Raman and infrared absorption spectra of ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ have been collected for $0<~x<~0.5$ in the frequency range of the phonon spectrum. The data show a remarkable dependence on the Al content. Raman spectra have 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.

Here we report the synthesis and characterization of ${\mathrm{Mg}}_{1-x}{\mathrm{Sc}}_x{\mathrm{B}}_2\left(0.12<x<0.27\right)$ system, with critical temperature in the range of $30>T_c>6\mathrm{K}$. We find that the $\mathrm{Sc}$ doping moves the chemical potential through the 2-D/3-D electronic topological transition (ETT) in the $\sigma$ band where the “shape resonance” of interband pairing occurs. In the 3-D regime beyond the ETT we observe a hardening of the ${\mathrm{E}}_{2g}$ Raman mode with a significant line-width narrowing due to the suppression of the Kohn anomaly over the range $0<q<2k_F$.