11 December, 2006 Neutron Scattering in Earth Sciences 1
Structure of glasses and meltsStructure of glasses and melts
Martin Wilding Martin Wilding
Institute of Mathematical and Physical Sciences, University of Institute of Mathematical and Physical Sciences, University of Wales, Aberystwyth, Ceredigion, SY23 3BZWales, Aberystwyth, Ceredigion, SY23 3BZ
Chris Benmore,Chris Benmore,
Intense Pulsed Neutron Source and the Advanced Photon Source,Intense Pulsed Neutron Source and the Advanced Photon Source,
9700 Cass Avenue,9700 Cass Avenue,
Argonne, IL 60439Argonne, IL 60439
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Outline Outline
The Liquid State
Neutron Scattering theory
Instrumentation and sample environment
Interpretation of glass structure
Studies of liquids and amorphous materials
Summary and future directions
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The Liquid StateThe Liquid State
Liquids lack long-range order
• Pair distribution function shows Short range order
Glasses formed by super-cooling a liquid.
• Show a glass transition
Amorphous materials by other routes
• Inherent polyamorphism
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The pair distribution The pair distribution
functionfunction
Average separation of atom pairs
Region where g(r) is zero
Pronounced first peak
Series of smaller peaks
g(r)=1 mean density of the system
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Glasses and amorphous Glasses and amorphous
materialsmaterials
Super-cooling
• Ergodicty
• Non-ergodicity
Configurational entropy and fragility
Pressure-induced amorphisation
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Neutron diffraction: Neutron diffraction:
Scattering theoryScattering theory
Double differential cross
section
Coherent scattering law
Static approximation
1
1
2
0 dEddEd
dII Ω
Ω=
σ
cohcohscattered
incident
dEd
d
k
k
NQS
1
241),(
Ω=
σ
σ
πω
)2cos1(2 22 θ−≈ incidentkQ
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Neutron diffraction: Scattering in Neutron diffraction: Scattering in
multimulti--component systemscomponent systems
Scattering:
• Sum of several atom-pairs
• Partial contributions
Faber-Ziman formalism
∑+=
Ω
n
incbcQFQd
d
N ααα
σ 2
,)()(1
[ ]1)()( ,
,
−=∑ QSbbccQFn
βαββα
αβα
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Neutron diffraction: the differential Neutron diffraction: the differential
cross sectioncross section
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Neutron diffraction: the pair Neutron diffraction: the pair
correlation functioncorrelation function
Sine Fourier transform of the Sα,β(Q):
• Total number density is ρ0
• Gα,β(r) is the probability of funding atom β at distance r from atom α.
Fourier transform of the total F(Q) is the weighted sum of all partial values.
Total correlation function, T(r).• Highlights correlation at high r
Differential distribution function D(r).
• Bulk density removed.
[ ] dQQrQSQr
rg )sin(1)(2
11)(
0
,
0
2, ∫∞
−=− βαβαρπ
[ ]1)()sin()(2
1)( ,
,00
2−== ∑∫
∞
rgbbccdQQrQQFr
rGn
βαβα
βαβαρπ
1)(4)(
)(4)(
,
0
,
0
−
+=
+=
∑
∑
n
n
bbccrGrrD
bbccrGrrT
βα
βαβα
βα
βαβα
ρπ
ρπ
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Neutron instrumentationNeutron instrumentation
Steady state (reactor type)
Beam of wavelength λ is scattered though angle 2θ.
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Neutron instrumentationNeutron instrumentation
Time-of-flight (spallation source)
Detectors at fixed angle record different wavelengths (Q)
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Neutron diffraction: Neutron diffraction:
correction procedurescorrection procedures
Structural information is in elastic, single scattering events.
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Sample environmentsSample environments
Pressure cells
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Sample environmentsSample environments
Containerless levitation
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Interpretation of glass Interpretation of glass
structurestructure
Based on the pair distribution function (PDF)
Continuous random networks (CRN)
Characteristic distance ranges
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Interpretation of glass Interpretation of glass
structurestructure
Short-range order and connectivity
Intermediate range Order
The first sharp diffraction peak (FSDP)
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Partial structure factor Partial structure factor
determinationdetermination
Isotopic substitution, e.g.
H/D
2222
2222
2
2
)()(2
)()(
OOOHOHHH
OOOO
OHOHOH
HHHH
OH
bcbbccbcA
rgA
bcrg
A
bbccrg
A
bcrG
++=
++=
)(441.0)(446.0)(113.0)(
)(091.0)(23.4.0)(486.0)(
)(190.0)(92.4.0)(318.0)(
2
2
2
rgrgrgrG
rgrgrgrG
rgrgrgrG
OOOHHHOH
OOOHHHOH
OOOHHHOH
++=
++=
+−=
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Partial structure factor Partial structure factor
determinationdetermination
Combined neutron and X-ray data
[ ] [ ]
−
−−−
=∆
)(1
1)()(
1)(
)(
QW
W
QSQW
WQS
QS
X
N
X
X
NN
-1
0
1
2
3
4
5
6
7
8
9
10
1 1.5 2 2.5 3 3.5 4
Radial distance (Angstrom)
T(r
)
Mg-O
O-O
Enstatite glass
Forsterite glass
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Partial structure factor Partial structure factor
determinationdetermination
Reverse Monte Carlo and empirical structural refinement
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Studies of liquids and amorphous Studies of liquids and amorphous
materials: Simple oxidesmaterials: Simple oxides
Confined to simple systems:
• SiO2
• Li2O-SiO2
• K2O-SiO2
• Na2O-SiO2
• CaO-Al2O3-SiO2
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Studies of liquids and amorphous Studies of liquids and amorphous
materials: Simple oxidesmaterials: Simple oxides
MgO-SiO2 glasses
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25
Momentum transfer (Angstrom-1)
S(Q
)
67% MgO 33% SiO2 (forsterite)
50% MgO 50% SiO2 (enstatite)
54% MgO 46% SiO2
58% MgO 42% SiO2
62% MgO 38% SiO2
-1
0
1
2
3
4
5
0.5 1.5 2.5 3.5 4.5
Radial distance (Angstrom)
T(r
)
Si-O
Mg-OO-O
Enstatite glass
Forsterite glass
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Studies of liquids and amorphous Studies of liquids and amorphous
materials: amorphous icesmaterials: amorphous ices
• Low and high-density forms of amorphous ice
• Demonstration of LDA-HDA transitions
• Fully hydrogen-bonded tetrahedral networks
• Interstitial water molecule
• Related to the high pressure structure of liquid water
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Studies of liquids and amorphous Studies of liquids and amorphous
materials: amorphous icesmaterials: amorphous ices
Changes in the first peak in the diffraction pattern
Changes in the O-O partial in real space
Change in O-O, moves to greater radial distance with HDA-LDA
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Studies of liquids and amorphous Studies of liquids and amorphous
materials: high pressure studiesmaterials: high pressure studies
Amorphous forms of GeO2
Classic “strong” network-forming glass
Tetrahedral-octahedral coordination change
Changes in height and position of FSDP
Shrinkage and collapse of open network structures
Intermediate 5-coordinate Ge-O stabilised
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MgOMgO--SiOSiO22 glasses: Neutron glasses: Neutron
diffractiondiffraction
In Situ high pressure neutron data 38% SiO2 glass
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1 2 3 4 5 6 7 8
Momentum Transfer / Angstrom -1
S(Q
)
Ambient
4.1 Gpa
5.7 Gpa
8.6 Gpa
6.7 Gpa
Abrupt changes in the S(Q) between 6 and 8 GPa
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Studies of liquids and amorphous Studies of liquids and amorphous
materials: high pressure studiesmaterials: high pressure studies
In Situ high pressure neutron data 38% SiO2 glass
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.0 2.0 3.0 4.0 5.0 6.0
r / Angstrom
g(r
)
Ambient
4.1 Gpa
5.7 Gpa
8.6 Gpa
6.7Gpa
• Change in first peaks
– Si-O
– Mg-O
• Increasing distortion of the Mg-O polyhedron?
• Non-linear change in structure
• Is it polyamorphic?
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Summary and future Summary and future
directionsdirections
Neutrons offer the opportunity to determine the structure of liquids directly by diffraction
Total structure factor related to the PDF by Fourier transform
Combined techniques can be used to extract partial S(Q)’s
Developments in sample environment:• Levitation studies
• High pressure studies
New neutron sources
More challenging experiments