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This note reports a study of the coating of a crystal with `ice' at temperatures below 45 K during single-crystal in-house diffraction experiments when using an open-flow helium cryostat. The `ice' consists mainly of crystalline oxygen and nitro­gen. This suggests completely different techniques for avoiding this type of icing compared with water icing. With appropriate choices of crystal mount, crystal position with respect to the nozzle and gas flow conditions, it is possible to avoid detectable condensation. However, sometimes this cannot be achieved in practice (poor diffraction from a smaller crystal, necessity of positioning the crystal in certain orientations to achieve desired data completeness, need to reduce helium consumption etc.). The problem of icing seems to be less common for powder experiments where the laminar gas flow is parallel to the capillary containing the sample, and for synchrotron experiments where the sample is comparatively small and almost continuously rotated, which facilitates the ice covering being removed by the gas flow. This last technique can in principle also be applied to single-crystal X-ray diffraction using laboratory diffractometers – periodic rapid rotation of the crystal can help to minimize any icing, but this technique will not work when the condensation rate is comparable to or faster than one frame of data collection. The coating around a sample crystal reduces the quality of the diffraction data, and the temperature at the sample below the coating may differ significantly from that at the cryostat nozzle reported by the instrument.

Supporting information

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Text file https://doi.org/10.1107/S1600576721006440/kc5127sup1.txt
Powder diffraction pattern generated using the first frame

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Text file https://doi.org/10.1107/S1600576721006440/kc5127sup2.txt
Powder diffraction pattern generated using the last frame

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Link https://dx.doi.org/10.5281/zenodo.4599800
Single-crystal XRD data for CoSO4*H2O at 15 K obtained using an open-flow helium cryostat. Diffraction pattern also has powder diffraction rings arising from solid air components, N2 and O2. The data can be viewed and treated with CrysAlisPro software


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