With increasing the Li doping level in La0.7Ca0.3−yLiyMnO3 (0⩽y⩽0.3), the system is driven from a higher-conductivity regime to a lower-conductivity regime, which is in sharp contrast with the behavior of Na or K doping. Compared to those of K or Na, the ionic radius of Li is much smaller than that of Ca. Therefore, substitution of Ca by Li results in a smaller average A-site radius, narrowing the bandwidth and hence enhancing the band gap and resistivity. The effect of random disorder of La3+ and A2+ also plays an important role leading to charge localization.
Bhattacharya, S., Mukherjee, RK., Chaudhuri, BK., & Yang, HD. (2003). Effect of Li doping on the magnetotransport properties of La0.7Ca0.3-yLiyMnO3 system: Decrease of metal-insulator transition temperature. Applied Physics Letters, 82, . https://doi.org/10.1063/1.1580650