Phonon natures in Raman spectra of Y-Ba-Cu-O compounds

Raman spectra of the superconducting phase and nonsuperconducting phases, which mainly include semiconducting-phase and impurity-phase ${\mathrm{BaCuO}}_2$, are examined in Y-Ba-Cu-O compounds. The doping effects from substitution of copper atoms by impurity atoms are discussed. The substitution of a lighter atom, e.g., vanadium, gives rise to local modes at 810 and 840 ${\mathrm{cm}}^{\mathrm{-}1}$, which are qualitatively elucidated by the theory of lattice dynamics in nonideal solids with a local perturbational operator related to the mass-defective coefficient. In tin substitution there is a critical doping $x_c$=1. Above $x_c$ the system becomes nonsuperconducting, which could be explained on the basis of band-structure analysis. The emergence of a Raman line at 646 ${\mathrm{cm}}^{\mathrm{-}1}$ below $T_0$=234 K shows a phase transition. The discrimination of this line from 640 ${\mathrm{cm}}^{\mathrm{-}1}$ in impurity-phase ${\mathrm{BaCuO}}_2$ is clearly made.