The crystal structures of CeSiO_4 and Ca_2Ce_8(SiO_4)_6O_2

摘要

Two new solubility-limiting phases relevant to nuclear waste disposal are reported, namely CeSiO_4 and Ca_2Ce_8(SiO_4)O_2, produced by hydrothermal synthesis at 180℃. X-ray diffraction data are presented for both compounds. Rietveld refinement was performed for each of these phases. CeSiO_4 was confirmed to be a zircon structure type, with space group I4_1/amd, unit cell a= 6.9564(3), c= 6.1953(4) A. Bond lengths for SiO_4 are in excellent agreement with published values; Ce~(4+) is coordinated to eight oxygen atoms with four regular and four short bonds. Ca_2Ce_8(SiO_4)O_2 was shown to have an apatite structure, with space group P6_3/m and unit cell a= 9.4343(3), c = 6.8885(4) A. The unit cell and bond lengths were found to be slightly smaller than would be expected from other lanthanide-containing analogs; possible reasons for this are discussed.