No abstract

No abstract

The in situ X-ray Pair Distribution Function (PDF) characterisation technique has been used to study the behaviour of (K x Na 1-x ) 0.5 Bi 0.5 TiO 3 , as a function of electric field. As opposed to conventional X-ray Bragg diffraction techniques, PDF is sensitive to local atomic displacements, detecting local structural changes at the Ångstrom to nanometre scale. Several field-dependent ordering mechanisms can be observed in x = 0.15, 0.18 and at the morphotropic phase boundary (MPB) composition x = 0.20. X-ray total scattering shows suppression of diffuse scattering with increasing electric field amplitude, indicative of an increase in structural ordering. Analysis of PDF peaks in the 3-4 Å range shows ordering of Bi-Ti distances parallel to the applied electric field, illustrated by peak amplitude redistribution parallel and perpendicular to the electric field vector. A transition from ‹110› to ‹112› type off-centre displacements of Bi relative to the neighbouring Ti atoms are observable with increasing x. Analysis of PDF peak shift with electric field show the effects of Bi-Ti 2 redistribution and onset of piezoelectric lattice strain. The combination of these field-induced ordering mechanisms is consistent with local redistribution of Bi-Ti distances associated with domain reorientation and an overall increase in order of atomic displacements.

The piezoelectric devices widespread in society use noncentrosymmetric Pb-based oxides because of their outstanding functional properties. The highest figures of merit reported are for perovskites based on the parent Pb(Mg 1/3 Nb 2/3 )O 3 (PMN), which is a relaxor: a centrosymmetric material with local symmetry breaking that enables functional properties, which resemble those of a noncentrosymmetric material. We present the Pb-free relaxor (K 1/2 Bi 1/2 )(Mg 1/3 Nb 2/3 )O 3 (KBMN), where the thermal and (di)electric behavior emerges from the discrete structural roles of the s 0 K + and s 2 Bi 3+ cations occupying the same A site in the perovskite structure, as revealed by diffraction methods. This opens a distinctive route to Pb-free piezoelectrics based on relaxor parents, which we demonstrate in a solid solution of KBMN with the Pb-free ferroelectric (K 1/2 Bi 1/2 )TiO 3 , where the structure and function evolve together, revealing a morphotropic phase boundary, as seen in PMN-derived systems. The detailed multiple-length-scale understanding of the functional behavior of KBMN suggests that precise chemical manipulation of the more diverse local displacements in the Pb-free relaxor will enhance performance.