The lithium ion conductor β-spodumene: an orientational glass

Abstract

Polycrystals of -spodumene were investigated using impadence spectroscopy for frequencies 0.1 Hz ( (235 GHz and for temperatures 5 K (T(500 K. At high temperatures the Li ions are mobile and lead to dc and ac conductivity phenomena. These are analyzed in terms of the universal dielectric response and using the modulus formalism. The results obtained by both procedures are critically compared. At low temperatures or very high frequencies dielectric loss due to a dipolar freezing process is observed which bears a close resemblance to an orientational glass transition. To further confirm the glassy character of -spodumene, we report measurements of the low temperature specific heat (3 K(T(50 K) which provide evidence for the existence of excess contributions that are usually observed in amorphous materials. Introduction Spodumene, LiAlSiO , belongs to the small class of crystals that exhibit a contraction of certain crystallographic axes upon heating. The negative thermal expansion coefficient has technological relevance and is for instance used in ''zero'' expansion materials [1], it is due to the peculiar network structure which is built up from an array of corner-sharing SiO and AlO tetrahedra. This structure is similar to that of keatite which itself is a modification of quartz [2]. In -spodumene the centers of the tetrahedra are however occupied randomly by Al> or by Si> and, to keep the charge balance, the network is ''stuffed'' by Li> ions which occupy interstitial sites.The structure of -spodumene involves 5, 7, and 8 membered rings of the SiO and AlO units which are

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