It was found that the Li ion conductivity of sintered samples involved 2 contributions. One of these arose from grain boundaries which perhaps contained amorphous silicate phases. The other arose from a bulk phase with an apatite structure. Although the conductivity of the bulk phase was lower than that of the grain boundary, it appeared to be increased by several orders of magnitude with respect to that of stoichiometric apatites. The conductivity enhancement of the apatite phase could be explained in terms of a model in which both Li ions and vacancies were introduced at the 4f sites of the apatite structure.

M.Sato, Y.Kono, H.Ueda, K.Uematsu, K.Toda: Solid State Ionics, 1996, 83[3-4], 249-56