Local structural coupling of A- and B-site disorder in perovskite bismuth-based piezoelectrics

The local and average structure of (1-x)Bi0.5K0.5TiO3-xBiFeO3 (BKT-xBFO, x = 0.25 and 0.5) solid solutions are studied by synchrotron X-ray total scattering from ambient to 773 K. Pair distribution functions (PDFs) demonstrate that disordered BKT-0.25BFO and BKT-0.5BFO show the same structural coherence length of ~16 A as pure Bi0.5K0.5TiO3, while their average structures are pseudocubic at all temperatures. Complementary density functional theory (DFT) calculations suggest distinctly different local structural distortions in BKT-0.25BFO and BKT-0.5BFO with random cations distribution on both A- and B-lattice. Based on the experimental and theoretical analysis, we propose that the optimal piezoelectric properties found at the structural phase boundary composition of x = 0.25 originate from tetragonal and rhombohedral polar nanoregions (PNRs) in an on average pseudocubic matrix. In contrast, for x = 0.5 there are only quasi-rhombohedral polar distortions in a pseudocubic matrix phase.