Effect of Interstitial Li on the Density of Si

Abstract

The densities of silicon and of silicon containing interstitial Li⁺ were determined. There is a volume expansion of 4.7×10^—24 cm³ per Li⁺ dissolved in silicon. This expansion is interpreted as arising from the Coulombic repulsion between the Li⁺ and the partially shielded Si⁴⁺ cores. Calculations based on a simple model of the charge distribution in silicon are in accord with this interpretation. The density change accompanying the room temperature deionization and aggregation of the lithium was also measured. During the initial stages of this process, the volume increases linearly by 6.0×10^—24 cm³ per lithium transferred from the ionized to the deionized state. This expansion is interpreted as arising from overlap repulsive forces between the Si and aggregates of neutral lithium in neighboring interstitial sites. Considerations based on the size of Li⁰ in these aggregates and the free energy change for the deionization and aggregation process support this interpretation. A final nonlinear region in the observed volume expansion is interpreted as being due to the precipitation of lithium metal so as to leave itself and the silicon strain free.