Two Sm-123 single crystals of composition Sm 1.06 Ba 1.94 Cu 2.65 Al 0.35 O 6+δ (0≤δ≤1) were annealed under reducing (T=750°C, P(O 2 )=10 −4 atm) and oxidizing (T=400°C, P(O 2 )=10 −1 atm) conditions. Their investigations by X-ray diffraction at room temperature evidence that both reduced and oxidized samples belong to the same space group, P4/mmm. Aluminum substitutes copper only at the (0, 0, 0) crystallographic position and it displaces to the (≅0.06, ≅0.06, 0) site. Two oxygen ions in the basal plane are bonded to each Al ion. Both (0, 12, 0) and (12, 0, 0) sites are occupied by oxygen ions so that Al ions are in a tetrahedral coordination. The local structural distortion, induced by aluminum substitution, inhibits the tetragonal→orthorhombic phase transition.