Special quasirandom structures description of the local structure of disordered Bi0.5K0.5TiO3

Abstract

Polar nanoregions (PNRs) are believed to play a decisive role in the local and macroscopic polarization in relaxor ferroelectrics. The limited microscopic understanding of the structure and dynamics of PNRs hampers the rational design of new lead-free materials. Here, the local structure of A-site disordered Bi0.5K0.5TiO3 (BKT) is investigated using synchrotron x-ray and neutron pair distribution function (PDF) analysis and density functional theory (DFT) optimized special quasirandom structures (SQSs). DFT-relaxed SQS with a 4 × 4 × 4 supercell size can reproduce the experimental PDFs of disordered BKT, as well as the partial PDFs and total polarization, with comparable results to those reported from a combined analysis of x-ray and neutron PDF data with large-box reverse Monte Carlo methods. We find that small Bi3+-rich polar clusters are likely to be the microscopic origin of relaxor behavior in disordered BKT, and that the existence of large polar nanoregions (PNRs) is not necessary to explain the relaxor properties. Our results also highlight the great potential of the SQS approach to gain a nanoscale-to-microscopic understanding of other relaxor solid solutions.

Influence of temperature-induced A-site cation redistribution on the functional properties of A-site complex polar perovskite K1/2Bi1/2TiO3

“…On the contrary, the slow cooling (0.1 K min −1 ) from 900 °C provides sufficient time to form a significantly higher degree of local Bi 3+ -rich polar clusters than that of the normally cooled and quenched KBT. However, within the scope of the current work, the driving force for the formation of Bi 3+ -rich clustering 49 or local displacement of Bi along 〈111〉 c directions 12 in average tetragonal structured KBT cannot be determined.…”

“…Considering that the Bi-based A-site complex perovskite oxide ceramics, among others, possess local scale intrinsic disorder, for example, related to the heterogeneous distributions of A-site cations and their polar displacements, 11,12,14 controlling the degree of this local disorder with post-sintering annealing is critical to tuning the functional response. It has been reported that the microscopic origin of relaxor nature at room temperature in the as-processed KBT is related to the existence of Bi 3+ -rich polar clusters, 49 which intrinsically stabilizes during the cooling step of the sintering process. Interestingly, at the higher symmetry cubic phase, the preferred displacement of Bi 3+ is the 〈100〉 c directions; however, during the transition to the tetragonal phase with cooling, the local Bi-displacement branch off to 〈111〉 c directions and induces local scale structural heterogeneity.…”

“…The thermally treated samples with dimensions of 1 mm × 5 mm × 1 mm (thickness × length × width) were used for high-energy XRD experiments. 47,49 42 Spectra are simulated using the program DMt. 43 A scanning electron microscope (SEM) (Helios NanoLab 600i Dual Beam, FEI Company, USA) was used to observe the difference in microstructure of the thermally treated KBT samples.…”

“…To further conrm the relaxor state of the SC 900 KBT, the 47,49 Ti MAS NMR spectra for SC 900 and NC 900 samples were collected. 47,49 Ti MAS NMR spectra for the SC 900 and NC 900 show two peaks (Fig. 7) with their maxima at −948 ppm and −1290 ppm.…”

“…7) with their maxima at −948 ppm and −1290 ppm. These maxima are interpreted as the presence of both 49 Ti and 47 Ti isotopes, respectively. The NMR frequency of both Ti isotopes lies in the same range, hence lines from both are usually observed.…”