Abstract
Six different rare-earth oxyapatites, including Ca2 RE 8(SiO4)6O2 (RE = La, Nd, Sm, Eu, or Yb) and NaLa9(SiO4)6O2, were synthesized using solution-based processes followed by cold pressing and sinter-ing. The crystal structures of the synthesized oxyapatites were determined from powder X-ray diffraction (P-XRD) and their chemistries verified with electron probe microanalysis (EPMA). All the oxyapatites were isostructural within the hexa-gonal space group P63/m and showed similar unit-cell parameters. The isolated [SiO4]4- tetra-hedra in each crystal are linked by the cations at the 4f and 6h sites occupied by RE 3+ and Ca2+ in Ca2 RE 8(SiO4)6O2 or La3+ and Na+ in NaLa9(SiO4)6O2. The lattice parameters, cell volumes, and densities of the synthesized oxyapatites fit well to the trendlines calculated from literature values.
Highlights
For immobilization of the radionuclides in the high-level waste (HLW) raffinate stream following aqueous reprocessing of used nuclear fuel, glass-ceramic waste forms are being developed as an alternative to borosilicate glass (Crum et al, 2012, 2014, 2016)
We report the synthesis method and crystal structures of oxyapatites, Ca2RE8(SiO4)6O2 (RE = La, Nd, Sm, Eu, Yb) and NaLa94)6O2
A-RE [e.g. Ca-La denotes Ca2La8(SiO4)6O2]. These apatites generally crystallize in space group P63/m; P63 (Lambert et al, 2006) and P3 (Sansom et al, 2004) spacegroup symmetries have been reported as well
Summary
For immobilization of the radionuclides in the high-level waste (HLW) raffinate stream following aqueous reprocessing of used nuclear fuel, glass-ceramic waste forms are being developed as an alternative to borosilicate glass (Crum et al, 2012, 2014, 2016). As a result of the chemical diversity in the HLW raffinate stream, several crystalline phases including powellite [(AE)MoO4], rare-earth borosilicate [(RE)3BSi2O10], cerianite (ZrxCe1-xO2), and oxyapatite [(AE)2(RE)8(SiO4)6O2], where AE and RE are alkaline earth and rare-earth metals, respectively, crystallize from the glass matrix upon slow cooling inside the waste canister during the waste form fabrication process (Crum et al, 2012, 2014, 2016). We report the synthesis method and crystal structures of oxyapatites, Ca2RE8(SiO4)6O2 (RE = La, Nd, Sm, Eu, Yb) and NaLa94)6O2. We compare the general structural parameters of these isostructural oxyapatites with different RE cations
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