empirical potential structure refinement
TRANSCRIPT
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Disordered Materials:
Lecture II
Finding and refining a structural model
Alan Soper
Disordered Materials Group
ISIS
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Summary of Lecture I
• Discussion of disorder in our world.• Concept of correlation in disordered systems.• Use of radial distribution function to characterise
the correlations in a disordered system.• Use of diffraction to count atoms as a function of
distance.• How to characterise structure in molecular
systems:–
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Summary of lecture II
• Extracting the structure factor from the diffraction experiment.
• Computer simulation as a tool to model disordered materials
• Molecular systems:– SDF, bond angle distributions, OPCF
• Use of computer simulation to go from measurements (D(Q), g(r)) to SDF, bond angle distribution, OPCF, etc.
• Some case studies: molten alumina, water, amorphous phosphorus, silica, silicon...
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ISIS SANDALS (liquids
diffractometer)
Incident neutron beam
Sample position
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The liquid structure factor:
Fd Q = ∑,α β≥α
2−δ αβ cα cβ bα bβ{4 πρ ∫ r 2 gαβ r −1 sin Qr
Qrdr }
The partial structure factors, Hαβ Q
The site-site radial distribution functions, gα β r
The atom scattering factor or “form factor”
Atomic fraction ofcomponent “”
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A much more tricky question:how do we interpret the data?
• For many years the next step was to simply invert our scattering equation:
d r =1
22 ρ
∫0
∞
Q2 D Q sin Qr
QrdQ
= ∑,α β≥α
2−δ α β cα cβ bα bβ gα β r −1
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This leads to many problems
• Truncation errors.• Systematic errors.• Finite measuring statistics.• Some site-site terms are more strongly weighted
than others.• These all make interpretation of the data
unreliable.• Radial distribution functions (g(r)) do not yield
the Orientational Pair Correlation Function (OPCF).
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Introduce: computer simulation
• Requires an atom-atom potential energy function.
• Place computer atoms in a (parallelpiped) box at same density as experiment.
• Apply periodic boundary conditions– the box repeats itself indefinitely throughout
space.
• Apply minimum image convention.
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Minimum image convention
D
Count atoms out to D/2
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Monte Carlo computer simulation
1.Using the specifed atom-atom potential function, calculate energy of atomic ensemble.2.Displace one atom or molecule by a random amount in the interval ±.3.Calculate change in energy of ensemble, ΔU.4.Always accept move if ΔU < 05.If ΔU > 0, accept move with probabilityexp[- ΔU/kT].6.Go back to 2 and repeat sequence.
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But there is a problem:
We don’t know the potential energy function!
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Introduce Reverse Monte Carlo, RMC
1. Build a box of atoms as before. Calculate χ2=[D(Q)-F(Q)]2/2
2. Displace one atom or molecule by a random amount in the interval ±.
3. Calculate change in χ2 of ensemble, Δχ2.4. Always accept move if Δ χ2 < 05. If Δ χ2 > 0, accept move with probability
exp[- Δ χ2].6. Go back to 2 and repeat sequence.
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This approach has problems, particularly with molecules.
• Molecules are usually introduced via unphysical coordination constraints.
• Especially with molecules the ensemble of atoms can get “stuck” i.e. it does not sample phase space correctly.
• Various reasons why this can occur.
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Introduce Empirical Potential Structure Refinement, EPSR
• Use harmonic constraints to define molecules.• Use an existing “reference” potential for the
material in question taken from the literature (or generate your own if one does not exist).
• Use the diffraction data to perturb this reference potential, so that the simulated structure factor looks like the measured data.
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• M measured datasets, N partial structure factors: (Usually M < N )
• Assign a “feedback” factor f for the data:
• and (1 – f ) for the simulation:
• Form inversion of
Introducing the data
F Q = ∑,α β≥α
2−δ α β c α c β bα bβ Hα β Q
wij' =fwij , 1≤i≤M
wij' = 1− f δ i−M ,j , M<i≤M+N
wij' , 1≤i≤M+N, 1≤ j≤N
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F i =1,M+N Q =[fw11 fw12 fw1N
fw 21 fw22 fw2N
fw M1 fw M2 fw MN
1− f 0 . 0 0 . 0 0 . 00 .0 1− f 0 . 0
0 .0 0 . 0 1− f
1− f 0 .0 0 . 0 0 . 0 1− f 0 . 0
0 .0 0 . 0 0 .0 1− f
] × [S1
S2
S N
]
Refining the potential: M datasets, N partial structure factors
ΔU j r =Fourier Transform of { ∑i=1, M
w ' ij−1 Di Q −F i Q }, j=1, N
Dat
aS
imul
atio
n
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What do we measure if there are molecules present?
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However, two issues need to be addressed:-
• Issue 1: Often not possible to measure all partial structure factors.
• Issue 2: Even if we could, what do they mean?
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Structure refinement of liquid water
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Waterdata
Afterstructurerefinement
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Water partial g(r)’s
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Water empirical potentials
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zL
yL
xL
φL
θLr
Beyond g(r): the spatial density function
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Bond angle distributions
1
23
θ Or:-
1
2
θ
H
H
θOr:-
2
2
2
1
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zL
yL
xL
xM
z
yM
φL
θLr
φMθMχM
A step further: the orientational pair correlation function
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The spatial density function of water...
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Water structure
(Courtesy Phil Ball, H2O: A Biography of Water)
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zL
yL
xL
φL
θLr
Beyond g(r): the spatial density function
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Choose distance range (0-5.7Å)
and a contour level
(% of all molecules in distance range)
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Water under pressure
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Water at 298K, 0kbar
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Water at 268K, 0.26kbar
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Water at 268K, 2.09kbar
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Water at 268K, 4.00kbar
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Bond angle distributions
Or:-H
H
θ
2
1
1
23
θ
22
1
2
θ
Or:-
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O-O-O angle distribution
1
23
θ
22
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O-O-H angle distribution
Or:-H
H
θ
2
1
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O-μ angle distribution
1
2
θ
Or:-
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zL
yL
xL
xM
z
yM
φL
θLr
φMθMχM
A step further: the orientational pair correlation function
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Dipole orientations in water
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Another example:
Molten Al2O3
[Courtesy of
Neville Greaves
(Aberystwth)
and
Claude Landron
(Orleans)]
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Molten Al2O3
FinalfitafterEmpiricalPotentialStructureRefinement:
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Partial g(r)’s for Al2O3
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The problem of tetrahedrally coordinated glasses and liquids
(water, a-MX2, a-Si, a-Ge)
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Amorphous SiO2
“First sharp diffraction peak” - FSDP
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Radial distribution functions:
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Coordination numbers:
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Spatial density function for a-SiO2:
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Spatial density function for a-SiO2:
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Spatial density function for a-SiO2:
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Compare with amorphous Si
FSDP is absent!
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Amorphous SiO2
“First sharp diffraction peak” - FSDP
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Coordination number- a-Si:
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Coordination numbers – a-SiO2:
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Triangle or “bond angle” distributions, a-Si:
1
23
θ
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Triangle or “bond angle” distributions, a-SiO2:
1
23
θ
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Compare structure factors…(renormalise Q to near-neighbour distance)
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A puzzle: a-(red)P
FSDP is present!
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Summary (1)
• Disorder is intrinsic to our existence, and occurs over a very wide range of length scales.
• We quantify disorder at the atomic level via the pair correlation function. For molecules this is the orientational PCF, which contains more information than the radial distribution functions, g(r).
• Structure factors measured in diffraction experiments derive from the site-site radial distribution functions.
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Summary (2)
• Computer simulation is used to generate a model of the scattering system.
• Diffraction data are introduced either– via χ2 (RMC),
or – via an empirical potential, (EPSR).
• Simulated ensembles are used to calculate a number of distribution functions not accessible directly from the experiment.
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Summary (3)
• Tetrahedrally bonded glasses and liquids show structural similarities.
• Relevance/role of the “FSDP” is unclear.• We can only really study these properties by
forming structural models consistent with the data.
Thank you for your attention!