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VIVALDI

VIVALDI (Very Intense Vertical Axis Laue DIffractometer) surveys large volumes of reciprocal space rapidly using the Laue technique with a cylindrical image-plate detector on a white neutron beam.

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Applications

The first years of operation of VIVALDI have proven that neutron Laue diffraction with image-plate detection on a thermal beam is a high-performance technique especially well suited to small crystals, rapid chemical crystallography, reciprocal-space surveys and studies of structural and magnetic phase transitions. The Laue experiment is becoming as easy as a powder diffraction experiment, and, for a large crystal, just as fast, but with the additional qualities of single-crystal diffraction.


Main applications:

  • Rapid structural studies
  • Reciprocal-space surveys; identification of twinning and incommensurability
  • Very small samples, particularly attractive for high-pressure experiments
  • Complete nuclear and magnetic structure determinations through phase transitions

Selected examples

Molecular structures

By far the most popular type of investigation on VIVALDI to date has been location of hydrogen atoms or precise characterisation of water molecules in molecular structures with unit cells up to 10000 Å3 in volume. The gain in efficiency of one-to-two orders of magnitude over a conventional monochromatic diffractometer allows detailed atomic structural information to be obtained from crystals previously considered to be too small for neutron diffraction. The left-hand figure below shows part of the molecular structure of [H4Co4(C5Me4Et)4] at 20 K, the first published result from VIVALDI. The existence of the hydrogen atoms around the Co cluster in this 8200 Å3 unit cell was proven from a three-day diffraction measurement on a crystal of just 0.6 mm3 in volume. The right-hand figure shows the first observation of Li-encapsulated H in [t-Bu2AlMe2)2Li]-[{Ph(2-C5H4N)N}6HLi8]+

R Bau, N N Ho, J J Schneider, S A Mason, and G J McIntyre, Inorg. Chem. 43 (2004) 555-558
S R Boss, J M Cole, R Haigh, R Snaith, A E H Wheatley, G J McIntyre and P R Raithby, Organometallics 23 (2004) 4527-4530

Magnetic diffuse scattering

The capability to survey large volumes of reciprocal space is illustrated by the study of the magnetic ordering in the mineral tapiolite, FeTa2O6. Named after the god Tapio of Finnish mythology, tapiolite is one of a group of naturally occurring minerals with general formula A2+B25+O6. Each layer of Fe2+ cations is separated from the next by two sheets of diamagnetic Ta5+ ions along the c axis. This results in a highly convoluted magnetic exchange pathway that favours 2-D magnetic ordering. The difference between Laue diffraction patterns just above and just below the 3-D antiferromagnetic ordering temperature (8K) reveals with striking clarity the 2-D ordering of the Fe moments which manifests itself as rods of scattering along c* in various zones of reflections.

E M L Chung, M R Lees, G J McIntyre, C Wilkinson, G Balakrishnan, J P Hague, D Visser and D McK Paul, J. Phys.: Condens. Matter 16 (2004) 7837-7852

Rapid structural studies

The variation of the chromium-oxygen bond lengths in the ammonium chromium Tutton's salt (ND4)2Cr(D2O)6(SO4)2 was followed as a function of temperature in a series of six-hour data collections, each of four Laue exposures. The variation of the bond lengths could be modelled only by assuming a rather specific vibronic coupling model that had been obtained earlier by fitting a Jahn-Teller Hamiltonian to the low-energy excitations of [Cr(OH2)6]2+. The orthorhombic strain diminishes with increasing temperature, which increases the number of thermally accessible vibronic states - and hence increases the observed averaging of the bond lengths.

C Dobe, C Noble, G Carver, P L W Tregenna-Piggott, G J McIntyre, A-L Barra, A Neels, S Janssen and F Juranyi, J. Am. Chem. Soc. 126 (2004) 16639-1665

High-pressure crystallography

Full crystallography in a pressure cell is quite feasible using the white-beam Laue technique. Both opposed-piston and anvil cells can be used, although attention must be paid during the construction of the cell to maximise the accessible solid angle and to reduce the amount of material in the incident beam in particular. For anvil cells the projected forms of the reflections from the sample and anvils facilitate location of the sample in the beam, and the wide wavelength band of the Laue technique allows recovery of reflections masked by the cell pillars, simply by rotation of the cell.

G J McIntyre, L Mélèsi, M Guthrie, C A Tulk, J Xu, J B Parise, J. Phys.: Condens. Matter, 17(2005) S3017-S3024

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