A NEUTRON POWDER DIFFRACTION STUDY OF (ND1.5CE0.5)(SR2-YNDY)CU2MOX (M=GA, TA)

The (Nd, Ce)2Sr2Cu2MOx (M = Ga, Ta) are structurally related to YBCO with yttrium replaced by a fluorite-type block of composition (Nd, Ce)2O2, and barium by strontium. Neutron powder diffraction studies on these materials have been performed to compare structural order within the layers containing the M atoms. For M = Ga, long-range ordering of the corner-linked GaO4 tetrahedra along the [110] direction of the simple perovskite cell results in an orthorhombic supercell. These zigzag chains of tetrahedra are of two types which are distinguished by the direction of atomic displacements in the chains; equal quantities of both types occur in (Nd, Ce)2Sr2Cu2GaOx. When the tetrahedral Ga ions are replaced by octahedral Ta, the oxygen positions in the basal plane are fully occupied, and the tetrahedral chains are converted into octahedral layers to give a smaller tetragonal unit cell. Cooperative rotation of the TaO6 octahedra within these layers has been confirmed.