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–
–
–
dS
dt> 0
–
p(r)
r
–
p(r)
r
–
p(r)
r
p(r)
r
p(r)
r
–
–
–
–
Structure factor for a gas
–
A- Determine the static structure factor for a monoatomic gas
of punctual non-interacting particles. B- Determine the static structure factor for a gas where the
particles have finite size and consequently an effective repulsion distance. Determine the maximum possible
density of such a gas. C- Study the low-Q limit of the structure factor. Should the
structure factor be null at the origin?
Exercise
–
θ ω ω
θ
σ Ω ω
–
ω
ρ(�r, t) =
N∑i=1
δ (�r − �ri(t))
–
ρ ρ
ρ ρ r
t ∼ τ
t » τ
t « τρ
ρ
ρ
0
α βα ≠ β
d2σ
dΩdω(2θ, ω) = N
k′
k
{|〈b〉|2 Sd( �Q, ω) + 〈|b|2〉Ss( �Q, ω)
}
Coherent and incoherent xs
–
Starting with the total scattering cross section and considering
the self and distinct contributions, derive the expression showing
explicitly the coherent and incoherent contributions.
Exercise
d2σ
dΩdω(2θ, ω) = N
k′
k{〈b〉|2 S( �Q, ω) +
(〈|b|2〉 − |〈b〉|2
)Ss( �Q, ω)}
–
σ
4π σ
4π
σ = σ σ
σcoh = 4π |〈b〉|2
σinc = 4π(〈|b|2〉 − |〈b〉|2
)
Static structure factor
–
Exercises
Find the relationship between the van Hove time-dependent
correlation function G(r,t) and the static structure factor S(Q).
Propose an expression for the static correlation function
using the macroscopic density of the material and the real
space pair distribution function g(r).
Using this expression, find the relationship between the static
structure factor S(Q) and and g(r).
Static structure factor
–
S( �Q) =
∫ +∞
−∞dω S( �Q, ω) =
∫d�r ei
�Q·�rG(�r, 0)
Static approximation
S(QQ)-1 & g(r)-1 become a FT pair
F ( �Q) = S( �Q)− 1
G(�r) = 4πρr [g(�r)− 1]
Definition
S( �Q)− 1 = ρ
∫V
d�r [g(�r)− 1] ei�Q·�r
ρ [g(�r)− 1] =1
(2π)3
∫d �Q
[S( �Q)− 1
]e−i�Q·�r
F ( �Q) =
∫V
d�rG(�r)
4πrei
�Q·�r
G(�r)
4πr=
1
(2π)3
∫d �Q F ( �Q) e−i�Q·�r
–
0 2 4 6 8 10 120
1
2
3
S(Q)
Q/Å-1
0 5 10 15 20 25
0
1
2
3
g(r)
r/Å
Isotropic structure factor
–
Exercises
Consider now the isotropic case and find the relationship
between the isotropic structure factor S(Q) and
the isotropic pair distribution function g(r).
A- Consider a well-ordered system with a single (sharp) peak
in the real space correlation function and calculate the corresponding structure factor. B- Discuss the changes you could expect in the structure factor if
you had a distribution of distances around a single mean-value. C- Discuss now what you should observe if you had multiple
distances, each one with a distribution around the mean value.
–
0 5 10 15 20 25
0
1
2
3
g(r)
r/Å
2D analogy 0 2 4 6 8 10 12
0
1
2
3
S(Q)
Q/Å-1
–
0 2 4 6 8 10 120
1
2
3
S(Q)
Q/Å-1
0 5 10 15 20 25
0
1
2
3
g(r)
r/Å
Δ
→
S(∞) = 1
S(0) = ρ χT kBT
–
0Q
4π/λ
f(Q) = b
f(Q)
f(Q) = b
f(Q)
Q =4π
λsin
(2θ
2
)Bragg law
(d2σ
dΩdE′
)=
k′
k
1
2π�
∑jj′
fj( �Q)fj′( �Q)
∫ ∞
−∞〈e−i �Q·�rj′ (0)ei �Q·�rj(t)〉e−iωtdt
Form factor
–
The Fermi’s pseudo-potential works well for the neutron-nucleus
interaction yielding to a constant scattering length.
Explain qualitatively the effect that a non-punctual potential
has on the structure factor.
Exercise
–
π ρ
0
1
g(r)
r
-2
0
D(r)
r
πρ
0
RDF(r
)
r
π ρ
∝ ρ
π ρ π ρ
Correlation functions
–
Exercises
Consider a system with a FSDP at 2.1Å−1 with a FWHM of 0.62 Å−1.
What could you say about the characteristics of the
corresponding pair distribution function?
How many coordination shells could you observe in such a system?
Taking into account the repulsion region in the interaction
potential, show that the low-r region of density function is
proportional to the density.
–
αβ
α
α α
b̄2
⎛⎝ FS1(Q)
FS2(Q)FS3(Q)
⎞⎠ =
⎛⎝ c2Xb
2X1 c2Yb
2Y1 2cXcYbX1bY1
c2Xb2X2 c2Yb
2Y2 2cXcYbX2bY2
c2Xb2X3 c2Yb
2Y3 2cXcYbX3bY3
⎞⎠
⎛⎝ FXX(Q)
FYY(Q)FXY(Q)
⎞⎠
–
Weighting factors
–
Exercise Consider a binary system for which you have performed three
different total scattering experiments with three different isotopic
compositions. A- Write down the expression of the weighting factors matrix B- Evaluate this matrix for the case of the Ag2Se, if you used the
following isotopic compositions:
107Ag-natSe, 109Ag-76Se and natAg-76Se.
Assume that for each substitution, you have replaced 100% of
the atoms by its corresponding isotopes.
C- Invert the matrix to obtain the partial structure factors
from the three experimental structure factors.
–
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡=
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
)()()(
3211.01654.01559.02272.01654.00780.02706.00706.02594.0
)()()(
AgSe
SeSe
AgAg
S3nat76
S210976
S1107nat
QFQFQF
QFQFQF
Samples: 107Ag2natSe, 109Ag2
76Se, natAg276Se
Isotope b (fm) σσa (b) σ
s (b)
nAg 5.922 24.6 4.99107Ag 7.64 14.6 7.44109Ag 4.19 35.4 2.55
nSe 7.97 4.55 8.3176Se 12.2 33.1 18.7
Silver chalcogenidesAg2X
Network formersAsX or As2X3
+
X= S, Te or Se
107F76 + 1.0
NF76 + 0.5
107FN
Fast-ion conductor or semiconductor glasses
–
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−
−
−
=
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
)()()(
30.2900.2509.1063.2922.3211.850.2231.1717.12
)()()(
S3nat76
S210976
S1107nat
AgSe
SeSe
AgAg
QFQFQF
QFQFQF
gSeSe(r) + 3
gAgAg(r)
gAgSe(r) + 6
2.6 Å
4.6 Å
2.8 Å
SSeSe(Q) + 3
SAgAg(Q)
SAgSe(Q) - 3
nAgSe = 9.3nSeSe = 13.0nAgAg = 5.3
–
0 4 8 12 16 200
1
2
3
g ij(r)
r (Å)
AgSe
AgAg
SeSe
Ionic
b̄2 ΔFγ(Q)
c2γ(b2γ1 − b2γ2)
= Fγγ(Q) +
∑nα �=γ cαbα Fαγ(Q)
cγ (bγ1 + bγ2)
First difference method
–
Substitution ----- γγ: γ1, γ2
Important! We change scattering lengths but nnot composition
b̄2 F (Q) =
n∑α=1
n∑β=1
cαcβbαbβ Fαβ(Q)
b̄2 Fγ1(Q) = c2γb2γ1 Fγγ(Q) + cγbγ1
n∑α �=γ
cαbα Fαγ(Q) +
n∑α,β �=γ
cαcβbαbβ Fαβ(Q)
b̄2 Fγ2(Q) = c2γb2γ2 Fγγ(Q) + cγbγ2
n∑α �=γ
cαbα Fαγ(Q) +
n∑α,β �=γ
cαcβbαbβ Fαβ(Q)
b̄2 ΔFγ(Q) = c2γ(b2γ1 − b2γ2) Fγγ(Q) + cγ(bγ1 − bγ2)
n∑α �=γ
cαbα Fαγ(Q)
Correlation function of atom γγ with all other
components
small
–
Ω
⇒
…
–
Second difference method
–
Partial structure factor for pairs γ and δ
New substitution δδ: δ1, δ2
b̄2 Δ2Fγδ(Q) = cγcδ(bγ1 − bγ2) (bδ1 − bδ2) Fγδ(Q)
Fγδ(Q) =b̄2 Δ2Fγδ(Q)
cγcδ(bγ1 − bγ2) (bδ1 − bδ2)
b̄2 ΔFγδ1(Q) = c2γ(b2γ1 − b2γ2) Fγγ(Q) + cγcδ(bγ1 − bγ2) bδ1 Fγδ(Q) + cγ(bγ1 − bγ2)
n∑α �=γ,δ
cαbα Fαγ(Q)
b̄2 ΔFγδ2(Q) = c2γ(b2γ1 − b2γ2) Fγγ(Q) + cγcδ(bγ1 − bγ2) bδ2 Fγδ(Q) + cγ(bγ1 − bγ2)
n∑α �=γ,δ
cαbα Fαγ(Q)
–
Δ×
Δ
→
Δ
–
Δ
τ = (θ ϕ)/60 × 100%
–
–
Ispin inc(Q) =3
2ISF(Q)
Icoh(Q) + Iisotope inc(Q) = INSF(Q)− 1
2ISF(Q)
–
–
→
→
→
→→
–
Q =4π
λsin
(2θ
2
)Face d-spacing (Å)
λ (Å) Flux (107 n cm–2 s–1)
Filter
Si111 1.807 0.7 5.0 Ir Cu220 1.278 0.5 4.5 Rh Cu331 0.829 0.35 0.3 Non
–
–
θ Φ π σ σ ε
∞
–
Beam stop
Void sample position
Sample-like Cd specimen
– = θ ≈ θ θ θ
σ
–
Θ
ω ω
�2Q2
2m= 2E + �ω − 2
√E2 + �ωE cos 2θ
I(2θ) = C Φ0 Nσ
4π
∫ Emax
−∞
k′
kS( �Q, ω) ε(k′) dω
θ
ε∞
–
ω ω ⎯→ ω ε ω ω
0.0 0.2 0.4 0.6 0.8 1.0 1.20.0
0.2
0.4
0.6
0.8
1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.00.0
0.2
0.4
0.6
0.8
1.0
λ (Å)
ε0
c1
c2
c3
3/8
C1 = 1− α/2
eα − 1
C2 =3
8− α (α+ 3)
8 (eα − 1)
C3 =α(α+ 1)
4 (eα − 1)
S(Q) =1
ε0
1
b2coh
(dσ
dΩ
)corr
+
(1 +
b2incb2coh
)(C1δ − C2δ
2 + C3δγ − m
2M(δ + γ)
)−− b2incb2coh
ε ε ∝ ε α
–
ω
IcorrS (2θ) =1
αS,SC(2θ)
(IS(2θ)− IBS (2θ)−
αC,SC(2θ)
αC,C(2θ)
(IC(2θ)− IBC (2θ)
))−Δ
–
θ
0 5 10 15 20 25
0
1
2
3
g(r)
r/Å
–
General D.I. Page, The structure of liquids by neutron diffraction, p. 173, Chap. 8 in Chemical applications of thermal neutron scattering, B.T.M. Willis (Ed.), Oxford University Press (1973). J.G. Powles, The structure of molecular liquids by neutron scattering, Advances in Physics 22, 1 (1973). A.C. Wright, The structure of amorphous solids by x-ray and neutron diffraction, Advances in Structure Research by Diffraction Methods 5, 1 (1974). P.A. Egelstaff, Classical fluids, p. 405, Chap.14 in Methods of experimental physics, vol. 23, part B, Academic Press (1987). P. Chieux, Introduction to accurate structure factor measurements of disordered materials by neutron scattering, J. Molec. Struct. 296, 177 (1993). G. J. Cuello, Structure factor determination of amorphous materials by neutron diffraction, J. Phys.: Condensed Matter 20, 244109 (2008) H. E. Fischer, A. C. Barnes, P. S. Salmon, Neutron and x-ray diffraction studies of liquids and glasses, Reports on Progress in Physics 69, 233-299 (2006) Examples
See the D4 web site (www.ill.fr/YellowBook/D4/pub/) for a list of publications arisen from D4 expts. Visit the Disordered Materials Group at ISIS (www.isis.stfc.ac.uk/groups/disordered-materials/)
–
Examples with useful information about data treatment D.M. North, J.E. Enderby and P.A. Egelstaff, The structure factor for liquid metals: I. The application of neutron diffraction techniques, J. Phys. C 1, 784 (1968). H. Bertagnolli, P. Chieux and M.D. Zeidler, A neutron-diffraction study of liquid acetonitrile: I. CD3C14N, Molec. Phys. 32, 759 (1976). J.L. Yarnell, M.J. Katz R.G. Wenzel and S.H. Koenig, Structure factor and radial distribution function for liquid Ar at 85 K, Phys. Rev. A 7, 2130 (1973). Corrections G. Placzek, Phys. Rev 86, 377 (1952).
A.K. Soper and P.A. Egelstaff, Multiple scattering and attenuation of neutrons in concentric cylinders: I. Isotropic first scattering, Nucl. Instr. Meth. 178, 415 (1980). I.A. Blech and B.L. Averbach, Multiple scattering of neutrons in vanadium and copper, Phys. Rev. 137, A1113 (1965). H.H. Paalman and C.J. Pings, Numerical evaluation of x-ray absorption factors for cylindrical samples and annular sample cells, J. Appl. Phys. 33, 2635 (1962).