The existence of alkali metal anions in interstitial cavities of crystalline silica and micro-porous materials (e.g., zeolites) was suggested based upon nuclear magnetic resonance and electron paramagnetic resonance measurements but had never been studied theoretically. Density functional theory calculations were made of the interaction of a Li– anion with cluster models of α quartz which contained up to about 100 Si and O atoms. The interaction of Li– with orthosilicic acid, Si(OH)4, was also considered. The properties of Li– incorporated in a silica structure were compared with those of the hydride anion, H–. It was found that, in agreement with previous theoretical studies, H– formed stable structures in which a Si atom became penta-coordinated with a bipyramidal trigonal structure. The Li– behaved very differently. Because of its larger size, Li– did not come close enough to the tetra-coordinated Si atom to induce the structural distortion observed for H–. On the contrary, the Li atom became incorporated into the silica structure, with formation of a Si-Oδ–-Liδ+ linkage. The negative charge was transferred to the host silica structure where it became trapped at pre-existing defects or induces additional structural changes in the network.

Comparative ab initio Study of the Structure and Stability of H– and Li– Anions in Silica Networks. S.Brazzelli, C.Di Valentin, G.Pacchioni: Physical Review B, 2004, 69[2], 024101 (7pp)