Hume-Rothery controlled formation of structurally complex alloy phases in the ternary Ga–Mg–Pd system

Abstract

The Hume-Rothery rule has been used as a guide to search for new approximants of the Mackay icosahedron and Bergman cluster type in the Ga–Mg–Pd system. Ternary alloys with target compositions close to the e/ a ratios 1.67 and 2.09 were synthesized by melting the elements and subsequent annealing in sealed tantalum ampoules. Six new intermetallic phases have been characterized: a cubic face-centered Mackay icosahedron approximant Ga 15− x− y Mg 38+ y Pd 13+ x , a rhombohedral Mackay icosahedron approximant Ga 30− x Mg 32+ x Pd 21, a cubic Bergman cluster approximant Ga 12+ x+ y Mg 11− y Pd 6− x , a cubic phase with interpenetrating I3 and Frank–Kasper networks Ga 32.6Mg 43.4Pd 29, an ordered ternary variant of the NiTi 2 structure type GaMg 3Pd 2 and a ternary Laves phase. The e/ a ratios for the approximants with Mackay icosahedra or Bergman clusters show a strong experimental correlation indicating that the valence electron concentration is an important factor concerning the stability of the phases. The approximant phases exist over a wide range of composition. The structure of the rhombohedral phase is based on the geometry of the canonical cell tiling CB 2C. The results support the proposal by Henley and Mihalkovic on the geometrical structures of Mackay icosahedron-type quasicrystals in ternary systems with one transition and two main group metals.