Andrew B. Cairns

Homologous critical behavior in the molecular frameworks Zn(CN)2 and Cd(imidazolate)2.

Using a combination of single-crystal and powder X-ray diffraction measurements, we study temperature- and pressure-driven structural distortions in zinc(II) cyanide (Zn(CN)2) and cadmium(II) imidazolate (Cd(im)2), two molecular frameworks with the anticuprite topology. Under a hydrostatic pressure of 1.52 GPa, Zn(CN)2 undergoes a first-order displacive phase transition to an orthorhombic phase, with the corresponding atomic displacements characterized by correlated collective tilts of pairs of Zn-centered tetrahedra. This displacement pattern sheds light on the mechanism of negative thermal expansion in ambient-pressure Zn(CN)2. We find that the fundamental mechanical response exhibited by Zn(CN)2 is mirrored in the temperature-dependent behavior of Cd(im)2. Our results suggest that the thermodynamics of molecular frameworks may be governed by considerations of packing efficiency while also depending on dynamic instabilities of the underlying framework topology.

Compositional dependence of anomalous thermal expansion in perovskite-like ABX3 formates

The compositional dependence of thermal expansion behaviour in 19 different perovskite-like metal-organic frameworks (MOFs) of composition [A(I)][M(II)(HCOO)3] (A = alkylammonium cation; M = octahedrally-coordinated divalent metal) is studied using variable-temperature X-ray powder diffraction measurements. While all systems show essentially the same type of thermomechanical response-irrespective of their particular structural details-the magnitude of this response is shown to be a function of A(I) and M(II) cation radii, as well as the molecular anisotropy of A(I). Flexibility is maximised for large M(II) and small A(I), while the shape of A(I) has implications for the direction of framework hingeing.

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.

Zero-strain reductive intercalation in a molecular framework.

Intercalation of potassium into the molecular framework silver hexacyanoferrate occurs with remarkably small volume strain.

Anomalous thermal expansion, negative linear compressibility, and high-pressure phase transition in ZnAu2(CN)4 : Neutron inelastic scattering and lattice dynamics studies

We present temperature dependent inelastic neutron scattering measurments, accompanied byab-initio calculations of phonon spectra and elastic properties as a function of pressure to understand anharmonicity of phonons and to study the mechanism of negative thermal expansion and negative linear compressibility behaviour of ZnAu2(CN)4. The mechanism is identified in terms of specific anharmonic modes that involve bending of the Zn(CN)4-Au- Zn(CN)4 linkage. The high-pressure phase transition at about 2 GPa is also investigated and found to be related to softening of a phonon mode at the L-point at the Brillouin zone boundary and its coupling with a zone-centre phonon and an M-point phonon in the ambient pressure phase. Although the phase transition is primarily driven by a L-point soft phonon mode, which usually leads to a second order transition with a 2 x 2 x 2 supercell, in the present case the structure is close to an elastic instability that leads to a weakly first order transition.

Interlayer bond formation in black Phosphorus at high pressure

Abstract Black phosphorus was compressed at room temperature across the A17, A7 and simple‐cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X‐ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple‐cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two‐step mechanism for the A7 to simple‐cubic phase transition, indicating the existence of an intermediate pseudo simple‐cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non‐layered simple‐cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene‐based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.

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].

HP–HT behavior of urea, a precursor to photocatalytic materials

Urea, CO(NH2)2, is an inexpensive and easily accessible compound, known as the principal end product of nitrogenous metabolism in mammals. Although it has been studied at high pressure for more than 100 years [1], the crystal structures of its non-ambient phases have been reported only recently [2]. Still, its behavior under high pressure-high temperature (HP-HT) conditions has not been studied so far. High pressure is known not only as a tool to access new crystal polymorphs, but also to synthesize novel materials starting from molecular low-Z substances. Pyrolysis of urea at the atmospheric pressure and 830 K without additive assistance is known to produce graphitic carbon nitride, an effective catalyst for water splitting. Therefore, we have undertaken a combined FT-IR absorption spectroscopy and angle dispersive XRD study to explore the phase diagram of urea up to 15 GPa and 600 K [3], and to characterize the products of its HP-HT decomposition in situ and after quenching to ambient conditions. The phase II, reported already in the volumetric study by Bridgman [1], was structurally characterized for the first time and found to be isostructural with the room-temperature HP phase IV. It was also noticed that the sluggish transitions and enhanced metastability regions for all the phases are a consequence of differences in hydrogen-bond patterns and the high energy barrier of their rearrangements. HP-HT studies in a hydrostatic pressure transmitting medium allowed to define the thermodynamic boundaries of crystal phases and draw the tentative phase diagram of urea (see the figure). The reaction boundary of urea has been also delineated. At pressure lower than 3 GPa and temperature around 420 K the 2D graphitic carbon nitride oxide of high crystallinity was formed. At higher pressure, where the reaction begins from the phase V, the selectivity is poor and the carbon nitride oxide is obtained with a lower yield, together with by-products like melamine and ammonium carbamate. This study shows that the solid-state reactions under high pressure are directed by the structure of the molecular crystal precursor, depending on the network of hydrogen bonds. It is known that a photoactivity of graphitic carbon nitride materials is improved by introducing heteroatoms in its structure, and the HP-HT one-pot synthesis of highly crystalline oxide of graphitic carbon nitride can be an efficient method to produce this important photocatalyst. Moreover, the moderate pressure conditions indicate the possibility to scale up the process, while decreasing the synthesis temperature suggests reduced energy cost. From a fundamental point of view the negative, anomalous slope of the instability boundary, which is presumably due to the strong hydrogen bonds between molecules in the urea crystal, requires further studies. [1] Bridgman, P. W. (1916). Proc. Am. Acad. Arts Sci. 52, 91-187. [2] Olejniczak, A. et al. (2009). J. Phys. Chem. C 113, 15761-15767. [3] Dziubek, K. et al. (2017). J. Phys. Chem. C 121, 2380-2387.

Complexity in supramolecular analogues of frustrated magnets at high pressure

Andrew Brian Cairns1, Matthew J. Cliffe2, Joseph A. M. Paddison3, Dominik Daisenberger4, Matthew G. Tucker5, François-Xavier Coudert6, Andrew L. Goodwin7, Mohamed Mezouar1 1ESRF The European Synchrotron, Grenoble, France, 2Department of Chemistry, University of Cambridge, Cambridge, United Kingdom, 3Department of Physics, University of Cambridge, Cambridge, United Kingdom, 4Diamond Light Source, Didcot, United Kingdom, 5Spallation Neutron Source, Oak Ridge, United States, 6CNRS & Chimie ParisTech, Paris, France, 7Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford, United Kingdom E-mail: andrew.cairns@esrf.fr