Phase transitions in the Tl2Ba2CuO6+δ superconductor

Abstract

Structural phase transitions in the “orthorhombic” form of the superconductor Tl2Ba2CuO6+δ have been investigated by synchrotron x-ray powder diffraction. High-resolution studies show that oxygen rich compositions with Tc’s up to ≃70 K are monoclinic (F112/m symmetry) with 90°<γ<90.2° whereas slightly reduced samples with Tc’s of 70–90 K have orthorhombic Fmmm symmetry. The peakwidths of the Bragg reflections show that the orthorhombic and monoclinic distortions induce microscopic strains in the ab plane. A high-temperature in situ synchrotron x-ray powder-diffraction experiment has been used to investigate the transitions between the monoclinic, orthorhombic, and thallium-deficient tetragonal forms of this material. The thermal variations of the macroscopic strains show that the 895 K monoclinic-orthorhombic phase transition, which is driven by loss of excess oxygen, is well described by Landau theory. However, the orthorhombic-tetragonal transition at 964 K, due to the loss of thallium oxide, shows a strong deviation from Landau behavior. A possible structural rearrangement of interstitial oxygen atoms is observed at 550–700 K.

Source-reported materials — not catalogue approval

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