Crystal structures and new perspectives on YAu and YAu.

YAu (triyttrium tetragold) and YAu (tetradecayttrium henpentacontagold), two binary representatives of Au-rich rare earth (R) systems crystallize with the space groups R-3 and P6/m, adopting the PuPd and GdAg structure types, respectively (Pearson symbols hR42 and hP65). A variety of binary R-Au compounds have been reported, although only a few have been investigated thoroughly. Many reports lack information or misinterpret known compounds reported elsewhere. The PuPd type is fairly common for group 10 elements Ni, Pd, and Pt, while Au representatives are restricted to just five examples, i.e. CaAu, PrAu, NdAu, GdAu, and ThAu. SmAu is suspected to be SmAu due to identical symmetry and close unit-cell parameters. The PuPd structure type allows for full substitution of the position of the rare earth atom by more electronegative and smaller elements, i.e. Ti and Zr. The GdAg type instead is more common for the group 11 metals, while rare representatives of group 12 are known. YAu can be represented as a tunnel structure with encapsulated cations and anionic chains. Though tunnels are present in YAu, this structure is more complex and is best described in terms of polyhedral `pinwheels' around the tunnel forming polyhedra along the c axis.