Surveying the higher dimensions of the aperiodic composite nonadecane/urea

We recently observed for the first time that there exist phase transitions where the structural changes correspond just to degrees of freedom hidden in the internal (super)space of an aperiodic material, here the composite nonadecane/urea [1]. A key factor in the discovery of this type of transition [2] was the examination of the diffraction pattern in 3D, only possible at the time on a four-circle triple-axis neutron spectrometer, the analyzer used in zero-energy transfer to reduce the background and improve resolution. Despite the greater accessibility in reciprocal space, the weak intensity of the superlattice reflections limited the volume of reciprocal space that could be explored. Modern neutron Laue diffractometers with large image-plate detectors permit rapid and extensive exploration of reciprocal space with high resolution in the two-dimensional projection and a wide dynamic range with negligible bleeding of intense diffraction spots [3]. Surveying nonadecane/urea with neutron Laue diffraction from 300K to 4K reveals further detail of the superspace-driven phase transition, notably an increase in misorientation in the plane perpendicular to the composite misfit axis, as well as a first-order transition to a new phase at lower temperature. These new observations shed further light on how nature can use the degrees of freedom hidden in the internal superspace to form states that cannot be envisaged in the usual 3D real space.

Neutron diffraction at high pressure, low temperature under light irradiation

The exploration of multi-dimensional phase diagrams is a topical subject. However, the simultaneous variation of several control parameters such as temperature, pressure and light irradiation requires a suitably optimized sample environment, particularly when the aim of the experiment is to obtain structural information. We report on a new such sample environment, developed in the context of neutron diffraction measurements, in which the sample can be submitted to pressures up to 7 kbar, temperatures down to 1.7 K and light irradiation in the 660 to 852nm wavelength range, simultaneously. The Ti-Zr alloy pressure cell combines a high mechanical resistance over a wide temperature range with an acceptable neutron background level. The pressure medium is helium gas, ensuring the best possible hydrostatic conditions over a very broad temperature range. The low-temperature environment is obtained from an ILL-type `orange cryostat'. After focusing into an optical fiber, laser light is transmitted to the sample through a sapphire optical window implemented in the pressure cell. The laser flux density at the sample position is of ~30mW/cm2. The geometry of the set-up is optimized to offer a wide optical access (+/- 50⁰ vertical, +/-165⁰ horizontal), particularly well suited for Laue neutron diffraction techniques. First results obtained on the pressure-photo-induced spin crossover of a model coordination complex will be presented.

Hydrogen bonding structure and polymorphism in agrochemicals

Polymorphism of crystals, crystal habit and crystal growth are important factors that must be controlled for any commercial crystallization process. Pharmaceuticals and agrochemicals are two of the most industrially-important, active-molecule systems for which the physical properties are strongly correlated to their crystal structure. While pharmaceuticals have attracted more academic interest to date, the market for agrochemicals is also very considerable, amounting to $15 bn annually. Given the potential significant toxicity of some agrochemicals, the ability to control physical properties such as solubility and dissolution rates, which depend on the crystal structure of the agrochemical itself, represents a way of optimizing the ratio between the amount of product used and its efficiency, improving its function and reducing its environmental impact. Hydrogen bonds play a crucial role in the spatial arrangement of the active molecules and the crystallization process. However, high accuracy and precision of the hydrogen atom positions can only be achieved through single crystal neutron diffraction (SND). SND experiments have been performed on three herbicides - isoproturon (IPU), pendimethalin (PDM), and diflufenican (DFF) - and the fungicide cyprodinil (CYP) [1][2]. All four structure refinements show a ten-time improvement in precision in the hydrogen atom positions compared to SXD with accurately determined nuclear positions. For cyprodinil, which crystallises as two polymorphs, A and B, differences in the hydrogen bonding network have been determined. Form A is governed by single, linear hydrogen bonds between two molecules, while the B form is characterized by the presence of dimers linked through pairs of hydrogen bonds, leading to a stable 8-membered ring. These differences in structure are reflected in the physical properties of the two polymorphs such as melting point and the observed slow inter-conversion that takes place during storage.

Fast Neutron Laue Diffraction with CCD Detectors

The large area neutron Laue diffractometer based on CCD detectors (CYCLOPS [1]) has been developed and recently completed at the ILL. High-quality Laue patterns covering an angular range of 360⁰ horizontally and 92⁰ vertically, can be obtained in only few seconds. The diffractometer excels for fast survey of reciprocal space and fast data collections through phase transitions as well as in-situ experiments on single crystals with time resolution similar to that obtained with powder diffraction. The detector is being upgraded with new faster CCD cameras having a larger dynamic range. A protocol for detector corrections from spatial distortions and uniformity response has been established which allows to obtain accurate integrated intensities leading to good structural refinements. Examples of results of phase transitions and structural investigations will be presented.