The postperovskite phase transition of Fe and Al-bearing MgSiO₃ bridgmanite, the most aboundant mineral in the Earth's lower mantle, is believed to be a key to understanding seismological observations in the D″ layer, e.g., the discontinuous changes in seismic wave velocities. Experimentally reported phase transition boundaries of Fe and Al-bearing bridgmanite are currently largely controversial and generally suggest wide two-phase coexistence domains. Theoretical simulations ignoring temperature effects cannot evaluate correctly two-phase coexistence domains under high-temperature. We show high-pressure and high-temperature phase transition boundaries for various compositions with geophysically relevant impurities of Fe²⁺SiO₃, Fe³⁺Fe³⁺O₃, Fe³⁺Al³⁺O₃, and Al³⁺Al³⁺O₃ derived from the ab initio finite-temperature free energies calculated combining the internally consistent LSDA+U method and a lattice dynamics approach. We found that at ∼2500K, incorporations accompanied by Fe³⁺ expand the two-phase coexistence domains distinctly, implying that D″ seismic discontinuities likely arise from the phase transition of Fe²⁺-bearing bridgmanite.