Correlations between the structure and dielectric properties of Pb (Sc 2/3 W 1/3) O 3–Pb (T i/Z r) O 3 relaxors

Solid solutions of (1−x)Pb(Sc2/3W1/3)O3–(x)PbTiO3 and (1−x)Pb(Sc2/3W1/3)O3–(x)PbZrO3 (PSW-PT and PSW-PZ) show remarkably different dielectric responses. Even though PT has a much higher Curie temperature (490°C) than PZ (230°C), addition of Ti up to x≈0.25 decreases Tε,max — in contrast to the increase of Tε,max for the substitution of PZ. Concentrations of Ti with x>0.25 lead to a strong increase in Tε,max. The structural origins of this behavior were studied by x-ray and neutron diffraction, pair distribution function (PDF) analysis and density functional theory (DFT) calculations. For x<0.25 the B cations form a 1:1 ordered doubled perovskite structure (space group Fm3¯m) in agreement with the “random site model,” where the ordered structure consists of one B sublattice occupied by Sc and the other by a random mixture of the remaining cations. The B site order is reduced by incorporation of Zr, but highly stabilized by Ti with the degree of order in excess of 95% for x<~0.25. The results of PDF analysis and DFT calculations show that locally the atoms are significantly displaced from their average lattice positions and that Tε,max is strongly correlated with the cation displacements. The initial anomalous decrease of Tε,max in PSW-PT is due to the suppression of ferroelectricity by a decrease in the perovskite volume and is related to reduced Pb displacements. For x<0.25 the contribution to ferroelectric polarization from Ti and W are restricted because of the high B-site ordering. However, as the order is reduced for x>0.25, the active Ti and W cations couple their displacements and dominate the dielectric response, driving Tε,max up. For PZ substitution, the lack of ordering leads to nearly linear growth of Tε,max corresponding to a uniform increase in Pb and B-cation displacements.