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

Abstract

The Si L2,3 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 calculation. A combination of Si s- and d-like partial density of states derived from this calculation resembles the Si L2,3 energy-loss near-edge structure (ELNES) of α-quartz. The Si L2,3 ELNES of the silicates are interpreted using the results of the band-structure calculation of α-quartz. The Si L2,3 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 L2,3 ELNES because of their similarity to the α-quartz spectrum. The similarities between the L2,3 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 L2,3-edge shapes are indicative of the number, distribution, and nature of the non-nearestneighbor atoms. Spectra of olivine glasses and metamict zircon more closely resemble the α-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 2,3-edge onsets and polymerization, Si 2p and 2s binding energies, and the