Neutron-diffraction study of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$ with various ${\mathit{T}}_{\mathit{c}}$’s from 0 to 73 K

We refined the crystal structures of four pseudotetragonal samples of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$ with ${\mathit{T}}_{\mathit{c}}$’s of 0 (metallic), 48, 58, and 73 K by Rietveld analysis of time-of-flight neutron-powder-diffraction data. The presence of excess oxygen atoms located at an interstitial site between double TlO layers was confirmed. The change in oxygen content and the corresponding one in hole carrier concentration in this compound are caused by incorporation and release of the excess oxygen, whose amount is about 0.1 per formula unit for the metallic nonsuperconducting sample. The highest ${\mathit{T}}_{\mathit{c}}$ value of about 85 K in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$ is achieved in the absence of excess oxygen. As the oxygen content is decreased, the c value increases, and apical ${\mathrm{O}}^{2\mathrm{-}}$ ions go away from the ${\mathrm{CuO}}_2$ layer, while ${\mathrm{Ba}}^{2+}$ ions approach it. It was also suggested that about 5% of a Tl site is substituted by Cu. A significant portion of hole carriers in this compound is attributed to substitution of Cu for Tl and incorporation of excess oxygen.