Local structure, pseudosymmetry, and phase transitions in Na1/2Bi1/2TiO

Abstract

The structural behavior of ceramic solid solutions (1-x)Na ½ Bi ½ TiO 3 -xK ½ Bi ½ 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 pseudo-tetragonal (T) phases was observed at x≈0.2. For x<0.2, both local and average room-temperature structures are similar to those in NBT. Simultaneous long-range anti-phase and short-range in-phase octahedral rotations average, resulting in effective anti-phase a -a -c -tilting, which yields monoclinic symmetry when probed by X-ray diffraction (XRD). For these compositions, polar ordering is coupled to anti-phase 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) 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 non-polar (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 pseudo-cubic 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.2

Relaxor-to-Ferroelectric Crossover and Disruption of Polar Order in “Empty” Tetragonal Tungsten Bronzes

“…[77][78][79] BNT compounds have a complex sequence of phase transitions and may include coexisting phases. [77][78][79][80] The presence of an AFE phase was proposed due to pinched/doubled P-E loops, however other explanations have since been offered involving coexisting phases, domains or nano-regions of different symmetries and electric field driven transitions between a weakly or non-polar relaxor phase and that of a polar ferroelectric phase. [77][78][79][80][81][82][83][84] The origin for these properties is still somewhat contested and may differ in each respective solid solution.…”

Ferroelectrics under the Synchrotron Light: A Review

“…Recent structural descriptions of ferroelectric perovskites often refer to their pseudo-symmetries, due to the existence of local deviations from the global symmetry, arising from the many possible local displacement of the ionic positions and chemical order [11,12]. …”

Cyclic electric field response of morphotropic Bi1/2Na1/2TiO3-BaTiO3 piezoceramics

“…9 Averaging over several domains yielded an a À a À c À antiphase tilting. 10 By means of in situ transmission electron microscopy (TEM) electric-field experiments, it was possible to determine the evolution of phases as a function of poling field. 11,12 Measurements of the piezoelectric coefficient d 33 provided a strong correlation between high d 33 and electric field induced creation of MPBs.…”