Trigonal Ir9Al28, a new structure type and approximant to decagonal quasicrystals

During an investigation of the binary system Al–Ir, a new phase with composition Ir9Al28 was identified. After annealing at 780 °C, its structure was determined by single-crystal X-ray diffraction. Trigonal Ir9Al28 is the first representative of a new structure type with Pearson symbol hP236-14, a = b = 12.2864(4) Å, c = 27.341(1) Å, γ = 120°, space group P31c, no. 159. The crystal structure can be described likewise as stacking of eight puckered and flat layers with a sequence …P0PFpp0piFiPi… along [0 0 1], as a six-layer stacking sequence along [1 0 0], or as packing of pseudo-Mackay icosahedra. The Ir substructure with pseudosymmetry P63/mmc resembles the V4Al23 structure type. Pentagonal columnar clusters running along [0 0 1] show close resemblance to decagonal quasicrystals with six-layer period along the 10-fold axis.