THE ROLE OF OXYGEN IN THE FORMATION OF TITANIUM-BASED ICOSAHEDRAL PHASES AND CRYSTAL APPROXIMANTS

The first report of an oxygen-stabilized quasicrystal is presented. Ox ygen is shown to be essential for the formation of the icosahedral and 1/1 approximant phases in Ti-Mn-Si and Ti-Cr-Si alloys. Recent X-ray and neutron diffraction structural studies by us of the 1/1 approximan t phase reveal specific sites for the oxygen in the Mackay icosahedral clusters. On the basis of these data, possible explanations for the r ole of oxygen in quasicrystal formation are considered. These include firstly the creation of a nucleation barrier for competing crystalline phases, secondly the stabilization of the fundamental Mackay icosahed ral cluster and thirdly an adjustment of the electron-to-atom ratio to shift the Fermi surface into a minimum in the density of states.