Irreversible Structural Transformation of five fold i-AlPdMn Quasicrystals after Ion Bombardment and Annealing
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Irreversible Structural Transformation of five fold i-AlPdMn Quasicrystals after Ion Bombardment and Annealing F. Schmithüsen1, G. Cappello1,4, S. Decossas1, G. Torricelli1, T.-L. Lee1, M. de Boissieu5, Y. Calvayrac6, T. Lograsso7, F. Comin1 and J. Chevrier1,2,3 1 2 3 4 5 6 7
ESRF, BP220, Grenoble cedex, France LEPES-CNRS, BP 166, Grenoble cedex 9, France Université Joseph Fourier, Grenoble, France Institut Curie - Section de Recherche, Paris Cedex 05, France LTPCM INPG BP 75 Saint Martin d'Hères France C.E.C.M._C.N.R.S., 15, rue G. Urbain, Vitry CEDEX, France Ames Laboratory/Iowa State University, Ames, IA 50011, USA
ABSTRACT Five fold i-AlPdMn surface prepared under UHV by ion bombardment and annealing was so far considered to be bulk terminated. This result was substantially based on a quantitative LEED analyses [1]. Analysis of the specular rod in a X ray diffraction experiment at grazing incidence supported this result [2]. We present a new study of this surface by high resolution X ray diffraction at normal incidence. In this Bragg configuration the diffraction peak 18 – 29 for instance is at a photon energy of 2.873keV, the 72 – 116 reflection at 5.725keV. This results in an analyzed thickness of the sample surface of a few micrometers. The surface was cleaned by ion bombardment. During annealing (T≅880K), we clearly observed the progressive disappearance of the initial Bragg peak characteristic of the as cast bulk sample. Conversely a new Bragg peak grows at an energy position shifted by 1eV compared to the position of the original Bragg peak. This is a clear signature for an irreversible structural transformation which takes place on at least the micron thickness. On the transformed surface, both, a LEED pattern and a RHEED pattern, characteristic for a five fold surface were easily obtained. This high resolution experiment (the relative Bragg peak shift is 3.10-4) was reproduced on samples from different initial compositions. This shows that five fold i-AlPdMn surface changes after preparation by ion bombardment and annealing at 900K on a micrometer thickness. This is not consistent with the conclusion that the surface is simply terminated by a cut of the original bulk. We conclude that a reorganization process of the quasicrystalline structure during annealing proceeds in the surface vicinity (probed depth is close to a few microns). INTRODUCTION Thanks to the possibility to produce icosahedral AlPdMn quasicrystal single grains at the cm scale with an almost perfect long range order [3-7] and short range order [8] this compound serves today also in surface science as a model system. Because of its chemical reactivity however, AlPdMn surfaces cannot be studied adequately in air where an aluminum oxide layer covers the whole sample surface [9] but requires a surface preparation under UHV (Ultra High Vacuum). Two different possible requirements on a clean surface intrinsic to the quasicrystal can be desirable: firstly a thermodynamic equilibrium between surface and the volume and secondly K11.2.1
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