Decomposition mechanisms in metal borohydrides and their ammoniates

We present results from \textit{ab initio} calculations to explain the mechanisms that cause ammoniation to lower decomposition temperatures in metal borohydrides ($\mathcal{M}$(BH$_4$)$_x$, MBs) with $\mathcal{M}$ of low electronegativity ($\chi_p < 1.6$), but raise decomposition temperatures of high-$\chi_p$ MBs ($\chi_p > 1.6$). Results include an examination of materials' dihydrogen bond networks, energy of formation calculations, and transition state searches for potential decomposition mechanisms. We find that ammoniation always causes thermodynamic destabilization, helping to explain 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. We describe two ways that B$_2$H$_6$ formation can be blocked, which are each valid in one of the ammoniated MBs ($\mathcal{M}$(BH$_4$)$_x\cdot$$y$NH$_3$, MBAs) studied. We also consider why these materials release either H$_2$ or NH$_3$ gas upon decomposition. We find that NH$_3$ is much more strongly coordinated with higher-$\chi_p$ metals, and that direct H$_2$ formation becomes easier in higher-$\chi_p$ materials, contributing to why higher-$\chi_p$ MBAs are more likely to produce H$_2$ upon decomposition.