Crystal structures of the RAlSi and RAlGe compounds

It was established by X-ray diffraction that the terbium alumosilicide TbAlSi crystallizes with a tetragonal, α-ThSi2-type structure (tI12, I41/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)Å, b=10.2983(2)Å, c=5.7085(1)Å; RB=0.072. The structures of all known RAlSi compounds are characterized by trigonal-prismatic coordination of the smaller atoms.