Interdisciplinary Science, 2013, Vol. 58, No. 5, pp. 17-21 This paper is available online at http://stdb.hnue.edu.vn
DOPANT EFFECT OF Y ON OPTICAL PROPERTIES OF ZnW04 CERAMICS Nguyen Manh An
Faculty of Physics, Hong Due University
Abstract. We investigate the effect of the rare earth ion, Y on the structure, absorption and Raman spectroscopy of ZnW04 ceramics. In the XRD patterns of Ce doped ZnW04, 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 ZnW04 and suggests a solubility limit of Ce in ZnW04 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 ZnW04 decreases with increasing Y content. The reasons for the above changes are discussed in this presentation.
Keywords: Effect, Y, structure, absorption, Raman spectroscopy, ZnW04 ceramics.
1. Introduction
1\ingstate crystals are perspective scintillating materials for x-ray detectors' 7-ray medical tomographs. In particular, the ZnW04 crystal is a well-known scintillator emitting light at 480 nm under UV, X-ray and 7-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 ZnW04 is needed in order to solve material science problems. Doping of rare-earth ions into ZnW04 lattice are expected to influence its chemical and physical properties. However few studies have been reported on doped-ZnW04 compared to that of other inorganic compounds [1,2]. Furthermore, the doping causes a disorder in structure.
The disorder in doped rare earth ZnW04 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 mterest 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: [email protected]
disorder, including that which is dynamic. There were some reports on Raman scattering of pure ZnW04 in single crystal or ceramic [3, 4] but there's been little concern about rare earth doped ZnW04 [5]. Therefore, in this presentation we investigate the role of rare earth ion doped ZnW04 on the strucmre, Raman spectroscopy and absorption of this compound.
2. Content 2.1. Experiment
Zni_^Ya.W04 (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 °C for 6 hrs. The resulting pellets were fm-ther 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 °C for 10 hrs with a heating rate of 10 "C/min and finally cooled at the rate of 5
•^C/min.
Structiu-al characterization was performed by means of X-ray diffraction using a D5005 diffractometer with Cu Ko; radiation and with 29 varied in the range of 20 - 70" at a step size of 0.02^*. The photoabsorption of Zni_iYj;W04 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 Zni_iYj:W04 (x = 0.0,0.05,0.10,0.15,0.20). The XRD patterns are in excellent accord witii 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 I) were observed.
However, the remaining peaks in the XRD traces are related to monoclinic sti"ucture while the second phase peaks apparently increase in the XRD data of the sample with x > 0.10.
For Y doped ZnW04, all peaks are indexed according to the P2/c (C^^) cell of ZnW04.
The lattice parameters deduced for the pure ZnW04 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 a; = 0.00 to 3: = 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.
23 30 40 93 eo 7D
Figure 1. XRD patterns ofZni-^Ce^W04 ceramics
QGO Q05 010 Q15 033
Yccria^
Figure 2. Cell parameter vs. Ce content
The further effect of Y substitution is described by the Raman specti-a of the Zni_a:Yj;W04 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 (Cl^) symmetry predict only 18 active Raman phonons with Ag and Bp symmetries, according to the rule of decomposition in terms of irreducible representations, rRaman/iR = 8Ag + 7Au + lOB^ + 93^. 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~\ most of the Raman studies have focused on this region. However, in this ease, the change appears in the range from 750 to 950 em"^ as shown in Figure 3.
600 700 800 900 1C00
V\&i«nLrrter(ari^)
Figure 3. Raman spectra of Zni-xYxW04 ceramics
19
Nguyen Manh An
In general, the internal vibrations of a tightiy 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 ZnW04, the WOg octahedra share oxygen atoms and it is more difficult to clearly differentiate between internal and external vibrational modes. Notice that there are 4Ag and 2B(, 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 m agreement with the result obtained from XRD data. This also suggests tiiat 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 ZnW04 ceramics is about 0.1.
In order to determine the optical band gap for the Y doped tungstate materials, diffuse refiectance measurements were carried out. Figure 4 shows the diffuse reflectance spectra of the Y doped ZnW04 samples. For a crystalline semiconductor, it was shown that optical absorption near the band edge follows formula ahv = A{hv — Eg)^^"^ [7]
where a, v. 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 ZnW04 was 1. The band gap of the pure ZnW04 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 Wsd states hybridized with Gap and they possess a very strong tendency for self-trapping [8]. The band gap of Zni_iYa:W04 decreases with the increase of Y content (the inset of Figure 4).
250 333 383 4CD 45C
\Aa^ength(tTT)
Figure 4. Absorption spectra ofZni^^Y^WO^ ceramics (The inset shows the band gap vs. Y content)
20
Dopant effect of Y on optical properties ofZnWOi ceramics
3. Conclusion
The solid-state approach has been employed to synthesize Y doped tungstate materials, Zni_iYiW04 ceramics. Optical absor])tion 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 witli structural analysis. The luminescent spectra exhibit broad blue-green emission bands which peaked at 495 nm with a shoulder at 505 mn. 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 ZnW04 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. Chen, Yu Zhou, G. Zhang, S. Yu, 2007. J. Phys. Chem. Clll, pp.
3927-3933.
[3] G. Huang, Y. Zhu, 2007. Materials Science and Engineenng, B139, pp. 201-208.
[4] X. Zhao, W. Yao, Y. Wu, S. Zhang, H. Yang^ Y. Zhu, 2006. Journal of Solid State Chemistry, 179, pp. 2562-2570. '
[5] F. Yang, C. Tu, H. Wang,Y. Wei, Z. You, G. Ji'a, J. Li, Z. Zhu, X. Lu, Y. Wang, 2008.
Journal of Alloys and Compounds, 455, pp. 269-273.
[6] H. Wang, F.D. Medina, YD. Zhou and Q.N. Zhang, 1992. Physical Review B45, pp.
10356-10362.
[7] M.A. Butler, 1977. J. Appl. Phys. 48, p. 1914.
[8) V. Nagirnyi, M. Kirm, A. Kotlov, A. Lushchik, L. Jonsson, 2003. J. Lumin. 102, p.
597.
