Compressive behavior and electronic properties of ammonia ice: a first-principles study

Stabilisation of solid-state cubic ammonia confined in a glass substance at ambient temperature under atmospheric pressure

Ammonia, a widely available compound, exhibits structural transitions from solid to liquid to gas depending on temperature, pressure, and chemical interactions with adjacent atoms, offering valuable insights into planetary science. It serves as a significant hydrogen storage medium in environmental science, mitigating carbon dioxide emissions from fossil fuels. However, its gaseous form, NH3(g), poses health risks, potentially leading to fatalities. The sublimation pressure (psub) of solid cubic ammonia, NH3(cr), below 195.5 K is minimal. In this study, we endeavoured to stabilise NH3(cr) at room temperature for the first time. Through confinement within a boric acid glass matrix, we successfully synthesised and stabilised cubic crystal NH3(cr) with a lattice constant of 0.5165 nm under atmospheric pressure. Thermodynamic simulations affirmed the stabilisation of NH3(cr), indicating its quasi-equilibrium state based on the estimated standard Gibbs energy of formation, . Despite these advancements, the extraction of H2(g) from NH3(cr) within the boric acid glass matrix remains unresolved. The quest for an external matrix with catalytic capabilities to decompose inner NH3(cr) into H2(g) and N2(g) presents a promising avenue for future research. Achieving stability of the low-temperature phase at ambient conditions could significantly propel exploration in this field.

Electron- and positron-driven molecular processes for H2O, CO2, and NH3 in their gas and ice phases

In this comprehensive study, we explored the molecular chemistry of H2O, CO2, and NH3 molecules in their gas and ice phases assisted by electron and positron interactions over a wide range of energy from threshold to 5000 eV. These simple molecules have immense applications in areas such as atmospheric sciences and astrochemistry. We employed the spherical complex optical potential (SCOP) model along with the complex scattering potential-ionization contribution (CSP-ic) model and modified them to quantify the probabilities of various charged particle-driven molecular processes through inelastic cross-sections (Qinel), total ionization cross-sections (Qion), direct ionization cross-sections (QD-ion), and positronium formation cross-sections (QPs) for these molecules in gas and ice phases. We propose a novel model that incorporates the band gap of a material for positron impact cross-sections with molecules in their condensed phase. This is the first study on electron interactions with NH3 (ice) and positron interactions with H2O, CO2, and NH3 in their condensed phase.

Hydrogen Bonding in Liquid Ammonia

The nature of hydrogen bonding in condensed ammonia phases, liquid and crystalline ammonia has been a topic of much investigation. Here, we use quantum molecular dynamics simulations to investigate hydrogen bond structure and lifetimes in two ammonia phases: liquid ammonia and crystalline ammonia-I. Unlike liquid water, which has two covalently bonded hydrogen and two hydrogen bonds per oxygen atom, each nitrogen atom in liquid ammonia is found to have only one hydrogen bond at 2.24 Å. The computed lifetime of the hydrogen bond is t ≅ 0.1 ps. In contrast to crystalline water-ice, we find that hydrogen bonding is practically nonexistent in crystalline ammonia-I.

Self-assembled rhomboidal ammonia monolayer confined in two vertically stacked graphene oxide/graphene nanosheets

Confined water molecules have attracted widespread research interest due to their versatile phase behaviors. Ammonia (NH₃, isoelectronic with water) molecules are also expected to realize the delicate self-assembled hydrogen-bonded network like water in confinement. Here, the structures and phase behavior of NH₃ monolayers confined in two structurally symmetrical graphene oxide (GO) or graphene (G) nanosheets are investigated using first-principles calculations and ab initio molecular dynamics simulations. A highly ordered new rhomboidal phase with all NH₃ molecules adopting a Y-shaped configuration, in which one N-H bond is parallel to the confining planes and two other N-H bonds point to the top/bottom GO/G layers, respectively, was discovered at low temperature, resulting from the symmetrical confinement and subtle interlayer/intermolecular interactions. Remarkably, this new phase is so stable that a quite large strain is needed to destroy it. At room temperature, these NH₃ monolayers behave like a liquid. These rhomboidal NH₃ monolayers confined in GO/G nanosheets not only offer diverse hydrogen-bonded networks but also possess potential piezoelectricity for future device applications.