X-ray Excited-Optical Luminescence of Lanthanides in Phosphors Prepared from Yttrium Oxide and Iron TransitionGroup Oxides: Application to the Determination of Lanthanides in Iron and Its Alloys Edward L. DeKalb and Velmer A. Fassel Ames Laboratory-ERDA and Department of Chemistry, Iowa State University, Ames, Iowa 500 10
A series of new phosphors has been prepared using yttrium oxide and the oxides of the iron transition group elements. These phosphors when excited by x-ray radlation are strongly luminescent in the visible spectral region if the oxides of the iron-group oxides contaln lanthanlde elements. These phosphors have been used for the determlnatlon of several lanthanides at the ppm level in Iron and several of its alloys.
Phosphors can provide convenient, sensitive means of detecting and measuring trace concentrations of those elements which “activate” the phosphor. The lanthanide elements are especially well suited to this analytical technique because they produce relatively sharp line luminescence when the phosphor host containing the lanthanide is excited by ultraviolet or x-ray radiation, or by energetic particle bombardment. Numerous phosphor compositions have been prepared, and many have been evaluated for possible analytical applications (1-7). Other than the SrO-ThOz. CrzO3 system we reported previously ( I ) , to our knowledge no phosphor containing large amounts of the iron group of transition elements has been reported. This absence is understandable because the presence of these elements, even at low concentration, tends to suppress the luminescence of the lanthanides (8-10). D’Silva and Fassel (11, 12) have recently described the x-ray excited optical luminescence (XEOL) of lanthanide activators in yttrium-based phosphate and vanadate-phosphor systems. An extension of these studies has shown that the incorporation of iron group transition elements as major constituents into these phosphor hosts suppressed the lanthanide activator luminescence only to a very limited extent. In fact, the relatively sharp line luminescence of the activators could be observed a t the low part per million (ppm) level. These observations suggested the potential application of such phosphors to the detection and determination of ppm amounts of the lanthanides in the iron group of transition elements. In this communication, we describe the preparation of these phosphors and their potential analytical application.
EXPERIMENTAL FACILITIES AND PROCEDURES The XEOL spectra were excited and observed under the same experimental conditions previously described ( 1 , 8 ) . The phosphors were prepared by heating a blend of Y203, (NH&HP04, and the transition metal oxide to a temperature of 600 to 1000 “C. In the special case of one of the Cu-containing phosphors, NaZC03 was also added for charge compensation; and in the case of the iron containing phosphors, Na4PzO.i was added to improve the nucleating properties of the iron-containing phosphors. The resulting phosphors may or may not be single phase compounds. However, the stoichiometric compositions of the phosphors prepared in this study were YCr(P04)2, YzMn3(P04)4, Y o . E F ~ o . z P O ~ O . ~ N ~Y~Co3(P04)4, ~PZO~, Y~Ni3(P04)4, y3Cu32354
(Po4)5, and NaYCu(PO&. Similar compositions with the phosphate ions replaced by vanadate ions (from NH4V03) also formed good phosphors. Only the iron-containing phosphate phosphor was optimized to the composition which provided the most intense lanthanide luminescence. To prepare the phosphors, the transition metals were dissolved in mineral acids to which were added 100 ppm of Sm, Eu, Gd, Tb, and Dy. The solutions were converted to the oxides, which were then combined with the Y Z O S - N H ~ H P O ~ mixture. The dry powders were blended and then heated for 30 min a t 600 “ C . They were then cooled, ground in an agate mortar, and the spectrum of the potential phosphor was recorded. This process was repeated using the same powders, but with successively higher temperatures of 700,800,900, and 1000 O C . However, the heating sequence was suspended when the material fused, since the luminescence intensity invariably declined upon fusion.
RESULTS AND DISCUSSION Spectra of the Phosphors. The XEOL spectra of the five lanthanides occurring at a concentration level of 100 ppm in the iron-transition element component of the phosphor are shown in Figure 1. It should be noted that all of these spectra were recorded at reduced x-ray power so that nearly all spectral features would remain on scale. The spectra therefore do not accurately reflect detectabilities. In these five spectra, T b generally provides the greatest line intensities, and except for the Cu-containing phosphor, the violet and blue lines are more intense than those in the green. Gadolinium showed relatively strong emission in the Ni phosphor, but considerably weaker emission in the other phosphors. Sm, Eu, and Dy also showed relatively strong emission in the Cu phosphor, and considerably weaker emissions in the other phosphors. The Y ~ C U ~ ( P O ~ ) ~ phosphor gave a spectrum qualitatively similar but somewhat weaker than the one shown in Figure 1. As indicated earlier, the composition Yo.sFeo.zPOp 0.2NadPz07 provided the most intense lanthanide luminescence of iron-based phosphors so far prepared. This phosphor was prepared by blending the dry components in an agate mortar, followed by heating of the blend to 400 “C for 30 min. The powder was again blended and heated to 900 OC for 30 min. The spectrum of this phosphor is shown in Figure 2. The iron portion of this phosphor contained 100 ppm each of Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy. To demonstrate that lanthanides can also be detected in high alloy steels such as stainless steel AIS1 type 303 (18% Cr, 9% Ni, and 71% Fe), two phosphors were prepared from this steel with the molar composition of YO,~(AISI type 303)0.3P04.0.2Na4P207 (assuming that the steel composition consisted entirely of Fe). The spectrum of the phosphor prepared from the original steel sample is shown in the lower portion of Figure 3 (labeled as “Pure” Steel). A trace of T b is observable in this spectrum because the Y203 phosphor base contained -4 ppm Tb. The spectrum of the phosphor obtained when the equivalent of 100 ppm each of Ce, Nd, Sm, Eu, Gd, Tb, and Dy was added to the dissolved steel sample is shown in the top portion of Figure 3.
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Figure 1. Spectra obtained from transition element phosphors The transition element oxides used to prepare the phosphors contained 100 ppm of Sm, Eu, Gd, Tb and Dy
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Figure 2. Spectrum obtained from an iron-containing phosphor The FepOBused to prepare the phosphor contained 100 ppm of Ce. Pr, Nd, Sm, Eu, Gd, Tb, and Dy
Gadolinium could not be detected in the iron-containing phosphors a t the 100-ppm level. The two P r bands in Figure 2, and the Ce band in Figures 2 and 3 are due to transitions from the 5d to 4f levels in those lanthanides All other lanthanide spectral features are attributed to transitions within the 4f levels. For the observations on steels discussed above, and also for the analytical studies described below, the samples were dissolved electrolytically, because many low alloy steels and all stainless steels are difficult to convert to oxides in a reasonable amount of time by more conventional techniques. The dissolution bath consisted of equal parts of concentrated HC104 and glacial acetic acid. The dissolved sample was evaporated to dryness on a hot plate. Although the dissolution bath is a potentially explosive mixture, we have not experienced any difficulties. Nevertheless, t h e evaporation s t e p should be u n d e r t a k e n w i t h d u e caution.
Figure 3. Spectra obtained from phosphors containing oxides from stainless steel type 303 Upper spectrum, 100 ppm of Ce. Nd, Sm, E .L I Gd, Tb and Dy were added to the dissolved steel prior to phosphor preparation
ANALYTICAL APPLICATION Determination of Lanthanides in Steel. The metallurgical properties of both low and high alloy steels can be modified and improved by the addition of small amounts of the lanthanide elements to the melt just prior to casting ( 1 3 ) . The lanthanides can be added individually, but are more often introduced as mixtures, such as mischmetal. The total lanthanide content of such steels range from 0.01 to nearly 1.0%. Of the many analytical techniques suggested for the determination of lanthanides in steels, only a very few have not required lengthy, time-consuming separation procedures prior to the actual determination. Direct spectrophotometric analyses on dissolved steels have been
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Table I. Comparison of Results on Ce and Pr in Low Alloy Steels Ce, p p m Sample
1
2 3
4 5 6 7 8 9 10
PI, p p m
XEOL
Plasma
REDOX
XEOL
Plasma
1350 1320 19 50 65 80 175 90 145 145
1350 1700
... ... ... ... ...
< 20 < 20
... ...
...
55 84 55 210 90 160 170
86 180 130
... ...
150 140 200 < 20
... ... ... ...
290 140 210
... ... ... ... ...
RARE EARTH CONCENTRATION IN Fe (ppml
Flgure 4. Analytical curves obtained for the determination of Ce, Pr, Nd, and Sm in low alloy steel Triplicate phosphor preparations provided the indicated precision
reported by Alykov and Cherkesov (14) and by Romanov (15) f6r lanthanides a t concentrations greater than 250 ppm. Instrumental neutron activation analysis was used by Burianova and Frana (16) for the determination of La and Ce down to 10 and 100 ppm in steels. Detection limits of IO to 100 ppm were also reported by Ryabova et al. (17, 18) for an optical emission technique based on the direct excitation of powdered steel samples. Emission spectral examination of oxide samples was used by Kawashima et al. (19) for the determination of La and Ce down to 50 and 80 ppm. X-ray fluorescence techniques have been used by Kawashima et al. (19), Ambrose et al. (20), Cohen and Bryan (21), and Tunney (22). Ambrose et al. used pressed steel millings to obtain detection limits down to 20 and 40 ppm, and borax-fused samples for detection limits down to 30 and 50 ppm. The x-ray excited optical luminescence method described by Kat0 et al. (23) is in some ways similar to the work described in this paper. However, a complicated separation procedure was used. The lanthanides were co-precipitated with thorium as a carrier, first as fluorides, next as hydroxides, and finally as oxalates. After ignition, Tho2 served as the host matrix for the lanthanide luminescence. Analytical Calibrations. Reference samples were prepared by adding known quantities of various lanthanides to solutions of pure iron. These solutions were then dried and ignited a t 900 OC. Phosphors were prepared from these oxides by blending 31.9 mg Fez03 with 264.1 mg (NH&HP04, 89 mg of Na4P207, and 180.6 mg Y203 containing 20 ppm of Er as an internal reference element ( 2 ) . The mixture was heated to 400 "C for 30 min, again blended and heated to 900 "C for an additional 30 min. The phosphor was then ground to a powder and compressed into a Yz-inch diameter aluminum planchette with a hand press. Typical analytical calibration curves for the determination of lanthanides in low alloy steels are shown in Figure 4. The precision of phosphor preparation and luminescence intensity measurement is indicated by bars through the plotted average values, which were obtained from triplicate phosphor preparations made on different days. Because the same Y2O3 was used in preparing sample phosphors, the analytical curves could be used directly without correcting for the residual lanthanide contents. Since no independently certified reference samples were
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available for accuracy verification, recourse was taken to a comparative analytical study on a series of low alloy steels obtained from the Lynchburgh Foundry and from the Molybdenum Corporation of America. Table I presents a summary of the data obtained on these samples by our XEOL technique, by plasma optical emission of the dissolved samples (24) and by reduction-oxidation titrimetry (REDOX). Overall, there is good agreement in the analytical results, but a few discrepancies are evident. Detailed analytical studies on the samples have definitely revealed inhomogeneities, but it has not been unequivocally demonstrated that the inhomogeneities account for all of the discrepancies.
LITERATURE CITED (1) E. L. DeKalb. A. P. D'Silva, and V. A. Fassel, Anal. Chem.. 42, 1246 (1970). (2) R. J. Jaworowski, J. F. Cosgrove, D. J. Bracco, and R. M. Walters, Specfrochim. Acta, Part B, 23, 751 (1968). (3) W. A. Shand, J. Mater. Sci., 3, 344 (1968). (4) L. I. Anikina and A. V. Karyakin, Russ. Chem. Rev., 33, 571 (1964). (5) D. P. Shcherbov, lnd. Lab., 34, 763 (1968). (6) T. Nakajima. Y. Ouchi. H. Kawaguchi, and K. Takashima, Jpn. Analyst, 19, 1183 (1970). (7) T. Nakajima, H. Kawaguchi, K. Takashima. and Y. Ouchi, Bunko Kenkyu, 18,210,262 and 299 (1969). (8) E. L. DeKalb, V. A. Fassel, T. Taniguchi, and T. R. Saranathan, Anal. Chem., 40, 2082 (1968). (9) J. Jaworowski and J. F. Cosgrove, Spectrochim. Acta, Part B, 23, 765 (1968). (10) P. Pringsheim, "Fluorescence and Phosphorescence," Interscience, New York, 1949, pp 322-328. (11) A. P. D'Silva and V. A. Fassel. Anal. Chem., 45, 542 (1973). (12) A. P. D'Silva and V. A. Fassel, J. Luminesc., 8, 375 (1974). (13) N. Kippenhan and K. A. Gschneidner, Jr., "Rare Earth Metals in Steels," Report No. IS-RIC-4, Rare-Earth Information Center, Iowa State Unlversity, Ames, Iowa. 1970. (14) N. M. Alykov and A. I. Cherkesov, Sovrem, Metody Khim. Spectral, Anal. Mater., 1967, 215. (15) P. N. Romanov, Ind. Lab., 40, 777 (1974). (16) M. Burianova and J. Frana, Radioisotopy, 14, 635 (1973). (17) D. 2 . Ryabova, L. S. Etelis, and M. I. Gladkov, Urd. Konf. Spektrosk., 7th 1, 111 (1971). (18) D. 2. Ryabova, M. I. Gladkov, L. S. Etelis, E. I. Kolodenskaya, and G. F. Stasyuk, hd. Lab., 37, 1710 (1971). (19) I. Kawashima, T. Miyazaki. I. Tanaka, and K. Tokiwa. Bunko Kenkyu, 18, 14 (1967). (20) A. D. Ambrose, D. W. Swingler, and G. Clasper, Metall. Met. Form., 39, 332 (1972). (21) S. Cohen and F. R. Bryan, Appl. Spectrosc., 22, 342 (1968). (22) A. A. Tunney, Report PB-226-972/8GA (1973); Available from National Technical Information Center, Springfleld, Va. (23) K. Kato, K. Takashima. and T. Nakajima, Bunseki Kagaku, 21, 1154 (1972). (24) C. C. Butler, R. N. Kniseley, and V. A. Fassel, Anal. Chem.. 47, 825 (1975)
RECEIVEDfor review April 10, 1975. Accepted August 25, 1975. Work performed for the U S . Energy Research and Development Administration under contract No. W-7405eng-82.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
