Abstract
The La4BaCu5O ?? with the orthorhombic structure has been prepared for the first time by the following heat treatments : firing in flowing N2 gas at 780°C or quenching into liquid N2 from 1000°C in air. The oxygen content of this phase (the high-temperature phase) is y=12 in La4BaCu5O ??, and that of the well-known tetragonal phase (the low-temperature phase) is y=12.5. We have proposed the new structure model for the tetragonal phase in which the structure is built up by only CuO5 pyramids and also for the orthorohmbic phase built up by CuO4 squares and CuO5 pyramids (regularly arranged). This orthorohmbic phase has also metallic conductivity down to 4.2K.
