Connections between the electron-energy-loss spectra, the local electronic structure, and the physical properties of a material: A study of nickel aluminum alloys

The local electronic structure of a material can be determined from the energy-loss spectrum of a swift electron beam scattered through it. When the electron beam is focused down to the width of an atomic column, the electronic density of states (DOS) at an interface, grain boundary, or impurity site can be decomposed by site, chemical species and angular momentum. Here we discuss the use of electron-energy-loss spectroscopy (EELS) fine structure to provide insight into the origin of grain boundary and interfacial properties reported earlier [D. A. Muller et al., Phys. Rev. Lett. 75, 4744 (1995)] for Ni${}_{3}$Al. We examine the electronic structure trends in Ni-Al compounds, both experimentally with the EELS measurements and theoretically, using ab initio band-structure calculations. The conditions under which the band-structure calculations can quantitatively reproduce the EELS measurements (and in particular, the question of just which local DOS is being measured) are addressed. Cyrot-Lackmann's moments theorem provides a framework to explain the systematic changes in the local DOS on alloying. The shape changes in the near-edge fine structure of both the Ni and Al $L$ edges are readily understood by the sensitivity of the fourth moment of the local DOS to the angular character of the Ni-Al bonding. The language of bond-order potentials proved useful in linking shape changes in the DOS to changes in cohesion. The consequences for formation energies and ordering trends in the transition-metal--aluminum alloys are also discussed.