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

摘要

Two Sm-123 single crystals of composition Sm_(1.06)Ba_(1.94)Cu_(2.65)Al_(0.35)O_(6+#delta#) (0 <= #delta# <= 1) were annelaed under reducing (T = 750 deg C, P(O_2) = 10~(-4) atm) and oxidizing (T = 400 deg 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 (approx= 0.06, approx=0.06,0) site. Two oxygen ions in the basal plane are bonded 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 structural distortion, induced by aluminum substitution, inhibits the tetragonal-> orthorhombic phase transition.