Evidence for peroxide formation in superconducting ${\mathrm{YBa}}_{2\mathrm{-}\mathit{x}}$${\mathrm{La}}_{\mathit{x}}$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\pm \mathrm{\delta }}$

The sysetm ${\mathrm{YBa}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{\mathit{x}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\pm \mathrm{\delta }}$(0≤x≤0.5), annealed to a maximum oxygen concentration under 1 atm of ${\mathrm{O}}_2$, exhibits three distinct compositional ranges. (1) For 0≤x<0.05, the maximum oxygen concentration is ${\mathrm{O}}_{6.94+0.5\mathrm{x}}$; the intercalated oxygen atoms are monomeric and $T_c$ increases to 99 K at x=0.05. (2) For 0.10≤x<0.5, the maximum oxygen concentration is ${\mathrm{O}}_{6.85+0.5\mathrm{x}}$; beyond x≊0.07 the additional oxygen appear to be intercalated as peroxide ions and $T_c$ decreases linearly with x through a smooth orthorhombic-tetragonal phase transition at x=0.30. (3) For 0.05≤x<0.10, a transition between the two regimes occurs.