In this study, multicomponent rare-earth zirconate ceramics (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Zr2O7 and (La0.2Eu0.2Gd0.2Yb0.2Y0.2)2Zr2O7 were synthesized via conventional sintering and reactive flash sintering, respectively. Single-phase (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Zr2O7 ceramics, with the defect fluorite structure, were successfully obtained via conventional sintering and reactive flash sintering, while secondary phase segregation and precipitation were observed only in conventionally-sintered (La0.2Eu0.2Gd0.2Yb0.2Y0.2)2Zr2O7 ceramics. This study proposes that the critical electric field of reactive flash sintering introduces defects to soften the lattice, which not only improves the mass transportation, but also relieves the lattice stress induced by the atomic radius difference, resulting in the single-phase defect fluorite structure of (La0.2Eu0.2Gd0.2Yb0.2Y0.2)2Zr2O7. Thus, reactive flash sintering is an efficient route for synthesizing and developing novel multicomponent oxides that cannot be synthesized via conventional sintering due to pronounced lattice stress.