  We investigated the infrared reflectivity of several Mg- and Si-substituted
CuGeO3 single crystals. The temperature dependent b-axis and c-axis optical
response is reported. For T<Tsp we detected the activation of zone-boundary
phonons along the b axis of the crystal on the pure sample and for 1% Mg and
0.7% Si concentrations. From a detailed analysis of the phonon parameters the
redshift of the B2u mode at 48 cm^-1 is observed and discussed in relation to
the soft mode expected to drive the spin-Peierls phase transition in CuGeO3.
Moreover, the polarization dependence of a magnetic excitation measured in
transmission at 44 cm^-1 has been investigated.
