The nominal Li concentration giving a maximum room temperature Li-ion conductivity sigma(1.i) similar or equal to 3.45 x 10(-4) Scm(-1) with an E-a similar or equal to 0.438 eV in the system Li7-xLa3Hf2-xTaxO12 fired in an alumina crucible at 1130 degrees C for 48h occurs in the narrow range of x = 0.45 +/- 0.05. The samples were prepared by solid-state reaction and characterized by XRD, SEM, electrochemical impedance spectroscopy and Li-7 MAS NMR measurements. Published by Elsevier B.V.