Local structure, pseudosymmetry, and phase transitions in Na1/2Bi1/2TiO3-K1/2Bi1/2TiO3 ceramics

Abstract

The structural behavior of ceramic solid solutions (1 - x)Na1/2Bi1/2TiO3-xK(1/2)Bi(1/2)TiO(3) (NBT-KBT) was studied using high-resolution powder diffraction and transmission electron microscopy. A temperature-independent morphotropic phase boundary (MPB) separating NBT-like pseudorhombohedral (R) and KBT-like pseudotetragonal (T) phases was observed at x approximate to 0.2. For x < 0.2, both local and average room-temperature structures are similar to those in NBT. Simultaneous long-range antiphase and short-range in-phase octahedral rotations average, resulting in effective antiphase a(-) a(-) c(-) tilting, which yields monoclinic symmetry when probed by x-ray diffraction (XRD). For these compositions, polar ordering is coupled to antiphase octahedral rotations so that tilting and ferroelectric (FE) domains coincide. Compositions with x > 0.2 exhibit a tetragonal-like distortion; however, complex splitting of reflections in XRD patterns suggests that the actual symmetry is lower than tetragonal. For 0.2 <= x <= 0.5, in-phase octahedral tilting a(0)b(+)a(0) (or a(+)b(0)b(0)) is present but confined to the nanoscale, while for x > 0.5 the structure becomes untilted. In-phase tilting evolves above the ferroelectric transition and occurs around a nonpolar (a or b) axis of the average T structure. The onset of polar order has no significant effect on the coherence length of in-phase tilting, which suggests only weak coupling between the two phenomena. The average symmetry of the T phase is determined by the effective symmetry (Imm2) of assemblages of coherent in-phase tilted nanodomains. Near the MPB, the coexistence of extended R-and T-like regions is observed, but lattice distortions within each phase are small, yielding narrow peaks with a pseudocubic appearance in XRD. The temperature of the FE phase transition exhibits a minimum at the MPB. The structured diffuse scattering observed in electron diffraction patterns for all the compositions suggests that polar order in NBT-KBT solid solutions is modulated away from the average displacements refined using powder diffraction data.