The atomic structure of the fivefold symmetric quasicrystal surface of icosahedral AlPdMn has been investigated by means of a dynamical low-energy-electron diffraction (LEED) analysis. Approximations were developed to make the structure of an aperiodic, quasicrystalline surface region accessible to LEED theory. A mix of several closely similar, relaxed, bulklike lattice terminations is favored, all of which have a dense Al-rich layer on top followed by a layer with a composition of about 50% Al and 50% Pd. The interlayer spacing between these two topmost layers is contracted from the bulk value by 0.1 Å, to a final value of 0.38 Å, and the lateral density of the two topmost layers taken together is similar to that of an Al(111) surface. The LEED structural result is qualitatively consistent with data from ion scattering spectroscopy, which supports an Alrich termination. Keywords Ames Laboratory, Physics and Astronomy Disciplines Biological and Chemical Physics | Physical Chemistry CommentsThis article is from Physical Review B 57, no. 13 (1998) The atomic structure of the fivefold symmetric quasicrystal surface of icosahedral AlPdMn has been investigated by means of a dynamical low-energy-electron diffraction ͑LEED͒ analysis. Approximations were developed to make the structure of an aperiodic, quasicrystalline surface region accessible to LEED theory. A mix of several closely similar, relaxed, bulklike lattice terminations is favored, all of which have a dense Al-rich layer on top followed by a layer with a composition of about 50% Al and 50% Pd. The interlayer spacing between these two topmost layers is contracted from the bulk value by 0.1 Å, to a final value of 0.38 Å, and the lateral density of the two topmost layers taken together is similar to that of an Al͑111͒ surface. The LEED structural result is qualitatively consistent with data from ion scattering spectroscopy, which supports an Al-rich termination. ͓S0163-1829͑98͒03713-8͔

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The structure of the ͑3 3 1͒ reconstructions of the Si(111) and Ge(111) surfaces induced by adsorption of alkali metals has been determined on the basis of surface x-ray diffraction and low-energy electron diffraction measurements and density functional theory. The ͑3 3 1͒ surface results primarily from the substrate reconstruction and shows a new bonding configuration consisting of consecutive fivefold and sixfold Si (Ge) rings in ͗110͘ projection separated by channels containing the alkali metal atoms. [S0031-9007(98)05973-0] PACS numbers: 61.10.Eq, 61.14. Hg, 68.35.Bs, 71.15.Mb Over the last decade there has been a large effort to understand the structure and properties of reconstructions on elemental semiconductor surfaces. The main driving force behind these reconstructions is the reduction of the number of dangling bonds without introducing too much strain in the surface region. Three structural elements meeting this principle have emerged so far. Particularly important are adatoms which saturate three dangling bonds on (111) surfaces while creating only one unsaturated bond. Adatoms stabilize the clean Ge(111)-c͑2 3 8͒ surface [1] and are also a major stabilizing factor in the dimer-adatom-stacking-fault model of the clean Si͑111͒-͑7 3 7͒ surface [2]. The second structural element that effectively reduces the number of dangling bonds is the dimer frequently found on the (001) surfaces [3]. The third structural element is the p-bonded chain which was first proposed for the clean Si͑111͒-͑2 3 1͒ reconstruction [4]. Yet this simple principle has been of no utility in predicting surface structures as demonstrated by the metal induced ͑3 3 1͒ reconstructions on the (111) surfaces of Si and Ge. Despite the small unit cell, the atomic geometry is still unknown and has been heavily debated over the last ten years [5][6][7][8][9][10][11][12][13]. The mere observation of the symmetry-breaking ͑3 3 1͒ unit cell calls for a unidirectional structural motif and it was, therefore, appealing to introduce p-bonded chains to explain the 3 3 1 periodicity. At present, there are two promising models for the ͑3 3 1͒ reconstruction that have been proposed on the basis of scanning tunneling microscopy (STM) [8], electronic properties [9], and total-energy calculations [10]. The Seiwatz model [see Fig. 1(a)] [7,11,12] consists of parallel p-bonded chains formed by fivefold rings of Si (Ge) atoms in ͗110͘ projection, separated by empty channels, with a top-site adsorbate saturating the surface dangling bonds. The second model is the extended Pandey model [10,13] [see Fig. 1(b)] which consists of a sevenfold ring carrying the p-bonded chain alternating with a five and six-member ring of Si. It is intuitive to describe the ring sequences from these models with the notation 567567 (extended Pandey) and 500500 (Seiwatz model). Unfortunately, neither of these structures is able to explain our surface x-ray diffraction data (SXRD) or low-energy electron diffraction (LEED) data.We determined the structure by using a multipletechnique ...