Cubic approximants in the Al–Pd–Fe and Al–Pd–Ru systems

The structures of cubic Al66.8Pd21.2Ru12.0 (Fm3; a=15.6058(6) إ), Al63.6Pd30.2Fe6.2 (Fm3; a=15.5187(9) إ) and Al68.9Pd17.1Fe13.9 (Im3; a=15.3755(8) إ) phases, which are referred as the derivatives of 1/0-rational approximants for an icosahedral phase, were determined by means of the single crystal X-ray structural analysis. The edge-shared connection by (Pd,Ru)- and (Pd,Fe)-icosahedra is one of applicable descriptions for these 1/0 related structures and a variety of filling motifs inside the icosahedra produce the crystallographic difference for each structure. The structure of an 1/1-rational approximant of Al57.3Cu31.5Ru11.3 (Pm3; a=12.3773(8) إ) indicates a disordered atomic arrangement inside (Cu,Al)- and Ru-icosahedra. These features are similar to those found in disordered atomic arrangements inside the (Pd,Ru)- and (Pd,Fe)-icosahedra of the 1/0 related approximant structures. Such variation of Mackay atom clusters is suggested to play an important role against the stability region of the fundamental atom clusters with local icosahedral symmetry, together with the chemical and topological ordering found in the icosahedral phases.