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High-T/sub c/ Tl-base oxides can be synthesized by a diffusion reaction within a short reaction time. The F addition apparently promotes the phase transformation from 2223 to 1223. The scanning electron microscope observation reveals that the 1223 phase formed by the F addition shows a dense and homogeneous structure, resulting in an increase in transport I/sub c/ at 77 K. Moreover, F addition significantly improves the I/sub c/ at 77 K under magnetic field, and shifts the irreversibility line to higher temperature. Meanwhile, V addition was found to enhance the diffusion reaction, resulting in the formation of 2223 layer of about 600 pm in thickness after the reaction at 850/spl deg/C for 2 h. The V is pushed out from the diffusion layer and accumulates on the surface of the specimen. The V addition causes almost no degradation in T/sub c/. I/sub c/-B performance is appreciably improved by annealing in O/sub 2/ after the reaction.