Determination of the Solubility of Ce and La in BaTi03 by Quantitative WDS Electron Probe Microanalysis Določanje topnosti Ce in La v BaTi03 s kvantitativno WDS elektronsko mikroanalizo Samardžija Zl, D. Makovec, D. Koiar, Jožef Štefan Institute, Ljubljana Ouantitative vvavelength dispersive (WDS) electron probe microanalysis was applied for determination of Ba, Ti, Ce and La concentrations in Ce and La doped BaTiO3 ceramics. The high resolution and analytical sensitivity of the vvavelength dispersive spectrometer allovved accurate quantitative measurements of cation concentrations in doped BaTiO3. Pure BaTiO3, Ce02 and La2Ti207 vvere used as verified standard compounds. The experimental parameters of an electron probe microanalyzer vvere optimized to achieve the maximum sensitivity and precision of analysis. K-ratios vvere determined for the BaLaf, TiKah CeLa1 and LaLa1 spectral lines. Because of multiple peak overlapping of the CeLa12 and BaLB14 lines, vvhich introduces substantial error in the measured CeLa1 intensity, k-ratios for CeLa1 vvere corrected using the empirical calibration curve method. Element concentrations vvere calculated by ZAF and/or PROZA matrix correction procedures. Results vvere calculated to the perovskite AB03 formula considering different modes of dopant incorporation and donor charge compensation. Key vvords: quantitative electron probe microanalysis, vvavelength dispersive analysis, solid solubility of Ce and La in BaTiO3 S kvantitativno valovno disperzijsko (WDS) elektronsko mikroanalizo smo določali koncentracije Ba, Ti, Ce in La v keramiki BaTi03 dopiran i s cerijem in lantanom. Visoka ločljivost in analitična občutljivost valovnega disperzijskega spektrometra omogoča natančno kvantitativno meritev koncentracij kationov. Kot standardi so bile uporabljene čiste spojine BaTi03, La2Ti207 in CeO?. Eksperimentalni parametri elektronskega mikroanalizatorja so bili optimizirani za dosego visoke občutljivosti in natančnosti analize. K-razmerja smo določali za spektralne linije BaLav TiKah CeLa1 in LaLaP Pri tovrstni analizi prihaja do prekrivanja spektralnih linij CeLa12 in BaLB14, kar povzroča napako pri merjenju intenzitete linije CeLav Z uporabo empirične umeritvene krivulje smo korigirali k-razmerja za CeLaKoncentracije elementov smo izračunali z uporabo ZAF in/ali PROZA matrične korekcijske metode. Podani rezultati so preračunani na formulo perovskita AB03 z upoštevanjem različnega načina vgradnje dopantov in kompenzacije donorskih nabojev. Ključne besede: kvantitativna elektronska mikroanal/za, valovna disperzijska analiza, trdna topnost Ce in La v BaTi03 1. Introduction BaTiO; ceramics modified vvith donor dopants. such as Lau or NbM, are extensively studied because of their interesting dielectric and semiconducting properties. Donor dopants may be incorporated into the BaTiO, perovskite lattice differently as re-gards their site of incorporation and donor charge compensation1. A knovvledge of the quantity and modality of dopant incorporation is of fundamental importance in the optimization of the processing of materials. Characterization methods usually applied such as XRD, TGA, DTA and electrical measurements Zoran samardžija. dipl. ti/. Institut J ož c t .Štefan Jamova 39. 6100(1 I jubljana give information about solid solubility indirectly by measuring changes in material properties. Ouantitative vvavelength dispersive (WDS) electron probe microanalysis (EPMA) is the best ap-propriate direct method2 for determination of the mode and amount of dopant incorporation. The information about sample composition is taken at the micro-level (from a micro-volume of approximately I pm!) giving the possibility of analyzing multi-phase ceramic samples. In this paper a detailed study of the optimal experimental procedure for WDS quantitation in the Ba-Ce-Ti-O and Ba-La-Ti-O svstems is presented. 2. Experimental La and Ce doped BaTiO, ceramic samples vvere prepared by conventional ceramic technologv. Oxides vvere mixed, pressed Samardžija Z. et al.: Determination of the Solubilitv of Ce and La in BaTiO by Quantitavie WDS Electron Probe Microanalvsis into pellets and sintered at 1400 C in air for a longer period (5 davs) with intermittent cooiing. crushing and mixing to promote homogeneit). Samples vvere polished and coated vvith a thin film of carbon in order to assure electrical conductivitv and prevent charging under the electron beam. A JEOL JXA 840A SEM/EPMA scanning electron micro-scope and electron probe microanalyzer equipped vvith a Tracor Z-Max 30 EDS analy/er and tvv o JEOL WD spectrometers con-trolled by a Tracor Series II X-Ray Microanalvsis Svstcm vvere used for overall analvsis of the samples. Preliminarv information about the ceramic microstructure and grain size of the doped BaTiO, samples vvas obtained by SEM observations. In the WDS microanalvsis it is important that the grain size of the phase analvzed is quite large (min. 5-10 um average diameten in order to avoid the influence of neighbour-ing phascs during the analvsis. Electron micrographs of the samples vvere recorded using compositional backscattered electron imaging based on atomic number contrast. Because of strong and multiple peak overlapping of the most intense Ba. Ti. La and Ba. Ti, Ce spectral lines in the EDS spectra. accurate quantitative analvsis based on EDS (resolution 150eV) vvas not possible. The better resolution achieved in the WD spectrometer (5-10 eV) allovved separate analvsis of BaLa,. TiKa,. LaLa, and CeLa, spectral lines vvithout spectral inter-ference. At the same time a high X-ray collection efficiency and higher peak to background ratios vvere achieved. The experimental parameters in EPMA vvere optimized according to data based on a Monte Carlo electron trajeetorv simulation, Kanaya-Okayama calculations for electron range and 4>(pz) curve calculations for X-rays generated by electron-solid interaetions. Takitig into account the excitation energies of characteristic X-rays. absorption and fluorescence processes in the material, the optimal conditions vvere found to be 20 kV voltage and 10/15 nA electron beam current. X-rays vvere diffracted on the high resolution and reflectivity crystals: PET (2dfl02=0.40267nm), was used for Ba, La, Ti and LiF (2d2(Hi=0.87420 nm). vvas used for the Ba, Ce, Ti element combi-nation. Diffracted X-rays vvere detected by a sealed xenon filled gas proportional counter. Accurate peak positions for the spectral lines analvzed as vvell as background positions vvere determined by slovv vvavelength scans. Counting time vvas set to assure less than IVr standard counting deviation. Peak to background ratios vv ere maximized setting the pulse height ana-lyzer parameters. bias and pulse voltage diseriminators, to the optimum values. This lead to improved analvtical sensitivitv due to the decreased minimum detectability limit, vvhich vvas calculated according to Ziebold eriterion and vvas found to be in the range of 100 to 300 ppm element concentration (0.01 to 0.03vvt'/f) in the samples. Homogeneitv of standards and samples is required in EPMA. It vvas checked and confirmed according to the eriterion2 that ali the measured peak intensity counts. N, from the analyzed points. should fall vvithin N+/-3N"2. vvhere N":=o is the standard counting deviation. Specimen surface contamination. vvhich may occur during the analysis, vv as reduced using a liquid nitrogen eold trap in the vacuum sv stem and mounting the eold finger above the samples. Analysis vvas performed point by point -on selected doped BaTiO, grains. The programmed softvvare controlled and auto-mated the multi-point analv sis. vvith a complete quantitative rou-tine vvas applied in the čase of long data-eollection times. In such cases the electron beam current štabiIity vvas confirmed to be better than 0.5'r of the nominal value. Measured intensities of generated characteristic X-rays in the samples vvere transformed into k-ratios relative to knovvn element concentrations in the appropriate standards consisting of BaTiO,. La2Ti,07 and CeO . The svstematic error vvhich ap-peared in the measurements of Ce concentration vvas due to BaLB, i and CeLa,; spectral interference causing overlap of the corresponding spectral peaks even in the WDS analvsis. This problem vvas solved by using the cmpirical calibration curve method for correction of the CeL« intensitv '. Corrected k-ratios for Ce vvere tlien processed in the quantitation routine. Quantitative analysis vvas performed through the ZAF and/or PROZA matrix correction programs. transforming the measured k-ratios into element concentrations. 3. Results and discussion Microstructures of typical Ce-doped and La-doped BaTiO, samples are shovvn in Figures 1 and 2. Figure 1: Backscattered electron micrograph of Ce-doped BaTiO,; phase identification: 1:1:4 lamellae - BaCe,Ti40P. BT4 - polytitanate BaTi40„ BCT - Ce-doped BaTiO, grains Slika 1: Slika odbitih elektronov mikrostrukture keramike BaTiO, dopirane s Ce: oznake faz: 1:1:4 lamele - spojina BaCe,Ti4Oj,. BT4 polititanat BaTi40,„ BCT - zrna BaTiO, dopirana s Ce Figure 2: Backscattered electron micrograph of La-doped BaTiO,; phase identification: 1:1:4 lamellae - BaLa,Ti4012, BT4 - polvtitanate BaTi40„ BLT - La-doped BaTiO, grains Slika 2: Slika odbitih elektronov mikrostrukture keramike BaTiO, dopirane z La; oznake faz: 1:1:4 lamele - spojina BaLa;Ti40,:. BT4 polititanat BaTi40,„ BLT - zma BaTiO, dopirana z La Samardžija Z. et al.: Determination of the Solubility of Ce and La in BaTiO, by Quantitavie WDS Electron Probe Microanalvsis ln every sample at least 10 doped BaTiO, grains vvere ana-lyzed. Data obtained from counting statistics confirmed the ho-mogeneity of the phases analy/ed within the range N+/-o, better than required N+/-3o. In Tables 1 and 2 the average results of measurements of cation coneentrations in 4 Ce-doped and 3 La-doped characteristic samples are presented. Data vvere calculat-ed to the perovskite ABO, formula considering different modes of Ce and La incorporation45. It should be noted that the oxygen content vvas not measured but calculated by difference from 100'i, vvhich is the usual approach in the analvsis of oxide com-pounds. Table 1: Results of WDS microanalvsis of chosen Ce-doped BaTiO, samples ple Bala«) Ce (al9r) Til al',) Calculated formula 1 19.5I+/-0.4I 6.79+/-0.43 12.31+/-0.30 BaTill.,Ccll ,0, 2 19.44+/-0.49 0.54+/-0.11 19.57+/-0.54 98 ;CeM .|,Tl,, .„,&„,(VTl) 0.004 3 3 17.55+/-0.40 1.60+/-0.07 18.66+/-0.50 Ba,(„,Ce.....Ti,,.,,1V, )"",,,.0, 4 18.72+/-0.44 1.02+/-(),09 19.44+/-0.46 0.L, .-Cc Ti.,,,,1 \ I,) , :, ,0, Table 2: Results of WDS microanalvsis of chosen La-doped BaTiO, samples Composition of La-doped BaTiO, phase: Ba, La^Ti, ,„( VTl)'"\/40, Sample Ba (at%) La(at%) Ti (at'/r ) X (mol % La) 1 17.35+/-0.28 2.53+/-0.04 19.43+/-0.25 13 2 16.24+/-0.20 3.97+/-0.04 19.58+/-0.25 20 14.69+/-0.23 5,84+/-0,09 18,63+/-0,24 2X The quoted standard dcviations from the average values re-late to data measured on the various grains analyzed in the samples. Ce could be ineorporated into the BaTiO, lattice in tvvo different modes; substitutionally at Ti sites. as vvell as at Ba sites. It is generally accepted that the large trivalent rare earth cations La1+ and Cei4 enter the BaTiO, lattice at Ba24 sites and act as donors. In the present investigation. it is assumed that donor charge compensation takes plače bv creation of Ti4+ vacancies. (VT,)'...... Ce4+ ions enter the Ti4< sites7. The distribution of Ce betvv een Ba and Ti sites. expressed in the formulas given in Table 1, is calculated on the basis of WDS microanalysis of the (Ba+Ce)/Ti ratio. The chemical formulas of La containing BaTiO, soliti so-lution listed in Table 2 take into account Lau on Ba2+ sites and the presence of Ti-site vacancies. Correction of the Ba-Ce interference allovvs the correct analysis of samples vvith a lovv Ce concentration. Without cor- rection the high Ba to Ce peak overlap ratio introduccs a large er-ror in the final quantitative analvsis. Use of the ZAF and PROZA quantitative matrix correction programs gave practicallv the same results for calculated element concentrations vvithin the range of statistical precision. Such a result is expected for the ex-perimental conditions applied. As an illustration of the analytical precision achieved the La-doped BaTiO, sample 3 can be presented as an example. Starting composition of the sample vvas expressed by the formula Ba,)7l5La(l,x5Tiii..PX(VTi)""0,. The sintered sample vvas monophasic and the calculated formula based on the results of WDS microanalvsis vvas Ba,17]5Lafl:)(5Ti09(l7(VTi)""O,. For the Ce-doped BaTiO, sample 4 can be used as an illustration of the analvtical precision. Starting composition of the sample vvas expressed by the formula Ba,,l,4,,Cc0.,K4(1Ti„..1!.^(VT,)""0015O3. The sintered sample vvas monophasic and the corresponding formula based on the results of WDS microanalysis vvas Ban„4!iC el)ll5;Tin,)iS,( V,,) n0L,O,. 4. Conclusions WDS quantitative microanalysis can be successfully applied for the investigation of oxide compound solid solutions. A de-tailed study and the optimization of WDS experimental EPMA procedure vvas performed for the analysis of Ce and La doped BaTiO, ceramics. The results allovv the determination of solid solubility and investigation of the mode of dopant incorporation in BaTiO,. The basic advantages of WDS microanalysis realized can be summarized as follows:direct analysis of phases of inter-est in the chosen samples, good analytical sensitivity and precision. 5. References 1 H. M. Chan, M. P. Harmer and D. M. Smyth, Compensating Defects in Highly Donor Doped BaTiO,, ,/. Ani. Ceram. Soc . 69. 1986, 6, 507-510 ; J. 1. Goldstein et al.. Scanning Electron Microscopy and X-Rav Microanalvsis. Plenum Press, Nevv York 1992 5 M. Čeh. Z. Samardžija. D. Makovec, Studv of Solid Solubility of Cerium in BaTiO, bv Quantitative WDS Microanalvsis. Scanning. Vol. 15. Supplement III, 1993,94-95 4 D. Makovec, Z. Samardžija and D. Kolar, Incorporation of Cerium into the BaTiO, Lattice, Third Euro-Ceramics Proc. I. 1993. 961-966 4 D. Makovec, Z. Samardžija, U. Delalut and D. Kolar, Defect Structure and Phase Relations of Highly La-Doped BaTiO,. to be published * G. H. Jonker and E. E. Havinga. The Influence of Foreign Ions on the Crvstal Lattice of Barium Titanate, Mat. Kes. Buli. 17. 1982. 345-350 7 J. P. Guha, D. Kolar. Subsolidus Equilibria in the System BaO-CeO,-TiO„ ./. Am. Cev Soc. 56, 1973. 1,5-6