Far-infrared vibrational mode in Cu1-xMxGe1-ySiyO3 (M=Zn,Cd,Ni)

We report on far-infrared measurements of Zn-, Cd-, and Ni-doped crystals of the spin-Peierls compound CuGeO3. Dopants that replace Cu have the effect of introducing several absorption lines, polarized in the ab plane, between 5 and 55 cm-1. The intensity of the absorption grows with dopant concentration but saturates above 2%. One line near 10 cm-1 loses intensity above 4 K, and a second line near 20 cm-1 is absent at low temperatures but grows to peak at about 40 K in agreement with a three-level model with two excited states 10 and 30 cm-1 above the ground state. As the doping is increased these lines broaden and a temperature-independent absorption develops over the entire range from 5 to 55 cm-1. These features are magnetic-field independent up to 16 T and are absent in samples doped with Si that replaces Ge instead of Cu. We suggest the new absorption is due to localized lattice modes of the dopant ion and the surrounding GeO4 tetrahedra.