Domain structure and phase evolution in quenched and furnace cooled lead-free Na1/2Bi1/2TiO3–BaTiO3 ceramics

Abstract

Relaxor ferroelectric Na1/2Bi1/2TiO3-based materials have gained considerable attention as a potential lead-free alternative in recent years and can be tailored to exhibit giant strain or superior high power properties. Quenching (1-x)(Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-BT) ceramics in air from the sintering temperature is beneficial in enhancing the depolarization temperature and the lattice distortion. Here, a comparative study using transmission electron microscopy (TEM) and X-ray diffraction is presented for unpoled, furnace cooled and quenched NBT-BT (3, 6, 9 and 12 ​mol. % BT) ceramics describing the domain structure and phase assemblage. In contrast to the furnace cooled sample, an enhanced lamellar domain contrast is observed for the quenched morphotropic phase boundary composition with 6 ​mol. % BT. The phase fraction obtained using high resolution X-ray diffraction changes from a near pseudocubic structure with small distortions towards a more pronounced rhombohedral and tetragonal phase assemblage. On the NBT-rich side (3 ​mol. % BT), a second rhombohedral phase emerges in addition to the R3c symmetry, exhibiting a long-range lamellar domain structure. Further, quenched and subsequently poled NBT-6BT features an increased tetragonal fraction associated with a highly lamellar domain contrast. The quenching treatment stabilizes the ferroelectric order, evidenced from the development of a long-range ferroelectric domain structure, which rationalizes the enhanced depolarization temperature.