The present study has been aimed to obtain fundamental aspects on the design for a room temperature superionic conducting material, Li_3M_2 (PO_4)_3 (M=Sc and In), which has a so called average structure at a high temperature. In order to obtain the superionic conducting phase Li_3M_2 (PO_4)_3 (M=Sc and In), several kinds of aliovalent ions such as Mg^<2+>, Ti^<4+>, Zr^<4+>, Sn^<4+>, Hf^<4+>, Nb^<5+>, and Ta^<5+> were substituted for the M site. The differential thermal analysis indicated that no phase transition existed in the divalent ion substituted samples with the substituted ratio over 5 mol%, the tetravalent ion substituted samples with the ratio over 5 mol%, and the pentavalent ion substituted samples with the ratio over 2.5 mol%. The highest ionic conducting materials, which were found in the Ti-and Zr-substituted samples, has a quite higher room temperature conductivity by three orders of magnitude than that of the unsubstituted samples. The neutron diffraction measurements confirmed the stabilization of the high temperature phase at room temperature in the substituted samples, and also confirmed a disorder of Li ion distribution among three kinds of available sites, giving a driving factor to the stabilization of the high temperature phase. The substituted materials could be a promising material for the solid electrolyte in a solid state lithium battery or a chemical sensor.
We also investigated the stabilization of the high temperature phase of Li_3V_2 (PO_4)_3, which has the same structure as that of Li_3M_2 (PO_4)_3. The stabilization of the high temperature phase was also observed in this compound by substituting 10 mol% of Zr for M sites. The charge-discharge capacity of an electrochemical lithium cell where this material was used as a cathode material was found to be improved by 20%.