Ordered and disordered structures characterized by energy-loss spectroscopy

N. Jiang, B. Jiang and J. C. H. Spence

Department of Physics and Astronomy, Arizona State University, Tempe, AZ85287, USA (Nan.Jiang@asu.edu)

Electron energy loss spectroscopy (EELS) is a powerful tool for studying electronic structure by probing the unoccupied density of states (DOS). For small angle scattering, the EELS intensities can be simplified as the product of the unoccupied DOS, r(E) and an atomic transition matrix: , in which | i > and | f > represent the initial and final states, and q is the momentum transfer of electron scattering. Therefore, EELS can be also used to measure the orientation dependence of electronic structure. It is known that the DOS is determined by short range rather than long-range atomic structure. Taking advantage of the small probe in electron microscope, the near-edge fine structure of EELS (ELNES) can also provide structural information on materials that lack translational symmetry. In this presentation, show examples of the use of EELS to study electronic structure in the anisotropic crystalline superconductor MgB₂, and measure structural fluctuation in the long-range order of several silicate glasses.

MgB₂ has a simple layered AlB₂ structure. The electronic structure, which is characterized by both the spatial and energy distribution of its electrons, is essential in determining the transition temperature T_c in this material. The calculated DOS in MgB₂ is compared with orientation-dependent EELS measurements, to verify the calculations. The electron density distributions are then given in the form of difference charge-density maps, computed by subtracting a superposition of isolated neutral atom densities on MgB₂ crystal sites from the electron density obtained in the calculations. The results are also discussed in comparison with other isoelectronic compounds.

Medium-range order (5 – 20Å) in glasses has traditionally been deduced indirectly from experimental evidence. Any structural information beyond this scale, however, is unknown. Here we introduce an alternative experimental approach using spatially resolved EELS for characterizing glass structures and electronic structure. Experimental evidence for long-range structural and compositional fluctuations in CaSiO₃ (Wollastonite) and CaAl₂Si₂O₈ (anorthite) glasses are given. The implications for electronic structure and stoichiometry-induced structural variations are analyzed. These fluctuations, on the scale of a few nanometers, were discovered by examining the spatial dependence of inner shell near-edge absorption spectra obtained using a 50nm diameter probe. This spectroscopy is sensitive to both angular and distance correlations in bonding. Results are obtained from the average of the short to medium-range structures around Al and Si, which are obtained by comparing experimental data in the glasses with calculations of the local density of states in compositionally equivalent Wollastonite and anorthite crystals, respectively. This work is supported by NSF DMR9814055.