Structural phase transformation in the superconducting YLa _xBa_2-xCu₃O₇ compounds

An orthorhombic-to-tetragonal phase transformation in YLa xBa2-xCu3O7 has been observed as a function of x, but with the same oxygen concentration, from x-ray powder diffraction data. Compounds with x≤0.37 have Tc in the 93-53 K range and are orthorhombic, while compounds with x≥0.45 have T c≤42 K and are tetragonal. An analysis of the YLa 0.6Ba1.4Cu3O6.99 diffraction pattern suggests that the tetragonal structure is closely related to those of the orthorhombic YBa2Cu3O7 and the tetragonal YBa2Cu3O6 with a partial substitution of La3+ ion on the Ba2+ site and a disorder of oxygen atoms at the midpoints of both a and b unit cell edges.