Chemistry and superconductivity in thallium-based cuprates

Following the discoveries of high temperature superconductivity in the rare earth copper oxide systems at 40 K by Bednorz and Muller in 1986 and at 90 K by other researchers in 1987, Sheng and Hermann, in 1988, discovered superconductivity in the thallium-alkaline-earth copper oxide systems with critical temperatures as high as 120 K. All of the Tl-based compounds can be described by the general formula, Tl(m)A2Ca(n-1)CuO2(n+m+2), where m=1 or 2; n=5; A=Ba, Sr. For convenience, the names of these compounds are abbreviated as 2223 for TlBa2Ca2Cu3O10, where each number denotes the number of Tl, Ba(Sr), Ca and Cu ions per formula, respectively. The compounds with m=1 and m=2 are usually referred to as single and double Tl-O layered compounds, respectively. The highest superconducting transition temperature known so far was found in Tl2BaCa2Cu3O10 at 125 K.