Studies on the Superconductivity in the System (Li_1-xM_x)Ti₂O₄ (M=Mg, Be) with the Spinel Structure

The system (Li_1-xM_x)Ti₂O₄ (M=Mg, Be) with the spinel structure was prepared by firing twice a mixture of the component oxides, i.e., Li₂O, MgO, BeO, Ti₂O₃, and TiO₂ at 1073K for 12 h in vacuo(ca. 10^-5 Torr). The lattice constants were obtained by the powder X-ray diffractometry whereas the molar fraction of Ti³⁺ ions in the total Ti atoms, i.e., f (%) was determined by thermogravimetry. The critical temperatute Tc(K, mid-point) was obtained by using a usual four-probe resistivity measurement in the range of 300 K to ca. 2 K. As a result, the cubic lattice constant a(A) of the system linearly increased as the molar ratio f increased. However, it was found that Be²⁺ with a much smaller ionic radius than that of Li⁺ can hardly replace the Li⁺ site of the spinel structure, as implied from the plots of the lattice parameter a and the chemical composition x. The highest Tc was observed at f=ca. 50 % in the composition Li_1+xTi_2-xO₄, and the Tc suddenly decreased as either the molar ratio f or the lattice constant a decreased. On the other hand, the Tc of the composition (Li_1-xMg_x)Ti₂O₄ gradually decreased as either the molar ratio f or lattice constant a increased.