Chemical Order and Dielectric Properties of Lead Scandium Tungstate Relaxors

摘要

The tungstate relaxor ferroelectric perovskite, Pb(Sc_(2/3)W_(1/3))O_3 (PSW), adopts a 1:1 B-site ordered "random site" structure with one cation position occupied by Sc~(3+) and the other by a 2:1 distribution of W~(6+) and Sc~(3+). The ordering is destabilized in solid solutions of PSW with PbZrO_3 (PSW-PZ), but stabilized by PbTiO_3 in the (1-x)PSW-(x)PT system. For PSW-PT complete structural order is retained up to x = 0.25 (Pb(Sc)_(1/2)(W_(1/2)Ti_(1/2))_(1/2)O_3). The different response of PSW to the two substitutions can be explained by the changes in the ion size/charge mismatch on the random site and the size difference of the two ordered positions. The changes in order are accompanied by differences in the dielectric response of the two systems. For PSW-PZ the temperature of the permittivity maximum (T_(ε,max)) increases linearly with x; however, in PSW-PT T_(ε,max) decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of PSW-25%PT; when the order parameter was reduced from ～1.0 to ～0.6, T_(ε,max) increased by more than 60℃.