OBSERVATION OF HENDRICKS-TELLER PARTIAL ORDER IN A TETRAGONAL CUPRATESUPERCONDUCTOR - LA1.68ND0.14NA0.10K0.082CUO4

The cuprate superconductor, La1.68Nd0.14Na0.10K0.082CuO4, was prepared by selective precipitation from a NaOH/KOH eutectic melt at 350-degre es-C. Synchrotron x-ray-diffraction data indicate that the structure i s related to the tetragonal T-structure type of doped La2CuO4, but the presence of superlattice reflections shows the unit cell to be triple d along the c direction [a = 3.763(1) angstrom and c = 39.63(2) angstr om]. The existence of this supercell is confirmed by electron microsco py and electron diffraction, which show that the tripling is due to ca tion ordering. Careful examination of the positions and widths of the superlattice reflections in the synchrotron data indicates that the do pant cations are in fact only partially ordered, and a simple Hendrick s-Teller model using an ordering scheme suggested by the electron micr oscopy data agrees well with the diffraction data. There is no structu ral change (orthorhombic distortion) between room temperature and 28 K , just above the onset temperature for superconductivity determined fr om magnetic-susceptibility measurements. The Meissner fraction, estima ted to be 80% at 5 K, indicates that this material is a bulk supercond uctor.