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 m easurements. As previously found, well defined acoustic modes are obse rved close to strong Bragg reflections. Transverse acoustic phonons re main resolution limited for wavevectors smaller than 0.35 Angstrom(-1) and a broadening occurs for larger wavevectors. Beside acoustic modes , the scattering function S(Q, omega) shows an extremely rich structur e. A series of broad (1 THz) dispersionless excitations appears with e nergies close to 1.8, 3, 4 and 5.5 THz. These excitations were found a t different points in reciprocal space and present intensity variation . Some of these 'optic' bands might be associated with crossings at qu asi-Brillouin zone boundaries. The system has also been investigated a t high temperatures up to 800 degrees C. Beyond a 10% reduction of the sound velocity, results are identical to those obtained at room tempe rature. In particular the transverse acoustic modes remain resolution limited for wavevectors smaller than 0.35 Angstrom(-1).