Interstitial anions in the T^* structure

Rietveld refinement of powder neutron diffraction data has been used to locate the excess anions in the T^* phase La_1.25Dy_0.75CuO_3.75F_0.5. The structure is orthorhombic, Cmma, a=5.4609(1), b=5.5089(1), c=12.4842(2) A ̊ and Z=4. The excess anion content is incorporated as an interstitial defect in the La₂O₂ rocksalt layers analogous to the defect identified in La₂NiO_4.18. Data on the stoichiometric T^* phase La_1.25Dy_0.75CuO₄, P4/nmm, a=3.8661(1) and c=12.4475(2) A ̊ show that the tetragonal structure is retained to 10 K. Both structures show complete ordering of the Dy into the fluorite layers of the T^* structure. An inability to distinguish between O and F in the refinement leaves unresolved the driving force for the orthorhombic distortion; it does not appear to be due to a cooperative tilting of the CuO_5-xF_x square pyramids.