The crystal structure of the binary iridium-aluminum IrAl_2.75 and rhodium-aluminum RhAl_2.63 phase

The crystal structures of IrAl_2.75 (“Ir₂Al₅”) and RhAl_2.63 (“Rh₂Al₅”) phases have been studied with single crystal methods: space group P23, IrAl_2.75, a = 7.674(1) Å, Z = 8, R_F = 0.053 (457 hkl, 29 parameters) and RhAl_2.63, a = 7.6692(1) Å, 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 IrAl_2.75 and 29.0(3) for RhAl_2.63. Ordering and full occupation lead either to the AuZn₃ (space group Pm[unk]n) or to the hypothetical “Ir₄Al₁₃” structure (space group Pm[unk]).