Polar phonons and central mode in antiferroelectric PbZrO3 ceramics

Infrared reflectivity spectra of lead zirconate ceramics were measured in the frequency region of 20-3000 cm(-1) and temperature range from 10 to 900 K. The data, extended to lower frequencies by coherent source submillimetre, time-domain terahertz transmission and microwave dielectric spectroscopies, were fitted with the factorized model of the dielectric function. Besides the polar optical phonons, one of which slightly softens in the vicinity of the antiferroelectric phase transition, a central-mode type dispersion was revealed in the 5-25 cm(-1) (10(11)-10(12) Hz) range. This central mode is found to be responsible for the strong dielectric anomaly in the paraelectric phase, whereas the lattice phonon contribution does not exceed epsilon' similar to 300 in the vicinity of T-c. We attribute the origin of the central mode to the lattice disorder caused by strongly anharmonic hopping of Pb ions.