Understanding the agglomerate crystallisation of hexamine through X-ray microscopy and crystallographic modelling

© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/The detailed molecular-scale mechanism of the growth of organic crystals underpins a diversity of phenomena, such as the isolation and purification of high-quality materials for the pharmaceutical and fine chemical sectors. Recent advances in X-ray Microscopy (XRM) and complementary diffraction contrast tomography (DCT) have enabled the detailed characterisation of the micro-structure of hexamine agglomerates. Detailed XRM analysis of the growth history and micro-structure of such agglomerates reveals a highly orientated epitaxial inter-relationship between their constituent micro-crystallites. This is found to be consistent with a secondary nucleation growth mechanism associated with re-growth at the 3-fold corner sites within the crystals’ dominant {1 1 0} dodecahedral morphology. The agglomeration appears to heal upon further growth as the aligned agglomerated micro-crystals connect and fuse together but, in doing so, pockets of inter-crystallite mother liquor become trapped forming a symmetric pattern of solvent inclusions. The mechanistic origin of this phenomenon is rationalised with respect to historical data together with an analysis of the solid-state chemistry of the compound through the development of a ‘snow flake’ model. The latter draws upon hexamine's propensity for edge growth instabilities with increasing crystal size as well as its tendency for unstable growth at the facet corners along the 〈1 1 1〉 directions, a situation compounded by the lack of growth-promoting dislocations at the centers of the {1 1 0} habit surfaces. The agglomerative mechanism presented here could apply to other high symmetry crystal systems, particularly those whose crystal structures involve centred Bravais lattices and where the dominant inter-molecular interactions are angled towards the facet edges.Peer reviewe

An evaluation of diffraction peak profile analysis (DPPA) methods to study plastically deformed metals

A range of diffraction peak profile analysis (DPPA) techniques were used to determine details of the microstructure of plastically deformed alloys. Four different alloys were deformed by uniaxial tension and compression to a range of strains. The methods we have considered include, the full-width, Williamson-Hall methods, Warren-Averbach methods, and van Berkum's alternative method. Different metals were chosen to understand the effect of the deformation microstructure and crystal structure, a nickel alloy, two stainless steel alloys and two titanium alloys.The dislocation density values found by Williamson-Hall and Warren-Averbach methods were found to be close to those expected from TEM results of similar metals. When using the Warren-Averbach methods the results suggest that systematic errors in the dislocation density are introduced by three main factors: (1) separation of instrumental broadening, (2) separation of size and strain broadening, and (3) separation of dislocation density and arrangement. Which suggests in many cases the simpler Williamson-Hall method may be preferable.The other main parameters that can be determined by DPPA are the crystal size and the dislocation arrangement. The work suggests that further investigation is needed to understand what use if any these parameters have for quantifying the deformed microstructure of plastically deformed metals