Quasicrystalline Ti-Ni-Fe-Si: Mössbauer indications for high transition-metal-site symmetry

Melt-spun ${\mathrm{Ti}}_{56}$ ${\mathrm{Ni}}_{28\mathrm{-}\mathrm{x}}$ ${\mathrm{Fe}}_{\mathrm{x}}$ ${\mathrm{Si}}_{16}$ shows the icosahedral phase recently reported for other rapidly quenched Ti-Ni-based alloys. ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer measurements in this alloy yield a quadrupole splitting Δ∼0. The value of Δ indicates a highly symmetric Fe site in the icosahedral structure. The relative intensities of the x-ray diffraction lines suggest a structure based on a three-dimensional Penrose tiling in which some of the transition-metal atoms occupy sites with icosahedral symmetry. This is the first experimental indication that a transition metal may occupy the icosahedral site in any quasicrystal.