Abstract
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.
Highlights
Derivatives obtained by substitution of a hydrogen proton in the NH3 group for a metal cation
In our recent work on potassium amidoborane we found that weak intermolecular interactions experienced significant enhancement under pressure[14]
It is in this context we have choosen to study sodium amidoborane using diamond anvil cell technique coupled with Raman spectroscopy for in situ probing vibrational properties under pressure
Summary
Synthesized samples of NaAB were used for our spectroscopic study. Details of synthesis procedure and variable pressure Raman measurements are described in Methods. Our recent high-pressure study revealed a spectacular ‘soft’ mode behavior for the one of N-H stretching vibrations indicating formation and continuous enhancement of conventional hydrogen bonding between molecular fragments of different layers[14]. Similar character of observed changes in experimental Raman spectra, namely, the “redshift” of B-N and N-H stretching modes as well as the higher anisotropy of N-H stretching vibrations, strongly suggests the experimentally found phase transition at about 3 GPa is of the same origin with the computed one at 10 GPa. The discrepancy in the value of the phase transition pressure might stem from the fact that total-energy calculations were performed at zero temperature. This work uncovers an example of intricate interplay between chemical interactions and mechanical response which opens up an exciting new direction for tailoring materials with rare functionality
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