Tables for
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. 213-215

Section Flat foils

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: Flat foils

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Although less commonly used with modern diffractometers, the foil-type transmission sample mounting was quite common in some older-style X-ray cameras. Sprinkling powders onto single-sided Scotch tape was sometimes used with instrumentation such as Hägg–Guinier cameras, but care should be taken as the quality of the tapes as diffraction substrates can vary wildly; the crystallinity of the polymer can be high or low, and the adhesive sometimes contains mineral inclusions, such as talc. In the modern diffractometer, foil-type transmission data can sometimes be collected using the same rotating sample stage as for reflection measurements. Simply turning the stage by 90° and using a different holder can be sufficient if the optical configuration is suitable for both reflection and transmission. For solid organic samples such as polymers this foil transmission geometry has significant advantages because of the lack of transparency effects. It is worth noting, however, that the processing of polymers can induce significant texture, such that the data collected from a film in reflection geometry will not necessarily be identical to those collected in transmission. Should a reproducible pattern independent of geometry be required, then steps should be taken to reduce the sample to a true random powder and/or a 2D detector should be used.

With powder samples the technique requires the use of a transparent substrate, usually in the form of a thin polymer film or foil. In an analytical laboratory the easiest place to find such a substrate is the X-ray fluorescence laboratory, where very thin X-ray transparent polymer films are used for both sample supports and covers for liquid cells. Some of the materials used in these applications are familiar in the diffraction community as windows, i.e. Mylar and Kapton, but others such as polypropylene are not. The substrate will obviously add to the background, but a good substrate from a diffraction standpoint combines transparency with a lack of sharp features in the diffraction pattern. This makes fitting the background much easier. Any holder must be capable of stretching or holding the film flat across an opening for the X-ray beam. A commercial version of a foil-type holder is shown prior to assembly in Fig. 2.10.37[link]. Example data from three different XRF films are shown in Fig. 2.10.38[link], together with that from a thicker Kapton foil commonly used as window material. It is notable that, despite the two 7.6 µm Kapton films being almost twice as thick as the Mylar or polypropylene films, the scattering from them is almost identical. The lack of any distinctive, sharp features above 6° 2θ in the Kapton films makes them attractive in this region, but for low-angle data Mylar is probably the better choice. Although giving a generally higher background, the thicker 50 µm Kapton foils can be used very successfully (see Fig. 2.10.39[link]). Despite the greater attenuation they are much easier to handle, as their greater stiffness and weight makes them less susceptible to static electricity.

[Figure 2.10.37]

Figure 2.10.37 | top | pdf |

Parts prior to assembly of a transmission foil sample in the holder. In this instance, micronized quartz is held as a loose powder between two 50 µm Kapton foils while the upper foil is stretched into place by the black clip.

[Figure 2.10.38]

Figure 2.10.38 | top | pdf |

Transmission data from double layers (as used for powder samples) of different polymer substrate films. They include 3.6 µm Mylar, 4.0 µm polypropylene and 7.6 µm XRF films, and a thicker 50 µm Kapton foil.

[Figure 2.10.39]

Figure 2.10.39 | top | pdf |

Diffraction pattern from loose SRM640c powder between two 50 µm Kapton foils.

One advantage of transmission foil mounts is the small amount of sample required. In a similar way to producing smear mounts for reflection geometry, there are a number of ways to prepare the thin layer required. Loose powders may be trapped between two foils as in Fig. 2.10.39[link], or alternatively a slurry or smear mount may be used in a similar way to reflection geometry. Although the sample may adhere sufficiently such that a single foil can be used, it may be necessary to use a sandwich in the same way as a loose powder. For instance, slurries do not usually adhere well to Kapton foils, so it is often better to sacrifice a little intensity from the additional Kapton attenuation and ensure the sample does not fall away during data collection. Lack of adhesion could be regarded as an advantage with regards to recovery of valuable samples. Where an adhesive is used, the same considerations as with a smear mount in reflection still apply with regards to background etc.

Ideally the sample thickness should be perfectly uniform, but in practice this will rarely be achieved. Commonly a specimen in visible light transmission will appear something like that seen in Fig. 2.10.40[link]. Rotation is used to average out inhomogeneity in the specimen.

[Figure 2.10.40]

Figure 2.10.40 | top | pdf |

Transmitted light view of a micronized quartz sample through 50 µm Kapton foils.

Sedimentation during slurry mounting and compression of powders between two foils can lead to preferential orientation in foil transmission samples just as with flat-plate reflection specimens. Although the physical effect is the same for plate-like crystallites, it should be remembered that the crystallite orientation with respect to the beam is rotated by 90°, so the resulting diffraction patterns will not look the same. This becomes very apparent when comparing the foil transmission and reflection patterns from the micronized mica in Fig. 2.10.41[link].

[Figure 2.10.41]

Figure 2.10.41 | top | pdf |

Comparison of data from micronized 40S mica taken in reflection and transmission geometry, and spray-dried material in reflection geometry. For improved clarity the spray-dried and transmission data sets are translated by +1° and +2° 2θ respectively.

Foil transmission specimens are usually rotated in a similar fashion to a reflection sample, but the improvement in statistics falls short of that found in the capillary geometry described in the next section.

One thing worth considering is that there is an inherent angular intensity aberration due to the plate transmission geometry. Owing to geometrical considerations, the path length through the specimen (and support) increases with angle with a resulting increase in absorption. For refinement work, a 1/cos θ correction can be applied.

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