Comparativeab initiostudy of the structure and stability ofH−andLi−anions in silica networks

Abstract

The existence of alkali metal anions in interstitial cavities of crystalline silica and microporous materials (e.g., zeolites) has been suggested based on NMR and EPR measurements but has never been studied theoretically. In this work we report results of density functional theory calculations on the interaction of a ${\mathrm{Li}}^{\ensuremath{-}}$ anion with cluster models of \ensuremath{\alpha} quartz containing up to $\ensuremath{\approx}100\mathrm{Si}$ and O atoms. We also consider the interaction of ${\mathrm{Li}}^{\ensuremath{-}}$ with orthosilicic acid, ${\mathrm{Si}(\mathrm{OH})}_{4}.$ The properties of ${\mathrm{Li}}^{\ensuremath{-}}$ incorporated in a silica structure have been compared with those of the hydride anion, ${\mathrm{H}}^{\ensuremath{-}}.$ We found that, in agreement with previous theoretical studies, ${\mathrm{H}}^{\ensuremath{-}}$ forms stable structures where a Si atom becomes penta-coordinated with a bipyramidal trigonal structure. ${\mathrm{Li}}^{\ensuremath{-}}$ behaves completely differently. Because of its larger size, ${\mathrm{Li}}^{\ensuremath{-}}$ does not come close enough to the tetra-coordinated Si atom to induce the structural distortion observed for ${\mathrm{H}}^{\ensuremath{-}}.$ On the contrary, the Li atom becomes incorporated into the silica structure, with formation of a $\ensuremath{\equiv}\mathrm{Si}\ensuremath{-}{\mathrm{O}}^{\ensuremath{\delta}\ensuremath{-}}\ensuremath{-}{\mathrm{Li}}^{\ensuremath{\delta}+}$ linkage. The negative charge is transferred to the host silica structure where becomes trapped at pre-existing defects or induces additional structural changes in the network.