Molecular dynamics study of the ordered Cu(3)Au - I. Vibrational and structural properties of the low indexed surfaces
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Lekka, C. E.
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Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
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Lekka, C. E.
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Using an effective potential model in analogy to the tight-binding scheme iu the second-moment approximation, we investigated the structural and vibrational properties of the low indexed Cu(3)Au faces in the ordered state. We found that for the three orientations studied, the Au rich faces are the energetically favored surfaces, exhibiting rippling with Au lying above Cu atoms. In the (0 0 1) surface the rippling effect is more pronounced and it is restricted in the surface layer, while in the (1 1 0) and (1 1 1) surfaces it occurs with smaller magnitude in the first seven layers, with the Au and Cu atoms occupying alternatively the higher position. In addition, these rippling effects persist up to the order-disorder transition temperature. Moreover, we found that the (0 0 1) and (1 1 1) surfaces exhibit small contraction that remains almost the same for all temperatures studied, while the (1 1 0) face differentiates its behavior above 500 K, manifesting an unexpected dilation on both Au and Cu interlayer relaxed positions. In addition, around this temperature, Au surface atoms penetrate in the second pure Cu layer exchanging their positions with Cu atoms. The phenomenon is accompanied by the spontaneous creation of adatoms. Furthermore, from the temperature dependence of the mean-square-displacements of Cu and Au surface atoms we found that the vibrational amplitudes in the normal to the surface direction are larger for the (0 0 1) surface followed by the (1 1 0) and (1 1 1) face. From the calculated phonon density of states and phonon spectral densities at room temperature, we found that the presence of the surface is manifested by new phonon modes, while some of the existing bulk modes are shifted towards lower frequencies. It came out that the Cu and Au surface atoms are, for all orientations studied, responsible for the high and low-energy modes respectively. (C) 2001 Elsevier Science B.V. All rights reserved.
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