Transcript Neutron and high energy X-ray diffraction: Applications

Synchrotron and neutron experiments
Angus P. Wilkinson
School of Chemistry and Biochemistry
Georgia Institute of Technology
Atlanta, GA 30332-0400
Thanks are due to Alan Hewat and Ian Swainson
for many of the slides
Outline
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Comparison of X-ray and neutron scattering
Applications of neutron diffraction
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“Light” elements
Magnetism
High Q data
Penetration
What is a synchrotron and why use one?
Resonant scattering and the determination of complex
cation distributions
Where X-rays meet neutrons – in the high energy
regimen
Summary
A comparison of X-rays and neutrons
X-rays
Neutrons
Atomic scattering power varies
smoothly with atomic number
Atomic scattering power varies
erratically with atomic number
Atomic scattering power decreases as
the scattering angle increases
Atomic scattering power is constant as
the scattering angle changes
Largely insensitive to magnetic
moments
Scattered by magnetic moments
Readily available as intense beams
Low intensity beams
Typically, strongly absorbed by all but Weakly absorbed by most materials
low Z elements
Relative Scattering Powers of the Elements
Locating “light elements”

Structure of the 90K high
Tc superconductor
– Left -by X-rays
(Bell labs & others)
– Right -by Neutrons
(many neutron labs)

YBa2Cu3O7 drawing from Capponi et al.
Europhys Lett 3 1301 (1987)
The neutron picture
gave a very different
idea of the structure important in the
search for similar
materials.
Hydrogen in metals

Hydrogen storage in metals
– Location of H among heavy
atoms
– No single crystals

Laves phases eg LnMg2H7 (La,Ce)
– Binary alloys with large/small
atoms
– Various arrangements of
tetrahedral sites can be occupied
by H-atoms
– Up to 7 Hydrogens per unit
Gingl, Yvon et al. (1997) J. Alloys Compounds 253, 313.
Kohlmann, Gingl, Hansen, Yvon (1999) Angew. Chemie 38, 2029. etc..
Hydrogen – a blessing and a curse
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Neutrons see hydrogen well – perhaps too well.

Neutron incoherent scattering is an isotropic “random” scattering
of neutrons. This is the basis of some techniques (quasi-elastic
neutron scattering) but is a killer for neutron, at least powder,
diffraction.
– Deuterate to avoid problems. This can be difficult and may change what you
want to examine. For example, cement hydration in H2O is different from
that in D2O
%
bc
bi
sc
si
ss
sa
H 99.985 -3.741 25.27
1.758
80.27
82.03
0.3326
D 0.015 6.671 4.04
5.592
2.051
7.643
0.000519
Unit of b is fm.
Unit of cross-section s is 4pb2 in barns (100 fm2). ss = si + sc
Form factor fall off

X-ray scattering amplitude is strongly dependent on sinq/l
making it very difficult to get good quality x-ray data at high
sinq/l
– This can give problems with determining “thermal parameters”

Neutrons give good signal at high sinq/l
High Q data

Time-of-flight neutron
diffraction facilitates
the collection of data to
very high Q (small dspacing)
– No form factor fall off
– Highest flux at short
wavelength

Similar experiments
can also be done with
very high energy
synchrotron radiation
Cu Ka
Mo Ka
Ni metal, synchrotron radiation, GE detector
From Peter Chupas
The magnetic structure of MnO
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
MnO, NiO and FeO order antiferromagnetically
After taking into account the arrangement of unpaired spins the
unit cell is twice as big as the atomic arrangement would suggest
– So you get extra peaks in the neutron diffraction pattern
Powder neutron diffraction data for MnO
 Extra
peaks are only
present in the neutron
diffraction pattern at
temperatures where
the unpaired spins are
ordered (below Neel
temperature).
Neutrons are penetrating
 Neutrons
can pass through a reasonable
thickness of metal. This makes it easier to
build sample environments
– No Be windows or other special approaches
needed
– V and some alloys such as TiZr have essentially
zero coherent scattering cross section and do not
give any Bragg peaks
Radiant Furnace
• Al vacuum body
• Water-cooled base
• W or Ta radiant elements
• Mo-foil heat shields
• 6 kW of power
• Turbo vac. 10-7 Torr base
pressure, 5e-6 at 2000K
• Gas inserts, static or
purge
Courtesy of I. Swainson
Cryomagnet
• 1.5K to RT
• 200mK-1.5K He3
• Up to 9T vertical field
Courtesy of I. Swainson
Pressure with neutrons
 Pressure
has always been the
most problematic for neutrons,
due to low flux
 Usually need large volume
 And P = F/A acts against you
Gas pressure cell made from aluminum.
Max P ~ 0.5 GPa
 But
improvements in neutron optics; e.g., neutron K-B
mirrors help compress beams, new sources (SNS), and
advances in synthetic diamonds (LARGE single
crystals) may mean neutrons make a significant step
forwards shortly
Courtesy of I. Swainson
Absorption – an isotopic problem
Neutron are not without absorption problems!
Element Mean sa
Ce
0.63
Pr
11.5
Isotope %
Nd
50.5
152
0.2
735
Pm
168.4
154
2.1
85
Sm
5922
155
Eu
4530
14.8 61100
156
Gd
49700
20.6
157
Tb
23.4
15.7 259000
158
Dy
994
24.8
2.2
Ho
64.7
160
21.8
0.77
Er
159
Tm
100
Yb
34.8
Lu
74
Hf
104.1
Gd
Gd
Gd
Gd
Gd
Gd
Gd
sa
1.5
• Other (non-REE) absorbers include Cd and B
• 11B, 7Li however are relatively cheap to buy.
Courtesy of I. Swainson
Synchrotron radiation
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High intensity
Plane polarized
Intrinsically collimated
Wide energy range
Has well defined time
structure
Advantages of using a synchrotron
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The high level of intrinsic collimation and high
intensity of the source facilitates the construction of
powder diffractometers with unrivaled resolution
– More information in the powder pattern
 Can
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do experiments with good time resolution,
although not combined with ultrahigh resolution
Can do experiments at short wavelengths
– Facilitates collection of high Q (small d-spacing) data, and
reduces or eliminates problems due to absorption
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Can do resonant scattering
– Chose a wavelength close to an absorption edge and tune the
scattering power of the elements in you samples
Diffractometer Geometry
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Crystal analyzer gives very good resolution, low count rate and is insensitive
to sample displacement, useable with flat plate or capillary
Soller slits give modest resolution, good count rate and insensitivity to sample
displacement
Simple receiving slits give good count rate, easily adjustable resolution, can be
used with flat plate or capillary
11BM high resolution diffractometer
12 channel analyzer system
Complex materials
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Many real materials do not have just one species on a
given crystallographic site
– YBa2Cu3O7-x
» Can have both oxygen and oxygen vacancies on a given site
– Zeolites, Mx[Si1-xAlxO2]
» Extraframework cations M occupy sites that may also have
vacancies and water present
» Al may not be randomly distributed over all available sites
– NiFe2O4
» What is the distribution of nickel and ion over the tetrahedral and
octahedral sites in the spinel?
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It can be difficult to pin down the distribution of
species over the available sites
Information from diffraction data
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Bragg scattering provides a measure of the scattering
density at a particular crystallographic site
Fhkl =  ni f i exp[8p 2U i (sin2 q / l2 )]exp[2pi(hxi + kyi + lzi )]
i
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With one diffraction data set it can be very difficult
/impossible to estimate, xi ni and Ui for multiple
species on nominally the same site
– typically we assume that the xi and Ui are the same for all
species at nominally the same site
» This may be a gross approximation!
– to estimate individual ni the species must differ in scattering
power, even then more than two species can not be handled
» Determining Mn/Fe distribution in MnFe2O4 using neutrons is easy
Scattering contrast
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In some cases lab x-ray data does not generate enough
contrast to solve a problem
– Ni/Fe distribution and other “neighboring element problems”
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Neutrons may generate the needed contrast
– But not for Ni/Fe!
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More than one data set with different scattering
contrast levels may be needed
– Differing scattering contrast data set per species on the site?
» constraints on composition and site occupancy reduce this
requirement
– Can get these additional data sets by isotopic substitution
and neutron scattering or by resonant x-ray scattering
Resonant x-ray scattering and
isotopic substitution
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Isotopic substitution is very expensive
Different isotopically substituted samples may not
be the same!
Resonant x-ray scattering makes use of the same
sample for all measurements
Reliable resonant scattering factors can be
awkward to get
Absorption and restricted d-spacing range can be a
problem with resonant scattering measurements
The X-ray scattering factor
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The elastic scattering is given by,
f ( E, Q) = f o (Q) + f ' ( E) + if " ( E)
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For a spherical atom,
 r (r) sin Qr
f (Q) = 4 p 
dr

Qr

o
0
2
Absorption and anomalous scattering
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f” “mirrors” the absorption
coefficient
 2pm c 0 
f "(E) = 
 E a
2
 e h 
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f’ is intimately related to
the absorption coefficient

 2  Ef " ( E )
f ' ( E ) =  
dE
2
2
 p 0 ( E0  E )
Examples – Cs8Cd4Sn42
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Cd location in the type I clathrate Cs8Cd4Sn42
– Is the Cd randomly distributed over all the available
framework sites?
– Distribution of Cd effects Seebeck coefficient and
thermoelectric performance
– Cd absorbs neutrons
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Cd and Sn have similar atomic number
– essentially indistinguishable by X-ray scattering unless Xrays have energy close to absorption edge
– collect data at 80 keV, Cd K-edge and Sn K-edge
» more good data improves reliability of the results
» Scattering factors estimated from absorption measurements
Chem. Mater. 14, 1300-1305 (2002).
Sn scattering factors in Cs8Cd4Sn42
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Anomalous scattering terms calculated from
Kramers-Kronig transformation of absorption data
4.5
-6
4
-6.5
3.5
-7
3
f"
-7.5
2.5
-8
2
-8.5
1.5
-9
1
0.5
29180
-9.5
29190
29200
29210
29220
Energy / ev
29230
-10
29240
f'
Resonant scattering and Cs8Cd4Sn42
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Selecting an X-ray energy close to an absorption
edge distinguishes Cd from Sn
Diffraction data recorded at up sinq/l ~0.7Å-1
Location of Cd in Cs8Cd4Sn42
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Cd is located only on 6c sites
– From analysis of data collected at 80 keV and both the Cd
and Sn K-edges
Type I framework. 6c site (red), 16i site (grey) and 24k site (green)
Powder XRD at high energy
 High
energy X-rays offer many of the advantages
associated with neutrons – along with a lot more
flux!
– Can use massive sample environment due to penetrating
nature of X-rays
– Can map out phase and stress distributions inside parts due
to penetrating power
– Systematic errors due to absorption and extinction are
eliminated
– Can make measurements to very high Q
» provides a lot of structural detail
Summary
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Synchrotron based instruments offer very high
resolution, excellent peak to background ratio, high
data rates, low absorption and the ability to tune an
elements scattering power
Synchrotron instruments are expensive and the data is
often harder to analyze than that obtained using
neutrons
Neutrons excellent for low Z element problems
Neutrons usually the tools of choice for magnetism