OPTICAL PHONONS OF ALTERNATING CHAINS WITH 3 4-FILLED BANDS AND INTERNAL DEGREES OF FREEDOM - A SIMPLE-MODEL FOR CA2CUO3/

The long-wavelength phonon spectrum of an alternating chain, consistin g of molecules with internal degrees of freedom and ions (atoms) in be tween, is investigated within a frozen-phonon-type approach for the pe riod-4 mixed charge-density-wave-bond-order-wave (CDW-BOW) ground stat e of an one-dimensional Hamiltonian with 3/4-filled bands, diagonal, a nd off-diagonal electron-phonon interaction. Raman and infrared data o f the recently synthesized quasi-one-dimensional system Ca1.8Sr0.2CuO3 are analyzed within this model. The occurrence of ''forbidden'' Raman modes is explained semiquantitatively as the consequence of the perio d-4 CDW-BOW ground state. The parameters derived from the experimental data are consistent with intermediately strong electron-electron inte raction. Necessary experiments to check the relevance of the proposed model are discussed.