Crystal structures of the compounds Sm2AlGe3 and Tb2AlGe3

The structures of two new rare-earth aluminum germanides, Sm2AlGe3 and Tb2AlGe3, were determined from X-ray powder diffraction data (RB = 0.066 and 0.064, respectively). These R2AlGe3 compounds crystallize with orthorhombic structures of the Y2AlGe3 type, oP24, (62) Pnma − c6, a = 6.8153(4), b = 4.2723(3), c = 17.9491(10) Å (R = Sm) and a = 6.7749(4), b = 4.1980(3), c = 17.7222(10) Å (R = Tb), Z = 4. Two-thirds of the Ge atoms are situated at the centers of R6 trigonal prisms, whereas the other Ge atoms occupy the centers of R4 tetrahedra. Four other compounds are known to crystallize with the same structure type, R = Dy, Ho, Er and Tm, but Sm2AlGe3 is the first example of an isotypic compound where R is a light rare-earth element.