The Characterization of High-Critical-Temperature Ceramic Superconductors by Vibrational Spectroscopy

The high-T c superconducting ceramics have been extensively investigated by Raman and infrared spectroscopy. Some of the k = 0 phonons of the La 2– x M x CuO 4 ( M = Sr, Ba), MBa 2 Cu 3 O 7–δ , (MBCO), Bi-(Ca,Sr)-Cu-O, Tl-(Ba,Sr)-Ca-Cu-O, Pb-Sr- Ln-Ca-Cu-O, and Ba 1– x K x BiO 3 (BKB) systems have been observed, and assignments made. The Raman measurements on single crystals have provided the more enlightening results, inasmuch as assignments are facilitated by the use of polarization techniques. Assignments have also been aided by oxygen stoichiometry experiments, substitution of rare earth cations for Y 3+ in YBa 2 Cu 3 O 7–δ , low-temperature measurements, and isotopic studies. Infrared studies and assignments for the Bi, Tl, Pb, and BKB superconductors are lacking. Many of the zone-center phonons for each of the high-T c materials (particularly those for non-totally-symmetric modes) remain undetected. Additionally, some disagreement in assignments (Raman and IR) persists, even for the more extensively studied La 2– x M x CuO 4 and MBCO superconductors.