Effects of doping on phonon Raman scattering in the Bi-based 2212 system

The polycrystalline samples of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{y}$ (Bi2212) with different oxygen content, ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{y}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{y},$ have been characterized by x-ray-diffraction and Raman-scattering measurements. It is found that the $c$-axis parameter decreases with increasing doping concentration (Pr or La) or oxygen content. The softening of the ${O(2)}_{\mathrm{Sr}}{A}_{g}$ mode corresponds to the decrease of the $c$-axis parameter. The effect of Pr substitution for Ca on this mode is smaller than that of La substitution for Sr. The shift of the ${O(1)}_{\mathrm{Cu}} {B}_{1g}$ mode is related to the ionic size of the doping element. The ${O(3)}_{\mathrm{Bi}}{A}_{g}$ mode does not exhibit remarkable frequency shift since the $\mathrm{Bi}\ensuremath{-}O(3)\ensuremath{-}\mathrm{Bi}$ force constant nearly remains unchanged upon doping. The effect of oxygen content on the ${O(2)}_{\mathrm{Sr}}{A}_{g}$ mode in the Bi2212 system is in contrast to that on the corresponding ${O(4)}_{\mathrm{Ba}}{A}_{g}$ mode in the Y123 system. Change in valence is the dominating effect causing the frequency shift of the ${O(2)}_{\mathrm{Sr}} {A}_{g}$ mode.