Soft mode spectroscopy in barium titanate

Abstract

Infrared reflectivity measurements are performed with a scanning interferometer on a single crystal over a wide range of temperature (300-1350K) especially in the cubic paraelectric phase of barium titanate (T_c=395K). The temperature dependence of TO and LO frequencies, dampings and TO oscillator strengths is reported. An overdamped soft mode, which follows a Curie-Weiss law over 800 degrees , is evidenced. A deviation from this law occurs over a hundred degrees above the tetragonal-cubic transition. The discrepancy of the value of dielectric constant obtained from optical mode frequencies and that obtained by direct dielectric measurements is discussed in terms of the possibility of a disorder which would appear progressively on cooling, in the cubic phase, and which complicates the displacive-type mechanism.