Bonding in silicates; investigation of the Si L (sub 2,3) edge by parallel electron energy-loss spectroscopy

Arizona State University, Department of Geology, Tempe, AZ, United States

The Si L (sub 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 (sub 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 (sub 2,3) edge spectrum of alpha -quartz is interpreted using the results of a recent pseudopotential band-structure calculation. A combination of Sis-and d-like partial density of states derived from this calculation resembles the Si L (sub 2,3) energy-loss near-edge structure (ELNES) of alpha -quartz. The Si L (sub 2,3) ELNES of the silicates are interpreted using the results of the band-structure calculation of alpha -quartz. The Si L (sub 2,3) edges of Q ⁴ , Q ³ , Q ² , some Q ¹ silicates, and amorphous materials have ELNES similar to that of alpha -quartz, and the Q ⁰ and some Q ¹ silicates have ELNES different from that of alpha -quartz. A "coordination fingerprint" is defined for Q ⁴ , Q ³ , and Q ² Si L (sub 2,3) ELNES because of their similarity to the alpha -quartz spectrum. The similarities between the L (sub 2,3) ? core-loss edge shapes of the third-row XO (super n-) ₄ (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 (sub 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 alpha -quartz spectrum 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 become less important with increase in polymerization. There is a positive linear correlation between the energies of the Si L (sub 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 (sub 2,3) -energy onsets correlate with increases in polarizing power of the next-nearest-neighbor cations. The Si L (sub 2,3) -edge shapes are affected by the types and coordinations of the next-nearest-neighbor cations. For example, andradite, ilvaite, fayalite, and gamma -Fe ₂ SiO ₄ have FeO ₆ bonded to SiO ₄ and exhibit similar ELNES. Topaz, dumortierite, staurolite, and kyanite have similar Si L (sub 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 (sub 2,3) and Al, Si, O, and FK 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.

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