Eels Studies of B2-Type Transition Metal Aluminides: Experiment and Theory
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Warrington, WA4 4AD, U.K., "Oak Ridge National Laboratory, Oak Ridge, TN 37831-6114, USA.
ABSTRACT The electronic structure and bonding character of intermetallic alloys are investigated by a combination of electron energy loss spectroscopy (EELS) experiments and ab initio electronic structure calculations. A detailed comparison is made between experimental spectra and calculations. The changes in electronic structure within a transition metal alurninide series and also due to alloying are studied using EELS spectra. The Korringa-Kohn-Rostoker coherent-potential-approximation method and large supercell models are used to investigate changes in composition and the effect of dopants on the electronic structure. INTRODUCTION Intermetallic alloys are a class of materials of potential interest for high performance structural applications. A serious difficulty limiting their current use, however, is their intrinsic brittle behavior at room temperature. Electron energy loss spectroscopy (EELS) is a potentially powerful local probe of the electronic structure of alloys. This locality is twofold. Firstly, EELS white lines result from the excitation of p-core electrons into unoccupied s- and d-conduction band states and, as such, probe the local (atom specific) densities of states. Secondly, because the EELS experiment is performed in a high resolution electron microscope, the spectra result from small regions of the sample. This microprobe capability then opens up the exciting possibility of simultaneously probing the chemistry and electronic structure of such important determinants of mechanical properties as grain boundaries. However, before EELS can be used in this way, the key question as to whether EELS spectra can be understood in terms of a single particle framework must be answered in the affirmative. Here, we show results of EELS experiments on bulk transition metal aluminides and corresponding electronic structure calculations based on the local density approximation (LDA) that suggest this is the case. MATERIALS AND METHODS Alloys were prepared by arc melting high purity starting materials (Al 99.999% pure and 99.98% pure transition metal (TM)). Transmission electron microscopy specimens were prepared by electropolishing 3mm diameter disks. Energy loss spectroscopy was carried out on a Philips CM30 electron microscope equipped with a Gatan model 666 spectrometer. At the operating voltage of 100kV an energy resolution, measured at the full width at half maximum of the no loss peak, of better than leV is typically obtained. Theoretical calculations using the linear muffin-tin orbital (LMTO) method [4], the Korringa-KohnRostoker coherent-potential-approximation (KKR-CPA) method [3] and the large system multiple scattering (LSMS) method [5] are used to calculate the density of states (DOS). The DOS are either compared directly to experiment or are combined with the transition 567 Mat. Res. Soc. Symp. Proc. Vol. 408 01996 Materials Research Society
matrix elements to calculate the EELS spectrum. According to Fermi'
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