OPTICAL CONDUCTIVITY OF THE ICOSAHEDRAL QUASI-CRYSTAL AL75.5MN20.5SI4 AND ITS 1/1 CRYSTALLINE APPROXIMANT ALPHA-AL72.5MN17.4SI10.1

The reflectance of the metastable icosahedral quasicrystal Al75.5Mn20.5Si4 and its 1/1 crystalline approximant alpha-Al72.5Mn17.4Si10.1 has been measured from 50 cm-1 to 46000 cm-1 at temperatures from almost-equal-to 10 K to 295 K. The optical conductivity of the icosahedral sample is similar to that reported for the stable quasicrystal Al63.5Cu24.5Fe12; starting out from a low DC value it rises linearly with frequency up to almost-equal-to 1 eV reaching a maximum at almost-equal-to 2.2 eV with a magnitude that is comparable with transition metals, above which it decreases slowly. The conductivity of the approximant phase is similar in shape, but the overall magnitude is lower. There is a broad structure at 200 cm-1 in the quasicrystal which is resolved into al least nine sharp phonon lines in the approximant. Band structure calculations of the crystalline approximant predict a pseudogap in the electronic density of states at the Fermi surface suggesting that the unusual optical properties of the icosahedral material may also be explained by band structure arguments.