EPR Investigations of Impurities in the Lanthanide Orthophosphates
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M. M. ABRAHAM, L. A. BOATNER, and M. RAPPAZ Solid State Division, Oak Ridge National Laboratory,* Oak Ridge, TN 37830
ABSTRACT Lanthanide orthophosphates formed from elements in the first half of the 4f transition series are analogs of the monoclinic mineral monazite. The known geologic properties of this mineral make the general class of lanthanide orthophosphate compounds attractive substances for long-term containment and disposal of u-active actinide nuclear wastes. EPR spectroscopy has been used to investigate the structural properties and solid state chemical properties of impurities in these materials and to compare the characteristics of single crystals and polycrystalline bodies.
INTRODUCTION The mineral monazite, a mixed lanthanide orthophosphate (LnPO4 with Ln La, Ce, Nd, ... ), exhibits a number of characteristics that make analogs of this substance attractive as potential primary high-level radioactive waste forms [1,2]. These characteristics include an established long-term stability (-,109 years) under different geological conditions, a known ability to contain relatively high percentages of thorium and uranium, and an apparently high degree of resistance to metamictization due to u-particle and a-recoil radiation damage. This potentially important application of lanthanide orthophosphates has provided the impetus for a series of investigations of the physical and chemical properties of mixed orthophosphate-impurity systems. In particular, the technique of electron paramagnetic resonance (EPR) has been applied to the determination of valence states and site symmetries for various impurities in both single crystals and powders [3] of this class of compounds. In studying the solid state properties of mixed lanthanide orthophosphateimpurity systems, the EPR results have been correlated with investigations using optical techniques, M6ssbauer spectroscopy, and x-ray diffraction. By using a probe ion whose spectrum reflects the properties of the crystalline electric field, it was also possible to obtain structural information by means of EPR spectroscopy. Orthophosphates of the first half of the lanthanide transition series (i.e. La through Gd) crystallize in a monoclinic form that is the direct analog of natural monazite while the orthophosphates of the second half of the lanthanide series (i.e. Tb through Lu), as well as YPO 4 and ScP04, crystallize in a tetragonal form analogous to the mineral zircon. The EPR investigations were extended to encompass both lanthanide orthophosphate structural types. EXPERIMENTAL The lanthanide orthophosphate single crystals employed in these investigations were grown in a lead-based flux (Pb 2 P 2 0 7 ) using a variation of the technique Research sponsored by the Division of Materials Science, U.S. Department of Energy, under contract W-7405-eng-26 with Union Carbide Corporation.
476 described by Feigelson [4]. Following the growth by slow cooling, the crystals were removed from the entraining flux by boiling in nitric acid for about three weeks. Orthophosphate powde
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