Experimental Studies of Bonding Related Properties in Binary Intermetallics by Convergent Beam Electron Diffraction
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Experimental Studies of Bonding Related Properties in Binary Intermetallics by Convergent Beam Electron Diffraction X. H. Sang, A. Kulovits, J. Wiezorek Department of Materials Science and Mechanical Engineering, Swanson School of Engineering, University of Pittsburgh, 648 Benedum Hall, 3700 O Hara Street, Pittsburgh PA 15261. ABSTRACT Accurate Debye-Waller (DW) factors of chemically ordered ȕ-NiAl (B2, cP2, Pm 3m ) have been measured at different temperatures using an off-zone axis multi-beam convergent beam electron diffraction (CBED) method. We determined a cross over temperature below which the DW factor of Ni becomes smaller than that of Al of ~90K. Additionally, we measured for the first time DW factors and structure factors of chemically ordered Ȗ1-FePd (L10, tP2, P4/mmm ) at 120K. We were able to simultaneously determine all four anisotropic DW factors and several low order structure factors using different special off-zone axis multi-beam convergent beam electron diffraction patterns with high precision and accuracy. An electron charge density deformation map was constructed from measured X-ray diffraction structure factors for Ȗ1-FePd. INTRODUCTION The intrinsic properties of ordered intermetallics are related to their electronic structure, which differs significantly from that of the chemically disordered solid solutions of equivalent composition. Successful support of application driven research on development of intermetallicsbased alloys demands detailed knowledge of fundamental aspects of the electronic structure, i.e., interatomic bonding, based on which intrinsic materials properties can be understood. Densityfunctional theory based materials computation is an attractive approach to develop understanding of intrinsic properties. Predictions from computations require validation by suitable experimental data, which often is difficult to obtain. We used convergent electron beam diffraction (CBED) to provide experimental data, e.g. Debye-Waller (DW) and structure factors, electron and charge density distributions, suitable for validation of computational findings for binary intermetallics. CBED is very attractive as it uses nano-scale crystal volumes and can be applied for a broad range of phases. Traditional CBED methods are insufficient for accurate and precise DW factors measurement, which is essential to obtain accurate and precise structure factors for electronic structure determination [1]. Although DW factors are known for most stable elemental crystals [2], interatomic bonding alters the DW factors of the constituent atom species in binary intermetallics and they can differ greatly from elemental crystal values [3]. Here we apply a new robust experimental CBED method for the simultaneous determination of multiple structure factors and DW factors of the binary intermetallic phases ȕ-NiAl (B2, cP2, Pm 3m ) and Ȗ1-FePd (L10, tP2, P4/mmm ). A conventional tP4 FePd unit cell is used in this paper for convenience. This new off-zone axis multi-beam QCBED (Quantitative Convergent Beam Electron Diffracti
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