Figure 6(a) shows the room temperature emission spectra of LiIn_1−𝑥Tm_𝑥(WO₄)₂ (𝑥 = 0, 0.5, 5, and 10 at.%), under 360 nm excitation wavelength (corresponding to the ³H₆→¹D₂ excitation transition). For all the Tm³⁺ concentration, the extended emission spectra between 370 and 900 nm are the same and exhibit two blue emission peaks lying between 435 nm and 455 nm. The maximum of these two bands are centered at 440 and 448 nm.

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(b)

Figure 6 

(a) Room temperature emission spectra of LiIn1-xTmx(WO4)2 (𝑥=0, 0.5, 1, 5, and 10 at.%), excited at 360 nm wavelength. (b) Integrated emission intensity of 1D2 →3F4 transition (447 nm) in LiIn(WO4)2 as a function of Tm3+ concentration.

Referring on the energy level positions in the literature, Pujol et al. [25], have investigated the spectroscopic properties of Tm³⁺-doped KYb(WO₄)₂ single crystals. Therefore, these two peaks are originating from the intra-configurational 4f¹²-4f¹² transitions of the Tm³⁺ ions and assigned them to the ¹D₂→³F₄ emission transition.

By changing the concentration of Tm³⁺ion in LiIn(WO₄)₂, we determine the optimum of compositions with the highest emission intensity. In the emission spectra, the peaks positions did not change as the Tm³⁺ concentration change, which indicate that the crystal structure of Tm³⁺-doped LiIn(WO₄)₂ does not change. It is well known that the emission intensity increases with the activator ions concentration and at defined point of concentration begins to decrease. This phenomenon can be also explained by the quenching concentration phenomenon of the emission. Indeed, the increase of rare earth activator concentration induces the shortening of the RE³⁺–RE³⁺ distance, and it is well known that if the distance of luminescent centers is short enough, energy can transfer among these luminescent centers following a great deal of nonradiative transitions, which leads to the quenching of luminescence [26]. In the other hand, the concentration behaviour of the luminescence is a complex matter, since different energy transfers (mainly cross relaxation) can be involved and several experimental parameters are involved.

The integrated PL intensity of ¹D₂→³F₄ peaks versus Tm content 𝑥 at room temperature is shown in Figure 6(b). It shows that there is a saturation range of PL intensity with the Tm content from 𝑥=0.5 to 5 at. %. As the Tm content 𝑥 increases from 5 to 10 at %, the integrated PL intensity decreases due to the concentration quenching. For this transition, we consider 𝑥=5 at % as the optimum concentration.

Under 360 nm excitation wavelength, for all the doped samples, the ¹G₄→³H₆ emission transition is not observed. It should be also noted that we did not observe this transitions even under the excitation directly of ¹G₄ energy level.

Multiphonon-relaxation from ¹D₂ to ¹G₄ is improbable in view of the large energy gap (around 7000 cm⁻¹), with the maximum phonon energy of 940 cm⁻¹ in LiIn(WO₄)₂ double tungstate (FTIR part); then, we need an important number of phonons (around 8 phonon). It is possible that the cross-relaxation process according the scheme: (¹D₂→³F₃) + (³H₆→³H₄) or (¹D₂ → ³H₄) + (³H₆→³F₃) is the most probably contributed to the relaxation of the ¹D₂ level. The same phenomena have been observed by Macalik et al. [27] in the KLa(WO₄)₂ single crystal doped with Tm³⁺ ions.

Figure 7 displays the excitation spectra of LiIn_1−𝑥Tm_𝑥(WO₄)₂ (𝑥 = 0, 0.5, 5, and 10 at.%), recorded at room temperature in the 230–475 nm region, for 447 nm emission wavelength (¹D₂→³F₄ transition). For all samples the excitation spectra show a wide and intense band centered on 285 nm. In addition, a sharp peak having weaker intensity located at 360 nm is observed. This peak is not observed for the nondoped sample and presents the same variation of their intensity against the Tm³⁺ concentration, identical to the emission spectra (see Figure 6(a)). Based on the energy scheme of Tm³⁺, we have assigned this peak to the ³H₆→¹D₂ excitation transition. The intense band around 285 nm should be assigned to the W-O ligand-to-metal charge transfer states (LMCT) from (WO₆) units in the LiIn(WO₄)₂ host lattice.

Figure 7 

Room temperature excitation spectra of LiIn1-xTmx(WO4)2 (𝑥=0, 0.5, 1, 5, and 10 at.%), monitoring 447 nm emission wavelength (1D2 →3F4 transition).

Room temperature emission spectra of LiIn_1−𝑥Tm_𝑥(WO₄)₂ (𝑥 = 0, 0.5, 5, and 10 at.%), under the excitation wavelength of 285 nm (corresponding to the W-O ligand-to-metal charge transfer) between 400 and 600 nm, are shown on the Figure 8(a). There are two kinds of emission for all doped phosphors observed, a narrow peak at 470 nm originates from the Tm³⁺ ion transition ¹G₄→³H₆, and a broad-band centred around on 450 nm. This broad emission band is ascribed to the radiative decay of the regular lattice centre (WO₄)⁻² or radiative decay of excitons self-trapped (localized) on regular (WO₄)⁻² groups. Tungstate complexes are usually characterized by a broad band emission with large Stokes shift (10000 cm⁻¹ (1.24 eV) to −20000 cm⁻¹ (2.28 eV)) from the absorption [28].

(a)

(b)

Figure 8 

(a) Room temperature emission spectra of LiIn1-xTmx(WO4)2 (𝑥=0, 0.5, 1, 5, and 10 at.%) excited at 285 nm, (b) Integrated emission intensity for 1G4→3H6 transition (470 nm) in LiIn(WO4)2 as a function of Tm3+ concentration.

With increasing Tm³⁺ concentration, the narrow peak intensity decreases with the increasing from 0.5 to 10 at.% (Figure 8(b)) due to the concentration quenching. The intensity of broad-band decreases also with Tm³⁺ concentration in full studied concentration region, which implies that the energy transfer from (WO₄)⁻² to Tm³⁺ increases. We believe that the optimum of compositions with the highest emission intensity for ¹G₄→³H₆ is lower than 0.5 at.%.

On Figure 9(a), the excitation spectra of LiIn_1−x Tm_x(WO₄)₂ (𝑥 = 0, 0.5, 5, and 10 at.%), recorded at room temperature in the 230–410 nm region, for 470 nm emission wavelength (¹G₂→³H₄ transition) are displayed. For all samples the excitation spectra show only a wide and intense band centered on 285 (W-O ligand-to-metal charge transfer states). The presence of the strong absorption band of (WO₆) groups in the excitation spectrum of Tm³⁺ indicates that there exists an energy transfer from (WO₆) groups to ¹G₄ energy level of Tm³⁺ ions in LiIn(WO₄)₂. The nonobservation of ³H₆→¹D₂ excitation transition confirms our suggestion above that there is no multiphonon relaxation between ¹D₂ and ¹G₄. As the Tm³⁺ concentration increases, the W-O ligand-to-metal charge transfer band intensity (285 nm) decreases as shown on Figure 9(b). This is confirming the suggestion that the increasing with the Tm³⁺ concentration, the energy transfer between (WO₄)⁻² excited state and the ¹G₄ level increases.

(a)

(b)

Figure 9 

(a) Room temperature excitation spectra of LiIn1-xTmx(WO4)2, (𝑥=0, 0.5, 1, and 5 at.%), monitoring 470 nm emission wavelength (1G4→3H6 transition) (b) W-O charge transfer excitation band intensity monitoring 470 nm emission (1G4→3H6 transition) as a function of Tm3+ concentration.

No emission transition ¹D₂→³F₄ (447 nm) is recorded despite the existence of the W-O ligand-to-metal charge transfer band in the excitation spectrum monitoring the emission wavelength of this emission transition (see Figure 7). The existence of this band is simply due to the position of the ¹D₂→³F₄ emission transition wavelength (447 nm), which is very close to the emission of (WO₄)⁻² group (maximum 450 nm). Therefore, no energy transfer is occurring between W-O ligand-to-metal charge state and ¹D₂ energy level. on Figure 10, we proposed a tentative energy level diagram and luminescence process mechanism to help in the understanding of the present spectroscopic results for Tm-doped LiIn(WO₄)₂.

Figure 10 

Energy level diagram and luminescence process mechanism of Tm3+ in LiIn(WO4)2.

4. Conclusion

Blue emitting LiIn_1-xTm_x(WO₄)₂ (𝑥 = 0, 0.5, 1, 5, and 10 at.%) polycrystalline powders phosphors have been successfully synthesized by means of classical solid-phase reaction by means of classical solid-phase reaction. For all Tm³⁺ concentrations, optical spectra recorded at room temperature exhibit the absorption of ^2S+1L_J energy levels of Tm³⁺ as well as the intrinsic W-O charge transfer bands. Blue intraconfigurationally emissions transitions ¹D₂ →³F₄ and ¹G₄→³H₆ of Tm³⁺ are observed. Optimum concentration quenching for the ¹D₂ → ³F₄ transition appears to be about 5 at % Tm³⁺, whereas the ¹G₄→³H₆ transition will probably be optimum for a concentration less than 0.5 at.% Tm³⁺. Blue radiative decay of the regular lattice centre (WO₄)^-2 is also observed. No energy transfer exists between the W-O charge transfer state and ¹D₂ level nor between ¹D₂ and ¹G₄; on the other side, it exists between W-O charge transfer state and the ¹G₄ level. Because LiIn_1-xTm_x(WO₄)₂ (𝑥 = 0, 0.5, 1, 5, and 10 at.%) phosphors exhibit intensive blue emission and under UV excitation, it is considered to be a new promising blue phosphor for field emission display application.

Acknowledgments

The authors wish to thank the help of O. Viraphong (ICMCB) and D. Tahtat (CRNA) for sample preparation, S. Boutarfaia (CRND) for the ATD analysis, H. Menari and S. Belhousse (UDTS) for performing the diffuse reflection and FTIR measurements, and Dr N. Souami (CRNA) for SEM images.