PHASE-TRANSITION AND FERROELECTRIC CHARACTERISTICS OF PB[(MG1 3NB2/3)(1-X)TI-X]O-3 CERAMICS MODIFIED WITH LA(MG2/3NB1/3)O-3/

The effects of 0-5 mol% addition of La(Mg2/3Nb1/3)O-3 (LMN) on the pha se transition and ferroelectric behaviors of Pb[(Mg1/3Nb2/3)(1-x)Ti-x] O-3 (PMNT) ceramics with compositions near the morphotropic phase boun dary (MPB) were studied. An evolution of structure from rhombohedral t o tetragonal was found with increasing PbTiO3 (PT) content across the MPB (at similar to 32.5 mol% PT), and a coexistence of both rhombohedr al and tetragonal phases was also found at the MPB. The dual-phase fie ld extended toward the lower PT content side of the MPB, and, moreover , the rhombohedrality or tetragonality was reduced, especially for the compositions near the MPB, by the addition of La in PMNT. The ferroel ectric transition was found to change from normal to diffuse as the La content increased and the compositions became more rhombohedral. In a ccordance with the structural evolution, the change of remanent polari zation (P-r) and coercive field (E(c)) also became gradually indistinc t, and both P-r and E(c) were reduced. For compositions near the MPB, both PMNT and La-modified PMNT had a similar electromechanical factor (k(p)) in a range around 0.55-0.60, but the mechanical quality factor (Q,) was significantly reduced for the La-modified PMNT The piezoelect ric coefficient (d(33)), however, was largely improved with increasing La content in PMNT of compositions at MPB. A high value of d(33) appr oximate to 815 pC/N was obtained for the 5-mol%-La-modified ceramics, but it was associated with a low value of Q(m).