Effect of the Al content on the optical phonon spectrum in Mg1-xAlxB2

Raman and infrared absorption spectra of Mg1-xAlxB2 have been collected for 0less than or equal toxless than or equal to0.5 in the frequency range of the phonon spectrum. The data show a remarkable dependence on the Al content. Raman spectra ha,e been carefully analized and the x dependence of the peak frequency, the width and the intensity of the phonon modes have been obtained. The results allow us to distinguish between two regimes in the low and high Al content regions. In particular the onset of the high Al content phase evidenced in previous structural studies is marked by new spectral components at high frequencies. Finally a connection between the whole of our results and the suppression of the superconducting phase at high Al content is established.