Volume 107, Number 6, November–December 2002Journal of Research of the National Institute of Standards and Technology[J. Res. Natl. Inst. Stand. Technol. 107, 693–701 (2002)]Contamination in the Rare-Earth ElementOrthophosphate Reference SamplesVolume 107 Number 6 November–December 2002John J. DonovanDepartment of Geological Sciences,The University of Oregon,Eugene, OR 97403-1272John M. HancharDepartment of Earth and EnvironmentalSciences,The George Washington University,Washington, DC 20006Phillip M. PicolliDepartment of Geology,The University of Maryland,College Park, MD 20742Marc D. SchrierDepartment of Chemistry,The University of California,Berkeley, CA 94720Lynn A. BoatnerSolid State Division, Oak RidgeNational Laboratory,Oak Ridge, TN 37831andEugene JarosewichDepartment of Mineral Sciences,Smithsonian Institution,Washington, DC [email protected] of the fourteen rare-earth element(plus Sc and Y) orthophosphate stan-dards grown at Oak Ridge National Labora-tory in the 1980s and widely distr ibutedby the Smithsonian Institution’s Departmentof Mineral Sciences, are significantlycontaminated by Pb. The or igin of this im-purity is the Pb2P2O7flux that is derivedfrom the thermal decomposition ofPbHPO4. The lead pyrophosphate flux isused to dissolve the oxide starting materialsat elevated temperatures (艐1360 ⬚C)prior to the crystal synthesis. Because theserare-earth element standards are ex-tremely stable under the electron beam andconsidered homogenous, they have beenof enormous value to electron probe micro-analysis (EPMA). The monoclinic, mon-azite st ructure, orthophosphates show ahigher degree of Pb incor poration thanthe tetragonal xenotime structure, or-thophosphates. This paper will attempt todescribe and rationalize the extent of the Pbcontamination in these otherwise excel-lent materials.Key words: EPMA; microanalysis; or-thophosphates; quantitative analysis; rareearth elements; rare earth phosphates; REE;standards.Accepted: August 22, 2002Available online: http://www.nist.gov/jres1. IntroductionHighly accurate analyses from the electron mi-croprobe analyzer (EMPA) are only (but not solely) ob-tainable through the use of well-characterized and stablestandards containing a major and/or known concentra-tion of the element in question. For the rare earth ele-ments (REE) this goal has, until recently, been elusivedue to the lack of specimens exhibiting these vital prop-erties.The lanthanide orthophosphates, consisting of com-pounds with the stoichiometry LnPO4where Ln repre-sents any of the REE in the series extending from La toLu (plus the related compounds YPO4and ScPO4), are693Volume 107, Number 6, November–December 2002Journal of Research of the National Institute of Standards and Technologychemically durable and radiation resistant refractorymaterials. During the early 1980s a variety of singlecrystal rare earth orthophosphate samples were synthe-sized at Oak Ridge National Laboratory and the struc-tures determined from x-ray refinements [1, 2, 3, 4, 5,and 6]. The primary purposes of these studies werevaried, but they included nuclear and actinide wastedisposal and scintillator material research as well asfundamental materials characterization investigations.The crystals were synthesized using a high-temperaturesolvent (flux-growth) technique, the details of which areavailable from the original papers, and a good overviewof the development of these orthophosphates is dis-cussed in Boatner and Sales [7], and references therein.One interesting fact is that although the starting mate-rials were carefully selected to be free from REE impu-rities, they were grown in a lead pyrophosphate(PbHPO4) flux. Pb contamination was not a concern forthe or iginal purposes of those experiments, however itspresence was detected early on, and the solid statechemistry (but not the concentration) of Pb in the or-thophosphate was character ized by means of electronparamagnetic resonance spectroscopy (EPR) [8]. Sub-sequently, these materials were investigated for possibleuse as standards for EPMA by the Smithsonian Institu-tion [9], and put through a series of tests. These includedhomogeneity testing and a comparison to the commonlyused REE doped aluminum silicate glass standards ofDrake and Weill [10] using the EPMA, and a check of10 selected REE contaminants on 7 of the compoundsusing instrumental neutron activation analysis. The ma-terials appeared to be robust under electron bombard-ment, did not oxidize or seem hygroscopic, and no seri-ous contamination or inhomogeneities were noted at thetime and these efforts were followed by a general distri-bution of the material to interested parties.In the late 1990s it was reported to one of us (JJD)that at least one investigator (E. J. Essene, University ofMichigan, personal communication) had raised the issueof the role of the Pb impurity in some of the REEphosphate standards. The Pb impurity is especially sig-nificant in the CePO4crystals whose black coloration isconsistent with possible mixed valence (Ce3+–Ce4+) ef-fects—the presence of which could alter the high-tem-perature solid-state chemical properties and lead to anenhanced incorporation of Pb during the crystal-growthprocess. Subsequent investigations of the materials re-vealed Pb ranging in concentration from less than 0.01mass fraction to more than 0.04 mass fraction in theCePO4, depending on the specific grains analyzed. It isthe intent of this paper to characterize the extent of thePb contamination in these otherwise extremely usefulstandards for EPMA.2. Experimental MethodsQuantitative wavelength dispersive spectrometry(WDS) analyses for the REEs Sc, Y, and Pb in each ofthe 16 orthophosphate samples were done using aCameca SX-511electron microprobe at 20 keV, 20 nA(2.0 ⫻ 10⫺8A), using a 10 m beam diameter at UCBerkeley. In addition, one of the Drake and Weill REEglasses [10], and two other REE doped calcium alu-minum silicate discussed in Roeder [11] and Roeder etal. [12] were analyzed. For quantitative analyses, the K␣x-ray line was used for Sc, L␣lines for Y and the otherREE elements, and the M␣line was used for Pb. Counttimes were 20 s on peak and 10 s on each off-peakposition except for Pb where the count times were dou-bled, respectively.A complete description of the analytical setup andsecondary standard accuracy for the analyzed elements(the composition of the REE phosphate primary stan-dards in these cases had been
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