Dopant effect of y on optical properties of ZnWO4 ceramics

JOURNAL OF SCIENCE OF HNUE Interdisciplinary Science, 2013, Vol. 58, No. 5, pp. 17-21 This paper is available online at DOPANT EFFECT OF Y ON OPTICAL PROPERTIES OF ZnWO4 CERAMICS Nguyen Manh An Faculty of Physics, Hong Duc University Abstract. We investigate the effect of the rare earth ion, Y on the structure, absorption and Raman spectroscopy of ZnWO4 ceramics. In the XRD patterns of Ce doped ZnWO4, some foreign peaks were found and an anormalous change in cell parameter appeared around x = 0.15. This indicates that the Ce ion has an effect on the structure of ZnWO4 and suggests a solubility limit of Ce in ZnWO4 ceramics. In addition, we also calculated the Raman active modes by using group theory and we received 18 modes in Raman active and 16 modes in IR active. The absorption measurement indicates the band gap of ZnWO4 decreases with increasing Y content. The reasons for the above changes are discussed in this presentation. Keywords: Effect, Y, structure, absorption, Raman spectroscopy, ZnWO4 ceramics. 1. Introduction Tungstate crystals are perspective scintillating materials for x-ray detectors’ γ-ray medical tomographs. In particular, the ZnWO4 crystal is a well-known scintillator emitting light at 480 nm under UV, X-ray and γ-ray excitation. An important parameter of scintillating material is light emission efficiency and that depends on the transparency of the host crystal in the visible spectral region. Parasite absorption is usually caused by various defects of the lattice. Therefore, investigating the nature of defects responsible for absorption in the ZnWO4 is needed in order to solve material science problems. Doping of rare-earth ions into ZnWO4 lattice are expected to influence its chemical and physical properties. However few studies have been reported on doped-ZnWO4 compared to that of other inorganic compounds [1, 2]. Furthermore, the doping causes a disorder in structure. The disorder in doped rare earth ZnWO4 ceramics is expected to vary strongly depending on the doping level and temperature. The dynamic disorder is directly related to electronic processes and localization in the insulating phase of this material. Therefore, monitoring the disorder is of significant interest in order to understand the interplay of structural and optical properties. Raman spectroscopy is an efficient tool for the study of structural Received April 17, 2013. Accepted June 2, 2013. Contact Nguyen Manh An, e-mail address: nguyenmanhan@hdu.edu.vn 17 Nguyen Manh An disorder, including that which is dynamic. There were some reports on Raman scattering of pure ZnWO4 in single crystal or ceramic [3, 4] but there’s been little concern about rare earth doped ZnWO4 [5]. Therefore, in this presentation we investigate the role of rare earth ion doped ZnWO4 on the structure, Raman spectroscopy and absorption of this compound. 2. Content 2.1. Experiment Zn1−xYxWO4 (x = 0.0, 0.1, 0.2, 0.3 and 0.4) samples were prepared using a modified solid-state-reaction method which adopted much faster heating and cooling rates in the sintering process than those employed in the conventional method. The initial powder material for the synthesis was prepared by mixing appropriate amounts of ZnO (Sigma-Aldrich, > 99.0%), WO3 (Sigma-Aldrich, > 99.9%) and Y2O3 (Sigma-Aldrich, > 99.9 %), which were ground for 4 hrs in isopropyl alcohol. The powders were thereafter pressed into disks 10 mm in diameter and calcined at 600 0C for 6 hrs. The resulting pellets were further treated with repeated grinding in isopropyl alcohol for 4 hrs. The powders were then pressed into disks 10 mm in diameter and 5 mm in thickness, sintered at 850 0C for 10 hrs with a heating rate of 10 oC/min and finally cooled at the rate of 5 0C/min. Structural characterization was performed by means of X-ray diffraction using a D5005 diffractometer with Cu Kα radiation and with 2θ varied in the range of 20 - 700 at a step size of 0.020. The photoabsorption of Zn1−xYxWO4 was measured by UV-visible diffuse reflectance spectrometry (Jasco 670 UV-vis spectrometer). Raman measurements were performed in a back scattering geometry using a Jobin Yvon T 64000 triple spectrometer equipped with a cryogenic charge-coupled device (CCD) array detector and operated with a 514.5 nm line Ar ion laser. The photoluminescence spectra were recorded using a measurement system for optical properties of materials (USA). 2.2. Results and discussions Figure 1 shows the x-ray diffraction patterns of Zn1−xYxWO4 (x = 0.0, 0.05, 0.10, 0.15, 0.20). The XRD patterns are in excellent accord with previous powder data of JCPDS Card No. 89-0447. Furthermore, for the samples with x > 0.1, second phase peaks attributed to the Y rich phase (asterisk in Figure 1) were observed. However, the remaining peaks in the XRD traces are related to monoclinic structure while the second phase peaks apparently increase in the XRD data of the sample with x > 0.10. For Y doped ZnWO4, all peaks are indexed according to the P2/c (C 4 2h) cell of ZnWO4. The lattice parameters deduced for the pure ZnWO4 monoclinic unit cell were found to have values a = 4.70, b = 5.70 and c = 4.90 A˚. This is in agreement with previous results [6]. In the range of x = 0.00 to x = 0.20, the cell parameters decrease with increasing Y content. This cell parameter increase (Figure 2) indicates that the Y ions have indeed replaced the ion site in the unit cell. 18 Dopant effect of Y on optical properties of ZnWO4 ceramics Figure 1. XRD patterns of Zn1−xCexWO4 ceramics Figure 2. Cell parameter vs. Ce content The further effect of Y substitution is described by the Raman spectra of the Zn1−xYxWO4 ceramics which are plotted in Figure 3 with respect to variation of Y concentration x at room temperature. The selection rules for the Raman active modes in monoclinic P2/c (C42h) symmetry predict only 18 active Raman phonons with Ag and Bg symmetries, according to the rule of decomposition in terms of irreducible representations, ΓRaman/IR = 8Ag + 7Au + 10Bg + 9Bu. In polarized Raman scattering, the Ag modes can be observed by parallel polarization while the Bg modes can be observed by both parallel and crossed polarizations. Since all these modes fall into the frequency range below ∼ 700 cm−1, most of the Raman studies have focused on this region. However, in this case, the change appears in the range from 750 to 950 cm−1 as shown in Figure 3. Figure 3. Raman spectra of Zn1−xYxWO4 ceramics 19 Nguyen Manh An In general, the internal vibrations of a tightly bound group of atoms have higher frequencies than the frequencies of vibrations which occur when the more loosely bound groups vibrate against each other. However, in the case of ZnWO4, the WO6 octahedra share oxygen atoms and it is more difficult to clearly differentiate between internal and external vibrational modes. Notice that there are 4Ag and 2Bg modes assigned to the internal vibrations, in agreement with the group theoretical analysis. Also notice that the remaining modes have frequencies that increase more rapidly with increased doped content. This is in agreement with the result obtained from XRD data. This also suggests that Y dopant causes a change in structure or a disorder in samples that affects the symmetry of the crystal. From this discussion, it is clear that the solubility limit of Y in ZnWO4 ceramics is about 0.1. In order to determine the optical band gap for the Y doped tungstate materials, diffuse reflectance measurements were carried out. Figure 4 shows the diffuse reflectance spectra of the Y doped ZnWO4 samples. For a crystalline semiconductor, it was shown that optical absorption near the band edge follows formula αhυ = A(hυ − Eg)n/2 [7] where α, υ, Eg, and A are the absorption coefficient, the light frequency, the band gap, and a constant, respectively. Among them, n decides the characteristics of the transition in a semiconductor. According to the equation, the value of n for ZnWO4 was 1. The band gap of the pure ZnWO4 was estimated to be 3.27 eV from the onset of the absorption edge. For W-based semiconductors, it was already found that excitons are formed due to transitions into the tungstate W5d states hybridized with O2p and they possess a very strong tendency for self-trapping [8]. The band gap of Zn1−xYxWO4 decreases with the increase of Y content (the inset of Figure 4). Figure 4. Absorption spectra of Zn1−xYxWO4 ceramics (The inset shows the band gap vs. Y content) 20 Dopant effect of Y on optical properties of ZnWO4 ceramics 3. Conclusion The solid-state approach has been employed to synthesize Y doped tungstate materials, Zn1−xYxWO4 ceramics. Optical absorption edge energies for the tungstates synthesized in this study decrease as the Y content increases. In Raman spectra, there appear to be some new peaks when Y content is about 0.05, suggesting the existence of a new phase or disoder. This is in agreement with structural analysis. The luminescent spectra exhibit broad blue-green emission bands which peaked at 495 nm with a shoulder at 505 nm. Doped samples with x < 0.2 exhibited a strong luminescence e and samples with x > 0.2 gave weak luminescence. The received result suggests a solubility limit of Y in ZnWO4 ceramics. REFERENCES [1] F. Wen, X.Zhao, H. Huo, J.-S. Chen, E. Lin, J. Zhang, 2002. Mater. Lett. 55, p. 152. [2] Q. Zhang, X. 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