Crystal structure of Ca4.78Cu6O11.60

The crystal structure of Ca4.78Cu6O11.60 (crystal system, monoclinic; space group P2/c, Z=4, rho=4.48(2) g/cm(3), a=10.9456(4) Angstrom, b=6.3192(2) A ngstrom, c=16.8408(5) Angstrom, and beta=104.952(2)degrees) has been solved and refined using X-ray and neutron powder diffraction combined with Rietv eld analysis. It is closely related to the NaCuO2-type structure. The phase stoichiometry and the displacements of atoms with respect to their positio ns in the previously reported substructure (crystal system, orthorhombic; s pace group, Fmmm, a=2.807(1) Angstrom, b=6.351(2) Angstrom, and c=10.597(3) Angstrom) are explained by the minimization of Ca-Ca repulsion and by a re laxation toward a more regular octahedral environment for Ca atoms, The sub stitution of Tl atoms for Ca atoms results in an increased thermal stabilit y and in incommensurate structures with modulation vectors depending strong ly on the Tl content. (C) 2000 Academic Press.