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Synchrotron and neutron experiments Angus P. Wilkinson School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA 30332-0400 Thanks.

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Presentation on theme: "Synchrotron and neutron experiments Angus P. Wilkinson School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA 30332-0400 Thanks."— Presentation transcript:

1 Synchrotron and neutron experiments Angus P. Wilkinson School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA Thanks are due to Alan Hewat and Ian Swainson for many of the slides

2 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

3 A comparison of X-rays and neutrons X-raysNeutrons 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 beamsLow intensity beams Typically, strongly absorbed by all but low Z elements Weakly absorbed by most materials

4 Relative Scattering Powers of the Elements

5 Locating “light elements”  Structure of the 90K high T c 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. YBa 2 Cu 3 O 7 drawing from Capponi et al. Europhys Lett (1987)

6 Hydrogen in metals  Hydrogen storage in metals –Location of H among heavy atoms –No single crystals  Laves phases eg LnMg 2 H 7 (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, etc..Angew. Chemie

7 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 H 2 O is different from that in D 2 O Unit of b is fm. Unit of cross-section  is 4  b 2 in barns (100 fm 2 ).  s  i  c %b c b i  c  i  s  a H D

8 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 

9 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 K  Ni metal, synchrotron radiation, GE detector From Peter Chupas

10 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

11 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).

12 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

13 Radiant Furnace Al vacuum body Water-cooled base W or Ta radiant elements Mo-foil heat shields 6 kW of power Turbo vac Torr base pressure, 5e -6 at 2000K Gas inserts, static or purge Courtesy of I. Swainson

14 Cryomagnet 1.5K to RT 200mK-1.5K He 3 Up to 9T vertical field Courtesy of I. Swainson

15 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

16 Absorption – an isotopic problem Other (non-REE) absorbers include Cd and B 11 B, 7 Li however are relatively cheap to buy. Neutron are not without absorption problems! Courtesy of I. Swainson

17 Synchrotron radiation  High intensity  Plane polarized  Intrinsically collimated  Wide energy range  Has well defined time structure

18 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

19 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

20 11BM high resolution diffractometer 12 channel analyzer system

21 Complex materials  Many real materials do not have just one species on a given crystallographic site –YBa 2 Cu 3 O 7-x »Can have both oxygen and oxygen vacancies on a given site –Zeolites, M x [Si 1-x Al x O 2 ] »Extraframework cations M occupy sites that may also have vacancies and water present »Al may not be randomly distributed over all available sites –NiFe 2 O 4 »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

22 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, x i n i and U i for multiple species on nominally the same site –typically we assume that the x i and U i are the same for all species at nominally the same site »This may be a gross approximation! –to estimate individual n i the species must differ in scattering power, even then more than two species can not be handled »Determining Mn/Fe distribution in MnFe 2 O 4 using neutrons is easy

23 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

24 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

25 The X-ray scattering factor  The elastic scattering is given by,  For a spherical atom,

26  f” “mirrors” the absorption coefficient  f’ is intimately related to the absorption coefficient Absorption and anomalous scattering

27 Examples – Cs 8 Cd 4 Sn 42  Cd location in the type I clathrate Cs 8 Cd 4 Sn 42 –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, (2002).

28 Sn scattering factors in Cs 8 Cd 4 Sn 42  Anomalous scattering terms calculated from Kramers-Kronig transformation of absorption data

29 Resonant scattering and Cs 8 Cd 4 Sn 42  Selecting an X-ray energy close to an absorption edge distinguishes Cd from Sn Diffraction data recorded at up sin  / ~0.7Å -1

30 Location of Cd in Cs 8 Cd 4 Sn 42  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)

31 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

32 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

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