Effect of grain size on the transition between ferroelectric and relaxor states in 0.8Pb(Mg1/3Nb2/3)O3-0.2PbTiO3 ceramics

The phase transition between the ferroelectric and relaxor states in 0.8Pb(Mg1/3Nb2/3)O-3-0.2PbTiO(3) ceramics with decreasing grain sizes in the submicron range, down to the nanoscale, has been studied with measurements of the temperature dependence of the dielectric and elastic properties across the transition. Results indicate that main size effects are the shift of the transition toward lower temperatures when grain size decreases and the sharp slowing down of its kinetics below a size of 0.36 mu m. This is consistent with the existence of a well defined temperature of slowing down of the transition in the temperature interval between 334 and 338 K for 0.8Pb(Mg1/3Nb2/3)O-3-0.2PbTiO(3). Rietveld analysis of x-ray diffraction patterns indicates that kinetics mainly control the scale of the Cm monoclinic distortions within an average R3m rhombohedral phase, which decreases with grain size, analogously to the effect of decreasing the amount of PbTiO3 in the solid solution. The scale of the monoclinic order strongly affects ferroelectric switching, which is hindered when it is short ranged.