Evidence for Al doping in the CuO2 planes of YBa2Cu4O8 single crystals

Transition temperatures of YBa2Cu4O8 (124) single crystals vary between 70 and 81 K, dependent on the crucible material used for crystal growth. Crystals grown in Al2O3 crucibles (Al-124) have the lowest T-c and incorporate 0.1-0.15% Al in the structure, but have an increased pinning as compared to undoped crystals. X-ray structure refinements of 23 YBa2Cu4O8 single crystals showed a significantly reduced electron density at the Cu(2) site of Al-124, corresponding to an incorporation of 1-2% Al in the superconducting CuO2 planes. The electron density at the Cu(1) chain site showed no difference between crystals grown in Al2O3, ZrO2, and Y2O3 crucibles, but it was generally lower than at the Cu(2) plane site. A significant increase of the anisotropic thermal factor U-33 of the apical oxygen atom is observed in Al-124, due to a shorter Al-O bondlength. Cu-63(1) and Cu-63(2) NQR in undoped and Al-doped 124 single crystals showed a much stronger additional broadening of the plane than the chain Cu NQR lines in Al-124, indicating an Al incorporation in the CuO2 planes.