Domain structure-dielectric property relationship in lead-free (1-x) (Bi_1/2 Na_1/2) TiO₃- x BaTiO₃ ceramics

The domain morphology and crystal structure of (1-x) (Bi_1/2 Na_1/2) 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 (o_oe) superlattice diffraction spots and absence of 1/2 (o_oo) ones (o stands for odd and e stands for even miller indices) seem to suggest a P_4bm 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) (Bi_1/2 Na_1/2) TiO₃-x BaTiO₃ binary solid solution system.