High-temperature in situ neutron powder diffraction study of monoclinic Tl2Ba2CuO6+δ

Abstract

The monoclinic form of the Tl2Ba2CuO6+δ superconductor (F112/m symmetry) has been studied using high-resolution high-temperature in situ time-of-flight neutron powder diffraction. The amount of excess oxygen controls both the crystal structure and electronic properties: the as-prepared material (a=5.4412 Å, b=5.5026 Å, c=23.1523 Å, γ=90.127 °, δ=0.15) is non-superconducting, whereas the 700 K vacuum-annealed sample (a=5.4874 Å, b=5.5288 Å, c=23.3076 Å, γ=90.034 °, δ=0.11) is superconducting below Tc=73 K. Thermal variations of atomic c-axial displacements and bond valence sums demonstrate an electronic charge transfer from the TlO layers to the CuO2 planes. The peak widths of the Bragg reflections indicate microscopic strains in the c-axial direction, due to disordering of interstitial oxygen in the Tl2O2+δ bilayer, and in the ab-plane, due to the monoclinic and orthorhombic distortions, or static displacements of the Tl and O atoms in the TlO layers. Superstructural diffraction peaks due to an incommensurate structural modulation decrease in intensity with oxygen loss over the monoclinic-to-orthorhombic progression.