La8−xSrxCu8O20: An oxygen-deficient perovskite built of CuO6, CuO5, and CuO4 polyhedra

A new oxygen-deficient perovskite corresponding to the formulation La8−xSrxCu8O20−ε (1.28 ≤ x ≤ 1.92) has been isolated. The results of chemical analysis show that copper exhibits a mixed valence in this oxide. This perovskite crystallizes in a tetragonal cell with parameters related to the ap parameter of the cubic perovskite by: a ⋍ ap 2√2, c ⋍ ap. The structure has been determined by neutron powder diffraction in the space group p4mbm. This study shows that the oxygen vacancies are ordered, forming rows parallel to 〈001〉. The [Cu8O20] framework is built of corner-sharing CuO6 ocatahedra, CuO5 pyramids, and CuO4 square planar groups forming hexagonal tunnels in which are located the La3+ and Sr2+ ions. The slight oxygen substoichiometry with regard to the formulation A8Cu8O20 is discussed in terms of a superstructure along the c axis (c ⋍ 2ap), which was observed in some crystals by electron diffraction.