International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk
International Tables for Crystallography (2018). Vol. H, ch. 2.10, pp. 218-219

Section 2.10.2.3. Specimen containment

P. S. Whitfield,a* A. Huqb and J. A. Kadukc,d,e

aEnergy, Mining and Environment Portfolio, National Research Council Canada, 1200 Montreal Road, Ottawa ON K1A 0R6, Canada,bChemical and Engineering Materials Division, Spallation Neutron Source, P.O. Box 2008, MS 6475, Oak Ridge, TN 37831, USA,cDepartment of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, IL 60616, USA,dDepartment of Physics, North Central College, 131 South Loomis Street, Naperville, IL 60540, USA, and ePoly Crystallography Inc., 423 East Chicago Avenue, Naperville, IL 60540, USA
Correspondence e-mail:  pamela.whitfield@psi.ch

2.10.2.3. Specimen containment

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The choice of materials for designing sample holders usually depends on the type of experiments, temperature or pressure conditions, the type of neutron source (constant-wavelength or time-of-flight), the presence or absence of fine radial collimators in the instrument and finally the sample. The scattering of vanadium (σcoh = 0.0184 b) or a TiZr alloy (a null scatterer, because of exact matching of the negative scattering length of titanium and the positive scattering length of zirconium) is almost purely incoherent, making it ideal for sample containment for diffraction measurements. Although the coherent scattering of vanadium is small, in careful work it should not be a surprise to find weak peaks from V (space group [Im\bar3m], a = 3.027 Å) in the powder pattern. Examples of vanadium sample cans are shown in Fig. 2.10.50[link](b). At a reactor source aluminium is even better if low-angle (large d-spacing) reflections are of interest, as in the case of many magnetic structural studies, because the aluminium incoherent scattering cross section is three orders of magnitude less than that of vanadium. At elevated temperatures, vanadium easily forms oxides or hydrides in the presence of air or hydrogen, making the cell brittle, so its use is limited to low-temperature studies or in vacuum furnaces. At temperatures higher than 1273 K one often has to use boron nitride caps and molybdenum screws for vanadium sample holders to avoid eutectic formation (an example is shown in Fig. 2.10.51[link]b). For samples that react with vanadium, a thin layer of a noble metal such as gold can be vacuum deposited inside a vanadium can to stop it from reacting with the sample (Turner et al., 1999[link]). If this approach is used, it should be remembered that the melting point of gold is 1337 K, and when it is irradiated by neutrons it becomes activated with a half-life of 2.7 days. For experiments requiring hydrogen pressure at elevated temperature, Inconel (Fig. 2.10.50[link]a) is often the material of choice (Bailey et al., 2004[link]). However, in this case for diffraction measurements one has to either exclude the Inconel peaks or make use of radial collimators to reduce the signal from the vessel itself. In spallation sources with large detector area coverage one can also do an experiment where only detectors at a scattering angle of 90° are used. For gas-absorption experiments at low temperatures, however, vanadium is still the material of choice. For pressure measurement in anvil-type cells, TiZr is used for the gasket.

[Figure 2.10.50]

Figure 2.10.50 | top | pdf |

Two examples of sample holders used in neutron powder diffraction. (a) A cell made of Inconel used for hydrogen absorption studies in Li3N (Huq et al., 2007[link]). (b) Vanadium holders that were specially made for a sample changer built for the Powgen diffractometer located at Oak Ridge National Laboratory.

[Figure 2.10.51]

Figure 2.10.51 | top | pdf |

Two sample holders used for high-temperature studies. (a) A cell made of quartz with frits at the bottom to allow gas flow through the sample. (b) A holder on the right made of vanadium but using a boron nitride top with molybdenum bolt, nuts and washers to avoid melting due to eutectic formation. The fitting on the far left, which is made of stainless steel, is used to attach the boron nitride cap to the stick.

For opposed-anvil pressure experiments the anvil materials can be either cubic tungsten carbide or boron nitride. The latter is preferred as boron is highly absorbing and does not contribute anvil reflections to the sample measurement, so therefore effectively works as an incident-beam collimator. The use of tungsten carbide is reserved for techniques where a `through-gasket' approach is required, such as furnace measurements with a graphite heater where the use of a null scattering alloy as a gasket material is not possible. A recent development in high-pressure neutron scattering is the use of sintered diamond anvils, also called PCDs (from polycrystalline diamond). They allow the accessible pressure range to be doubled at the cost of adding very strong diamond reflections to the pattern.

However, for gas pressure cells aluminium is often used, as it can withstand higher pressure and Al absorbs neutrons only weakly as the absorption cross section of aluminium, σabs = 0.231 b for a wavelength of 1.8 Å, is small. For high-temperature gas-flow experiments fused silica (`quartz') glass is generally used for sample containment. These holders can also have glass frits attached at one or both ends for easy flow of gas through the sample, as shown in Fig. 2.10.51[link](a).

A few very high intensity instruments are now able to carry out powder diffraction from milligramme quantities of sample. For these measurements vanadium cans produce too much background, as there is more vanadium in the beam than the sample. The use of thin-walled silica/glass or Kapton capillaries maybe more appropriate in those circumstances.

It is also important to remember that an exchange medium is used for low-temperature (heat transfer) and pressure (pressure transfer) measurements. Helium gas is generally used as a low-temperature exchange medium. Typically, cans are sealed with a flange and lid that supports an indium (or other soft metal) gasket. If the sample is air sensitive and has to be loaded in a glove box, one should try to use a helium-filled glove box. Argon- or nitrogen-filled glove boxes are more common but the freezing temperatures of argon and nitrogen are 84 K and 77 K, respectively. They will no longer work as exchange gases below these temperatures and, because of their rather large neutron-scattering lengths, new diffraction peaks will emerge at or below these temperatures. Similarly one should ensure that a pressure medium will remain hydrostatic for the pressure range for which it is being considered (Varga et al., 2003[link]).

It is also worth noting that cooling powder samples below 1 K relies entirely on thermal conduction through the walls of the sample holder and to the specimen itself. If great care is not taken, the specimen temperature may be far higher than that reported by a thermometer attached to the sample holder. At a minimum, the holder lid should be made from copper, as it is expected that the superconducting transitions in aluminium and vanadium would cause the walls of the sample holder to become thermally insulating and greatly reduce their ability to cool the sample. Of course, properly sealing the loose powder under an atmosphere of 4He is equally important. It is essential that the indium seal be installed correctly, as 4He undergoes a transition to a superfluid at 2.17 K and has effectively zero viscosity, and can easily escape from a poorly sealed can.

Levitation methods (e.g. gas flow, acoustic, electrostatic) as shown in Fig. 2.10.52[link] offer a containerless method, which eliminates altogether sample–container reaction problems and diffraction or additional background scattering from a sample container (Weber et al., 2014[link]). Levitated samples are typically used in conjunction with laser heating to achieve high temperatures, in situ melting of samples and prevention of heterogeneous nucleation.

[Figure 2.10.52]

Figure 2.10.52 | top | pdf |

Aerodynamic levitation system to suspend melts at temperatures to 2773 K and beyond for neutron diffraction measurements.

References

Bailey, I. F., Done, R., Dreyer, J. W. & Gray, E. M. (2004). A high-temperature high-pressure gas-handling cell for neutron scattering measurements. High Press. Res. 24, 309–315.Google Scholar
Turner, J. F. C., Done, R., Dreyer, J., David, W. I. F. & Catlow, C. R. A. (1999). On apparatus for studying catalysts and catalytic processes using neutron scattering. Rev. Sci. Instrum. 70, 2325–2330.Google Scholar
Varga, T., Wilkinson, A. P. & Angel, R. J. (2003). Fluorinert as a pressure-transmitting medium for high-pressure diffraction studies. Rev. Sci. Instrum. 74, 4564–4566.Google Scholar
Weber, J. K. R., Benmore, C. J., Skinner, L. B., Neuefeind, J., Tumber, S. K., Jennings, G., Santodonato, L. J., Jin, D., Du, J. & Parise, J. B. (2014). Measurements of liquid and glass structures using aerodynamic levitation and in-situ high energy X-ray and neutron scattering. J. Non-Cryst. Solids, 383, 49–51.Google Scholar








































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