STRUCTURE OF YALGE AND ISOTYPIC RARE-EARTH ALUMINUM GERMANIDES

YAlGe, M(r) = 188.48, orthorhombic, oS12, (63) Cmcm - c2a, a = 4.0504 (5), b = 10.440 (1), c = 5.7646 (9) angstrom, V = 243.77 (4) angstrom-3, Z = 4, D(x) = 5.136 Mg m-3, lambda-(Mo K-alpha) = 0.71073 angstrom, mu = 37.481 mm-1, F(000) = 336, T = 300 K, wR = 0.049 for 153 contributing unique reflections. This new structure type, a substitution variant of the UPt2 type, is characterized by sheets of Ge centred trigonal prisms formed by Y and Al atoms via common Al edges. The YAlGe type is found with RAlGe compounds when R is a small rare-earth element [R = Gd (low-temperature modification), Tb, Dy, Ho, Er, Tm, Lu and Sc]. With large rare-earth elements [R = La, Ce, Pr, Nd, Sm, Eu and Gd (high-temperature modification)] the tetragonal alpha-ThSi2 type (or an ordered variant) is observed.