Crystal structures and structural modulations of Pb 2SrLaCu 2O 6+δ (δ≈0, δ≈1.5) by neutron powder diffraction and HREM

Abstract

The crystal structures of Pb 2SrLaCu 2O 6+δ with δ≈0 and δ≈1.5 have been refined from neutron powder diffraction data recorded at T=4.2 K and room temperature. The structural modulation in Pb 2SrLaCu 2O 7.5 as reported earlier has been investigated in a HREM study. The unit cell of the δ≈0 phase is orthorhombic with space group Pman. The basic unit cell of the oxidized material (δ≈1.5) is slightly distorted from tetragonal, having space group Pmmm. The incommensurate structural modulation of approximately √2 a p×3.4√2 a p×2 c is confirmed by the HREM study. We report a new intergrown structure for the δ≈1.5 phase, with the lattice constants a p×4.05 a p× c p. Both types of deviation from the basic unit cell appear to coexist in a single crystallite of the material. Some unexplained reflections in the refinement of the basic unit cell of Pb 2SrLaCu 2O 7.5 from the neutron data have been assigned to intergrowth reflections.