Structure of Al defect in high-temperature superconductor, Al-doped Sm-123: An electron density study

Two Sm-123 single crystals of composition Sm1.06Ba1.94 Cu2.65Al0.35O6+delta (0 less than or equal to 8 less than or equal to 1) were annealed under re ducing (T = 750 degreesC, P(O-2)=10(-4) atm) and oxidizing (T = 400 degrees C, P(O-2) = 10(-1) atm) conditions. Their investigations by X-ray diffracti on at room temperature evidence that both reduced and oxidized samples belo ng to the same space group, P4/mmm. Aluminum substitutes copper only at the (0, 0, 0) crystallographic position and it displaces to the (congruent to 0.06, congruent to 0.06, 0) site. Two oxygen ions in the basal plane are bo nded to each Al ion. Both (0, 1/2, 0) and (1/2, 0, 0) sites are occupied by oxygen ions so that Al ions are in a tetrahedral coordination. The local s tructural distortion, induced by aluminum substitution, inhibits the tetrag onal --> orthorhombic phase transition. (C) 2001 Academic Press.