![]() ![]() ![]() The Paris–Edinburgh (PE) cell developed in the 1990s has become a standard workhorse of high-pressure neutron powder diffraction and is capable of experiments up to 10 GPa with sample volumes of approximately 100 mm 3 (Besson et al., 1995 ). ![]() Amongst larger molecular systems it has been applied to the study of polymorphism in amino acids (Moggach et al., 2006 Funnell et al., 2010 ) and explosives (Davidson et al., 2008 ). High-pressure neutron powder diffraction has been successfully applied to the discovery of new phases of simple hydrates (Loveday et al., 2001 Fortes et al., 2007 ) and ices (Nelmes et al., 2006 Fortes et al., 2012 ). However, neutrons can have several advantages over X-rays, the most relevant to high-pressure crystallography being their greater sensitivity to low- Z atoms, particularly hydrogen, and their greater penetrability through extreme sample environments. In many ways neutron diffraction is the reverse all sources are located at central facilities and are weak compared even to laboratory X-ray tubes. In particular, improvements in synchrotron technology have led to the development of dedicated high-pressure beamlines (McMahon, 2015 ). X-ray diffraction benefits from the strong photon–electron interaction as well as excellent and relatively inexpensive laboratory sources which can be complemented by very-high-intensity synchrotron sources. The way is now open for joint X-ray and neutron studies on the same sample under identical conditions.ĭiffraction methods can provide the highest-quality structural information about a crystal on the atomic scale and much work has been carried out to adapt X-ray and neutron diffraction techniques to a variety of challenging sample environments, including high pressure (McMahon et al., 2013 Guthrie, 2015 ). This technique is shown to be suitable for low-symmetry crystals, and in these cases the transmission of diffracted beams through the cell body results in much higher completeness values than are possible with X-rays. Despite the smaller crystal size and dominant parasitic scattering from the diamond-anvil cell, the data collected allow a full anisotropic refinement of hexamine with bond lengths and angles that agree with literature data within experimental error. Data collections for two sizes of hexamine single crystals, with and without the pressure cell, and at 300 and 150 K, show that sample size and temperature are the most important factors that influence data quality. The cell body does limit the range of usable incident angles, but the crystallographic completeness for a high-symmetry unit cell is only slightly less than for a data collection without the cell. An unexpected finding is that even reflections whose diffracted beams pass through the cell body are reliably observed, albeit with some attenuation. This is made possible by modern Laue diffraction using a large solid-angle image-plate detector. The first high-pressure neutron diffraction study in a miniature diamond-anvil cell of a single crystal of size typical for X-ray diffraction is reported. ![]()
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