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High-resolution two-axis diffractometer D2B

D2B is very high-resolution powder diffractometer designed to achieve the ultimate resolution, limited only by powder particle size (Δd/d ~ 5x10-4), but is was built so that an alternative high flux option, with resolution comparable to that of D1A, but much higher intensity, could be chosen at the touch of a button.

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Applications

  • The structural chemistry of non-rigid molecules
  • Ab-initio structure solution from powders
  • Crystal and magnetic structure determination of powder compounds (even small samples)
  • Dependance in temperature/pressure/magnetic field structural (or magnetic) studies for powders

The formation of a cyclic water hexamer in zeolite NaX

Neutron powder diffraction is a valuable tool for the study of hydrogen and water molecules in materials. This is demonstrated here with a study of the adsorption of water on zeolites which is a subject of great scientific and technological interest due to the broad spectrum of zeolite applications as catalysts, adsorbents for separation processes, ion exchange, heat storage units, cooling devices and heat pump systems.
X-ray diffraction studies on fully hydrated faujasites, gave no convincing results about the arrangement of water molecules and extraframework cations in dependence on the water content. Thus, in order to understand the host-guest interactions in zeolite-water systems, neutron powder diffraction measurements were performed on D2B on faujasite NaX loaded with D2O. These were combined with diffuse reflectance infrared fourier transform spectroscopic (DRIFTS) measurements for determination of the adsorption complex structures.
It has been shown that there is strong evidence to the formation of cyclic hexamers of water molecules localised in the 12-ring windows between faujasite supercages (Fig. 1) which are stabilised by hydrogen bonds to zeolite framework oxygen atoms.

Ref.:

"Adsorption structures of water in NaX studied by DRIFT spectroscopy and neutron powder diffraction", Hunger J., Beta I.A., Böhlig H., Ling C., Jobic H., Hunger B., J. Phys. Chem. B 110, 342-353 (2006).

Figure 1: Cyclic hexamer of adsorbed water in NaX. Water molecules (in red) are located on W1 sites.

Magnetism chills out

Neutron powder diffraction is especially good at magnetic structure determination. This is illustrated here by the study of materials that behave as magnetic refrigerants.
When a magnetic field is applied to a magnetic material, the constituent magnetic moments align, and the material warms up. When the field is switched off, the moments become disorderly again and the material cools. In certain alloys, this magnetocaloric effect is substantial – large enough to suggest a novel type of refrigeration based on cycling the magnetic field-switching and allowing the heat given off to escape. Companies are already interested in applying this environmentally-friendly technology to air-conditioning and supermarket chillers.

The familly of the R5(Si1-xGex)4 compounds present a giant magnetocaloric effect. The extent of this magnetocaloric effect is believed to depend on an intimate coupling between the material’s crystal structure and its magnetism. X-ray and magnetisation measurements on Tb5Si2Ge2 indicate that, as the temperature drops, a strong magnetic moment appears alongside a change in crystal structure from monoclinic (M) to orthorhombic (O1) (fig. 1). The obvious question now is whether the structural transition (about 105 K) strictly coincides with the onset of magnetism, and if so which triggers which. This was studied in detail on D2B. Contrary to the expectations, applying a field as high as 5 tesla induced only a partial transition from the monoclinic to the orthorhombic structure (fig. 2). Even when lowering the temperature to 105 K, only half the sample changed. An unexpected result was that the remaining monoclinic phase had become magnetic as well. It was realised that even in a zero field magnetism first emerges at 112 K in phase (M) and only at 104 K in phase (O1). The monoclinic phase is largely ferromagnetic, similar in nature to the magnetic structure of the orthorhombic phase.
Thus the magnetic phase transition happens at a higher temperature (about 112 K) than the structural phase transition (about 105 K). The interplay between crystal structure and magnetic properties is more complex than previously thought. Neutron studies are the only way that could distinguish between the two magnetic phases.

Ref.:

"Observation of a Griffiths-like phase in the magnetocaloric compound Tb5Si2Ge2", Magen C., Algarabel P.A., Morellon L., Araujo J.P., Ritter C., Ibarra M.R., Pereira A.M., Sousa J.B., Phys. Rev. Letters 96, 167201-1-167201-4 (2006)

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Figure 1: The transition from monoclinic to orthorhombic structure in the alloy, Tb5Si2Ge2. Red balls: silicon or germanium atoms ; yellow balls: terbium atoms ; arrow marks: where a strong shear movement leads to breaking of the silicon or germanium bonds.

Figure 2: The percentage of monoclinic phase induced by increasing the magnetic field at selected temperatures as determined from high-resolution D2B data.

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