Structural, vibrational study and UV photoluminescence properties of the system Bi(2−x)Lu(x)WO6 (0.1 ≤ x ≤ 1)

The bismuth lutetium tungstate series Bi(2−x)Lu(x)WO6 with 0.1 ≤ x ≤ 1 were synthesized by solid state reaction of oxide precursors at 1000 °C for 3 h. The as-prepared polycrystalline compounds were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) analyses. Biphasic samples were obtained in the composition range of 0.1 ≤ x ≤ 0.3. Solid solutions were obtained in the composition range of 0.4 ≤ x ≤ 1, and their monoclinic crystal structure was refined using the Rietveld method. SEM micrographs showed that solid solutions presented homogeneous morphologies. Attributions of Raman vibrational modes were proposed. A shift in the vibrational wavenumber depending on the lutetium composition was observed. The specific broadening of the spectral bands was interpreted in terms of long range Bi/Lu disorder and local WO6 octahedron distortions in the structure. The PL experiments were performed under UV-laser light irradiation. Each PL band was decomposed into three Gaussian components with energies close to 1.25, 1.80 and 2.1 eV. Their integrated intensities increased with the value of x. The presence of the near infrared band at 1.25 eV is discussed.


INTRODUCTION
In the general framework of the development of multifunctional materials for various applications, we focus our attention on the correlations between structure and luminescence properties, in tungstate based materials. Generally, it is well established that extrinsic and intrinsic point defects can play a prominent role in photoluminescence properties.
Tungstate materials were extensively studied and investigated because of their potential applications in many fields as functional materials in high=performance luminescent materials 1 4 , catalysts 5 6 , scintillators 7 , laser hosts 8 as well as microwave applications 9 10 and humidity sensors 11 . Metal tungstates with general formula AWO 4   UV Photoluminescence. The equipment used to perform the measurements of photoluminescence (PL) under UV excitation is a spectrometer Horiba Jobin=Yvon HR800 LabRam. The entrance slit, positioned behind the filter, is a diaphragm whose diameter can

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range from 50 to 500 Om. The irradiated zone was limited to 1 µm in diameter for all samples.
The polycrystalline samples were in form of compacted pellets obtained under a fixed pressure of 5 kbar. The spherical mirror, characterized by an 800 mm focal length, allows reflecting the scattered radiation from the input to the dispersive grating to obtain spectra slot.
The 364.5 nm (3.40 eV) line of an Ar=ion laser was used as the excitation source. The power applied to the samples was fixed to 0.005 mW with an acquisition time set to 100 ms. Figure 1 illustrates the XRD patterns of the synthesized compounds from x = 0.1 to x=1. A biphasic system is observed in the composition range 0.1≤ x≤0.3. Characteristic peaks of the orthorhombic bismuth tungstate Bi 2 WO 6 were detected in addition to the peaks of the monoclinic structure BiREWO 6 (RE=Lu) phase. It should be recalled that all the as=prepared ceramics were thermally treated at 1000°C and were highly crystallized. The substituted compounds ranging from 0.4≤ x≤1 present a monoclinic structure and constitute a solid solution. This structural modification (orthorhombic/monoclinic) can be explained from the two electronic configurations of Bi 3+ and Lu 3+ : in the case of the Bi 3+ cations (electron configuration 6s 2 6p 0 ), the lone pair (6s 2 ) could play a spatial role in the structure organization, which is not the case for the Lu 3+ cations. In addition, the ionic size of Lu 3+ cations, smaller than the one of Bi 3+ cations, argues in favor of the formation of a solid solution having the structure of the limit phase BiLuWO 6 .

III.1. X Ray Diffraction analyses
The identification of these compounds was firstly obtained from the standard JCPDS files (Joint Committee standards for Powder Diffraction) 37 in which the standard phases BiYWO 6 , BiNdWO 6 , BiGdWO 6 and H=Bi 2 WO 6 were referenced. Figure 2 shows that the diffraction peaks of the monoclinic solid solution are shifted to higher angles as x increases.  The structural parameters of the solid solution ranging from 0.4≤ x≤1 were refined using FullProf suite software [38] which allows refinement of atomic coordinates, site occupancies and atomic displacement parameters as well as profile parameters (instrument parameters, background parameters, lattice constants and peak shape). The disordered model based on the work of Berdonosov and co=workers 39,40 on the series Bi 2=x Ln x WO 6 (Ln=Lanthanide) was used in our Rietveld calculations. In this model, Lu atoms are assumed to be distributed on the two bismuth sites of the H=Bi 2 WO 6 monoclinic structure characterized by a centrosymmetric A2/m space group. Such a substitution should involve specific RSC Advances Accepted Manuscript disordered distortions of the octahedral WO 6 groups, due to alternation of short Lu=O and long Bi=O bonds.

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We refined the atom coordinates of the heavy atoms (Bi/Lu) and W, including their individual Debye factors and keeping fixed the oxygen coordinates, using a monoclinic A2/m space group. The occupancy factors of all atoms were fixed in agreement with the global composition x. We obtained a significant goodness of fit and the factors R p , R exp , R wp and R B are quite reliable. Table 1a reports the final calculated cell parameters for the solid solution samples. Table 1b        The monoclinic structure of bismuth lutetium tungstates can be considered as being close to a wolframite=type structure. Figure 6 shows the alternating (Bi,Lu) 2 O 2 layers and the WO 6 edge sharing octahedra. Crystallographic data obtained from our Rietveld calculations showed that the WO 6 octahedral complexes have irregular shapes with short, medium and long bonds ranging from 1.65 Å to 2.13 Å.

III.2. Scanning electron microscopy
The SEM micrographs reported in Figure 7 show the morphology of the synthesized Bi (2= x) Lu (x) WO 6 : • The EDS microanalysis is congruent with the nominal chemical composition in heavy atoms of Bi (2=x) Lu (x) WO 6 and no significant variation in composition can be observed at a local scale.

III.3. Raman spectroscopy analyses
In the case of Bi 2=x RE x WO 6     involving increasing disorder in oxygen positions of WO 6 octahedra. This should be a major argument for the disorder model used in the Rietveld refinements to interpret our XRD data.

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As already reported for the BiLuWO 6 sample, the Raman bands can be highlighted as follows: • The Raman peaks at 904 and 760 cm =1 can be ascribed to the symmetric and antisymmetric stretching modes of the octahedra WO 6 , which involve the apical motion of oxygen atoms perpendicular to the layers.
• The peaks in the region of 720=640 cm =1 can be associated to the asymmetric stretching of octahedra involving equatorial motions of oxygen atoms within layers 43 47 .
• The bands in the mid=region 370=589 cm =1 represent the bending modes of WO 6 and stretching=bending modes of (Bi, Lu)O n .
• Some bands are well defined in the spectral range 180=370 cm =1 and are related to bending modes of the oxygen in Bi=O polyhedra of the Bi 2 O 2 layers.

III.4. Photoluminescence properties
By the past, the luminescence in scheelite tungstates was interpreted in terms of electronic charge transfers in the WO 4 2= complex oxyanions or in WO 3 defect centers 50 . Using a molecular orbital model for the octahedral WO 6 6= oxyanion, Von Oostehort et al. 51 showed that the excited state consisted of an electron initially on an oxygen 2p orbital (O2p) and occupying the tungsten 5d orbitals (W5d) with t 2g symmetry. The photoluminescence bands obtained under UV excitation (energy of 3.40 eV) are reported on Figure 9. The emission bands present energies ranging between 1.1 and 2.8 eV.
The PL bands have been decomposed into three Gaussian components (G1, G2, G3). The resulting fit is quite satisfactory. The multi=Gaussian decomposition analyses are shown in Figure 9. Table 4 gives the characteristics of the fitted gaussian functions: energies of maximum, integrated intensities and FWHM.  In Figure 10, each Gaussian is characterized by its intensity and its energy as a function of the composition x. The energies of the components G1 (close to 1.27eV) and G2 (close to 1.78eV) are quasi=independent of the composition x. As x increases, G3 presents a shifting to lower energies from 2.16 eV to 1.92 eV.

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The G1 component appears as being a narrow band for high lutetium content, strongly related to the increasing lutetium composition.

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The high energetic component G2, which has a maximum at x=0.8, is attributed to the allowed transition 1 T 1u 1 A 1g of the tungstate groups. Whereas the G3 component presents the same behavior as the G2, it increases with x till a maximum at x =0.8, this component could be linked to the 3 T 1u 1 A g transitions.
The integrated intensities of the experimental data are shown in Figure 11. It should be noted that the intensities of all components increase strongly in the composition range 0.1≤x≤0.4 corresponding to the biphasic system. The total intensity (G1+G2+G3) is quasi= constant for compositions x>0.4 corresponding to the solid solutions, despite the fact that the intensity of the small G1 component increases slowly as x increases. In the biphasic system, this can be due to the progressive formation of the second (Bi=Lu) phase coexisting with the inactive Bi 2 WO 6 phase, while, in the (Bi=Lu) monoclinic solid solution, the activation of luminescence (G1, G2 and G3 components) should occur. As the G2 and G3 intensities remain quasi=constant, this luminescence could be ascribed to charge transfers (or defect centers) linked to the WO 6 groups of the monoclinic structure.

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In this study, we have investigated the complex tungstate system (1=x)Bi 2 WO 6 = xBiLuWO 6 or Bi 2=x Lu x WO 6 and observed the existence of a mix system for x<0.4 and a solid solution for 0.4≤x≤1. Rietveld refinements of the monoclinic structures of the solid solution system showed that the Lu 3+ cations are distributed on the Bi sites. X=ray diffraction analyses coupled with Raman spectroscopy results showed the existence of disordered distributions of Lu and Bi atoms, associated with local distortions.
The photoluminescence under monochromatic UV excitation seems to be strongly related to the formation of the monoclinic structure induced by the substitution of bismuth by lutetium.
It has been decomposed into two types of emissions: the classical emission of tungstate groups WO 6 6= with two components due to charge transfers "W5d O2p" in the case of octahedral coordination, and a specific emission (narrow band at 1.25 eV) strongly related to the presence of lutetium. The exact origin of this NIR emission is not clearly established. If we refer to literature results on tungstates, this NIR emission might be due to defect centers due to oxygen vacancies or defect centers due to bismuth species with different valences or different molecular clustering.