Lattice modes in ferroelectric perovskites. III. Soft modes in BaTiO3

We report careful right-angle Raman measurements, in the tetragonal phase of BaTiO3, of the lowest overdamped "soft" E(TO) mode. The measurements are from 22°C to the tetragonal-to-cubic phase transition, which occurs at Tc=135°C. We fit the data to the usual damped-harmonic- oscillator form, and obtain the harmonic frequency o and the damping constant. The results are in reasonable agreement with other measurements (which, however, have not gone as high in temperature). Unexpectedly, we find that 0 increases up to Tc, which means that the dielectric constant perpendicular to the ferroelectric axis, a, decreases right up to Tc. However, this result agrees with capacitance measurements of a to T=Tc-8°C. We also comment on the results of the A1 modes and reemphasize that all the A1 and E modes disappear abruptly above Tc and thus obey the appropriate selection rules. There are, however, broad features at about 270 and 520 cm-1 that can be observed above as well as below Tc. These broad features are not any of the 3n-3=12 optical modes at the center of the Brillouin zone. © 1978 The American Physical Society.