Local structure, pseudosymmetry, and phase transitions in Na1/2Bi1/2TiO3–K1/2Bi1/2TiO3ceramics

Abstract

The structural behavior of ceramic solid solutions (1\ensuremath{-}$x$)Na${}_{1/2}$Bi${}_{1/2}$TiO${}_{3}$--$x$K${}_{1/2}$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 $x$ \ensuremath{\approx} 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 ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{c}^{\ensuremath{-}}$ 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 $0.2\ensuremath{\le}x\ensuremath{\le}0.5$, 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.