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SURFACE SEGREGATIONS IN RANDOM ALLOYS FROM FIRST-PRINCIPLES THEORY
Igor A. Abrikosov
Department of Physics and Measurements Technology,
Linköping University
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ACKNOWLEDGEMENTS
• A. Ponomareva• N. Skorodumova• A. V. Ruban • S. I. Simak • L. Pourovski• S. Shallcross
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CONTENTS :
• Introduction: first principles calculations at T=0 and at T>0
• Ordering effects in the bulk: multicomponent alloys
• Surface segregations in NiPt and NiPd.• Segregation energies in Fe-Cr system.• Cr-rich part of the diagram: importance of
correlation effects.
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Density Functional Theory(DFT)
∑=
++=
+∇−=
−=
occr
vvvreffv
reffvm
effH
relonerH
XCEXTH
ψψσρ
ρρρσ
σ
σψεσψ
*),...RR,R,,(
)()()(),...RR,R,,(
),...RR,R,,(2
),...RR,R,,(),...RR,R,,(
M21
M21
M212
2
M21M21
h
)(),...RR,R,,( M21 rjj
jCr ϕσψ ∑=
)()R,...,R,R,,,,(* M21 rrrrGH ′−=′ δσσ
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F F
C
A
BC
D
Structures:
A
B
CD
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}{ is σσ =r
⎩⎨⎧
=otherwise
atomby occupied is site if
1-
1 Aiiσ
1 1 -1 1 1
1 -1 1 1 -1
1 1 -1 1 1
-1 1 1 1 -1
,... ,s
kji
s
ji σσσσσ
∑ ⎟⎠
⎞⎜⎝
⎛−=−=s
sB kT
EZTkF exp Zln
...),3(),2(
)1()0(
++
++=
∑∑s
kjis
sji
s
tot
VV
VVE
σσσσσ
σ
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Calculations of effective interatomic potentials
The Connolly-Williams method
1. Choose structures fcc L12 L10 DO22
2. Calculate Etot: E(fcc) E(L12) E(L10) E(DO22)
with predefined correlation functions [ ],... , ,
)1,3()2,2()1,2(
kjijiji σσσσσσσ
...),2()1()0( ∑++=s
jis
tot VVVE σσσ
min
2
...}{ :L.S.Mby found are thenIf =∑⎥⎥
⎦
⎤
⎢⎢
⎣
⎡∑−≤
m f fVf
mEVNN strpot
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The Monte Carlo method
Calculations of averages at temperature T:Z
Tk
EA
A s B
ss∑ ⎟⎟
⎠
⎞⎜⎜⎝
⎛−
=exp
Create the Marcov chain of configurations: ⎟⎟⎠
⎞⎜⎜⎝
⎛−=
Tk
E
ZP
B
ss exp
1
Balance at the equilibrium state: ⎟⎟⎠
⎞⎜⎜⎝
⎛−→=⎟⎟
⎠
⎞⎜⎜⎝
⎛−→
Tk
EssW
Tk
EssW
B
s
B
s 'exp)'(exp)'(
EΔ )10( exp 0
0
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
≤≤>⎥⎦
⎤⎢⎣
⎡ Δ−>Δ
≤Δ
rrTkE
E
E
B
Atoms exchanged
ΔE
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Example: ordered phases in Cu2NiZn• 21 concentration and volume dependent effective
cluster interactions
• Electronic structure calculations using O(N) LSGF method
• 32 different atomic distributions at fixed concentration (144 atom supercell)
• VNi-Zn(nn)=12.8mRy > VCu-Zn(nn)=5.0 mRy >
VCu-Ni(nn)= 2.5 mRy
• VNi-Zn(4nn)= -2.5mRy, all other ECI are small
• Cluster expansion represents total energy calculations with average accuracy better than 0.015 mRy, and with the maximal error 0.2 mRy (or 4% of the ordering energy)
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Example: ordered phases in Cu2NiZn• 21 concentration and volume dependent effective
cluster interactions
• Electronic structure calculations using O(N) LSGF method
• 32 different atomic distributions at fixed concentration (144 atom supercell)
• VNi-Zn(nn)=12.8mRy > VCu-Zn(nn)=5.0 mRy >
VCu-Ni(nn)= 2.5 mRy
• VNi-Zn(4nn)= -2.5mRy, all other ECI are small
• Cluster expansion represents total energy calculations with average accuracy better than 0.015 mRy, and with the maximal error 0.2 mRy (or 4% of the ordering energy)
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Calculations of effective interatomic potentials
The generalized perturbation method
1. Calculate electronic structure of a random alloy (for example, use the CPA):
)( ,~ BAtg
...2
1)(
)'(
''
RR
jiRR
RRoneone VcEE σσ∑+= -determine a perturbationof the band energy due to small varioations of the correlation functions
2.
where the effective interatomic interactionsare given by an analytical formula:
{ }∫−= ARRR
BRRR
ARRR tttdEV ''''
1γγ
π
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Example: bulk ordering in NiPt
Method Δrnd(UR)
(mRy/atom)
Δrnd(R)
(mRy/atom)
Δ´L10(UR)
(mRy/atom)
CPA-GPM
1.57 -1.03 -6.06
FP-CWM
Lu et al.1.72 -2.23 -5.71
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The new surface Monte Carlo methodIn order to represent the bulk chemical potential, the obtained by bulk MCfixed reservoir of atoms is used:
Bulk reservoirSurface sample
Vacuum
)()( ABEBAEE surfbulk →Δ+→Δ=Δ
)10( exp 0
0
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
≤≤>⎥⎦
⎤⎢⎣
⎡ Δ−>Δ
≤Δ
rrTkE
E
E
B
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The new surface Monte Carlo method
Only one fixed bulk configuration of the reservoir is used.
How do the results depend on the size of the reservoir?
The dependence of the surface layer energy on size of reservoir in NiPd(100)
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Configuration of the (111) surface of the Ni49Pt51 substoichometric ordered alloy
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Surface segregations in the NiPt and NiPd alloys
1. A segregation reversal phenomenon has been observed at the surfaces of
NiPt random alloys: Pt segregates towards the (100) and (111) surfaces, Ni
segregates towards the (110) surface.
2. No such effect has been found for the isoelectronic NiPd alloys. The strong
Pd segregations have been observed on all low-indexed surfaces.
There are bulk ordered phases NiPt(L10) and Ni3Pt(L12)
in the Ni-Pt system
No bulk ordering occur in NiPd down to T=400K
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SGPM surface potentials for Ni50Pt50(Ni50Pd50) in K (σPt(Pd)=1)
Layer 1 2 3 4 5
V(1)V(1)
bulk167(-1613) -883(-648) -265(-126) 163(35) 0(0)
(110)V(2,1)
483(224) 555(262) 571(279) 586(280) 556(279)
V(2,1)
2433(1017) 2286(1097) 2379(1132) 2225(1116) 2225(1116)
V(2,1)
781(261) 686(296) 556(279) 556(279) 556(279)
V(1)V(1)
bulk-705(-1118) 231(2) -219(-60) 0 -
(111) V(2,1)
1373(820) 1852(854) 1979(871) 1668(837) -
V(2,1)
1927(921) 1857(861) 1668(861) 1668(837) -
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Example: bulk ordering in NiPtTransition to L10 in Ni50Pt50: Tc
exp=917 K, Tcteor=925 K
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Segregation profiles in the Ni50Pt50 and Ni50Pd50 random alloys
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Ni-Pt (111)
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Ni-Pt (111)
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Configuration of the (111) surface of the NiPt stoichometric and substoichometric ordered alloys
Ni50Pt50 ordered alloyNi49Pt51 ordered alloy
(111) surface
[ ]kind" wrong" of neighboursnearest theofnumber ≈antiE
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}{ is σσ =r
⎩⎨⎧
=otherwise
atomby occupied is site if
1-
1 Aiiσ
1 1 -1 1 1
1 -1 1 1 -1
1 1 -1 1 1
-1 1 1 1 -1
,... ,s
kji
s
ji σσσσσ
∑ ⎟⎠
⎞⎜⎝
⎛−=−=s
sB kT
EZTkF exp Zln
...),3(),2(
)1()0(
++
++=
∑∑s
kjis
sji
s
tot
VV
VVE
σσσσσ
σ
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EMTO vs Full-potential: c/a ratio in ordered alloys
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CONCLUSIONS :
• There are problems.
• We are here to solve them!