THE CRYSTAL-STRUCTURES OF LNBA2CU4O8 AND LNBA2CU3O7-DELTA (LN=ER, HO, DY) - A HIGH-RESOLUTION POWDER NEUTRON-DIFFRACTION STUDY

The structures of LnBa2Cu4O8 (124) and LnBa2Cu3O7-delta (123), (Ln=Er, Ho, Dy), have been refined from very high resolution powder neutron diffraction data in order to accurately delineate changes in the superconductor structure which occur as a function of the lanthanide ion size. Accurate positional parameters and anisotropic temperature factors, determined by this oxygen-sensitive technique, are presented for the structures of non-yttrium 124 compounds. Refined parameters demonstrate that the dimensions of the CuO2 planes alter in a similar fashion in the two series of materials with a reduction of the orthorhombic distortion with increasing lanthanide size and a slight flattening of the planes. Similar changes are seen in the copper-oxygen sublattice for both structure types: the apical bond length of the CuO2 square pyramids is found to decrease slightly despite the increase in the c-parameter as Er --> Ho --> Dy. No occupation of the inter-chain site is observed in the Ln 1 24 materials.