Nicholas P. Funnell

Correlated defect nanoregions in a metal–organic framework

Throughout much of condensed matter science, correlated disorder is key to material function. While structural and compositional defects are known to exist within a variety of metal–organic frameworks, the prevailing understanding is that these defects are only ever included in a random manner. Here we show—using a combination of diffuse scattering, electron microscopy, anomalous X-ray scattering, and pair distribution function measurements—that correlations between defects can in fact be introduced and controlled within a hafnium terephthalate metal–organic framework. The nanoscale defect structures that emerge are an analogue of correlated Schottky vacancies in rocksalt-structured transition metal monoxides and have implications for storage, transport, optical and mechanical responses. Our results suggest how the diffraction behaviour of some metal–organic frameworks might be reinterpreted, and establish a strategy of exploiting correlated nanoscale disorder as a targetable and desirable motif in metal–organic framework design.

Negative area compressibility in silver(I) tricyanomethanide

The molecular framework Ag(tcm) (tcm(-) = tricyanomethanide) expands continuously in two orthogonal directions under hydrostatic compression. The first of its kind, this negative area compressibility behaviour arises from the flattening of honeycomb-like layers during rapid pressure-driven collapse of the interlayer separation.

Destabilisation of hydrogen bonding and the phase stability of aniline at high pressure

Two crystalline phases of aniline have been investigated by a combination of single crystal X-ray diffraction data on aniline-h7 and neutron powder diffraction data on aniline-d7. Phase-I, which is formed on cooling the liquid at ambient pressure, is monoclinic (P21/c). Orthorhombic (Pna21) phase-II was crystallised at 0.84 GPa at room temperature and structurally characterised at pressures up to 7.3 GPa. The strongest intermolecular interactions in both structures are NH⋯π contacts and NH⋯N H-bonds. These interactions occur within layers in both phases, and the phases differ in the way the layers are stacked. The structures of both phases have been obtained under two sets of identical conditions, at 0.84 GPa and 0.35 GPa and studied at room temperature by neutron powder and X-ray single-crystal diffraction. At 0.84 GPa phase-II is the thermodynamically stable form because it has a lower molar volume than phase-I, but as the pressure is reduced the volume of phase-I becomes less than that of phase-II, and at 0.35 GPa phase-II partially transformed into phase-I. PIXEL calculations indicate that the intermolecular interaction energy for pairs of molecules connected by H-bonds is −9 to −16 kJ mol−1 in phase-I and II at 0.84 GPa, but one of these becomes destabilising in phase-II at 7.3 GPa, with an energy of +1 kJ mol−1, making it similar to several compressed CH⋯π contacts. The results demonstrate how the hierarchy of intermolecular interaction energies can be manipulated with pressure, driving a H-bond beyond its ambient-pressure distance limit into repulsive region of its potential, and trapping it within a compressed crystal structure.

Mesoscale Polarization by Geometric Frustration in Columnar Supramolecular Crystals

Abstract Columnar supramolecular phases with polarization along the columnar axis have potential for the development of ultrahigh‐density memories as every single column might function as a memory element. By investigating structure and disorder for four columnar benzene‐1,3,5‐trisamides by total X‐ray scattering and DFT calculations, we demonstrate that the column orientation, and thus the columnar dipole moment, is receptive to geometric frustration if the columns aggregate in a hexagonal rod packing. The frustration suppresses conventional antiferroelectric order and heightens the sensitivity towards collective intercolumnar packing effects. The latter finding allows for the building up of mesoscale domains with spontaneous polarization. Our results suggest how the complex interplay between steric and electrostatic interactions is influenced by a straightforward chemical design of the molecular synthons to create spontaneous polarization and to adjust mesoscale domain size.

Competing hydrostatic compression mechanisms in nickel cyanide

We use variable-pressure neutron and X-ray diffraction measurements to determine the uniaxial and bulk compressibilities of nickel(II) cyanide, Ni(CN)2. Whereas other layered molecular framework materials are known to exhibit negative area compressibility, we find that Ni(CN)2 does not. We attribute this difference to the existence of low-energy in-plane tilt modes that provide a pressure-activated mechanism for layer contraction. The experimental bulk modulus we measure is about four times lower than that reported elsewhere on the basis of density functional theory methods [Phys. Rev. B 83 (2011) 024301].