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 (epsilon') increases and considerable reduction in the low frequency (10(-2) to 10 Hz) dielectric losses and in dc conductivity (sigma(dc)) are seen with lanthanum substitution. A critical La content of x similar to 0.20 in BaBi4-xLaxTi4O15 exhibits a well-defined relaxor behavior as seen from the temperature and frequency dependence of the dielectric parameters epsilon'(T) and epsilon''(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 similar to 0.20. The observed increase in dielectric loss and sigma(dc) 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. (C) 2014 Elsevier B.V. All rights reserved.