INFRARED REFLECTIVITY OF PURE AND DOPED CUGEO3

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 re sponse is presented for different temperatures between 4 and 300 K. Mo reover, a full group theoretical analysis of the lattice vibrational m odes 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 p honons alone the b-axis of the crystal below the spin-Peierls transiti on temperature. Published by Elsevier Science B.V.