Influence of the electron concentration on the superconducting properties of the spinel system Li1-yTi2O4

In the spinel system Li_1-y Ti₂O₄ the Li content was reduced by oxidative extraction by means of a solution of I₂ as well as Br₂ in CH₃CN or by oxygen. The extraction is associated with a transition of Ti³⁺ to Ti⁴⁺: the system is Li_1-y Ti³⁺_1-yTi⁴⁺_1+y O₄. Consequently, with increasing y the charge carrier concentration decreases from the starting value of 0.5 electrons/Ti ion for LiTi₂O₄. The extraction proceeds in two steps. At the beginning, the Ti₂O₄ framework remains intact and the transition to superconductivity increases with decreasing charge carrier concentration to a maximal value of 13.2 K. Separated by a two-phase region for medium extraction levels for higher y values, a Li depleted material of approximate composition Li_0.3Ti₂O₄ is obtained. The crystal structure of Li_0.3Ti₂O₄ can be deduced from the original spinel lattice by a partial Ti migration from 16d to 16c and 8 a sites (space group Fd3m). Consequently, the Ti₂O₄ framework of Li_0.3Ti₂O₄ is disconnected and the material is no longer superconducting.