PHONONS IN PURE AND DOPED CUGEO3 SPIN-PEIERLS CRYSTALS - RAMAN AND ULTRASONIC STUDIES

Elastic anomalies have been identified in pure and doped (Zn, Si) CuGe O3 single crystals by studying the evolution of the Raman phonons and the elastic constant C-33 as a function of temperature and magnetic fi eld. At high temperatures phonon softening centered around 100 K is ob served for pure and doped crystals, brushing aside any relation with t he spin-Peierls transition. Below 20 K, all the Raman-active phonons s tiffen while the C-33 elastic constant softens as the result of magnet o-elastic coupling correlated to the spin-Peierls transition and a thr ee-dimensional Neel state.