CRYSTAL-STRUCTURES AND STRUCTURAL MODULATIONS OF PB2SRLACU2O6-APPROXIMATE-TO-0, DELTA-APPROXIMATE-TO-1.5) BY NEUTRON POWDER DIFFRACTION ANDHREM(DELTA(DELTA)

The crystal structures of Pb2SrLaCu2O6+delta with delta almost-equal-t o 0 and delta almost-equal-to 1.5 have been refined from neutron powde r diffraction data recorded at T = 4.2 K and room temperature. The str uctural modulation in Pb2SrLaCu2O7.5 as reported earlier has been inve stigated gated in a HREM study. The unit cell of the delta almost-equa l-to 0 phase is orthorhombic with space group Pman. The basic unit cel l of the oxidized material (delta almost-equal-to 1.5) is slightly dis torted from tetragonal, having space group Pmmm. The incommensurate st ructural modulation of approximately square-root 2a(p) X 3.4 square-ro ot 2a(p) X 2c is confirmed by the HREM study. We report a new intergro wn structure for the delta almost-equal-to 1.5 phase, with the lattice constants a(p) X 4.05a(p) X c(p). Both types of deviation from the ba sic unit cell appear to coexist in a single crystallite of the materia l. Some unexplained reflections in the refinement of the basic unit ce ll of Pb2SrLaCu2O7.5 from the neutron data have been assigned to inter growth reflections.