Nanoscale Polar Heterogeneities and Branching Bi-Displacement Directions in K_0.5Bi_0.5TiO₃

Abstract

K0.5Bi0.5TiO3 (KBT) -one of the few perovskite-like ferroelectric compounds with room-temperature tetragonal symmetry -differs from other members of this family (BaTiO3 and PbTiO3) by the presence of a disordered mixture of K and Bi on cuboctahedral sites. This disorder is expected to affect local atomic displacements and their response to an applied electric field. We have derived nanoscale atomistic models of KBT by refining atomic coordinates to simultaneously fit neutron/X-ray totalscattering and extended X-ray absorption fine-structure data. Both Bi and Ti ions were found to be offset relative to their respective oxygen cages in the high-temperature cubic phase; in contrast, the coordination environment of K remained relatively undistorted.In the cubic structure, Bi displacements prefer the 〈100〉 directions and the probability-density distribution of Bi features six well-separated sites; a similar preference exists for the much smaller Ti displacements although the split sites for Ti could not be resolved. The cation displacements are correlated, yielding polar nanoregions, while on average the structure appears as cubic. The cubic↔tetragonal phase transition involves both order-disorder and displacive mechanisms. A qualitative change in the form of the Bi probability-density distribution occurs in the tetragonal phase on cooling to room temperature because Bi displacements "branch off" to 〈111〉 directions. This change, which preserves the average symmetry, is accompanied by the development of nanoscale polar heterogeneities that exhibit significant deviations of their polarization vectors from the average polar axis.

Neutron Total Scattering Studies of Group II Titanates (ATiO3, A2+ = Mg, Ca, Sr, Ba)

“…Macroscopically, this is observed in BaTiO 3 ceramics as large relative permittivities, large polarizations, and large field induced electromechanical strains. This is also clearly relevant to other systems where these contrasting structural length scales influence the material response such as in relaxor ferroelectrics 24 . Thus, materials systems in which there is a disconnect between the long-range average structure and local structural arrangements are of both scientific and technological interest.…”

Piezoelectricity in perovskite-type pseudo-cubic ferroelectrics by partial ordering of off-centered cations

“…Recently, in solid solutions including Bi ions, such as BF-BT 46,47 and BNT 48 , offcentering of the Bi ions was observed. It was reported that the change in the Bi sites from six to ten split sites involved a phase transition from paraelectric (cubic) to ferroelectric phases (tetragonal) in K 0.5 Bi 0.5 TiO 3 22 . However, the partial ordering under an applied electric field is not discussed in the ferroelectric phase.…”

“…Besides, detailed studies of local structures in Bi-based solid solutions were reported. It was suggested that the Bi displacements change as a function of temperature, and that a phase transition could be associated with the ferroelectric response 22 . Moreover, it was reported that the high piezoelectric response appeared by the local distortion in pseudocubic symmetry under an electric field 23 .…”

Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in CsLn2Ti2NbO10 Dion-Jacobson Phases

“…Relaxor ferroelectrics, with frequency dispersion of their permittivity, 62 are usually described in terms of polar nanoregions caused by modulations on the cation sublattice. 63 Experimental and theoretical work on other layered materials has suggested a new class of relaxor ferroelectrics, “digital relaxors”. 15 , 64 In these systems, it is the disorder on the oxide sublattice due to the relatively shallow energy landscape (giving several structures of similar energies due to the 2D nature of these materials with perovskite blocks relatively decoupled) that gives rise to short-range polar regions.…”