Optic modes in the AlPdMn icosahedral phase

The dynamics of the icosahedral phase AlPdMn have been reinvestigated on a centimetre-size single grain using inelastic neutron scattering measurements. As previously found, well defined acoustic modes are observed close to strong Bragg reflections. Transverse acoustic phonons remain resolution limited for wavevectors smaller than 0.35 AA-1 and a broadening occurs for larger wavevectors. Beside acoustic modes, the scattering function S(Q, omega ) shows an extremely rich structure. A series of broad (1 THz) dispersionless excitations appears with energies close to 1.8, 3, 4 and 5.5 THz. These excitations were found at different points in reciprocal space and present intensity variation. Some of these 'optic' bands might be associated with crossings at quasi-Brillouin zone boundaries. The system has also been investigated at high temperatures up to 800 degrees C. Beyond a 10% reduction of the sound velocity, results are identical to those obtained at room temperature. In particular the transverse acoustic modes remain resolution limited for wavevectors smaller than 0.35 AA-1.