Temperature dependence of the Raman spectrum of the high T_c superconductor YBa₂Cu₃O₇

The Raman spectrum of polycrystalline sintered YBa2Cu3O7 has been measured at several temperatures between 10K and 300K. Ten Raman active phonon modes are observed with frequencies of 153, 217, 291, 309, 335, 441, 493, 506, 601 and 640 cm-1. The 335 cm-1 mode shows anomalous behavior: Above Tc it increases in frequency on cooling but below Tc its frequency shift changes sign as the mode begins to soften. The fact that most of the mode frequencies we observe do not coincide with the measured infrared frequencies supports the assignment of the symmetry of the crystal to a centrosymmetric space group. © 1993.