New stable decagonal quasicrystal in the system Al–Ir–Os

A stable decagonal quasicrystal with nominal composition Al73Ir14.5Os12.5 has been grown from the aluminum-rich melt. It has an incongruent melting temperature of Tm = 1283 °C and does not transform into a periodic phase within the maximum applied annealing time of 8 weeks at 1000 °C. The phase equlibria in the aluminum-rich part of the system Al–Ir–Os have been explored. The X-ray diffraction patterns can be indexed by five reciprocal basis vectors with lengths ai*=0.2588(2) Å−1, i = 1, …, 4, and a5*=0.05945(3) Å−1. This corresponds to a quasilattice parameter (i.e. the edge length of the related Penrose tiling) of 2.501(2) Å and a translation period along the 10-fold axis of 16.821(8) Å. The 5D space group is P105mc. The structure of the decagonal phase has been solved by two different approaches, the 3D pseudo-approximant technique and the 5D charge-flipping method. It is closely related to those of decagonal Al–Os–Pd and Al–Ni–Ru and can be described as quasiperiodic packing of columnar clusters with 20.150 Å diameter. The clusters consist of eight quasiperiodic layers with stacking sequence …ABCDA′B′C′D′…, where the prime (′) denotes the layers generated by a 105 operation.