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BaGa₂O₄ ceramics doped with Eu³⁺ ions (1, 3 and 4 mol.%) were obtained by solid-phase sintering. The phase composition and microstructural features of ceramics were investigated using X-ray diffraction and scanning electron microscopy in comparison with energy-dispersive methods. Here, it is shown that undoped and Eu³⁺-doped BaGa₂O₄ ceramics are characterized by a developed structure of grains, grain boundaries and pores. Additional phases are mainly localized near grain boundaries creating additional defects. The evolution of defect-related extended free volumes in BaGa₂O₄ ceramics due to the increase in the content of Eu³⁺ ions was studied using the positron annihilation lifetime spectroscopy technique. It is established that the increase in the number of Eu³⁺ ions in the basic BaGa₂O₄ matrix leads to the agglomeration of free-volume defects with their subsequent fragmentation. The presence of Eu³⁺ ions results in the expansion of nanosized pores and an increase in their number with their future fragmentation.