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The thermodynamical stabilities for the series of metal borohydrides M (B{H₄) }₍ (M=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 B{H₄}^- anions exists in M (B{H₄) }₍, and the charge transfer from M^n+ cations to B{H₄}^- anions is a key feature for the stability of M (B{H₄) }₍. A good correlation between the heat of formation H₁₎ₑ₎ of M (B{H₄) }₍ and the Pauling electronegativity of the cation can be found, which is represented by the linear relation, H₁₎ₑ₎=248. 7-390. 8 in the unit of kJ/mol BH₄. In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for M (B{H₄) }₍ (M=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 LiBH₄, NaBH₄, and KBH₄ desorb hydrogen to hydride phases. Mg (B{H₄) }₂, Sc (B{H₄) }₃, and Zr (B{H₄) }₄ show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, Zn (B{H₄) }₂ desorbs hydrogen and borane to elemental Zn due to instabilities of Zn hydride and boride. A correlation between the desorption temperature T₃ and the Pauling electronegativity is observed experimentally and so is an indicator to approximately estimate the stability of M (B{H₄) }₍. The enthalpy change for the desorption reaction, H₃₄ₒ, is estimated using the predicted H₁₎ₑ₎ and the reported data for decomposed product, H₇ₘ₃∕₁₎ₑ₈₃₄. The estimated H₃₄ₒ show a good correlation with the observed T₃, indicating that the predicted stability of borohydride is experimentally supported. These results are useful for exploring M (B{H₄) }₍ with appropriate stability as hydrogen storage materials.
Nakamori et al. (Fri,) studied this question.