Synchrotron and neutron experiments

Angus P. WilkinsonSchool 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 Comparison of X-ray and neutron scattering Applications of neutron diffraction

– “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 low Z elements

Weakly absorbed by most materials

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)

The neutron picture gave a very different idea of the structure -important in the search for similar materials.YBa2Cu3O7 drawing from Capponi et

al. Europhys Lett 3 1301 (1987)

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 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

Unit of b is fm. Unit of cross-section is 4b2 in barns (100 fm2). si c

% bc bi c i s a

H 99.985 -3.741 25.27 1.758 80.27 82.03 0.3326D 0.015 6.671 4.04 5.592 2.051 7.643 0.000519

Form factor fall off X-ray scattering amplitude is strongly dependent on sin

making it very difficult to get good quality x-ray data at high sin– This can give problems with determining “thermal parameters”

Neutrons give good signal at high sin

High Q data Time-of-flight neutron

diffraction facilitates the collection of data to very high Q (small d-spacing)– No form factor fall off– Highest flux at short

wavelength Similar experiments

can also be done with very high energy synchrotron radiation

Cu K Mo KNi metal, synchrotron radiation, GE detector

From Peter Chupas

The magnetic structure of MnO 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 is problematic for

neutrons, due to low flux Usually need large sample

volume and P = F/A acts against you

But improvements in neutron optics, new sources (SNS etc) combined with advances in preparing large high strength single crystals (diamonds and Moissanite) for large volume gem anvil cells and the availability of devices such as the Paris-Edinburgh cell are expanding the accessible area of PT space

Gas pressure cell made from aluminum. Max P ~ 0.5 GPa

Element Mean a

Ce 0.63

Pr 11.5

Nd 50.5

Pm 168.4

Sm 5922

Eu 4530

Gd 49700

Tb 23.4

Dy 994

Ho 64.7

Er 159

Tm 100

Yb 34.8

Lu 74

Hf 104.1

Absorption – an isotopic problem

• Other (non-REE) absorbers include Cd and B• 11B, 7Li however are relatively cheap to buy.

Isotope % a

152Gd 0.2 735 154Gd 2.1 85 155Gd 14.8 61100 156Gd 20.6 1.5 157Gd 15.7 259000 158Gd 24.8 2.2 160Gd 21.8 0.77

Neutron are not without absorption problems!

Courtesy of I. Swainson

Synchrotron radiation

High intensity Plane polarized Intrinsically collimated Wide energy range Has well defined time

structure

Advantages of using a synchrotron

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 achieve 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 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

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 diffractometer12 channel analyzer system

Complex materials 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?

It can be difficult to pin down the distribution of species over the available sites

Information from diffraction data Bragg scattering provides a measure of the scattering density at a

particular crystallographic site

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

)](2exp[)]/(sin8exp[ 222iiii

iiihkl lzkyhxiUfnF

Scattering contrast In some cases lab x-ray data does not generate enough

contrast to solve a problem– Ni/Fe distribution and other “neighboring element problems”

Neutrons may generate the needed contrast– But not for Ni/Fe!

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

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

The elastic scattering is given by,

For a spherical atom,

)(")(')(),( EifEfQfQEf o

f” “mirrors” the absorption coefficient

f’ is intimately related to the absorption coefficient

Absorption and anomalous scattering

aEhe

mcEf

202

)("

dEEE

EEfEf

022

0 )(

)("2)('

Examples – Cs8Cd4Sn42

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

Cd and Sn have similar atomic number – essentially indistinguishable by X-ray scattering unless X-rays

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

Anomalous scattering terms calculated from Kramers-Kronig transformation of absorption data

1.08

1.1

1.12

1.14

1.16

1.18

1.2

29.18 29.19 29.2 29.21 29.22 29.23 29.24

Sn K-edge scan for Cs8Cd

4Sn

42

Energy / keV

Data collected at 29.194 keV

0.5

1

1.5

2

2.5

3

3.5

4

4.5

-10

-9.5

-9

-8.5

-8

-7.5

-7

-6.5

-6

29180 29190 29200 29210 29220 29230 29240

f" f'

Energy / ev

Resonant scattering and Cs8Cd4Sn42

Selecting an X-ray energy close to an absorption edge distinguishes Cd from Sn

0

10

20

30

40

50

0 0.2 0.4 0.6 0.8 1

Real part of scattering factors

Cd 80 keVSn 80 keVCd at Cd edgeSn at Cd edgeCd at Sn edgeSn at Sn edge

sin(theta)/lambda

Diffraction data recorded at up sin/ ~0.7Å-1

Location of Cd in Cs8Cd4Sn42

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 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