A series of compounds with the general composition Sm_2-xDy_xZr₂O₇ (where 0≤ x≥ 2.0) were synthesized by chemical route and characterized by powder X-ray diffraction (XRD) analysis. The rare earth ion namely Sm⁺³ in the compound was gradually replaced with another smaller and heavier ion, Dy⁺³ of the 4f series, there by resulting in order–disorder structural transition, which has been studied by positron annihilation lifetime and Doppler broadening spectroscopy. This study reveals the subtle electronic micro environmental changes in the pyrochlore lattice (prevalent due to the oxygen vacancy in anti-site defect structure of the compound) toward its transformation to defect fluorite structure as found in Dy₂Zr₂O₇. A comparison of the changes perceived with PAS as compared to XRD analysis is critically assayed.