Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Decomposition mechanisms in metal borohydrides and their ammoniates   Ammoniation in metal borohydrides (MBs) with the form
$\mathcal{M}$(BH$_4$)$_x$ has been shown to lower their decomposition
temperatures with $\mathcal{M}$ of low electronegativity ($\chi_p \lesssim
1.6$), but raise it for high-$\chi_p$ MBs ($\chi_p \gtrsim 1.6$). Although this
behavior is just as desired, an understanding of the mechanisms that cause it
is still lacking. Using \emph{ab initio} methods, we elucidate those mechanisms
and find that ammoniation always causes thermodynamic destabilization,
explaining the observed lower decomposition temperatures for low-$\chi_p$ MBs.
For high-$\chi_p$ MBs, we find that ammoniation blocks B$_2$H$_6$
formation---the preferred decomposition mechanism in these MBs---and thus
kinetically stabilizes those phases. The shift in decomposition pathway that
causes the distinct change from destabilization to stabilization around
$\chi_p=1.6$ thus coincides with the onset of B$_2$H$_6$ formation in MBs.
Furthermore, with our analysis we are also able to explain why these materials
release either H$_2$ or NH$_3$ gas upon decomposition. We find that NH$_3$ is
much more strongly coordinated with higher-$\chi_p$ metals and direct H$_2$
formation/release becomes more favorable in these materials. Our findings are
of importance for unraveling the hydrogen release mechanisms in an important
new and promising class of hydrogen storage materials, allowing for a guided
tuning of their chemistry to further improve their properties.

11
The categories of this paper are: hep-lat
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con
19
The categories of this paper are: eess
32
The categories of this paper are: atom-ph
27
The categories of this paper are: comp-gas
29
The categories of this paper are: funct-an
0
The categories of this paper are: hep-ph
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
13
The categories of this paper are: quant-ph
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
34
The categories of this paper are: chem-ph
35
The categories of this paper are: mtrl-th