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 degrees 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) angstrom, c= 27.341(1) angstrom, gamma = 120 degrees, space group P31c, no. 159. The crystal structure can be described likewise as stacking of eight puckered and flat layers with a sequence... P(0)PFpp(0)p(i)F(i)P(i)... along [001], as a six-layer stacking sequence along [100], or as packing of pseudo-Mackay icosahedra. The Ir substructure with pseudosymmetry P6(3)/mmc resembles the V4Al23 Structure type. Pentagonal columnar clusters running along [001] show close resemblance to decagonal quasicrystals with six-layer period along the 10-fold axis. (c) 2005 Elsevier B.V. All rights reserved.