Crystal structure of a new 1/1-rational approximant for the Al-Cu-Ru icosahedral phase

The structure of a cubic Al57.3Cu31.4Ru11.3 phase, which is referred to as an 1/1-rational approximant of an Al-Cu-Ru icosahedral phase, was determine d by means of a single crystal X-ray structural analysis: space group Pm3 ( No. 200), a=12.3773(8) Angstrom, V=1896.2(2) Angstrom(3), MoK alpha (lambda =0.71073 Angstrom); atoms/cell=122.6, F(000)=2645, D-calc=5.04 Mg/m(3), R=0 .0495 for the observed 1140 reflections with F-obs>4.0 sigma(F-obs). Cu- an d Ru-rich icosidodecahedral atom clusters are recognized at the In and Lb s ites of the unit cell, respectively. The atom clusters possess Ru centers a nd the surrounding first shells of about 2.5 Angstrom in diameter show heav ily disordered features with partial breakdown of the local icosahedral sym metry. These chemically and topologically decorated Mackay-like atom cluste rs should be classified as the fundamental structural units for the F-type Al-Cu-Ru icosahedral phase. (C) 2000 Elsevier Science S.A. All rights reser ved.