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Liquids and Amorphous Diffractometer D4

D4, a two-axis diffractometer, uses short-wavelength neutrons from the hot source, and measures diffraction patterns over a large Q-range. This feature allows characterisation of local atomic order of non-crystalline materials (glasses, liquids, amorphous solids and solutions) with excellent accuracy.

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Applications

  • Short and intermediate range order in liquids and amorphous materials.
  • Isotopic substitution for the determination of local atomic environments.
  • Magnetic structure studies on very absorbent systems, as those containing Gd, Eu, etc.
  • Pair-Distribution Function (PDF) analysis of powder diffraction patterns.

Selected examples

Liquid para-hydrogen

This is the first direct determination of the centre-of-mass structure factor of liquid para-hydrogen. This is a very difficult experiment because of the relevance of inelastic scattering events (making the standard correction techniques unsuitable to hydrogen), and because of the overwhelming ratio between the incoherent and coherent neutron scattering cross section of protons (making extremely difficult to extract the intermolecular response).

More details in: The microscopic structure of hydrogen liquids, M. Zoppi, U. Bafile, M. Celli, G. J. Cuello, F. Formisano, E. Guarini, R. Magli, and M. Neumann, J. Phys.: Cond. Matter 15, S107 (2003)

Rare-earth ions in phosphate glasses

The method of isomorphic substitution (Dy/Ho) has been employed to study the relative distribution of rare-earth ions in the phosphate glass RAl0.25P2.83O9.53. It is found that 8.1 R-R nearest-neighbours reside at 5.6 Å in a network made from interlinked PO4 tetrahedra.

 

More details in: Identification of the relative distibution of rare-earth ions in phosphate glasses, R. A. Martin, P. S. Salmon, H. E. Fischer, and G. J. Cuello, Phys. Rev. Lett. 90, 185501 (2003)

Ion coordination in clays

First difference isotopic substitution allows obtaining a composite radial distribution function centred on the interlayer water hydrogens, showing that interlayer water adopts an extensively H-bonded liquid structure. Isotopic substitution of the interlayer lanthanide ion allows obtaining the partial pair-distribution function gLnH(r), which shows lanthanide ions being only partially hydrated.

More details in: Neutron diffraction studies of ion coordination and interlayer water structure in smectite clays: Lanthanide(III)-exchanged Wyoming montmorillonite, C. Pitteloud, D. H. Powell, M. A. González, and G. J. Cuello, Colloids Surf. A 217, 129-136 (2003)

Solvation structure of aqueous solution

This experiment on the solvation structure of aqueous CaCl2 (6.4 molal) at ambient temperature used the technique of second-difference isotopic substitution (44Ca/natCa and H/D). Contribution to not only the first but also the second hydration shells of Ca2+ are clearly resolved using the second difference G(r) results.

 

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