Oxygen-ordering superstructures in NdBa₂Cu₃O_6.5 single crystals investigated by hard-X-ray diffraction

Single crystals of NdBa2Cu3O6+x and LaBa2Cu3O6+x at oxygen concentrations x approximate to 0.4, 0.5, and 0.76 have been investigated by hard-x-ray diffraction. Intricate oxygen superstructures are found in nonsuperconducting NdBa2Cu3O6.5 crystals. By numerical calculations we demonstrate that three unique coexisting oxygen configurations are required to obtain the observed diffraction pattern. The oxygen configurations found are unlikely to give rise to hole doping of the CuO2 planes, explaining why a higher oxygen doping is necessary in NdBa2Cu3O6+x than in YBa2Cu3O6+x to make the compound superconducting.