The thermodynamical stabilities for the series of metal borohydrides M͑BH 4 ͒ n ͑M = Li, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf; n =1-4͒ have been systematically investigated by first-principles calculations. The results indicated that an ionic bonding between M n+ cations and ͓BH 4 ͔ − anions exists in M͑BH 4 ͒ n , and the charge transfer from M n+ cations to ͓BH 4 ͔ − anions is a key feature for the stability of M͑BH 4 ͒ n . A good correlation between the heat of formation ⌬H boro of M͑BH 4 ͒ n and the Pauling electronegativity of the cation P can be found, which is represented by the linear relation, ⌬H boro = 248.7 P − 390.8 in the unit of kJ/mol BH 4 . In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for M͑BH 4 ͒ n ͑M = Li, Na, K, Mg, Zn, Sc, Zr, and Hf͒, where the samples of the later five borohydrides were mechanochemically synthesized. The thermal desorption analyses indicate that LiBH 4 , NaBH 4 , and KBH 4 desorb hydrogen to hydride phases. Mg͑BH 4 ͒ 2 , Sc͑BH 4 ͒ 3 , and Zr͑BH 4 ͒ 4 show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, Zn͑BH 4 ͒ 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 P is observed experimentally and so P is an indicator to approximately estimate the stability of M͑BH 4 ͒ n . The enthalpy change for the desorption reaction, ⌬H des , is estimated using the predicted ⌬H boro and the reported data for decomposed product, ⌬H hyd/boride . The estimated ⌬H 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 M͑BH 4 ͒ n with appropriate stability as hydrogen storage materials.

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The formation condition of an intermediate compound of LiBH 4 during the partial dehydriding reaction and its local atomistic structure have been experimentally investigated. LiBH 4 changes into an intermediate compound accompanying the release of approximately 11 mass % of hydrogen at 700-730 K. The Raman spectra indicate that the B-H bending and stretching modes of the compound appear at lower and higher frequencies, respectively, as compared to those of LiBH 4 . These features are consistent with the theoretical calculation on the monoclinic Li 2 B 12 H 12 , consisting of Li + and ͓B 12 H 12 ͔ 2− ions, as a possible intermediate compound of LiBH 4 .

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