The structure of the clean Pt(l 11) surface has been studied between 300 K and 0.92T m (T m =2045 K) via x-ray scattering. The surface is unreconstructed for temperatures less than 0.65T nu but reconstructs at higher temperatures to form a layer isotropically compressed and incommensurate with the underlying bulk (111) planes. A disordered arrangement of discommensurations separates regions with ideal fee stacking from regions with faulted stacking. With increasing temperature both the compression of the surface layer and the orientational order of the discommensurations increase.
The valence-band structure of nickel aluminum was measured by use of angle-resolved photoemission with synchrotron radiation and calculated using the local-density approximation. The overall agreement between theory and experiment is remarkably goodmuch better than for pure nickel. This means that the "self-energy" corrections are significantly less in NiA1 than in pure nickel. The core-level binding energies in NiAl are compared experimentally and theoretically with the equivalent levels in Ni and Al. Surprisingly, the Ni core shifts to higher binding energy and the Al to lower as if charge were transferred from Ni to Alopposite to the direction predicted from electronegativity. These observations are discussed in terms of bonding in NiA1. 42 1582 1990 The American Physical Society 42 ELECTRONIC STRUCTURE OF NiAl 1583 from Ni to Al), leading to a net transfer from nickel to aluminum.This is consistent with the observed corelevel shifts, which are in the direction of larger binding energy for the nickel and smaller binding energy for the aluminum. Shifts to the lower binding energy are usually attributed to an increase in electron number at a site which increases the Coulomb repulsion with the cores and reduces the binding energy.
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