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

Abstract Transition temperatures of YBa 2 Cu 4 O 8 (124) single crystals vary between 70 and 81 K, dependent on the crucible material used for crystal growth. Crystals grown in Al 2 O 3 crucibles (Al-124) have the lowest T c and incorporate 0.1-0.5% Al in the structure, but have an increased pinning as compared to undoped crystals. X-ray structure refinements of 23 YBa 2 Cu 4 O 8 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 CuO 2 planes. The electron density at the Cu(1) chain site showed no difference between crystals grown in Al 2 O 3 , ZrO 2 and Y 2 O 3 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. 63 Cu(1) and 63 Cu(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 CuO 2 planes.