Chemically driven negative linear compressibility in sodium amidoborane, Na(NH2BH3)

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.

Mono- and Bimetalic Amidoboranes

“…A comprehensive high pressure study of NaAB using in situ Raman spectroscopy and DFT calculations was performed by Magoś-Palasyuk et al [103]. An isosymmetric phase transition to α / -NaAB was observed at pressure about 3 GPa [103].…”

Experimental and theoretical evidence of dihydrogen bonds in lithium amidoborane

“…Our earlier studies on potassium and sodium amidoborane (KNH 2 BH 3 , NaNH 2 BH 3 ) 25 , 26 revealed a significant role of hydrogen bonding determining structural and spectroscopic properties of those monometallic amidoboranes. Data available in the literature on possible hydrogen bonding in lithium amidoborane are rather controversial.…”

“…Based on observations of bonding schemes of sodium and potassium compounds, NaAB 26 and KAB 25 , revealed under high-pressure conditions, an alternative hypothesis about the role of hydrogen mediated secondary interactions in the process of thermal decomposition of alkali metal amidoboranes might be tentatively proposed. We now know that in the solid state alkali metal amidoboranes form extended hydrogen-bonded networks presenting either conventional (redshift) or unconventional (blueshift) types of hydrogen bonding.…”

Negative Compressibility in the Monoclinic Octahedron Model Constructed by Hinging Wine‐Rack Mechanism

“…In general, the materials will shrink in all directions when subjected to hydrostatic pressure so that the compressibility is usually positive. However, there are a lot of materials [ 1–23 ] with unexpected mechanical behavior giving us some surprises, i.e., negative compressive materials. They will expand in one or two directions when subjected to hydrostatic pressure, which are called negative linear compressibility (NLC) and negative area compressibility (NAC), respectively.…”