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) angstrom. The subcell does not fully describe the long-range structural features. The structure is periodic only m a statistical sense and contains long-period commensurate modulations that have a dominant periodicity (superlattice) or square-root 2a(p) x square-root 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), almost-equal-to 100 angstrom) 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.