Ammonium fluoride's analogy to ice: Possibilities and limitations

Superconductivity in Dilute Hydrides of Ammonia under Pressure

The past decade has witnessed great progress in predicting and synthesizing polyhydrides that exhibit superconductivity under pressure. Dopants allow these compounds to become metals at pressures lower than those required to metallize elemental hydrogen. Here, we show that by combining the fundamental planetary building blocks of molecular hydrogen and ammonia, conventional superconducting compounds can be formed at high pressure. Through extensive theoretical calculations, we predict metallic metastable structures with NHn (n = 10, 11, 24) stoichiometries that are based on NH4+ superalkali cations and complex hydrogenic lattices. The hydrogen atoms in the molecular cation contribute to the superconducting mechanism, and the estimated superconducting critical temperatures, Tc's, are comparable to the highest values computed for the alkali metal polyhydrides. The largest calculated (isotropic Eliashberg) Tc is ∼180 K for Pnma-NH10 at 300 GPa. Our results suggest that other molecular cations can be mixed with hydrogen under pressure, yielding superconducting compounds.

Understanding the impact of ammonium ion substitutions on heterogeneous ice nucleation

Understanding the mechanisms underpinning heterogeneous ice nucleation in the presence of ionic inclusions is important for fields such as cryopreservation and for improved models of climate and weather prediction. Feldspar and ammonium are both present in significant quantities in the atmosphere, and experimental evidence has shown that feldspar can nucleate ice from ammonium-containing solutions at temperatures warmer than water alone. In recent work, Whale hypothesised that this increase in nucleation temperature is due to an increase in configurational entropy when an ammonium ion is included in the ice hydrogen bond network (T. F. Whale, J. Chem. Phys., 2022, 156, 144503). In this work, we investigate the impact of the inclusion of an ammonium ion on the hydrogen bond network by direct enumeration of the number of structures found using Rick's algorithm. We also determine the energy of these systems and thus compare the effects of enthalpy and entropy to test Whale's hypothesis. We find that the inclusion of an ammonium ion increases the total number of configurations under conditions consistent with a realistic surface charge. We also find that the enthalpic contribution is dominant in determining the location of the ammonium ion within the structure, although we note that this neglects other practicalities of ice nucleation.

Comparison of the phase transitions of high-pressure phases of ammonium fluoride and ice at ambient pressure

The phase diagrams of water and ammonium fluoride (NH₄F) display some interesting parallels. Several crystalline NH₄F phases have isostructural ice counterparts and one of the famous anomalies of water, the fact that the liquid is denser than ice Ih, is also found for NH₄F. Here, we investigate the phase transitions of the pressure-quenched high-pressure phases of NH₄F upon heating at ambient pressure with x-ray diffraction and calorimetry, and we compare the results with the corresponding ices. NH₄F II transforms to NH₄F Isd, which is a stacking-disordered variant of the stable hexagonal NH₄F Ih polymorph. Heating NH₄F III gives a complex mixture of NH₄F II and NH₄F Isd, while some NH₄F III remains initially. Complete conversion to NH₄F Isd is achieved above ∼220 K. The NH₄F II obtained from NH₄F III persists to much higher temperatures compared to the corresponding pressure-quenched NH₄F II. Quantification of the stacking disorder in NH₄F Isd reveals a more sluggish conversion to NH₄F Ih for NH₄F Isd from NH₄F III. In general, the presence of stress and strain in the samples appears to have pronounced effects on the phase transition temperatures. NH₄F shows a complete lack of amorphous forms at low temperatures either upon low-temperature compression of NH₄F Ih or heating NH₄F III at ambient pressure. The amorphous forms of ice are often used to explain the anomalies of water. It will, therefore, be interesting to explore if liquid NH₄F displays more water-like anomalies despite the apparent lack of amorphous forms at low temperatures.