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+ (1.34Å) compared with that of Sr2+ (1.44Å) 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 (Te) at lower temperatures, and the ferroelectric to paraelectric phase transition (Tc) at higher temperatures. With increasing Ca2+ substitution, Te and Tc shifted towards higher temperature regions, while the maximum values of dielectric constant (εme and εm), Pr, Ec and d33 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.