_Local Structure, Pseudosymmetry, and Phase Transitions in Na{1/2}Bi_{1/2}TiO3_K{1/2}Bi_{1/2}TiO_{3} Ceramics
Abstract
The structural behavior of ceramic solid solutions (1-x)Na Bi TiO3-xK Bi TiO3 (NBT-KBT) was studied using high-resolution powder diffraction and transmission electron microscopy. A temperature-independent morphotropic phase boundary (MPB) separating NBT-like pseudo-rhombohedral (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 ferroelastic and non-180 ferroelectric (FE) domains coincide. Compositions with x>0.2 exhibit a tetragonal-like distortion associated with polar cation displacements similar to KBT; 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 a0b+a0 (or a+b0b0) is present but confined to the nanoscale down to low temperatures, while for x>0.5, the structure becomes untilted. This in-phase tilting evolves above the ferroelectric transition and occurs around a non-polar (a or b) axis of the average T structure. The polar ordering has no significant effect on the coherence length of this tilting, which suggests weak coupling between the two phenomena. The average symmetry of the T phase is determined by the effective symmetry 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 31, 2013
- Accession Number
- ADA596160
Entities
People
- E. Anton
- H. Kleebe
- I. Levin
- I. M. Reaney
- J. Pokorny
- J. Roedel
- Jacob L. Jones
- L. A. Schmidt
- M. Hinterstein
- Wook Jo
Organizations
- University of Florida