The phonon density of states in the archetypical icosahedral quasicrystal Al62Cu25.5Fe12.5

We study the vibrational density of states (VDOS) in the i-AlCuFe quasicrys tal measured by different methods: (i) Fe-57 inelastic nuclear-resonant abs orption (INA) of X-rays (ii) and time-of-flight inelastic neutron scatterin g (INS). The samples studied were i-Al62Cu25.5Fe12.5 with different isotopi c substitutions. INA measures the Fe-partial VDOS while INS the neutron-wei ghted 'generalised' GVDOS. We find the unexpected result that the iron-part ial VDOS has a single sharp maximum. The neutron-weighted GVDOS for differe nt isotopes (natural Cu and Cu-65; natural Fe and Fe-57 shows that the copp er-partial VDOS is strongly peaked as well but at significantly lower energ y. These results are discussed in light of the icosahedral quasicrystal mod el of Katz and Gratias. as well as specific heat results which indicate a d iscrepancy between the speed of sound, the specific heat and the low energy limit of the VDOS. (C) 2000 Elsevier Science B.V. All rights reserved.