Contrasting compressibility trends in layered structures Ag(tcm) and Ni(CN)₂

Ag(tcm) (tcm = tricyanomethanide) and Ni(CN)2 are layered structures. Both these materials exhibit area negative thermal expansion (area-NTE) due to 'rippling' of the layers - a displacement pattern that causes the interlayer separation to increase and the effective layer area to decrease (Fig. 1a). We have shown that Ag(tcm) shows negative area compressibility (NAC) under hydrostatic pressure. The latter can be attributed to the rippling phenomenon: when hydrostatic pressure is applied, the effective layer area increases (Fig. 1a). On the contrary, Ni(CN)2 shows shows positive linear compressibility in all directions, albeit much more strongly along the stacking axis than in any direction parallel to the square-grid sheets. This is attributed to transverse phonon modes within the Ni(CN)2 sheets, which can be visualised as 'tilting' of the rigid unit modes (RUMs) (Fig. 1b). A discussion of relating such behaviour to the Grüneisen parameters is undertaken in this study.

Hydrogen bond order/disorder in a polar one-dimensional confined ice

Ice is one of the most well-studied substances yet forming a polar, bulk phase, where the water molecule dipoles are all aligned, has proved to be challenging. One approach is to confine the water in nanotubes, such that the molecules are restricted in orientation [1], however this hinders their use as ferroelectric devices - the primary application of polar ices. An alternative is to exploit the voids in nanoporous crystalline hosts but there are, as yet, no reported confined ice systems where the water molecules are able to switch between paraelectric and ferroelectric states whilst retaining a crystalline structure. One such porous system is 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) tetrahydrate, where columnar stacks of the HHTP molecules are arranged in a pseudo-square arrangement and the water molecules occupy the pores, forming one-dimensional, meso-helical chains. The structure, determined previously [2], crystallises in Pbcn, however this places inversion centres within the water chains leading to unphysical intermolecular H...H contacts. A variable-temperature single-crystal X-ray diffraction investigation is described where careful consideration of the data shows how long-range dipole ordering within and between the water chains is induced, by varying temperature, transforming the crystal structure to the polar Pna21 space group [3]. Subsequent reassessment of the ambient temperature structure reveals a superposition of opposing polarisation states.

S-SAD phasing on SOLEIL Beamline PROXIMA 1

"PROXIMA 1, a beamline for macro-molecular crystallography at the 3rd generation synchrotron source SOLEIL, is equipped with a multi-circle goniometer (alpha 50 degrees) as well as a PILATUS 6M detector. These features, along with the extended energy range of the beam line towards the low energies (down to 5.5 keV) and the possibility to adapt the source size to the sample in order to optimize signal to noise ratio, have made the beam line very attractive for S-SAD phasing with more than seven examples of successful de novo phasing achieved over the last two years. The use of low energies has also proved a significant aid in assisting with MODEL building. The technical capabilities of the beam line for low energy data collections will be presented, along with a number of examples of the successful use of low wavelengths on the beam line. The importance of combining data from multiple sample orientations in order to achieve ""true multiplicity"" will be highlighted, as well as the importance of combining data from multiple crystals in order to achieve high multiplicity."

Mapping topology and structural disorder in [M(CN)₂]- coordination polymers

Dicyanometallate coordination polymers show a number of desirable properties: piezoelectricity, record-breaking negative thermal expansion (NTE) and negative linear compressibility (NLC), vapochromism, and luminescence. Despite this, the rules governing the structural trends arising from the use of this roughly 10 Å linker, [M(CN)2]- where M = Cu, Ag, or Au, are relatively poorly explored. This talk is concerned both with mapping the composition-connectivity relationship in these systems and understanding the role of guest cation size/shape mismatch. We show that using guest cations that would be a poor fit for the void-space in commonly adopted topologies can lead to two structural phenomena exemplified in compounds reported here. Firstly, the adoption of unusual topologies, such as 5-connected frameworks e.g.[Bu4N]0.5Cd[Ag(CN)2]2.5 and secondly, correlated structural disorder as seen in molecular perovskite analogues [Bu4N]M'[Au(CN)2]3 where M' = Cd, Mn, Fe.

Big Methods for Small Molecules

Since the early 70's, multiple wavelength experiments have been used to determine phases of proteins containing anomalous scatterers. The small molecule single crystal beamline, I19,[1] at Diamond Light Source, is designed to carry out single crystal anomalous dispersion studies using tunable wavelength. These experiments can differentiate between oxidation states; discriminate between atoms with near-identical X-ray scattering factors; and solve the phase problem for very low resolution X-ray data. We describe the application of MAD phasing (Karle and Hendrickson [2]) to determine the structure of large `small molecules' where only low-resolution data is available. Initial studies were carried out on a known, (well diffracting) centrosymmetric bromide containing compound. The wavelength dependence of the anomalous signal from the bromide was calculated from fluorescence absorption data in DetOx.[3] Datasets were then collected at 4 wavelengths chosen to maximize differences in the anomalous signal. Using the MAD phasing equations we obtained estimates for the anomalous scattering contribution from all atoms in the structure and a phase difference between that and the normal scattering component. This allowed us to reduce noise in the Patterson map and locate only the heavy atom scatterers. We then use phase estimates from the heavy atom substructure to locate the rest of the atoms. Initial proof of concept experiments will now be extended to larger structures where data is not of sufficient resolution to be solved by direct methods alone.