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) study of phase transition of these compounds provided its value from 1.59 to 1.78 indicating the degree of the disordering in the system.