Structure of the YBa₄Cu₂(CO₃)O_5.5±δ Oxycarbonate

The crystal structure of the YBa₄Cu₂(CO₃)O_5.5±δ (`142') oxycarbonate has been investigated by X-ray and neutron powder diffraction techniques. X-ray diffraction patterns indexed as a tetragonal P4/m unit cell with lattice parameters a = 8.1973 (4) and c = 8.0360 (5) Å. Rietveld refinements of powder neutron diffraction data indicated a distorted cubic perovskite cell in which yttrium and copper are arranged to yield a superlattice double that of the typical perovskite unit. Refinements indicate CO²⁻₃ ions substituted on two of the four Cu sites in the proposed cell. Infrared spectroscopy results were consistent with neutron structural analysis, giving strong supporting evidence to the presence of two unique carbonate sites in the unit cell.