Lattice modes in ferroelectric perovskites. II. Pb1-xBaxTiO3 including BaTiO3

Abstract

The Raman measurements of the ferroelectric PbTiO3 reported previously are extended to the ferroelectric system (Pb1-xBax)TiO3 including the end member BaTiO3. The measurements were made on crystals of BaTiO3, very small crystals for several x, and powders using the powder Raman technique. The expected 3 n-3 optic modes are accounted for at all x including BaTiO3. These modes in BaTiO3 obey the proper selection rules and disappear and reappear abruptly at the transition temperature Tc when approached from below or above, respectively. This is contrary to what is often suggested in the literature. Of course there are other broad bands observed in BaTiO3 that do not disappear at Tc. These broad bands are only found for x0.9 to 1. Similarly, the room-temperature data show that the lowest soft E (TO) mode is overdamped only for x0.9 to 1. For smaller x it is underdamped at room temperature. However, the linewidth of the underdamped soft E (TO) mode behaves in a singular manner as Tc is approached from below as is found in PbTiO3. For x=0.9 and 1 this singular behavior is not observed. The data on the soft E (TO) mode in BaTiO3 show that the shape of the response is practically independent of temperature in agreement with earlier work. Also, the intensity is practically independent of temperature in strong disagreement with earlier work. We cannot unambiguously determine the undamped frequency and linewidth of this mode, and we feel that the temperature dependences of these quantities, for the soft E (TO) mode in BaTiO3, are not as yet determined. The frequency of the modes is a fairly continuous function of x even though there are several phase transitions on the BaTiO3-rich end. Lastly, some discussion of the temperature dependence of the dielectric constant in terms of impurities and the concept of dirty displacive ferroelectrics is given. Very large enhancements of the static dielectric constant can be obtained due to interaction with the soft modes. © 1974 The American Physical Society.