Powder neutron-diffraction profile refinement techniques have been used to investigate the oxygen dependence of the crystal structure and its effect on the superconducting phase transition in high-quality samples of the superconductor HgBa2CuO4+ (0.040.23). The system remains tetragonal (space group P4/mmm) over the full range of temperature (10 300 K) and oxygen concentration explored. The a-axis lattice parameter decreases smoothly with increasing, while the c-axis lattice parameter exhibits a maximum. The extra oxygen in the material is found to randomly occupy the centered O(3) site (1/2, 1/2, 0) in the Hg layer, and no other additional site for the extra oxygen was found in the structure. There is also no mixing of the cations on the Cu and Hg sites. Tc is observed to vary strongly with, increasing from 0 K for =0.04 to a maximum Tc=95 K at 0.18, and then decreasing rapidly for larger. The relationship of Tc() to the rate of hole doping on the CuO2 planes indicates that this doping level is only half of that expected for O2- ions. Our structural studies reveal an interesting correlation between the occupation of the O(3) site and the movement of the Ba ions towards the O(3) and away from the Cu-O planes. The Ba layer thus becomes structurally disordered, and this disorder is found to closely mirror the Tc() behavior. Valence and/or distance arguments for the O(3) then suggest that this oxygen is only singly ionized.
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