Structural Disorder and Coherence across the Phase Transitions of Lead-Free Piezoelectric Bi_0.5K_0.5TiO₃

Abstract

Relaxor ferroelectrics exhibit superior properties for converting mechanical energy into electrical, and vice versa, but the structural disorder hampers an understanding of structureproperty relationships, and impedes rational design of new, lead-free materials. Bi0.5K0.5TiO3 (BKT) is a prototypical lead-free relaxor ferroelectric, but the microscopic origins of polarization, nature of the ferroelectric transition (TC) and structural changes across the tetragonal to pseudo-cubic transition (T2) are poorly understood. Here the local and intermediate structure of BKT is studied from room temperature to above TC by pair distribution functions (PDF) from synchrotron X-ray total scattering experiments and complemented by ab initio molecular dynamics (AIMD) simulations. The local structure varies smoothly across T2 as well as TC, in contrast to the abrupt changes at TC inferred from conventional diffraction.Ferroelectric distortions are larger on the local scale than in the average structure, with polar Ti 4+ displacements prevailing above TC. We find that local polar regions partly cancel each other below TC, while completely averaging out above, implying that BKT goes through a transition from partial to complete disorder across TC.

Nanoscale Polar Heterogeneities and Branching Bi-Displacement Directions in K0.5Bi0.5TiO3

“…As such, it has attracted attention as a component in piezoelectric solid solutions, several of which yield technologically relevant properties [11][12]. Despite multiple studies [13][14][15][16][17][18][19][20][21], the details of the KBT structure and the exact nature of its ferroelectric cubic↔tetragonal phase transition remain uncertain. Analysis of the literature reveals two different results.…”

Excellent sintering ability of Bi(Mn_2/3Sb_1/3)O₃-added (Bi_1/2K_1/2)TiO₃ lead-free piezoceramics with high density and low dielectric loss

“…6(c), compared with the cell volume of pure BKT prepared by us, the cell volume of BKMS is closer to the theoretical value of pure BKT (61.48 ¡). 24) And the cell volume of BKMS presents an obvious shrinkage at low and high sintering temperature regions. It is suggested that the cell shrinkage may be the result of the sintering ability of the BKMS ceramics.…”

A simple correction for the parallax effect in X-ray pair distribution function measurements

“…the same row) in Table 1, it can be seen that there are small differences. These are most noticeable in the LaB 6 and Ni samples, which is a known artefact (Jiang et al, 2018;Hong et al, 2016). However, this difference decreases after the correction is applied, which is most notable for CeO 2 .…”

“…Total scattering and pair distribution function (PDF) analysis has served as an excellent tool for understanding the local structures of disordered materials, offering new insights that traditional powder diffraction methods have been unable to achieve (Hou et al, 2018;Keen & Goodwin, 2015;Young & Goodwin, 2011;Proffen & Kim, 2009;Petkov, 2008;Billinge, 2008Billinge, , 2019Egami, 2007). This field has been rapidly evolving with the developments in computing power, brighter radiation sources and detector efficiencies, allowing more complex experiments and modelling to be achieved (Eremenko et al, 2017;Jiang et al, 2018;Jensen et al, 2012Jensen et al, , 2015Usher et al, 2015;Hu et al, 2013;Roelsgaard et al, 2019;Pramanick et al, 2018).…”