The relationship of oxygen content, structural phase transition and T_c in Tl₂Ba₂CuO_{6 ± δ}

Polycrystalline samples of Tl₂Ba₂CuO_{6 ± δ} were prepared by solid state reaction technique, with different heat treatments. Samples that were quenched from 860°C in air were pseudo-tetragonal and exhibited superconducting onsets at ∼ 84 K. Samples slow cooled from 860°C to room temperature in air showed a clear orthorhombic distortion and exhibited onsets at ∼ 20 K. No evidence of superconductivity was observed for samples prepared in sealed tubes at 860°C which were orthorhombic. Post annealing the 20 K and non-superconducting samples in argon atmosphere at 400-450°C for 1-2 h resulted in superconductivity at ∼ 50 K. A reversible structural phase transition from pseudo-tetragonal to orthorhombic symmetry accompanied by oxygen uptake/loss was observed in controlled oxygen atmosphere at ∼ 300°C as evidenced by DTA, TGA and high temperature X-ray diffraction studies. Variations in the oxygen content, although very small, have dramatic effects on the superconducting properties of Tl₂Ba₂CuO_{6 ± δ}. The dependence of superconductivity on structure, oxygen content/ordering is discussed.