Neutron scattering studies of the mechanism of ferroelectricity in 68% PbMg1/3Nb2/3O3-32%PbTiO3

Neutron scattering has been used to study the relaxor ferroelectric 0.68PbMg(1/3)Nb(2/3)O(3)-0.32PbTiO(3) (i. e. (1-chi) PbMg1/3Nb2/3O3-chi PbTiO3 with chi=0.32 denoted as 0.68PMN-0.32PT). On cooling, 0.68PMN-0.32PT exhibits a cubic-tetragonal ferroelectric phase transition at T-c similar to 425 K. It was found that well above T-c, the transverse optic mode and the transverse acoustic mode are strongly coupled, and a model was used to describe this coupling that gave similar parameters to those obtained for the coupling in PbMg1/3Nb2/3O3 (PMN). At a lower temperature additional quasi-elastic (QE) scattering was found for which the susceptibility follows the temperature dependence of the dielectric susceptibility, as found also in PMN. QE scattering is associated with the dynamic polar nanoregions that occur below the Burns temperature. In addition to QE scattering a strictly elastic central peak (CP) was observed which at wavevectors away from the Bragg peak was much weaker than the CP in pure PMN. On cooling below the ferroelectric phase transition the intensity of the CP decreased. Our results show that the QE scattering and CP observed both in PMN and in the present study result from scattering by dynamic and static nanoregions and can be explained qualitatively by a random-field model.