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 La1.25Dy0.75CuO3.75F0.5. The structure is orthorhombic, Cmma, a=5.4609(1), b=5.5089(1), c=12.4842(2) Aand Z=4. The excess anion content is incorporated as an interstitial defect in the La2O2 rocksalt layers analogous to the defect identified in La2NiO4.18. Data on the stoichiometric T∗ phase La1.25Dy0.75CuO4, 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 CuO5−xFx square pyramids.