Unlike other ferroelectric perovskites, PbTi${\mathrm{O}}_3$ displays temperature-dependent Raman spectra consisting of underdamped modes, including the "soft" transverse-optical (TO) modes. Moreover, the softest TO mode shows linewidth divergence at the ferroelectric-to-paraelectric transition. Also in contrast to previously studied perovskites, Raman selection rules are rigorously obeyed in both the ferroelectric and paraelectric states.

• Received 24 February 1970

DOI:https://doi.org/10.1103/PhysRevLett.25.167