We investigated the far- and mid-infrared reflectivity (20-6000 cm(-1)) of several Cu1-deltaMgdeltaGeO3 (delta = 0, 0.01) and CuGe1-xSixO3 (x = 0, 0.007, 0.05, 0.1) single crystals. The b- and c-axis optical response is presented for different temperatures between 4 and 300 K. Moreover, a full group theoretical analysis of the lattice vibrational modes of CuGeO3 in the high-temperature undistorted phase as well as in the low-temperature spin-Peierls phase is reported and compared with the experimental results. We observe the activation of zone boundary phonons alone the b-axis of the crystal below the spin-Peierls transition temperature.
