FOLDED MODES IN THE INFRARED-SPECTRA OF THE SPIN-PEIERLS PHASE OF CUGEO3

Polarized far-infrared transmittance spectra of CuGeO3 single crystals were measured at different temperatures (6 K<T<300 K). Two spectral l ines, at 284.2 cm(-1) in E parallel to c polarization and at 311.7 cm( -1) in E parallel to b polarization, appear at the temperature of the spin-Peierls transition and grow in intensity with decreasing temperat ure. We assign these spectral features to the folded modes of the dime rized lattice. We discuss a possible role of the spin-phonon interacti on in the formation of the 311.7 cm(-1) feature.