Transport in glassy fast-ion conductors. A stud of LiAlSiO₄ glass

The mechanism of ionic motion in LiAlSiO₄ glass is investigated using a combination of structural (vibrational spectroscopy, X-ray diffraction) and transport (ionic conductivity) methods. We find that the material is quite a good Li⁺ conductor (σ ≈ 10^-3 μ^-1 cm^-1 at 500 °C), and that the vibrational density of states deduced from spectroscopic studies agrees very well with the continuous random network model. Moreover, the tendency towards ordering seen in many aluminosilicates and described by Loewenstein's rule is discussed in the context of our spectroscopic observations, and we find that the Loewenstein rule also holds approximately in this glassy material. The rigidly alternating potential seen by Li⁺ ions in crystalline LiAlSiO₄ (β-eucryptite) is substantially altered by the disorder present in the glass. This disorder strongly reduces correlation effects on ionic motion in the glass, as opposed to the crystal, resulting in a smaller activation energy for conductivity in the glass.