(Li<SUB>1-x</SUB>M<SUB>x</SUB>)Ti<SUB>2</SUB>O<SUB>4</SUB>系スピネル(M=Mg,Be)の超電導に関する研究
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The system (Li<SUB>1-x</SUB>M<SUB>x</SUB>)Ti<SUB>2</SUB>O<SUB>4</SUB> (M=Mg, Be) with the spinel structure was prepared by firing twice a mixture of the component oxides, i.e., Li<SUB>2</SUB>O, MgO, BeO, Ti<SUB>2</SUB>O<SUB>3</SUB>, and Ti0<SUB>2</SUB> at 1073K for 12 h in vacuo(ca. 10<SUP>-5</SUP> Torr). The lattice constants were obtained by the powder X-ray diffractometry whereas the molar fraction of Ti<SUP>3+</SUP> 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 Be<SUP>2+</SUP> with a much smaller ionic radius than that of Li<SUP>+</SUP> 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<SUB>1+x</SUB>XTi<SUB>2-x</SUB>O<SUB>4</SUB>, 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<SUB>1-x</SUB>Mg<SUB>x</SUB>)Ti<SUB>2</SUB>O<SUB>4</SUB> gradually decreased as either the molar ratio f or lattice constant a increased.