FIELD EFFECT ON COCHRAN MODES IN SRTIO3 AND KTAO3

In connection with the Cochran theory of ferroelectricity, the thermal conductivity has been investigated in SrTiO3 between 5 and 300A°K and in KTaO3 between 2.5 and 300A°K. For temperatures above its maximum value, the conductivity shows an anomaly in SrTiO3 which is absent in KTaO3. This behavior is explained in terms of the degeneracy between the transverse optic (Cochran modes) and the longitudinal acoustic phonon branches. Measurements were also taken in electric fields up to 55 kV/cm, applied perpendicular to the temperature gradient. A considerable increase in thermal conductivity is observed in SrTiO3 below 50A°K, while the effect is much smaller in KTaO3. An interpretation of this effect is given by assuming the transverse optic mode at q=0 to shift in frequency upon application of an electric field. An analysis of the experimental data in terms of a simple model is presented which leads to quantitative values for the frequency shifts in SrTiO3. The results obtained for KTaO3 indicate the presence of a phase transition of the nonferroelectric type at 4.5A°K, in analogy to the 103A°K transition known for SrTiO3. © 1968 The American Physical Society.