Neutron powder diffraction – new opportunities in hydrogen location in molecular and materials structure

Abstract

The potential of neutron powder diffraction in the location of hydrogen atoms in molecular materials and inorganic-molecular complexes is reviewed. Advances in instrumentation and data collection techniques that have made this field accessible are reviewed, along with a wide range of applications carried out by our collaboration investigating functional materials, hydrogen-containing minerals and molecular compounds. Some of the limitations in this area, particularly for molecular systems, are also addressed. IntroductionHydrogen is a key element whose functionality in inorganic material systems stretches from geochemical materials (e.g. clay minerals, cements), through key inorganic compounds (e.g. catalysts, co-ordination complexes and hydroxides/hydrates) and materials (e.g. polymers, hydrogen storage media, proton conductors/fuel cell components) to supramolecular and framework systems (e.g aluminosilicates, solvates and clathrates). The role of hydrogen in the structures (and hence derived applications) of these materials is thus of the utmost importance but one that, until recently, experimental methods generally failed to address, or could only do so in selected cases at great expense and considerable effort.Determination of the position of hydrogen in a material, whether it be a naturally occurring hydrate/hydroxide, a cement, a hydride/hydrogen storage material, a solvate or an organometallic compound has traditionally been difficult, time-consuming, expensive and imprecise, even where possible. However, knowledge of the distribution of hydrogen is central to understanding behaviours such as polymorphism, hydrogen bonding and three-dimensional structure definition, proton diffusion 2 pathways and levels of uptake in hydrogen storage materials, physical properties ranging from ferroelectricity to the elastic properties of polymers, and reaction mechanisms. Therefore the implementation and application of methodology that allows routine hydrogen position definition from readily available material (i.e. small easily synthesised quantities and in polycrystalline/small single crystal (≤50µm) form)is both of widespread application and key importance.In the work described here, development of the application of powder neutron diffraction methods in an optimised fashion to such materials is shown to have the capability to reveal structural information in a range of hydrogen-containing materials. A range of recent studies will be summarized, to illustrate the potential of this new capability. The powerful complementary use of both X-ray and computational methods with neutron powder diffraction in these studies will also be highlighted. The aim in this Review is not to provide a detailed account of the different materials studied, but to highlight the power of the methods of approach we have been systematically developing to access more accurate and different chemistry. It draws on a range of examples from our own collaborative work, which has recently focused on this theme [1].In this review, we have deli...

Understanding Gas Storage in Cuboctahedral Porous Coordination Cages

“…Given that complete removal of excess DMF and Me-bdc was not possible without structural degradation, there is considerable incoherent scattering due to hydrogen. However, Weller et al, 70,71 as well as our previous MOF work, 72 have shown that neutron powder diffraction is quite possible with various amounts of hydrogen in crystalline materials, with hydrogen positions readily accessible due to their negative contrast. It is understandable that due to the amount of hydrogen, as well as the large unit cell, the patterns do not contain much information beyond Q = 2 Å −1 , despite containing sharp crystalline peaks.…”

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“…These yield data with low diffraction peak intensities on very large backgrounds requiring very long data collection times and, generally, yielding structural models with lower precision. Neutron source and instrument developments have resulted in new possibilities to study hydrogenous materials without the requirement, cost and associated problems to deuterate, [44][45][46] while also offering relatively short counting times 47 and prospects to take advantage of the scattering length contrast of H cf. D (−3.739 fm and 6.671 fm respectively).…”

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