Raman scattering in (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 mixed crystal system II

The temperature-concentration dependences of the Raman spectra in the high-temperature region above the Burns temperature and the electron diffraction at room temperature were investigated in the (1 - x)Pb(Zn1/3Nb2/3)O-3-xPbTiO(3) mixed crystal system. It was found that in the concentration range below x = 0.8, the Raman active modes exist in the cubic phase and the intensities of the Raman lines in the cubic phase decrease with increasing temperature. A finite region where the Raman active modes are observed in the cubic phase was determined in the temperature-concentration phase diagram. On the other hand, no (1/2, 1/2, 1/2) superlattice diffraction was observed in the mixed crystal with the content above x = 0.2. The present result indicates that the Raman active modes in the high-temperature phase cannot be explained by the zone folding for the superstructure with the Fm (3) over barm symmetry.