The crystal structure of the binary iridium-aluminum IrAl2.75 and rhodium-aluminum RhAl2.63 phase

The crystal structures of IrAl2.75 (''IrAl5'') and RhAl2.63 (''Rh2Al5'') phases have been studied with single crystal methods: space group P23, IrAl2.75, a = 7.674(1) Angstrom, Z = 8, R-F = 0.053 (457 hkl, 29 parameters) and RhAl2.63, a = 7.6692(1) Angstrom, R-F = 0.042 (684 hkl, 29 parameters). The unit cell contains three transition metal sites and five aluminum sites. Most aluminum sites reveal strong defects in the occupations, some of them are described as being split. The overall number of atoms in the unit cell is 30(1) for IrAl2.75 and 29.0(3) for RhAl2.63. Ordering and full occupation lead either to the AuZn3 (space group <Pm(3)over bar n>) or to the hypothetical (''Ir4Al13'') structure (space group Pm(3) over bar).