OPTICAL CONDUCTIVITY OF THE ICOSAHEDRAL QUASI-CRYSTAL AL75.5MN20.5SI4AND 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-e qual-to 10 K to 295 K. The optical conductivity of the icosahedral sam ple is similar to that reported for the stable quasicrystal Al63.5Cu24 .5Fe12; starting out from a low DC value it rises linearly with freque ncy 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, abov e which it decreases slowly. The conductivity of the approximant phase is similar in shape, but the overall magnitude is lower. There is a b road structure at 200 cm-1 in the quasicrystal which is resolved into al least nine sharp phonon lines in the approximant. Band structure ca lculations of the crystalline approximant predict a pseudogap in the e lectronic density of states at the Fermi surface suggesting that the u nusual optical properties of the icosahedral material may also be expl ained by band structure arguments.