Nb9PdAs7: A unique arrangement in the (Mn+3n+2Xn+nY)-X-2-Y-2 family of hexagonal structures

The ternary transition-metal arsenide Nb9PdAs7 has been prepared through re action of the elements, and its structure has been determined by single-cry stal X-ray diffraction methods. It adopts a new structure type (Pearson sym bol hP51, hexagonal, space group P (6) over bar, Z = 3), with unit cell par ameters a = 16.6955(6) and c = 3.5582(l) Angstrom. The structure contains a ssemblies of As-centered trigonal prisms that extend as triangular columns through sharing of the triangular faces. Not only does Nb9PdAs7 extend a fa mily of hexagonal structures with general formula (Mn+3n+2Xn+nY)-X-2-Y-2 to n = 4, the highest member known thus far, but it also displays the unique feature in which there are two distinct types of triangular columns, one ha ving corner atoms (Pd) different from the other atoms (Nb). Structural rela tionships between members of the (Mn+3n+2Xn+nY)-X-2-Y-2 family are presente d. The chemical bonding in Nb9PdAs7 was analyzed through an extended Huckel band structure calculation.