Temperature-Dependent Optical Phonons in Lead Titanate

Ceramic platelets and oriented single crystals of PbTiO3 have been investigated by Raman and infrared spectroscopy. The symmetries of the zone-center optical phonons are described and their temperature dependences from 20–760°K are studied, demonstrating that the doubly degenerate modes are relatively the “softest.” It is shown that their behavior accounts for the variation of the static dielectric constant in the ceramic between those temperatures if singly and doubly degenerate modes coexist as pairs of a two-mode type of behavior. It is found that the short-range crystal anisotropy predominates over the long-range electrostatic force in determining phonon frequencies. Above the 760°K Curie temperature the Raman spectrum is entirely second order and quite indistinct. No low-temperature phase transition is found.