Bernard Legrand1 and Guy Trglia2

1
Atomistic modelling of bimetallic surfacesfrom
many-body potentials to lattice models

• Bernard Legrand(1) and Guy Tréglia(2)
• (1) Service de Recherche en Métallurgie Physique
  (SRMP-CEA), Saclay, France
• (2) Centre de Recherche en Matière Condensée et
  Nanosciences (CRMCN), Marseille, France

based on works by Isabelle Meunier, Hazar
Guesmi, Laurent Lapéna, Pierre Müller CRMCN,
Marseille Isabelle Braems, Fabienne Berthier,
Robert Tétot, Jérôme Creuze LEMHE
Orsay François Ducastelle ONERA, Châtillon
2
Atomistic approach (tight-binding)
SMA
TBIM
Consistency TBIM-SMA
Atomic rearrangements
Chemical rearrangements
3
How to study chemistry on a rigid lattice
including atomic relaxation !
Segregation energy at the surface of alloy AcB1-c
?Hseg ?Hsite ?Hsize ?Halloy
?Hinter
interaction
impurity
SMA
SMA
SMA
Quenched Molecular Dynamics relaxation
VR
B(B)
B(A)
B(B)
4
Choice of a reference system Cu(Ag)

• Silver concentrates on the surface
• higher surface energy of Cu Dg.14eV/at
• larger atomic radius of Ag rAg  1.13rCu
• miscibility gap in the bulk Vblt0

Expected surface behaviour
Cu(Ag) (100)
c(10x2)
(1x1)
chemical transition structural transitions
also (111), (110)
5
Influence of relaxation on TBIM parameters

• size effect

• failure of elasticity (asymetry)
• ? Anisotropy (relaxation)

• alloying effect VR

Relaxation (bulk) ? phase separation tendency ?
? long range interactions
Surface (001) ? ordering tendency ? ?
anisotropy
6
Relaxation ? surface ordering Cu1-cAgc

• surface effective
• pair interactions

Bond breaking ? phase separation tendency ?
(ordering ?) ? ordering reversal depending of
orientation and concentration
Cu(Ag) (100) surface ordering whatever Ag
surface concentration ?
7
Structuraldependence of VR ?
Superstructure ? surface ordering
Cu1-cAgc(100), c?0

• STM Ag/Cu(100)
• Sprunger et al. PRB 1996
• q lt 0.1 mixed Cu-Ag phase
• with (1x1) structure
• q ? 1.0 pure Ag island
• with c(10x2) structure

T 300 K
Nad1/10Nlayer
1x1
c(10x2)
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Superstructure ? surface ordering
Cu1-cAgc(100), c?0
c(10x2) structure
(1x1) structure
V1100(1x1) gt 0
V1100(10x2) lt 0
O.K. with STM experiment Ag/Cu(100) Sprunger et
al. 1996 q lt 0.1 mixed Cu-Ag phase with (1x1)
structure q ? 1.0 pure Ag islands with c(10x2)
structure
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Lattice analysis validity Cu1-cAgc(100), c?0
DHseg
mixed Monte Carlo exchange displacements
Beginning of expulsion
V gt 0
V lt 0, Tc300 K
V1x1
Vc10x2
c0
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Experimental validation Sb/Si(111)
Mass Spectrometry Thermodesorption
Bragg-Williams P P0. ?/(1-?). exp
(DHads/kT) DHads VSb-Si ?? VSb-Sb
Langmuir-like behaviour
but not exactly
Two regimes q lt 0.7 V 0 q gt 0.7 V lt
0, Tc 870 K
Adsorption isotherms
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Adsorption Mode of Sb/Si(111) FP-LAPW study

• nature of interactions dépend on site
• Ternary positive slope ? Sb-Sb repulsive
• On top negative slope ? Sb-Sb attractive
• Adsorption site transition (q?0.7)
• ? change in the
  sign of V

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Conclusion
when ab initio meets classical simulations ?
Decomposition into main driving forces ? Tight
Binding Ising Model ? Ab Initio-Ising Model to
be used for long scale (time, space) modeling of
phenomena at complex material interfaces