Single Grains Hosting Two Space Groups — A Transmission Electron Microscopy Study of a Lead-Free Ferroelectric

In this letter, conclusive experimental evidence is presented for the coexistence of two phases within individual grains in the system ( Bi 0.5 Na 0.5 TiO 3)0.91-( BaTiO 3)0.06-( K 0.5 Na 0.5 NbO 3)0.03. Electron diffraction data unequivocally reveal the presence of a rhombohedral and a tetragonal phase with space group R3c and P4bm, respectively, both on the nanoscale level. Superlattice reflections, observed in the corresponding selected area electron diffraction, were used to identify the rhombohedral and tetragonal phase present at different regions within one grain. It is thought that the rhombohedral phase acts as a nucleation site for the phase transformation monitored under applied electrical field. The coexistence of these two phases in conjunction with the triggered tetragonal to rhombohedral phase transformation rationalizes the high bipolar and unipolar strain recorded for this lead-free ferroelectric material.