INFRARED SIGNATURES OF THE SPIN-PEIERLS TRANSITION IN CUGEO3

We investigated the infrared reflectivity of several Mg-and Si-substit uted CuGeO3 single crystals. The temperature-dependent b-axis and c-ax is optical response is reported. For T < T-SP we detected the activati on of zone-boundary phonons along the b axis of the crystal on the pur e sample and for 1% Mg and 0.7% Si concentrations. From a detailed ana lysis of the phonon parameters the redshift of the B-2u 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 pola rization dependence of a magnetic excitation measured in transmission at 44 cm(-1) has been investigated. [S0163-1829(97)51942-4].