Influence of lanthanum distribution on dielectric and ferroelectric properties of BaBi4-xLaxTi4O15 ceramics

Structural and electrical properties of Lanthanum substituted barium bismuth titanate BaBi4-xLaxTi4O15 (0 ≤ x ≤ 0.50) ceramics prepared by conventional solid-state reaction method have been investigated. Raman spectra reveals the distribution of lanthanum into the perovskite layers and (Bi2O2)2+ layers of BaBi4Ti4O15 ceramics. Room temperature dielectric constant (ε′) increases and considerable reduction in the low frequency (10−2 to 10 Hz) dielectric losses and in dc conductivity (σdc) are seen with lanthanum substitution. A critical La content of x ∼0.20 in BaBi4-xLaxTi4O15 exhibits a well-defined relaxor behavior as seen from the temperature and frequency dependence of the dielectric parameters εʹ(T) and ε″(T). The dielectric data fit well to the modified Curie-Weiss law and the Lorentz-type relation and show increasing diffuseness in the phase transition with increasing La content. The temperature dependence of the characteristic relaxation time obtained from the Cole-Cole model shows a good fit to the non-linear Vogel-Fulcher relation. Improvements in the remnant polarization and a stable piezoelectric charge coefficient are seen up to a La content of x ∼0.20. The observed increase in dielectric loss and σdc in addition to the diminished ferroelectric/piezoelectric properties for higher La content are explained in terms of changing oxygen vacancy concentration and structural relaxation due to the preferential incorporation of La into the (Bi2O2)2+ layers as evidenced through the Raman spectroscopy.