Neutron powder diffraction on Ce11B2C2 and Nd11B2C2:: A new model for the tetragonal crystal structure

Crystal structures of CeB2C2 and NdB2C2 were investigated by means of neutron powder diffraction using B-11 enriched samples. Diffraction patterns of (CeB2C2)-B-11 and (NdB2C2)-B-11 are very similar to each other, and Rietveld analyses reveal that the crystal structure belongs to a symmetry group of tetragonal P4/mbm. where the rare earth atoms occupy 2(a) sites and the B and C atoms occupy two 4(h) sites with different positional parameters. The newly determined structure differs from previously reported ones with P4/mbm and P (4) over bar 2c mainly in the site positions occupied by the B and C atoms. The B and C atoms form network planes consisting of non-regular squares and octagons.