Structure - property relations in lead tungstate based perovskite relaxor ferroelectrics

The structure and dielectric properties of (1 − x)Pb(Sc _2/3W_1/3)O₃-(x)PbTiO₃ (PSW-PT), (1 − x)Pb(Sc_2/3W_1/3)O ₃-(x)PbZrO₃ (PSW-PZ) and (1 − x)Pb(Sc_1/2W_1/4Ti_1/4)O₃-( x)Pb(Sc_1/2Nb_1/2)O₃ (PSWT-PSN) ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 the periodicity of the lattice is doubled due to a 1:1 ordered distribution of the B-site cations. The structural order in PSW can be described by a “random site model” with one cation site occupied by Sc3+ and the other by a random distribution of the remaining ions ($Sc3+1/3W6+2/3$). This ordering is destabilized in solid solutions of PSW with PbZrO ₃, but highly stabilized by PbTiO₃, with a degree of order in excess of 95% for x ≤ 0.25 in (1 − x )PSW-(x)PT. The distinct effects of Zr and Ti on the B-site order are accompanied by remarkably different alterations in the dielectric response of the two solid solutions. Both systems are relaxor ferroelectrics, and for the substitution of PZ the temperature of permittivity maximum T_ε,max increases with x. However, T_ε,max decreases for additions of Ti up to x ≈ 0.25, even though PT has a much higher Curie temperature ( T_C = 763 K) than PZ (503 K). Variations of T _ε,max in the PSW-PT system could be also induced by altering the degree of order of a given composition; changes as high as 50°C were observed for x = 0.25. Strong effects of order were also observed in (0.25)PSWT-(0.75)PSN, where the introduction of partial order induced a relaxor to ferroelectric transition. To understand the relations between the atomic structure and dielectric properties, the effects of PT and PZ on the structure and ordering of PSW were studied using synchrotron x-ray and neutron diffraction. Rietveld refinement was carried out on the PSW-PT and PSW-PZ systems to determine their average long-range crystallographic structure, along with Pair Distribution Function (PDF) analysis to probe the local displacements of the atoms. For x < 0.25 the B-cations form a 1:1 ordered doubled perovskite structure (space group Fm3¯ m). (Abstract shortened by UMI.)...