Bismuth (Bi) and lanthanum (La) doped lead manganese antimoine zirconate titanate (PZMST) ceramic powders have been synthesized by high temperature solid-state reaction method. Preliminary X-ray structural analysis of the compounds shows the formation of tetragonal structure. Scanning electron micrographs (SEM) shows a uniform grain distribution and grain size of the order of ~2.28 μm. Detailed dielectric studies of the Pb_0.95(La_1-z Bi_z)_0.05[(Zr_0.6Ti_0.4)_0.95(Mn_1/3Sb_2/3)_0.05]O₃ samples as a function of the temperature (from 25°C to 450°C) at frequency 1 kHz suggest that the compounds undergo a diffuse phase transition. The transition temperature shifts increase with increasing the Bi ratio. Diffusivity (γ