Analysis of surface and bulk behavior in Ni–Pd alloys

Abstract

-The most salient features of the surface structure and bulk behavior of Ni-Pd alloys have been studied using the BFS method for alloys. Large-scale atomistic simulations were performed to investigate surface segregation profiles as a function of temperature, crystal face, and composition. Pd enrichment of the first layer was observed in (111) and (100) surfaces, and enrichment of the top two layers occurred for (1 10) surfaces. In all cases, the segregation profile shows alternate planes enriched and depleted in Pd. In addition, the phase structure of bulk Ni-Pd alloys as a function of temperature and composition was studied. A weak ordering tendency was observed at low temperatures, which helps explain the compositional oscillations in the segregation profiles. Finally, based on atom-by-atom static energy calculations, a comprehensive explanation for the observed surface and bulk features will be presented in terms of competing chemical and strain energy effects.

Segregation and interlayer relaxation at the NiPd(111) surface

“…5 [13][14][15][16][17][18][19] and is consistent with simple arguments based on surface energies. 5 [13][14][15][16][17][18][19] and is consistent with simple arguments based on surface energies.…”

“…5 [13][14][15][16][17][18][19] and is consistent with simple arguments based on surface energies. More sophisticated theoretical treatments by Noebe and co-workers, 17,18 Poyurovskii et al, 15 and Rousset et al 13 have, however, predicted high Pd concentrations in the NiPd͑111͒ surface, comparable to that of NiPd͑100͒, in good agreement with the present findings. More sophisticated theoretical treatments by Noebe and co-workers, 17,18 Poyurovskii et al, 15 and Rousset et al 13 have, however, predicted high Pd concentrations in the NiPd͑111͒ surface, comparable to that of NiPd͑100͒, in good agreement with the present findings.…”

“…Looking in more detail, however, it is interesting to note that the amount of segregation measured in this study is almost the same as the 80% Pd found in the NiPd͑100͒ surface, 10 in disagreement with simple arguments based on the number of broken bonds. Theoretical work done since then has resolved this puzzle, showing clearly that Ni and Pd have a weak attractive interaction and that Ni enrichment of the second layer lowers the energy of the system, results that are found consistently using many different theoretical frameworks [13][14][15][16][17][18][19] ͑though one study disagrees 28 ͒. The Ni enrichment in the second layer of NiPd͑111͒ observed here, resulting in an oscillatory segregation profile, is also qualitatively similar to the results previously found for NiPd͑100͒, 10 though the amount of Ni is less in the present case ͓the second layer of NiPd͑100͒ is 100% nickel͔.…”

“…17 Hence, a consistent picture regarding oscillatory near-surface segregation in NiPd alloys has emerged in both experiment and theory. 17 Hence, a consistent picture regarding oscillatory near-surface segregation in NiPd alloys has emerged in both experiment and theory.…”

Composition, structure and stability of surfaces formed by Ni deposition on Pd(111)

“…Derry et al [16] observed an oscillatory depth profile of a Pd 50 Ni 50 (100) surface using low energy electron diffraction (LEED), in which the Ni concentration in the first three layers was 20%, 100% and 36%, respectively. Theoretical simulations predict oscillatory depth profiles for all faces [19][20][21].…”