High lithium ion conductivity solid electrolytes of Li1+xCrxTi2 (-) (x)(PO4)(3) (x = 0-0.5) and Li1.4AlxCr0.4 - xTi1.6(PO4)(3) (x = 0-0.4) with the NASICON-type structure were synthesized using a precursor prepared by the sol-gel method. The highest total electrical conductivity of 3.55 x 10(-4) S cm(-1) at 25 C was obtained for x = 0.4 in Li1+xCrxTi2 - x(PO4)(3) (LACTP) sintered at 1120 degrees C for 6 h in air. Higher electrical conductivity was observed by the partial substitution of Cr3+ with Al3+. The highest total electrical conductivity of 1.06 x 10(-3) S cm(-1) and bulk conductivity of 1.77 x 10(-3) S cm(-1) at 25 degrees C were obtained for x = 0.3 in Li1.4AlxCr0.4 - xTi1.6(PO4)(3) sintered at 1070 degrees C for 6 h in air. The contribution of electronic conductivity to the total conductivity was negligibly small. Li1.4Al0.3Cr0.1Ti1.6(PO4)(3) was unstable in water, but stable in saturated LiOH with saturated LiCl aqueous solution. (C) 2015 Elsevier B.V. All rights reserved.