Structural and microstructural description of relaxor-ferroelectric transition in quenched Na1/2Bi1/2TiO3BaTiO3

Quenching lead-free Na1/2Bi1/2TiO3-based ceramics from sintering temperature is established to increase the depolarization temperature, Td and the lattice distortion. In situ synchrotron X-ray diffraction measurements were carried out on furnace cooled and quenched Na1/2Bi1/2TiO3 - BaTiO3 (NBT-BT) with 6 and 9 mol. % BT to discern the field-induced ferroelectric order. Phase fractions were determined from full pattern Rietveld refinements and utilized together with the change in unit cell volume to calculate volumetric strain resulting from phase transformations. NBT-6BT demonstrates a cubic symmetry in the furnace cooled state but quenching stabilizes the rhombohedral R3c phase and delays the formation of a field-induced, long range-ordered tetragonal phase, thereby shifting the onset of macroscopic strain to higher fields. A field-induced phase transition from a weakly distorted rhombohedral to tetragonal phase can be observed in furnace cooled NBT-9BT. However, this phase transition cannot be detected in quenched NBT-9BT, since the ferroelectric tetragonal P4mm phase is stabilized in the initial state. In contrast to the furnace cooled materials, both the quenched compositions exhibit overall negligible volumetric strain as a function of electric field. Furthermore, scanning electron micrographs of chemically etched, poled and unpoled samples reveal an increased lamellar domain contrast in the quenched materials. All these findings strengthen the hypothesis of a stabilized ferroelectric order resulting in the absence of a field-induced phase transformation in quenched NBT-BT.