Ternary Rare Earth Osmium AluminidesR7+xOs12Al61+yBelonging to a Structural Family with Layered Topology

Abstract

The 10 intermetallic compounds R(7+)(x)Os(12)Al(61+)(y) (R = Y, Nd, Sm, Gd-Tm) were prepared by arc-melting of the elemental components. They crystallize with a hexagonal structure very similar to that of Y(7.28)Re(12)Al(61.38). The structure was determined from four-circle diffractometer data of Y(7+)(x)Os(12)Al(61+)(y): P6(3)/mcm, a = 1301.5(2) pm, c = 903.0(2) pm, Z =1. Four atomic sites, all located on the 6(3) axis, show fractional occupancy, resulting in the composition Y(7.86(1))Os(12)Al(61.51(4)), corresponding to the Pearson symbol hP90-8.63. The structure may be viewed as consisting of alternating atomic layers of two kinds, although chemical bonding within and between the layers is of similar character as can be judged from the near-neighbor environments, where all of the 11 atomic sites have high coordination numbers. One kind of layers (A). is relatively loosely packed and contains the yttrium and some aluminum atoms. The other kind (B). consists of the osmium and the remaining aluminum atoms in a nearly hexagonal close-packed arrangement. These layers are stacked in the sequence ABAB. A similar building principle has recently been recognized for several other structures of ternary intermetallic compounds of rare earth and transition metals with a high content of aluminum or gallium, where the structures of CeOsGa(4), Ho(3)Ru(4)Ga(15), and Y(2)Pt(6)Al(15) are the most recent examples. This structural family is briefly reviewed. The cell volume of Yb(7+)(x)Os(12)Al(61+)(y) indicates a mixed or intermediate valence character +2/+3 for the ytterbium atoms of this compound.