Raman scattering from A g and B 1 g phonons in Bi 2 Sr 2 Ca n − 1 Cu n O 2 n + 4 ( n =1,2)

Raman measurements were performed on intergrowth-free ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ (2:2:0:1) and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (2:2:1:2) crystals. The phonon spectra of both phases are similar; however, two lowest-frequency phonon lines corresponding to the 28- and 47-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ lines in 2:2:1:2 were observed at 20 and 41 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in 2:2:0:1, and, in both cases, these two Raman features appear exclusively in the z(yy)z\ifmmode\bar\else\textasciimacron\fi{} configuration. In addition, two weak ${\mathit{A}}_{\mathit{g}}$ lines at 109 and 129 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which are absent in spectra of 2:2:0:1, were resolved for 2:2:1:2. An overall assignment can be achieved by attributing the two lowest-frequency lines to the first- and second-order scattering features of the amplitude mode of the superstructural modulation, and the other lines to the optical-phonon modes of the distorted orthorhombic subcell. A careful comparison between the spectra of the single- and double-${\mathrm{CuO}}_{2}$-layer Bi compounds enables us to clarify some controversies on assigments of certain phonon modes, especially those in the ${\mathrm{CuO}}_{2}$ layer.