Chemistry and localization effects in the C-KVV Auger line shapes of transition metal carbides and related compounds
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The C-KLL Auger line shapes of the transition metal carbides and some related compounds are examined as a function of the metal and nonmetalloid components (C, N, and O). They are discussed in the context of hole-hole repulsion and screening effects. The effect of ir versus cr-bonding and the proportion of ionic versus covalent bonding are correlated with various aspects of the X-CVV Auger line shape, such as relative peak shifts, intensity of a high energy shoulder, and the multiplet splitting of the sp component.
I. INTRODUCTION Much theoretical effort has been directed toward understanding the C-KVV Auger line shapes of carbon in its various chemical forms, including graphite,1 diamond,2 silicon carbide,3 solid and gas phase hydrocarbons,4 and several transition metal carbides.5"8 Some of this work emphasized the impact on the Auger line shape of the hole-hole repulsion in the doubly ionized Auger final state, and the role of electron screening by various chemical bonding types, for example covalent versus ionic, 77 versus a, or bulk versus surface bonding. Hole-hole repulsion significantly affects the Auger line shape by distorting the self-convoluted density-of-states (DOS), i.e., the one-electron model for the line shape. Houston et al.1 and others9"11 have shown that different types of carbon bonds, even within the same material, exhibit large variations in the effective hole-hole repulsion. For example, the TT-electrons in the ab plane of graphite are relatively delocalized so that little repulsion-based distortion occurs in the convoluted DOS originating with the 77 electron density. However, significant correlation effects occur in the convoluted DOS from more localized cr-orbital electrons. Likewise, for diamond the surface and bulk contributions to the spectrum exhibit varying amounts of distortion because of diminished screening by other electrons near the surface. Transition metal carbides have many useful applications, e.g., in superconductivity and tribology,12'13 and thus they comprise a good system for study. On a more basic level, the metal carbides are a significant component of the carbon-based materials, which taken together provide a rich data set for Auger line shape analysis. The monocarbides are a series of compounds in which only the transition metal species vary and are therefore well suited to studies examining the effects of bonding and electron density on the Auger spectrum. Metallic bonding increases across a row of the transition metals as additional valence electrons fill the d orbitals. 2716
In addition, the number of electrons on the anion changes with the transition metal or the nonmetalloid component across a series of the transition metal carbides, and therefore alters the bond ionicities and the orbital radii. Ramqvist15 attributed the progressive shift in the C Is XPS binding energies of the transition metal carbides to increasing C-Me bond ionicity, as defined by the metal-nonmetal electronegativity difference. He correlated the respective energy shifts with the heats of
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