BONDING IN SILICATES: INVESTIGATION OF THE SI L2,3 EDGE BY PARALLEL ELECTRON ENERGY-LOSS SPECTROSCOPY

Abstract

The Si L 2,3 core-loss edge can be used to probe the crystal chemistry around Si, providing information on the s-and d-like partial density of unoccupied states of the Si-O bonds. We present Si L 2,3 edges from 59 silicates, glasses, and amorphous materials acquired by parallel electron energy-loss spectroscopy (PEELS) with a transmission electron microscope (TEM) at an energy resolution of 0.7 eV. The Si L 2,3 edge spectrum of α-quartz is interpreted using the results of a recent pseudopotential band-structure cal-culation. A combination of Si s-and d-like partial density of states derived from this calculation resembles the Si L 2,3 energy-loss near-edge structure (ELNES) of α-quartz. The Si L 2,3 ELNES of the silicates are interpreted using the results of the band-structure calculation of α-quartz. The Si L 2,3 edges of Q 4 , Q 3 , Q 2 , some Q 1 silicates, and amorphous materials have ELNES similar to that of α-quartz, and the Q 0 and some Q 1 silicates have ELNES different from that of α-quartz. A "coordination fingerprint" is defined for Q 4 , Q 3 , and Q 2 Si L 2,3 ELNES because of their similarity to the α-quartz spectrum. The similarities between the L 2,3 core-loss edge shapes of the third-row XO 4 n– (X = Al, Si, S, and P) series attests to a common molecular–orbital picture of their bonding. For Q 0 and some Q 1 spectra a "structure fingerprint" is defined because the Si L 2,3 -edge shapes are indicative of the number, distribution, and nature of the non-nearest-neighbor atoms. Spectra of olivine glasses and metamict zircon more closely resemble the α-quartz spec-trum than their crystalline analogs. In contrast to previous studies, we show that distortion of the SiO 4 tetrahedron is of secondary importance as an ELNES-modifying parameter. Polyhedral distortions be-come less important with increase in polymerization. There is a positive linear correlation between the energies of the Si L 2,3 -edge onsets and polymerization, Si 2p and 2s binding energies, and the 29 Si NMR isotropic chemical shifts. The shift to higher energies of the edge onsets with polymerization corresponds to an increase in effective charge on the Si atom with higher Q n . For silicates with isolated SiO 4 tetrahedra, increases in L 2,3 -energy onsets correlate with increases in polarizing power of the next-nearest-neighbor cations. The Si L 2,3 -edge shapes are affected by the types and coordinations of the next-nearest-neighbor cations. For example, andradite, ilvaite, fayalite, and γ-Fe 2 SiO 4 have FeO 6 bonded to SiO 4 and exhibit similar ELNES. Topaz, dumortierite, staurolite, and kyanite have similar Si L 2,3 ELNES, with AlO 6 bonded to the SiO 4 . Their edge shapes are distinct from those of silicates with SiO 4 bonded to AlO 4 , as in the feldspars. A comparison of the Al and Si L 2,3 and Al, Si, O, and F K core-loss edges of topaz illustrates the influence of neighbor effects and mixing of unoccupied states. This mixing illustrates the limitations of ab initio methods that model core-loss edges that neglect non-nearest-neighbor interactions.