The thermodynamical stabilities for the series of metal borohydrides $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ ($M=\mathrm{Li}$, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf; $n=1–4$) have been systematically investigated by first-principles calculations. The results indicated that an ionic bonding between $M^{n+}$ cations and ${\left[\mathrm{B}{\mathrm{H}}_4\right]}^{-}$ anions exists in $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$, and the charge transfer from $M^{n+}$ cations to ${\left[\mathrm{B}{\mathrm{H}}_4\right]}^{-}$ anions is a key feature for the stability of $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$. A good correlation between the heat of formation $\Delta H_{\mathrm{boro}}$ of $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ and the Pauling electronegativity of the cation ${\chi }_P$ can be found, which is represented by the linear relation, $\Delta H_{\mathrm{boro}}=248.7{\chi }_P-390.8$ in the unit of kJ/mol $\mathrm{B}{\mathrm{H}}_4$. In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ ($M=\mathrm{Li}$, Na, K, Mg, Zn, Sc, Zr, and Hf), where the samples of the later five borohydrides were mechanochemically synthesized. The thermal desorption analyses indicate that $\mathrm{Li}\mathrm{B}{\mathrm{H}}_4$, $\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$, and $\mathrm{K}\mathrm{B}{\mathrm{H}}_4$ desorb hydrogen to hydride phases. $\mathrm{Mg}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$, $\mathrm{Sc}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$, and $\mathrm{Zr}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_4$ show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, $\mathrm{Zn}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$ desorbs hydrogen and borane to elemental Zn due to instabilities of Zn hydride and boride. A correlation between the desorption temperature $T_d$ and the Pauling electronegativity ${\chi }_P$ is observed experimentally and so ${\chi }_P$ is an indicator to approximately estimate the stability of $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$. The enthalpy change for the desorption reaction, $\Delta H_{\mathrm{des}}$, is estimated using the predicted $\Delta H_{\mathrm{boro}}$ and the reported data for decomposed product, $\Delta H_{\mathrm{hyd}∕\mathrm{boride}}$. The estimated $\Delta H_{\mathrm{des}}$ show a good correlation with the observed $T_d$, indicating that the predicted stability of borohydride is experimentally supported. These results are useful for exploring $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ with appropriate stability as hydrogen storage materials.

• Received 16 February 2006

DOI:https://doi.org/10.1103/PhysRevB.74.045126