Structures, dielectric and ferroelectric properties of Sr2-xCaxNaNb5O15 lead-free ceramics

The high-density tungsten bronze (TB) Sr2-xCaxNaNb5O15 (SCNN, 0.05 <= x <= 030) lead-free ceramics were prepared by two-step solid-state reaction method. With increasing Ca2+ substitution, the crystal structure of SCNN ceramics slightly distorted from the TB tetragonal phase and became orthorhombic phase at room temperature. The smaller ionic radius of Ca2+ (1.34 angstrom) compared with that of Sr2+ (1.44 angstrom) contributed to the shrinkage of the crystal structure. Dielectric spectra of all compositions displayed two phase transitions: the ferroelastic orthorhombic to ferroelectric tetragonal phase transition (T-e) at lower temperatures, and the ferroelectric to paraelectric phase transition (T-c) at higher temperatures. With increasing Ca2+ substitution, T-e and T-c shifted towards higher temperature regions, while the maximum values of dielectric constant (epsilon(me) and epsilon(m)), P-r, E-c and d(33) increased at first and then decreased. The ceramics with most homogeneous microstructure and highest density were obtained at x = 0.15, resulting in optimized properties.