ENERGY-DISPERSIVE X-RAY FLUORESCENCE ANALYSIS OF MONO- AND POLYCRYSTALS OF SELENIDE SPINELS BY FUNDAMENTAL PARAMETER METHOD

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1 322 ENERGY-DISPERSIVE X-RAY FLUORESCENCE ANALYSIS OF MONO- AND POLYCRYSTALS OF SELENIDE SPINELS BY FUNDAMENTAL PARAMETER METHOD ABSTRACT Rafa Sitko, Beata Zawisza, Ewa Malicka Institute of Chemistry, Silesian University, Katowice, Poland The analysis of mono- and polycrystals of selenide spinels of general formula M x N y Cr z Se 4 (where M +2 and N +3 are cations of metals, e.g. Zn +2,V +3,Ga +3 ) by energy-dispersive X-ray fluorescence spectrometry (EDXRF) is presented. The monocrystals of spinels (sizes from ca. 1 to 5 mm) were analyzed without any treatment. The polycrystals were powdered and pressed into pellets of 10 mm in diameter with liquid binder (polyvinylpyrrolidone/methylcellulose solution). A tungsten pinhole collimator was placed in front of the X-ray tube to reduce the size of the analyzed area. The monocrystals were analyzed using pinhole collimators of 200 or 400 m holes (280, 581 m focal spot sizes) depending on the size of the analyzed crystal, whereas the polycrystalline spinels pressed into pellets were analyzed using 1000 m collimator (1469 m focal spot size). The quantitative analysis was performed using the fundamental parameters method with and without reference samples. The agreement between the results of standardless analysis and the results obtained using reference samples was quite satisfactory and the average relative difference between both quantification methods was ca. 3%. The synthesized monocrystals have well-formed flat faces. Nevertheless, various positions of a monocrystal can strongly influence EDXRF analysis. Thus, the dependence of quantitative analysis results on the monocrystal position was studied. The accuracy of the EDXRF results was also verified using inductively coupled plasma optical emission spectrometry (ICP-OES). INTRODUCTION Spinel solid solutions are already applied and are also promising materials in electronics and other branches of technology. The general chemical formula of selenide spinels can be expressed as follows: M x N y Cr z Se 4, where M +2 and N +3 are cations of metals, e.g. Zn +2, V +3, Ga +3. Polycrystalline spinels can be synthesized by means of solid-state reaction, whereas single crystals are grown by the chemical vapour transport method. The stoichiometry strongly influences the electric and magnetic properties of the synthesized spinel (Rudolf et al., 2007; Gro et al., 2007; Malicka et al., 2008). Thus, the chemical composition of synthesized spinels has to be determined. In previous investigations, the selenide spinels were digested prior to the X- ray fluorescence (XRF) analysis (Jurczyk et al., 1993; Jurczyk et al., 1999). Nevertheless, a lot of the synthesized single crystals have unique properties. Therefore, nondestructive methods are much more appropriate for the analysis of selenide spinels, especially in the case of monocrystals. Sitko et al. (2008) presented nondestructive standardless fundamental parameter (FP) analysis of spinels using an EDXRF spectrometer with monochromatized primary radiation. Because samples were excited using monochromatized radiation, the quantitative calculations were much simpler and the analysis error resulting from the uncertainty of the X-ray tube spectral distribution was completely eliminated.

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3 323 In this paper FP analysis of mono- and polycrystals of selenide spinels performed by means of a home-constructed EDXRF spectrometer is presented. The designed spectrometer with pinhole collimators of sizes ranging from 50 to 2000 m is described in details in a previous paper (Sitko et al., 2009). The serious advantage of pinhole collimators over capillary optics is the fact that X-ray tube spectral distribution is not modified, which is important in theoretical matrix correction methods, especially in standardless analysis. In this paper, quantitative analyses of selenide spinels performed using reference samples and also standardless method are presented and compared. EXPERIMENTAL Mono- and polycrystals of selenide spinels of general formula M x N y Cr z Se 4 (where M +2 and N +3 are cations of metals, e.g. Zn +2, V +3, Ga +3 ) were analyzed by EDXRF spectrometry. The measurements were performed by means of a home-constructed EDXRF spectrometer (Sitko et al., 2009). The samples were excited by an air-cooled side-window Rh target X-ray tube of 125 m thickness Be window and ca. 100 m nominal focal spot size (XTF 5011/75, Oxford Instruments, USA). The X-ray tube was supplied with an XLG high-voltage generator (Spellman, USA). A W pinhole collimator was used to reduce the size of the analyzed area. For the collimators with hole sizes of 100, 200, 400, 1000 and 2000 m, the following focal spot sizes are obtained: 169, 280, 581, 1469, 2888 m. The X-ray spectra of the samples were collected by thermoelectrically cooled Si-PIN detector (XR-100CR Amptek, Bedford, MA, USA) of 6 mm 2 active area, 500 m crystal thickness and 12.5 m Be window thickness. The Si-PIN detector cooled to the temperature of ca. -55 C reaches a resolution of 145 ev at 5.9 kev. The Si-PIN detector was coupled to a multichanel analyzer (PX4 Amptek, Bedford, MA, USA). In the constructed spectrometer, the incidence and take-off angles were 45. The position of the sample moved using the X-Y stage was monitored by CCD camera and two laser pointers. RESULTS AND DISCUSSION Polycrystalline spinels were synthesized by means of solid-state reaction. Although this method of synthesis allows obtaining polycrystals of stoichiometry close to the assumed one, the composition has to be controlled. The polycrystals of spinels were powdered and pressed into pellets of 10 mm in diameter with liquid binder (polyvinylpyrrolidone/methylcellulose solution) and analyzed using pinhole Fig. 1. EDXRF spectrum of ZnVCrSe 4 pressed collimator of the size hole of 1000 m into pellets of diameter 1cm. giving focal spot size of 1469 m. Fig. 1 shows an exemplary EDXRF spectrum of ZnVCrSe 4 collected in air using a 1000 m pinhole collimator. Table 1 presents the EDXRF analysis of ZnV x Cr 2-x Se 4 polycrystalline spinels performed by means of FP method with and without reference samples. Multielement reference samples were prepared by mixing an exact amount of powdered metals (Zn, V, Cr, Se) with liquid binder (polyvinylpyrrolidone /methylcellulose

4 324 solution) to obtain a homogeneous mixture and then pressing in the form of pellets of 1 cm in diameter. The agreement between the results of standardless analysis and the results obtained using reference samples is quite good and the average relative difference between both quantification methods is 2.8%. In some cases, the differences can reach 6% rel. or even 8.5% rel. (determination of Cr in Zn 1.00 V 1.75 Cr 0.25 Se 4 ). Nevertheless, the high relative difference observed for Cr results from small amounts of this element. The results of EDXRF analysis were also compared with inductively coupled plasma optical emission spectrometry (ICP-OES). The agreement between EDXRF and ICP-OES results is at least good. The average relative difference between ICP-OES and EDXRF using reference samples is ca. 1%, although in some cases, the differences reached 2.5% rel. Some poorer agreement is observed for standardless EDXRF - the average relative difference between ICP-OES and EDXRF is ca. 3.2%. Table 1. Analysis of ZnV x Cr 2-x Se 4 polycrystalline spinels (Measurement conditions: 30 kv; 200 A, 1000 m collimator, air, measurement time 5 min). Results are in % (m/m). Element Assumed concentration ICP-OES Standardless EDXRF EDXRF with standard (a) Zn 1.00 V 1.00 Cr 1.00 Se 4 Zn (b) Zn 1.01 V 0.98 Cr 1.09 Se 4 V (c) Zn 0.98 V 1.03 Cr 1.03 Se 4 Cr (d) Zn 0.99 V 1.05 Cr 1.01 Se 4 Se (a) Zn 1.00 V 1.50 Cr 0.50 Se 4 Zn (b) Zn 1.11 V 1.45 Cr 0.51 Se 4 V (c) Zn 1.08 V 1.45 Cr 0.49 Se 4 Cr (d) Zn 1.05 V 1.44 Cr 0.48 Se 4 Se (a) Zn 1.00 V 1.75 Cr 0.25 Se 4 Zn (b) Zn 1.02 V 1.73 Cr 0.27 Se 4 V (c) Zn 0.99 V 1.73 Cr 0.25 Se 4 Cr (d) Zn 0.98 V 1.75 Cr 0.26 Se 4 Se (a) assumed stoichiometry, (b) stoichiometry determined by standardless EDXRF, (c) stoichiometry determined by EDXRF using reference samples, (d) stoichiometry determined from ICP-OES a) b) Fig. 2. The relationchips between assumed concentration and concentration determined by standardless EDXRF in polycrystals of ZnV x Cr 2-x Se 4 ; (a) V (b) Cr.

5 325 a) b) Fig. 3. EDXRF spectrum of ZnGaCrSe monocrystal (a) and synthesized monocrystals (b). The chemical formulas of synthesized ZnV x Cr 2-x Se 4 polycrystalline spinels calculated from determined element concentrations are also included in Table 1. The agreement between the assumed and determined composition is quite good. Fig. 2 presents the relationships between the assumed and determined concentration of V and Cr in a series of spinels of general formula ZnV x Cr 2-x Se 4 for x ranging from 0.25 to 1.9. Apart from the polycrystalline spinels, the single crystals synthesized by chemical vapour transport method were analyzed by EDXRF. The chemical vapour transport process leads to a growth of a few dozen monocrystals of sizes from ca. 0.5 to 5 mm. The constructed spectrometer allows selecting an appropriate pinhole collimator depending on the size of the analyzed monocrystal. Moreover, the applied pinhole collimators do not modify the X-ray tube spectral distribution, which is important in FP methods, especially in standardless analysis. Fig. 3 presents the EDXRF spectrum of a ZnCr 2 Se 4 monocrystal doped with Ga collected using 400 m pinhole collimator. The chemical composition of the synthesized monocrystal determined by standardless EDXRF ( % Zn, Fig. 4. Analysis of ZnGaCrSe at various positions.

6 % Ga, % Cr, % Se) was in good agreement with the ICP-OES analyses (14.5% Zn, 0.27% Ga, 22.6% Cr, 62.6% Se). High quality results of standardless FP method can be obtained if the assumed measurement geometry, i.e. the incidence and take-off angles of 45, is assured. Fortunately, the synthesized monocrystals have well-formed flat faces. Nevertheless, various positions of a monocrystal can strongly influence EDXRF analysis. Thus, the dependence of the quantitative analysis results on the monocrystal position was studied. Fig. 4 presents the relationship between fluorescent radiation intensity (Zn, Cr, Se and Ga, values on Y-axis are divided by 100) and the position of the Zn 1.07 Ga 0.02 Cr 2.09 Se 4 monocrystal with a size of 1.4 mm for 400 m pinhole collimator (581 m focal spot size). Fig. 4 also presents uncertainty intervals ( 3S) calculated from five measurements at the same position of the monocrystal. The measurement results are within or are very close to uncertainty intervals when the X-ray beam coverstheanalyzedmonocrystal.fig.4also shows the concentration of Zn, Cr, Se and Ga calculated by standardless FP at each position of the Zn 1.07 Ga 0.02 Cr 2.09 Se 4 monocrystal. It can be observed that analysis results are within uncertainty intervals in a wider range of monocrystal position even if X-ray beam covers the analyzed monocrystal only partially. This can be explained by the fact that all elements in monocrystal are quantified and results are normalized during FP calculations. Tables 2 and 3 present a standardless analysis of Zn 1.07 Ga 0.02 Cr 2.09 Se 4 monocrystal at the central position and at positions 800 m and 1000 m off center, i.e. when monocrystal is partially excited by the primary X-ray beam. In such cases, fluorescent radiation is emitted not only from the front face of the monocrystal but also from its slanted face. Thus, the contribution of fluorescent radiation from the slanted face of the monocrystal can be significant and the real measurement geometry can significantly differ from the assumed geometry (the incidence and take-off angles of 45 ). Table 2. Standardless analysis of Zn 1.07 Ga 0.02 Cr 2.09 Se 4 monocrystal of size of 1.4mm at positions 800 m (400 m pinhole collimator) At center m off center m off center Element % (m/m) % (m/m) Relative Zn Ga Cr Se % (m/m) Relative Table 3. Standardless analysis of Zn 1.07 Ga 0.02 Cr 2.09 Se 4 monocrystal of size of 1.4mm at positions 1000 m (400 m pinhole collimator) At center m off center m off center Element % (m/m) % (m/m) Relative Zn Ga Cr Se % (m/m) Relative

7 327 CONCLUSIONS The quantitative analysis of selenide spinels was performed using the FP method with and without reference samples. The agreement between the results of both methods was quite satisfactory and the average relative difference was ca. 3%. The accuracy of the EDXRF results was also checked using inductively coupled plasma optical emission spectrometry (ICP-OES). The average relative difference between ICP-OES and EDXRF using reference samples was ca. 1%, although in some cases, the differences reached 2.5% rel. Bigger differences were observed between ICP-OES and standardless EDXRF - the average relative difference was ca. 3.2%. The analysis of monocrystals showed that accurate results can be obtained even if X-ray beam covers the analyzed monocrystal only partially. It can be explained by the fact that all elements in a monocrystal are quantified and results are normalized during FP calculations. Nevertheless, if a monocrystal is not correctly positioned, the contribution of fluorescent radiation from the slanted face of a monocrystal can be significant and the real measurement geometry significantly differs from the assumed geometry (the incidence and take-off angles of 45 ). In consequence, worse results can be expected. ACKNOWLEDGMENTS The work was supported by the Ministry of Science and Higher Education by the Grant No. N N (1784/B/H03/2007/33). REFERENCES Gro -Kowalski, J., and Malicka, E. (2007). Effect of double exchange on thermoelctric power of Cu x Ga y Cr z Se 4 Phys. B 391, Jurczyk, J., Buhl, F., and Wilczek, I. (1993). Semimicro quantitative X-ray fluorescence solution method for the analysis of spinels. Determination of Cu, Ga, Zn, In, Cr and Se in mono- and polycrystals Chem Anal. (Warsaw) 38, Jurczyk, J., Sitko, R., Zawisza, B., Buhl, F., and Malicka, E. (1999). XRF analysis of microsamples of semiconductor type multielement materials by the thin layer method. Determination of Cr, Co, Ni, Cu, Zn, Ga, Se, Sb, Yb Microchim. Acta 132, Malicka, E., Wa 2008). gnetic properties of Zn 1-x Sb x Cr 2 Se 4 (x = 0.11, 0.16 and 0.20) single crystals Solid State Chem. 181, Rudolf, T., Kant, Ch., Mayr, F., Hemberger, J., Tsurkan, V. and Loidl, A. (2007). Spin-phonon coupling in antiferromagnetic chromium spinels Phys.Rev.B76, Sitko, R., Zawisza, B., and Malicka E. (2008). Standardless energy-dispersive X-ray fluorescence analysis using primary radiation monochromatized with LiF(200) crystal Spectrochim. Acta Part B 63, Sitko, R., Zawisza, B., and Malicka E. (2009). -dispersive X-ray fluorescence spectrometer for analysis of conventional and micro-samples: Preliminary assessment Spectrochim. Acta Part B 64,