No abstract

The absorption coefficient of CuGeO 3 was measured in the energy range of 1-4 eV, in a temperature interval between 300 and 20 K, for light polarized along the b and c crystal axes. A weak absorption band at 1.7 eV is attributed to copper d-d transitions partially allowed by electron-phonon interactions; this is followed by a region of transparency leading to a strong exponential edge above 3 eV, which is identified with the onset of charge-transfer transitions. ͓S0163-1829͑96͒53040-7͔

A comparative study of first-order Raman scattering in K 0.984 Li 0.016 TaO 3 and KTa 0.976 Nb 0.024 O 3 single crystals has been performed. The soft-phonon line shows marked differences in the two systems, revealing a distinctive behavior with regard to the scale of polar order. Analysis of the low-frequency spectrum allows for an estimate of the polar correlation length in the Li-doped sample; this is limited to the nanometric scale, in agreement with the view of a dipole-glass transition. No evidence of a similar situation is found in the Nb-doped sample, where the general trend of soft-mode frequencies and intensities suggests a transition to a state of long-range order, for the quoted concentration. The intensities of the hard-phonon spectra and the TO 4 linewidth are shown to confirm the different evolution of the polar transition in the two systems.