Crystal structures of the RAlSi and RAlGe compounds

It was established by X-ray diffraction that the terbium alumosilicide TbAlSi crystallizes with a tetragonal, alpha-ThSi2-type structure (tI12, I4(1)/amd). It has a narrow homogeneity range TbAl1.05-0.99Si0.95-1.01. The equiatomic alumosilicides of Y, Dy, Ho, Er, Tm and Lu crystallize with orthorhombic, YAlGe-type structures (oS12, Cmcm) with constant composition. The unit cell parameters decrease smoothly with increasing atomic number of the rare-earth metal atom. The crystal structure of YAlSi was refined from powder X-ray diffraction data: a = 3.99476 (7) Angstrom, b = 10.2983 (2) Angstrom, c = 5.7085 (1) Angstrom; R-B = 0.072. The structures of all known RAlSi compounds are characterized by trigonal-prismatic coordination of the smaller atoms. (C) 2003 Elsevier B.V. All rights reserved.