Ternary Rare Earth Osmium Aluminides <i>R</i>₇₊<i>_x</i>Os₁₂Al₆₁₊<i>_y</i> Belonging to a Structural Family with Layered Topology

The 10 intermetallic compounds R7+xOs12Al61+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 Y7.28Re12Al61.38. The structure was determined from four-circle diffractometer data of Y7+xOs12Al61+y: P63/mcm, a = 1301.5(2) pm, c = 903.0(2) pm, Z =1. Four atomic sites, all located on the 63 axis, show fractional occupancy, resulting in the composition Y7.86(1)Os12Al61.51(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 CeOsGa4, Ho3Ru4Ga15, and Y2Pt6Al15 are the most recent examples. This structural family is briefly reviewed. The cell volume of Yb7+xOs12Al61+y indicates a mixed or intermediate valence character +2/+3 for the ytterbium atoms of this compound.