LOCAL ATOMIC-STRUCTURE AND THE VALENCE-BAND STRUCTURE OF THE RHOMBIC-TRIACONTAHEDRAL QUASI-CRYSTAL, ITS 1 1 APPROXIMANT, AND THE MACKAY-ICOSAHEDRAL QUASI-CRYSTAL IN THE AL-MG-PD ALLOY SYSTEM/

The rhombic-triacontahedral-type quasicrystal (RT-QC), its (1/1, 1/1, 1/1) approximant (RT-1/1) and the Mackay-icosahedral-type quasicrystal (MI-QC) can be formed in the Al-Mg-Pd system. The radial distribution function RDF(r) spectra of the three compounds is derived from neutro n diffraction experiments. Powdered x-ray diffraction Rietveld refinem ent is also carried out to determine the atomic structure of the RT-1/ 1, and the refined data are converted to the RDF(r). The interatomic d istance of the AI-Pd pair in the RT-1/1 and RT-QC is found to be remar kably shortened relative to that calculated from the hard sphere model with Goldschmidt radii. In the MI-QC, the interatomic distance of the AI-Pd pair is further shortened. In addition, the shortening of the i nteratomic distance is also observed in the Mg-Pd pair. The valence ba nd spectra studied by the x-ray photoemission spectroscopy and soft x- ray spectroscopy clearly show the formation of the bonding and antibon ding states associated with the Al-Pd and Mg-Pd atomic pairs in the MI -QC. The results are consistent with the shortening of the interatomic distances derived from the structure analysis. The reason for the pos session of the highest resistivity in the MI-QC among the three compou nds is attributed to the formation of the covalent bonding between AI- Pd and Mg-Pd atomic pairs. [S0163-1829(98)06841-6].