The lithium ion conductor beta-spodumene: An orientational glass

Polycrystals of beta-spodumene were investigated using impadence spectroscopy for frequencies 0.1 Hz < nu < 235 GHz and for temperatures 5 K < T < 500 K. At high temperatures the Li ions are mobile and lead to de 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 beta-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.