a ut SIR: The high neutron absorption cross section of the rare earths Gd, Sm, Eu, and Dy has necessitated their determination at the fractional ppm level in nuclear materials such as U, Zr, Th, and Pu. Since direct determination at these levels has not been feasible, preconceritration of the rare earths using chemical separation techniques has been the preferred analytical procedure ( I > . The successful application of X-ray excited optical fluorescence of the rare earths to their quantitative determination at the fractional pprn level in Thoz, one of the fertile nuclear materials, has been described (2). The determination of ultra trace amounts of the rare earths in uranium, the primary fertile nuclear element, and in zirconium, a prominent structural and cladding material in reactors, is of equal importance. Our studies showed that the simple oxides of these elements were not suitable hosts for the rare earth ““activators” at the fractional parts per million level. The oxides of e3 and Zr may, however, be incorporated into ternary oxide type hosts, in which optical fluorescence of the particular rare earth elements of interest is supported at the fractional ppm level (3). Our continuing studies on the chemical modification of QXidetype hosts has resulted in the synthesis of a new system of quaternary oxide phosphor materials which provide even greater powers of detection. This short communication discusses the preparation, spectral characteristics, and some analytical potentialities of these compounds. The quaternary oxide phosphor materials were prepared by simple, solid-state reactions at appropriate temperatures and have the general formula: a Rzl+O b Wz+O b R4+02 c WOs. This symbolism is frequently used in identifying phosphor systems and compositions (4,5). The formation of a definite compound or a single phase is not implied by this symbolism. The identity of the various cationic species represented by the formula is evident from the phosphor genesis schematically presented in Figure 1. The COIIIPOS~fions that gave the superior powers of detection were 2Liz0 SrO U 0 2 .2 W 0 3 and NazO 2Sr0.2Zr02 3wo3,respectively. For the preparation of the uranium-containing phosphor, reactor grade UOz or WsOs was used. Since commercial samples of ZrOz contained substantial amounts of rare earth impurities, the base material used was prepared from Ames Laboratory purified, electron-beam melted zirconium metal. Other chemicals were of reagent grade. The uraniurn-containing phosphor was prepared by grinding together stoichio-
PHOSPHOR GENESIS b R3+0 , C W 0 3
I R * * = Co, Sr,Bo 1
l3
- RARE
b R2*0. b R4’02. C W03
s r2+
EARTH TUNGSTATES
( R4+ = T i , Z r , Hf ,PIA, U , T h )
u4+
Sr 0 ,U02.2W 0 3
oRr
1
(R’*=
Li, N a , K, R b , C s )
2L i20,Sr 0 U 0 2 , 2 W O3 I
Figure 1. Quaternary oxide phosphor genesis 4 c
8 :n
4
a
-
e
0
WAVELENGTH
a
(1) 6.0. Goldbeck in “Analysis of Essential Nuclear Reactor Materials,” C. J. Rodden, Ed., U.S. Government Printing Office, Washington, D. C., 1964, pp 959-86. (2) T. R. Saranathan, V. A. Fassel, and E. L. DeKalb. ANAL. CHEM., 42, 325 (1970). (3) E. L. DeMalb. A. P. D’Silva, and V. A. Fassel, ibid.. P 1246. (4) P. D. Johnson in “Luminescence of Inorganic Solids,” P. Goldberg, Ed., Academic Press, New York, 1966, Chapter 5. ( 5 ) H.W. Leverenz, “An Introduction to Luminescence of Solids,’’ Wiley, New York, 1950, pp 60-61.
46
Cdj
(A!
Figure 2, X-ray excited optical fluorescence spectra of rare earths in U3Qs. Phosphor: 2L&O SrO .UOz. 2 W 0 3 metric amounts of &izC03,Sr(NO&, WOz or an equivalent amount of W 3 0 s , and WO,. The mixture was heated initially for an hour at 825 “C,reground, and heated for an additional period of two hours. For the zirconium-containing phosphor, the heating temperature was 1000 “C. Although extended heating cycles were employed to provide optimal rare earth line intensities, it is of interest to note that reactions among the phosphor components were relatively rapid. For example, the grayish colored, powdered mixture of the uranium phosphor components changed to a deeporange material within five minutes of heating. The constituent compounds for this material [Li2C03,Sr(NOs)?,UO?, and WO,] were white, colorless, brownish-black, and yellow,
ANALYTICAL CHEMISTRY, VQL. 42, NO. 14, DECEMBER 1978
c
z
t w
c a
x
co, 3000
,
,
--
1
4000
530C WAVELENGTH t a l
6000
Lc
c
A’-
Figure 4. X-ray excited optical fluorescence spectra of residual rare earths present in high purity U3Q8. Phosphor: ;!Li20.Sir0 .UOZ.2WQ3
7000
Figure 3. X-ray excited optical fluorescence spectra of rare earths in ZrOa. Phosphor: NazQ.2 S r 0 . 2Zr02.3WOa respectively. The deep-orange color of the phosphor indicates that the components reacted with each other to form a new material, which at this stage has not been characterized structurally or chemically. The primary consideration has been to achieve reproducibility of phosphor preparation and of optimal rare earth line intensities. The features of the rare earth line spectra observed suggest that a scheelite tungstate structure was formed. The optical emission spectra of the uranium- and zirconiumcontaining phosphors, with and without rare earth additions, are shown in Figures 2 and 3. These spectra were obtained with instrumental facilities already described ( 4 ) . The simplicity of the spectra permitted the use of 250-micron slits with an appreciable gain in sensitivity. A recording obtained under more sensitive instrumental conditions of the residual Dy and G d impurity in the UOz base material is shown in Figure 4. This recording and other observations indicated that these host materials will allow the detection of these rare earth elements at the 1 to 10 part per giga (part per lo9) level. Preliminary experiments have shown that an internal reference element, which affords internal spectral intensity control over preparation and instrumental variations (6), can be conveniently added to the quaternary oxide host with the Sr(NO&. Thus, dissolution of the sample is not required prior to analysis. (6) E. L. DeKalb, V. A. Fassel, T. Taniguchi,and T. R. Saranathan, ANAL.CHEM., 40, 2082 (1968).
Figure 1 shows that the R4+ion can be Ti, Zr, Hf, Pu, U, and Th, the elements being arranged in the ascending order of their ionic radii. Quaternary oxide phosphors containing Ti, Hf, and Th have also been prepared but their compositions have not been optimized. However, preliminary indications are that the exceptional sensitivity ends with uranium in this series. Plutonium compounds were not investigated because of lack of suitable facilities. Since the ionic radius of Pu4+is slightly less than that of U4+it should be possible to substitute PuOz for UO, in these compounds. Also, it is known that trivalent actinides exhibit sharp line fluorescence spectra in striking similarity to the trivalent lanthanide ions (7, 8). Thus, ultra trace amounts of the actinide elements should be detectable in UOz,PuOz,and other members of this series. A more detailed investigation on the analytical potentialities of these phosphors is in progress and will be discussed in further communications. ARTHURP. D’SILVA EDWARDL. DEKALB A.FASSEL VELMER Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Iowa 50010 RECEIVED for review July 28, 1970. Accepted September 21, 1970. Work was performed in the Ames Laboratory of the U.S. Atomic Energy Commission. (7) J. B. Gruber, “’Spectroscopic Studies of Actinide Ions in Crystalline Solids,” UCLA-34P120-3, 1968. (8) W. F. Krupke and J. B. Gruber, J. Chem. Plzys., 46, 542 (1967).
ANALYTICAL C H E M I S T R Y , VOL. 42, NO. 14, DECEMBER 1970
e
1847
