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. 2.6, pp. 153-154

Section 2.6.7. Low-temperature sample stages

C. A. Reissa*

aNoordikslaan 51, 7602 CC Almelo, The Netherlands
Correspondence e-mail: Celeste.Reiss@PANalytical.com

2.6.7. Low-temperature sample stages

| top | pdf |

2.6.7.1. Cryogenic cooling stages/cryostat

| top | pdf |

For cryogenic experiments, liquid nitrogen (boiling point 77.4 K at 1 atm, where 1 atm = 101 325 Pa) or liquid helium (boiling point 4.3 K at 1 atm) is required (Weast, 1980[link]). The most common types of chambers for medium-to-low temperatures are chambers with continuous-flow cooling. Here, a continuous flow of liquid nitrogen is provided from a Dewar storage vessel and the cooling process is controlled by a liquid-nitrogen controller. For lower temperatures helium is used. Helium is an expensive gas, and therefore a more economic method is to use a closed-cycle cryostat. This has the disadvantage that it consumes relatively large amounts of energy, but it does not need a continuous flow of helium and is also easy to use. The PheniX cryostat from Oxford Cryosystems is an example of such a chamber, which makes it possible to cool flat-plate powder samples to 20 K in just 35 min and to as low as about 12 K after a further 25 min. Recently, a group using beamline L11 at the Diamond Light Source synchrotron made some modifications to the PheniX cryostat to enable it to perform low-temperature Debye–Scherrer powder diffraction (Potter et al., 2013[link]). The original flat-plate sample holder in the cryostat was changed to a capillary sample holder.

2.6.7.2. Cryogenic cooling stages/cryostream

| top | pdf |

The cryostream from Oxford Cryosystems (Cosier & Glazer, 1986[link]) cools the sample in a different way (Fig. 2.6.8[link]). Originally developed for single-crystal X-ray diffraction experiments, it is currently also used to cool/heat capillaries in Debye–Scherrer experiments (Fig. 2.6.9[link]). To prevent atmospheric moisture from freezing on the capillary, the cryogenic nitrogen-gas stream is shrouded in a second dry gas stream. When the two flows are balanced, the outer stream protects the inner nitrogen stream and temperatures as low as 80 K can be reached without ice formation on the capillary. The main advantage of measuring a powder sample in a capillary (transmission geometry) in contrast to flat-plate reflection geometry is minimization of preferred orientation of the sample. Attention must be paid, as in all non-ambient measurements, to temperature gradients in the sample. A short capillary is therefore advisable to minimize the occurrence of a gradient along the capillary.

[Figure 2.6.8]

Figure 2.6.8 | top | pdf |

Schematic drawing of the Oxford Cryosystems cryostream setup.

[Figure 2.6.9]

Figure 2.6.9 | top | pdf |

Oxford Cryostream (A) mounted on a PANalytical diffractometer for cooling a capillary (B).

For absorbing samples, hard radiation must be used to completely penetrate the capillary. Another possibility is to use capillaries with very small diameters, but this is not always very easy and filling them can be time consuming.

Example: buckminsterfullerene. An example that demonstrates the capability of the pair distribution function (PDF) method to independently probe the short-range and long-range atomic ordering in materials is C60, also known as (C60-Ih)[5,6]fullerene, fullerene or buckyball (Egami & Billinge, 2003[link]). A buckyball molecule consists of 60 C atoms arranged on the vertices of a soccer-ball-like frame. At room temperature the C60 molecules are arranged in a face-centred cubic (f.c.c.) lattice and they assume completely random orientations as a result of thermal energy.

For long-range atomic order to be present, the atoms of the C60 molecules must remain in the same crystallographic position, which is not the case at room temperature. The time-averaged structure of the material can be represented as an f.c.c. structure, with space group [Fm\bar 3m], of uniform hollow balls with a diameter of about 7.1 Å. On cooling through 260 K a first-order structural phase transition occurs; the random rotation of each C60 molecule becomes slower and is now best described as a librational motion (Brown et al., 2005[link]). The phase transition is accompanied by a sudden contraction of the cubic lattice parameter and the long-range order can be described with a primitive cubic lattice (space group Pa[\bar 3]).

Fig. 2.6.10[link] shows the atomic PDF at room and low temperature; only the short distances within the balls are clearly observed at room temperature (Reiss et al., 2012[link]). The correlation between atoms of neighbouring molecules cannot be seen, but ball–ball correlations are visible at larger distances. The low-temperature measurement shows similar peaks below 7.1 Å as the ambient measurement, but above 7.1 Å peaks are visible that result from distances from C atoms in one C60 molecule to C atoms in another C60 molecule.

[Figure 2.6.10]

Figure 2.6.10 | top | pdf |

Atomic pair distribution function of C60 at room temperature (red) and at 100 K (blue).

References

Brown, C., Copley, J. & Qiu, Y. (2005). The orientational order/disorder transition in buckminsterfullerene (C60): an experiment using the NCNR Disk Chopper Spectrometer. Gaithersburg: NIST Center for Neutron Research. https://www.ncnr.nist.gov/summerschool/ss05/C60_EXPT_05.pdf . Google Scholar
Cosier, J. & Glazer, A. M. (1986). A nitrogen-gas-stream cryostat for general X-ray diffraction studies. J. Appl. Cryst. 19, 105–107.Google Scholar
Egami, T. & Billinge, S. J. L. (2003). Underneath the Bragg Peaks: Structural Analysis of Complex Materials. Oxford: Pergamon.Google Scholar
Potter, J., Parker, J. E., Lennie, A. R., Thompson, S. P. & Tang, C. C. (2013). Low-temperature Debye–Scherrer powder diffraction on Beamline I11 at Diamond. J. Appl. Cryst. 46, 826–828.Google Scholar
Reiss, C. A., Kharchenko, A. & Gateshki, M. (2012). On the use of laboratory X-ray diffraction equipment for pair distribution function (PDF) studies. Z. Kristallogr. 227, 256–261.Google Scholar
Weast, R. C. (1980). CRC Handbook of Chemistry and Physics, 61st ed. Boca Raton: CRC Press.Google Scholar








































to end of page
to top of page