The structure of O-AlFePd has been analyzed by single-crystal X-ray diffraction, yielding space group Cmca (No. 64), a = 0.8130(1) nm, b = 1.5474(2) nm, c = 2.3868(4) nm, atoms/cell = 204, F(000) = 3339, D calc = 5.621Mgm -3 and R = 0.0294 for the 2117 observed reflections with F obs 4.0·(F obs). The structure consists of four Fe sites, one Fe/Pd mixed site, and fifteen Al sites, and the corresponding structural formula is Al 13(Fe,Pd) 4 (Z = 8). The O-AlFePd phase is a new type of approximant of decagonal quasicrystals with a periodicity of 0.8 nm, and the alternation of flat and puckered layers at an interval of about 0.2nm is the fundamental structural feature. The structure could be also described by a linkage of pentagonal atom columns similar to that found in monoclinic Al 13Fe 4. Nevertheless, the columnar linkage for the O-AlFePd phase is unique, and this allows us to advance an idea explaining the fine (001) glide twins frequently found in the Al 13Fe 4 phase.
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