Christopher J. Ridley

Ammonia/water mixtures are key constituents of icy planetary interiors, yet their amorphous phases, structures, and formation pathways remain completely unexplored. Neutron diffraction measurements, interpreted using empirical potential structure refinement (EPSR), have been used to characterize the local structure of liquid and two distinct amorphous ammonia dihydrate phases. We find that flash-freezing at ambient pressure produces an amorphous solid that is structurally distinct from the liquid, with stronger water-water coordination, enhanced five-membered hydrogen-bonded rings, and a more ordered and less homogeneous local environment. The O-O pair distribution function resembles expanded high-density amorphous ice, while the overall coordination remains closer to that of low-density amorphous ice with ammonia being seen to stabilize local 4-fold hydrogen bonding while enabling denser medium-range packing. A second amorphous formation pathway emerges when flash-freezing occurs alongside compression to 0.3 GPa, producing a second distinct amorphous solid to flash-freeze at ambient pressure. Increasing pressure leads to a pressure-induced structural change indicated by the collapse of all of the second coordination shells (O-O, O-N, and N-N), leading to much denser packing and a reorganization of the hydrogen-bond network, despite the overall number of H-bonds remaining constant across all different conditions. These results demonstrate that ammonia/water mixtures exhibit multiple, pathway-dependent amorphous states, extending known water polyamorphism to heterogeneous H-bonded binary mixtures. This has fundamental implications for ice formation, stability, and dynamics in planetary and astrophysical environments.

High-pressure neutron diffraction shows that arsenolite forms a stable 1 : 2 deuterium inclusion compound (As ₄ O ₆ ·2D ₂ ), where D ₂ occupies one crystallographic site and becomes rotationally disordered on decompression without additional site occupancy.

A pressure-induced isostructural phase transition in Mn ₇ C ₃ is shown to lead to a significant increase in its bulk modulus.

The locations and occupancies of deuterium atoms in molybdenum deuteride were studied using time-of-flight neutron powder diffraction under pressures up to ∼6.2 GPa. We confirmed the P6₃/mmc space group and determined the overstoichiometric deuterium content to give a composition of MoD_1.15, showing that our data are sensitive to deuterium positions and occupancies. In MoD_1.15, the majority of the interstitial deuterium atoms occupy the octahedral sites, and the remainder occupy the tetrahedral sites and exhibit relatively short interatomic distances of 1.49 and 1.90 Å to molybdenum atoms.

Single v s . double networks: Cu(dca) ₂ shows NLC = −6.5 TPa ^{− 1} via flexible dca ⁻ then transitions to monoclinic at 1.11 GPa; Cu(tcm) ₂ exhibits ZLC through rigid tcm ⁻ maintaining structure to 3.75 GPa.

Pyrazine (1,4-diazine) is a widely used linker in coordination polymers, molecular magnets, and metal–organic frameworks, yet its solid-state polymorphism is controlled by weak C–H···N interactions. Using variable-temperature and high-pressure neutron diffraction, a previously unrecognized low-temperature transition to phase IV is identified, showing that the form stabilized under pressure can also be accessed below ∼90 K at ambient pressure in the perdeuterated compound. The transition from phase III → IV involves a symmetry-breaking lattice distortion driven by the reorganization of C–H···N hydrogen bonds, accompanied by negative linear compressibility, which is primarily accommodated through the reduction in void-space within the structure. Structural analysis reveals that while the overall monoclinic framework of phase IV is preserved under both temperature and pressure routes, the evolution of hydrogen-bonded network is distinct depending on the pathway taken. These results establish pyrazine as a model molecular solid for studying pressure–temperature polymorphism and demonstrate how weak directional interactions can produce anomalous mechanical responses in organic crystals.

Ice VII is thought to play a role in the water-rich interiors of Jupiter's moon Europa and Saturn's moon Enceladus and other planetary bodies. Our understanding of the physical properties of this phase therefore has far-reaching consequences both in planetary research and the wider physics and chemistry community. From its average structure, ice VII is seemingly simple, however the local structure reveals hidden complexity, namely individual positions of water molecules forming a complex network via hydrogen bonds. Through coupling Pair Distribution Function and Reverse Monte Carlo modelling to high pressure neutron scattering data, we have quantified the atomic and molecular structures of disordered ice VII. The decomposition of the average structure of ice VII into the individual positions of water molecules within the crystal lattice reveals that the D2O molecules are displaced along the direction of the polarization vector of each molecule. By applying this displacement, the structural model more accurately matches the D–O distances and D–O–D angles determined from the other ordered ice structures. Our experimental results are also supported by DFT calculations confirming that deviations of water molecules from their average crystallographic positions energetically stabilize the structure of ice VII. Our studies open new perspectives for structural studies of different forms of ice, their phase transitions treating them as vast clusters of molecules with an average periodic structure but symmetry-free local arrangements.