On the structure of the superconducting ortho-II phase of YBa2Cu3O6.51

Abstract

From the intensities of superstructure reflections, the structure of the ortho-II phase in YBa2Cu3O5.51(5) (Tc=55.8K) was determined by single crystal x-ray diffraction. In the basal plane of the 2a×b×c supercell, the oxygen atoms order to alternating Cu−O−Cu and Cu−Cu chains. Due to the oxygen order, the barium and yttrium atoms are displaced in (100) direction in an antiferrodistortive way (Ba-shift: 0.046A toward the Cu−O−Cu chains, Y-shift: 0.008A toward the Cu−Cu chains). The apex oxygen and copper atoms of the superconducting planes are displaced in (001) direction. The Ba displacements are confirmed by anomalous scattering near the Ba−K absorption edge observed by synchrotron x-ray diffraction. A rigid ion model shows that the cation displacements stabilize the ortho-II structure by reducing its lattice energy.