[en] It is evident that the promise of the new high temperature ceramic superconductors will not be realized without an intense materials science and engineering effort focused on converting these oxide powders into useful bulk forms. The general limitations of the conventional press and sinter ceramic processing approach are likely to be amplified by the sensitivity of the superconductivity transitions of YBa₂Cu₃O/sub 7-x/ to oxygen content. The crystal structure of the superconducting phase has been determined from Rietveld refinement of powder neutron diffraction data and the tetragonal crystal structure of YBa₂Cu₃O₆ has been determined from single crystal x-ray diffraction data. The two structures are illustrated and the respective calculated x-ray powder diffraction patterns are shown. The extent of the orthorhombic and tetragonal phase fields are reported as a function of the oxygen composition