The domain morphology and crystal structure of (1-x) (Bi1/2 Na1/2) TiO3 x BaTiO3 lead-free piezoelectric ceramics were systematically studied with transmission electron microscopy for compositions x=0.04 through 0.11. It was found that the ceramics with compositions x<0.06 display a R3c symmetry with ferroelectric domains of ∼100 nm forming complex structures at room temperature. Only nanodomains with faint contrast were observed in the compositions of 0.07 ≤x ≤0.09. The presence of weak 1/2 (ooe) superlattice diffraction spots and absence of 1/2 (ooo) ones (o stands for odd and e stands for even miller indices) seem to suggest a P4bm symmetry at room temperature. The morphotropic phase boundary composition x=0.06 showed mixed R3c and P 4bm phases. Large lamellar ferroelectric domains with P4mm symmetry were found to dominate in the ceramic of x=0.11. The observed domain structure correlates extremely well with the frequency dispersion of dielectric constant at room temperature and a new concept "relaxor antiferroelectric" was proposed to describe the dielectric behavior of compositions 0.07≤x ≤0.09. These results are summarized in a phase diagram for unpoled ceramics in the (1-x) (Bi1/2 Na1/2) TiO3-x BaTiO3 binary solid solution system.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (NSF) through Grant No. DMR-1037898 and the U.S.-Israel Binational Science Foundation through Grant No. 2006235. TEM experiments were performed at the Ames Laboratory which is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358.