Crystal Structure Investigations and Thermal Behavior of the Five‐Leg Spin Ladder Compound La8Cu7O19

La₈Cu₇O₁₉ was synthesized by solid state reaction of the oxides La₂O₃ and CuO at 1288 K in air. The crystal structure was determined by a joint Rietveld refinement of X‐ray and neutron powder diffraction data. La₈Cu₇O₁₉ crystallizes in the monoclinic space group C2/c (No. 15) with the lattice parameters a = 13.8310(4)Å, b = 3.75827(9)Å, c = 34.5917(8)Å and β = 99.332(2)°. La₈Cu₇O₁₉ is the n = 3 member of the homologous series La_4+4nCu_8+2nO_14+8n. The Cu—O sub‐structure in La₈Cu₇O₁₉ contains infinite ribbons, which can be described as perovskite type layers with a width of n = 3 Jahn‐Teller‐elongated octahedra, and Cu—O planes of complex geometry. DSC/TG‐measurements in different gas atmospheres show peritectic decomposition of La₈Cu₇O₁₉. The anisotropic thermal expansion of the lattice parameters was investigated using synchrotron radiation. The Madelung part of lattice energy was calculated and compared with the corresponding values of other lanthanum cuprates.