Ba₂La₂Cu₂Sn₂O₁₁: A new layered perovskite with a modulated structure

The average structure of the oxygen-deficient perovskite Ba₂La₂Cu₂Sn₂O₁₁ can be described by a tetragonal 1a_p x 1a_p x 4a_p subcell that has P4/mmm space group symmetry and cell parameters a = 3.9893 (3), c = 16.232 (1) Å. The subcell does not fully describe the long-range structural features. The structure is periodic only in a statistical sense and contains long-period commensurate modulations that have a dominant periodicity (superlattice) or √2a_p x √2a_p x 24a_p. The structure was determined from a combination of powder X-ray diffraction, time-of-flight neutron diffraction, and transmission electron microscopy. The structure contains copper-oxygen double layers that are interleaved by tin-oxygen double layers. The oxygen vacancies order between copper-oxygen layers and result in the now-familiar square pyramidal coordination of copper. The tin-oxygen octahedra are rotated around [001], and the short-range order (24a_p, ≈100 Å) of the sense of the rotations is believed to give rise to the structural modulation. The magnetic susceptibility versus temperature profile indicates the compound is paramagnetic to 4 K.