Raman spectrum of the superconductor ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$

Two Raman setups, one with high resolution, and the other for spatially resolved (1×1 μ${\mathrm{m}}^2$) experiments, have been used to investigate the polarized Raman spectra of a superconducting single crystal of the (2:2:1:2) phase ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$Ca${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$, and the unpolarized micro-Raman spectra of three ceramics, two of which have zero resistance below ∼105 K and contain substantial proportions of the (2:2:2:3) phase (∼15%-20%). Our spectra differ in some aspects from those recently published for the (2:2:1:2) phase. The occurrence of several low-frequency lines ($\nu \le 65$ ${\mathrm{cm}}^{-1\mathrm{}}$) is confirmed. The width of the lines and their shape is discussed with regard to the occurrence of disorder in the structure and of an electron-phonon coupling. The spectra of the ceramics reveal the presence of four minority phases in addition to the main phases (2:2:1:2) and (2:2:2:3). The ceramics containing the (2:2:2:3) phase display the same lines as the ceramic not containing this phase. This result is discussed in the light of the structural information available and of the grain-growth habit of the (2:2:1:2) phase.