New Li + ion conductors in the system Li 4SiO 4−Li 3AsO 4

Abstract

In the system Li 4SiO 4-Li 3AsO 4, Li 4SiO 4 forms a short range of solid solutions containing up to 14 to 20% Li 3AsO 4, depending on temperature, and γ-Li 3AsO 4 forms a more extensive range of solid solutions containing up to ≈55% Li 4SiO 4. The Li 4SiO 4-Li 3AsO 4 phase diagram has been determined and is of binary eutectic character. The ac conductivity of polycrystalline samples was measured over the range 0 to at least 300°C for nine different compositions. The two solid solution series have much higher conductivity than the pure end-members; maximum conductivity was observed in the γ-Li 3AsO 4 solid solutions containing ≈40 to 55% Li 4SiO 4, with values of ≈2×10 −6 Ω −1 cm −1 at 20°C rising to ≈0.02 Ω −1 cm −1 at 300°C. These values are comparable to those found in the system Li 4SiO 4-Li 3PO 4. The variation with composition of the Arrhenius prefactor and activation energy has been interpreted in terms of the mechanisms of conduction. Li 3AsO 4 is a poor conductor essentially because the number of mobile Li + ions is very small. This number, and hence the conductivity, increases dramatically on forming solid solutions with Li 4SiO 4, by the creation of interstitial Li + ions. At ≈40 to 55% Li 4SiO 4, the number of mobile Li + ions appears to be optimised. An explanation for the change in activation energy of conduction at ≈290°C in Li 4SiO 4 and at higher temperatures in Li 4SiO 4 solid solutions is given in terms of order-disorder of the Li + ions.