Mechanically induced decrease of the Li conductivity in an alumosilicate glass

In the present work β-spodumene LiAlSi2O6 and the corresponding glass form a model system highly suitable to study the influence of particle size on Li ion dynamics. The nanostructured samples were prepared by high-energy ball milling of the coarse grained starting material and the corresponding glass, respectively. Diffusion parameters and electrical conductivity were measured by 7Li spin-lattice relaxation nuclear magnetic resonance (NMR) and impedance spectroscopy. As reported previously, the Li diffusivity in the glassy sample is larger than that in the coarse grained crystalline material of the same chemical composition [Franke et al., Ber. Bunsenges. Phys. Chem. 96, 1674 (1992).] which is quite often observed also for other materials. Decreasing the particle size down to the nm-regime causes an enhancement of the Li conductivity of β-spodumene LiAlSi2O6 reaching an upper limit which is, however, still lower than the conductivity of the corresponding glass. Surprisingly, when the glassy material is mechanically treated under the same conditions, the Li diffusivity is slowed down. The Li conductivity of such a sample resembles that of nanocrystalline LiAlSi2O6. This is astonishing since one might expect that mechanical treatment of a glassy sample does not further influence the transport parameters. A possible structural description trying to explain the observed convergence of the transport parameters of the crystalline and glassy materials as a result of milling is briefly presented.