Exploration in the Sc-Al-Cu system: evidence for i-ScAl_2.4Cu_3.2 quasicrystal and new crystal data for five intermetallic phases

The ternary system Sc-Al-Cu system has been investigated in this work. Several alloys with various compositions have been prepared by arc-melting from the elements and studied by X-ray diffraction. An icosahedral quasicrystal of formula ScAl_2.4Cu_3.2 has been identified using the reciprocal space reconstructed from its diffraction images. The diffraction data collected for several single crystals led to solve and refine five crystal structures: cubic ScAlCu₂ (Fm $3¯$m), hexagonal ScAl_1.1Cu_0.9 (P$6¯$m2), hexagonal ScAl_2.4Cu_2.9 (P6₃/mmc), tetragonal ScAl_5.3Cu_6.7 (I4/mmm) and orthorhombic ScAl_2.2Cu_3.4 (Cmmm). Almost all these structures are characterised with copper/aluminum atomic mixing at most of the crystallographic sites and the atomic arrangements are favourable to the existence of stoichiometry ranges for the compounds. These results, providing new single crystal data sets for these ternary compounds, clarify and confirm the structures previously reported (from crystal or powder data).