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

Abstract

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.