Neutron scattering study of the vibrational density of states in icosahedral and crystallineAl0.80Mn0.20

We present an inelastic neutron scattering study of the vibrational density of states, $\overline{g}\mathrm{}\left(E\right)\mathrm{}$, for the icosahedral and crystalline phases of ${\mathrm{Al}}_{0.80}$ ${\mathrm{Mn}}_{0.20}$. At low energy transfers we find nearly identical $\overline{g}\mathrm{}\left(E\right)\mathrm{}\propto E^2$ dependences indicating that the two materials are elastically similar. In the intermediate energy range, 20-35 meV, the crystalline $\overline{g}\mathrm{}\left(E\right)\mathrm{}$ shows weak structure while that of the icosahedral phase remains smooth. Above 40 meV there is an excess of the icosahedral $\overline{g}\mathrm{}\left(E\right)\mathrm{}$ compared with that of the crystalline material. These results provide a direct test for models of the interatomic forces and dynamics of the icosahedral phase.