Synchrotron and neutron experiments

1. 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

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

4. Relative Scattering Powers of the Elements

5. 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)

6. 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. Chemie38,2029.etc..

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 H2O is different from that in D2O % bc biscsisssa 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

8. 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

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 Ka Mo Ka 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. 10-7 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 He3 • Up to 9T vertical field Courtesy of I. Swainson

15. 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

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

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, 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

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

27. 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).

28. Sn scattering factors in Cs8Cd4Sn42 • Anomalous scattering terms calculated from Kramers-Kronig transformation of absorption data

29. Resonant scattering and Cs8Cd4Sn42 • Selecting an X-ray energy close to an absorption edge distinguishes Cd from Sn Diffraction data recorded at up sinq/l ~0.7Å-1

30. 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)

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