EFFECT OF A SITE SUBSTITUTION ON ORDER-DISORDER TRANSITION IN PB(FE1 2NB1/2)O-3 (PFN) AND PB(SC1/2TA1/2)O-3 (PST)/

The effect of A site substitution of Pb (Fe1/2Nb1/2)O-3 (PFN) and Pb ( Sc1/2Ta1/2)O-3 (PST) with Na or La on the order-disorder transition an d dielectric characteristics was examined. It was clarified that the d egree of ordering at the B site did not change by the substitution tho ugh it has been reported that in case of Pb (Mg1/3Nb2/3)O-3 (PMN), the degree of ordering can be controlled by the A site substitution, The reason may be due to the difference in cation size ratio of B site, th at is, Mg: Nb = 1 : 2 for PMN and Fe: Nb and Sc: Ta = 1 : 1 for PFN an d PST, By partial substitution of A site by La or Na, the dielectric c haracteristics of PFN and PST drastically changed from normal ferroele ctric to ''relaxor ferroelectric'', This indicates that ''relaxor ferr oelectrics'' can not be necessarily derived from the order-disorder st ate at the B site sublattice as understood so far, The ''relaxor ferro electric'' property of Na or La doped PST and PFN would be classified into the same group of (Pb, La) (Zr, Ti)O-3 (PLZT) and La doped (Sr, B a)Nb2O6 (SBN).