The conductivity was found to depend upon the composition. Thus, the replacement of (SiO4)4- by (PO3)3- increased the conductivity whereas the introduction of excess Na+ ions by replacing one Y3+ ion with three Na+ ions did not give rise to a large decrease in conductivity. The latter effect was explained by assuming that the Y3+ lattice sites accommodated some of the Na+ ions.

K.Yamashita, T.Kakuta, B.Sakurai, T.Umegaki: Solid State Ionics, 1996, 86-88[1], 585-8