Electron energy-loss spectroscopy at incommensurately modulated crystalline and glassy Ba₂TiGe₂O₈

Abstract

Based on cluster molecular orbital calculations, high-energy resolution (ΔE ∼ 0.4 eV) Ti–L_2,3 electron energy loss near-edge structures of single-crystalline and glassy Ba₂TiGe₂O₈ are interpreted. The finding that the Ti–L_2,3 near-edge structure of the Ba₂TiGe₂O₈ single crystal possesses less pronounced peaks than the glass under identical experimental conditions can be attributed to distinct distortions of the titanium environment caused by the very strong one-dimensional structural modulation hosted by the Ba₂TiGe₂O₈ crystal lattice. As lattice periodicity is absent in the glass, the titanium environment is more regular in the vitreous surroundings. Moreover, the modulation in crystalline Ba₂TiGe₂O₈ is responsible for the virtually indiscernible O–K near-edge structures of the glassy and crystalline samples.

Acknowledgement

All calculations have been carried out at the Research Institute of Software Technology (RIST) of the TechnoZ in Salzburg.

Notes

It is important to note that such fitting in either case can only help support the interpretation, since the large number of individual titanium ions with individual environments (in the single crystal provoked by the modulation and in the glass by the disorder dominating the medium-range order) will result in numerous individual near-edge structures and their superposition forms the observed ELNES. It is therefore not astonishing that, for example, the full width at half maximum of peak E in figure 4 (a) amounts to 1.5 eV, a value that is significantly larger than the experimental spectral resolution. Particularly for low-intensity peaks, such as peak D (most likely representing the d _{x 2 −y 2} orbital) in the single-crystal spectrum depicted in figure 4 (a), it is critical to discuss the peak positions in terms of their relation to the molecular orbital calculations (although, in the present case, it is tantalising to discuss the larger distance between the d _{x 2 −y 2} orbital and the d _{z 2} orbital in terms of a symmetrization of the TiO₅ pyramid, making the term scheme more similar to that of Ba₂TiSi₂O₈ shown in figure 3).