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High-temperature face-centered cubic bismuth oxide phase is a material of great interest given its unique properties. In the present study, α-Bi₂O₃ and tantalum powders were used as the starting powders for the formation of high-temperature bismuth oxide phase via mechanochemical synthesis by high energy ball milling. (Bi₂O₃)₈₀(Ta)₂₀ and (Bi₂O₃)₉₅(Ta)₅ in weight concentrations were milled in either an oxygen-free argon-filled glove box environment or an ambient atmosphere to investigate the effects of oxygen concentration and tantalum addition. The as-milled powders were examined using X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and differential scanning calorimetry to reveal the structural evolution. The experimental results showed that for (Bi₂O₃)₉₅(Ta)₅ powder mixtures milled within the glove box, tantalum gradually reacted with the α-Bi₂O₃ phase and formed a β-Bi_7.8Ta_0.2O_12.2 phase. For (Bi₂O₃)₈₀(Ta)₂₀ milled under the same conditions, Ta and α-Bi₂O₃ mechanochemically reacted to form δ-Bi₃TaO₇ and bismuth after 10 min of high energy ball milling, whereas milling (Bi₂O₃)₈₀(Ta)₂₀ under the ambient atmosphere with a much higher oxygen concentration accelerated the mechanochemical reaction to less than five minutes of milling and resulted in the formation of high-temperature δ-Bi₃TaO₇ phase.