Surface Characterization of Clean Gd 5 Ge 4 (010)
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Surface Characterization of Clean Gd5Ge4(010) Chad D. Yuen,1,2 Gordon J. Miller,1,2 and Patricia A. Thiel1,2,3 1
Ames Laboratory, Iowa State University, Ames, Iowa, 50011 Department of Chemistry, Iowa State University, Ames, Iowa 50011 3 Department of Material Science and Engineering, Iowa State University, Ames, Iowa 50011 2
Abstract Based on X-ray photoelectron spectroscopy, Gd5Ge4(010) does not show evidence of surface segregation. Scanning tunneling microscopy reveals two types of terraces which alternate laterally on the surface. From the step heights, these two surface terminations are assigned as dense, Gd-pure layers in the bulk structure. There is evidence of reconstruction on one type of terrace. Introduction Intermetallics have properties that are of real and potential use. Sometimes, those properties are surface or interface properties, such as catalytic activity, low adhesion, or oxidation resistance [1; 2]. This motivates investigations of their basic surface structure and composition. The most extensively-investigated intermetallic surfaces, by far, are those of metallic quasicrystals and related periodic phases. From the studies of clean surfaces of these materials in ultrahigh vacuum, some interesting generalizations have emerged [3]. First, quasicrystals and related periodic phases are surprisingly stable against surface segregation and surface reconstruction, i.e. they tend to be bulk-terminated. Exceptions exist, but the overall picture is that surfaces of these intermetallics are less prone to segregation and reconstruction than those of typical metallic alloys. Second, because of their chemical and structural complexity in the bulk, the surfaces are intrinsically heterogeneous. As a result, they present a variety of adsorption sites. If the surface is bulkterminated, these sites can be predicted and exploited, e.g. in heterogeneous catalysis [4]. Finally, surfaces of quasicrystals and related intermetallics can contain metastable terraces or regions of terraces. These metastable regions develop and then evolve or disappear during the high-temperature treatment by which well-ordered surfaces are typically prepared. This again is thought to be a consequence of chemical and structural complexity, wherein different bulk terminations may be separated by relatively small energetic differences. It is not known whether these generalizations may apply to surfaces of intermetallics other than quasicrystals and related phases. In this paper, we report a study of the surface of an intermetallic, Gd5Ge4, which allows some interesting comparisons. Gd5Ge4 is a prototype for a class of materials that have promising magnetoresponsive properties. More specifically, the rare earth metal Gd, and its alloys or compounds with certain post-transition elements such as Ge and Si, are considered promising materials for magnetic refrigeration below or near room temperature [5; 6].
Figure 1. Bulk structure of Gd5Ge4 [7-9].
The bulk structure of Gd5Ge4 is represented in Figure 1. The crystal symmetry is orthorhombic w
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