We report a novel family of hydrogen-rich materials - alkali metal di(amidoborane)borohydrides, M(BH3NH2BH2NH2BH3). The title compounds are related to metal amidoboranes (amidotrihydroborates) but have higher gravimetric H content. Li salt contains 15.1 wt% H and discharges very pure H2 gas. Differences in thermal stability between amidoboranes and respective oligoamidoboranes explain the release of the ammonia impurity (along with H2) during the thermal decomposition of light alkali amidoboranes, LiNH2BH3, NaNH2BH3 and NaLi(NH2BH3)2, and confirm the mechanism of the side decomposition reaction.

Modern ab initio calculations predict ionic and superionic states in highly compressed water and ammonia. The prediction apparently contradicts state-of-the-art experimentally established phase diagrams overwhelmingly dominated by molecular phases. Here we present experimental evidence that the threshold pressure of B120 GPa induces in molecular ammonia the process of autoionization to yet experimentally unknown ionic compoundammonium amide. Our supplementary theoretical simulations provide valuable insight into the mechanism of autoionization showing no hydrogen bond symmetrization along the transformation path, a remarkably small energy barrier between competing phases and the impact of structural rearrangement contribution on the overall conversion rate. This discovery is bridging theory and experiment thus opening new possibilities for studying molecular interactions in hydrogen-bonded systems. Experimental knowledge on this novel ionic phase of ammonia also provides strong motivation for reconsideration of the theory of molecular ice layers formation and dynamics in giant gas planets.

Over the past few years we have been witnessing a surge of scientific interest to materials exhibiting a rare mechanical effect such as negative linear compressibility (NLC). Here we report on strong NLC found in an ionic molecular crystal of sodium amidoborane (NaAB) – easily-accessible, optically transparent material. In situ Raman measurements revealed abnormal elongation of B-N and N-H bonds of NaAB at pressure about 3 GPa. Ab initio calculations indicate the observed spectroscopic changes are due to an isostructural phase transition accompanied by a stepwise expansion of the crystal along c axis. Analysis of calculated charge density distribution and geometry of molecular species (NH2BH3) univocally points to a chemically driven mechanism of NLC – pressure-induced formation of hydrogen bonds. The new H-bond acts as a “pivot screw” coupling N-H covalent bonds of neighbor molecular species – a system resembling a two-lever “jack device” on a molecular scale. A mechanism based on formation of new bonds stands in apparent contrast to mechanisms so far reported in majority of NLC materials where no significant alteration of chemical bonding was observed. The finding therefore suggests a qualitatively new direction in exploration the field towards rational design of incompressible materials.