RARE-EARTH AND URANIUM TRANSITION-METAL PNICTIDES WITH LAFE4P12 STRUCTURE

The new compounds NdFe4Sb12, SmFe4Sb12, EuFe4Sb12 and SmRu4Sb12 were prepared by reaction of the binary lanthanoid antimonides LnSb or EuSb2 respectively, with the transition metals and antimony in sealed silica tubes. Single-crystal structure refinements of NdFe4Sb12 as well as of the previously reported compounds EuFe4P12, UFe4P12, EuRu4Sb12 and NdOs4Sb12 led to residuals of R=0.014, 0.023, 0.013, 0.025 and 0.042 respectively. Systematic differences in the interatomic distances of CeFe4P12, EuFe4P12 and ThFe4P12 suggest that the Fermi level cuts through a phosphorus-phosphorus antibonding band. The thermal parameters of the rare earth components increase with the size of their transition metal-pnictogen cages. The cell volumes of the LaFe4P12 type pnictides suggest that cerium and europium have intermediate valencies. In agreement with the higher electronegativity of phosphorus, cerium is the most tetravalent in the phosphides and europium is the most divalent in the antimonides.