Li Ion Dynamics in Lanthanum Lithium Tantalate Studied by NMR and Impedance Spectroscopy

Abstract

A conduction mechanism of Li ions in lanthanum lithium tantalate was investigated by ⁷Li NMR and impedance spectroscopy. Conductivity spectra of La_1/3-xLi_3xTaO₃ with x=0.06 and 0.09 exhibited Jonscher's universal dynamic response behavior. A scaling analysis revealed that the concentration of Li ions responsible for the long-range migration remained constant in the temperature range from -80°C to 30°C. Correlation times for the local motion of Li ions were estimated from the measurement of NMR spin-lattice relaxation times to be in the order of 10^-9 s at room temperature. Activation energies of the correlation times, which correspond to the barrier-height to the elementary hop of Li ions, were 8 kJ mol^-1 for x=0.06 and 10 kJ mol^-1 for x=0.09. These activation energies were confirmed to be related through power-law exponents to those of dc conductivities. In spite of the similar crystal structure and the same Li content, the dc conductivity at room temperature of La_1/3-xLi_3xTaO₃ with x=0.06 was one order of magnitude lower than that of La_2/3-xLi_3xTiO₃ with x=0.06; this may be ascribed to the longer correlation time and the higher activation energy of La_1/3-xLi_3xTaO₃.