Highly tuneable icosahedral orders in complex Al-Pd-W-Fe alloys

We report the formation of novel complex intermetallics in Al-Pd-W-Fe alloys with varying icosahedral orders. The quaternary system is akin to the Al-Pd-Cr-Fe and Al-Pd-Mo-Fe systems, in which high-order periodic approximants associated with the face-centred icosahedral quasicrystal, i-Al70Pd20Mn10, have been previously reported. Herein, an analogous high-order periodic approximant is identified at compositions of Al67–68Pd24W1–2Fe6–9, along with an icosahedral quasicrystal at approximately Al69Pd22W3Fe6. For the periodic approximant, millimetre-sized single grains were successfully obtained through slow cooling method combined with a post isothermal treatment at a sub-melting temperature. To our surprise, single grains annealed at 1223 K and 1248 K showed different crystal symmetries, suggesting a structural phase transition analogous to one observed in the corresponding approximant of the Al70Pd21Mo1Fe8 alloy. High-quality monophase samples of all three phases, with varying icosahedral orders, were obtained using several heuristic approaches. These monophase samples were finally subjected to a comparative assessment of their thermoelectric properties, which reveal contrasting behaviour associated with the position and shape of the pseudogap at the Fermi level.