First-principles study on the intermediate compounds of LiBH4_4

We report the results of the first-principles calculation on the intermediate compounds of LiBH$_4$. The stability of LiB$_3$H$_8$ and Li$_2$B$_n$H$_n (n=5-12)$ has been examined with the ultrasoft pseudopotential method based on the density functional theory. Theoretical prediction has suggested that monoclinic Li$_2$B$_{12}$H$_{12}$ is the most stable among the candidate materials. We propose the following hydriding/dehydriding process of LiBH$_4$ via this intermediate compound : LiBH$_4 \leftrightarrow {1/12}$Li$_{2}$B$_{12}$H$_{12} + {5/6}$ LiH $+ {13/12}$H$_2 \leftrightarrow$LiH $+$ B $+ {3/2}$H$_2$. The hydrogen content and enthalpy of the first reaction are estimated to be 10 mass% and 56 kJ/mol H$_2$, respectively, and those of the second reaction are 4 mass% and 125 kJ/mol H$_2$. They are in good agreement with experimental results of the thermal desorption spectra of LiBH$_4$. Our calculation has predicted that the bending modes for the $\Gamma$-phonon frequencies of monoclinic Li$_2$B$_{12}$H$_{12}$ are lower than that of LiBH$_4$, while stretching modes are higher. These results are very useful for the experimental search and identification of possible intermediate compounds.Comment: 7 pages, 5 figures, submitted to PR

Magnetic Phase Transition of MnBi under High Magnetic Fields and High Temperature

Magnetization measurements and differential thermal analysis (DTA) of polycrystalline MnBi were carried out in magnetic fields up to 14 T and in 300-773 K, in order to investigate the magnetic phase transition. The magnetic phase transition temperature (T t ) at a zero magnetic field is 628 K and linearly increases with increasing fields up to 14 T at the rate of 2 KT À1 . A metamagnetic transition between the paramagnetic and field-induced ferromagnetic states was observed just above T t . The exothermic and endothermic peaks were detected in the magnetic field dependence of DTA signals in 626-623 K, which relates to the metamagnetic transition. The obtained results were discussed on the basis of a mean field theory

Dehydriding reaction of metal hydrides and alkali borohydrides enhanced by microwave irradiation

Effects of microwave irradiation on metal hydrides (LiH, NaH, MgH 2, CaH2, and TiH2) and alkali borohydrides (LiBH4, NaBH4, and KBH4) were systematically investigated for the first time. TiH2 was heated to 600 K by microwave irradiation for 3.5 min, at which less than 0.16 mass % of hydrogen was desorbed from surface of the powder. On the other hand, LiBH4 was heated rapidly above 380 K, at which almost all hydrogen, 13 mass %, was desorbed. The rapid heating of TiH2 is mainly due to conductive loss, while that of LiBH4 is related to a structural transition at approximately 380 K

Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: First-principles calculations and experiments

The thermodynamical stabilities for the series of metal borohydrides M(BH₄)n (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 Mⁿ⁺ cations and [BH₄]⁻ anions exists in M(BH₄)n, and the charge transfer from Mⁿ⁺ cations to [BH₄]⁻ anions is a key feature for the stability of M(BH₄)n. A good correlation between the heat of formation ΔHboro of M(BH₄)n and the Pauling electronegativity of the cation ϰP can be found, which is represented by the linear relation, ΔHboro=248.7ϰP–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(BH₄)n (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(BH₄)₂, Sc(BH₄)₃, and Zr(BH₄)₄ show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, Zn(BH₄)₂ desorbs hydrogen and borane to elemental Zn due to instabilities of Zn hydride and boride. A correlation between the desorption temperature Td and the Pauling electronegativity ϰP is observed experimentally and so ϰP is an indicator to approximately estimate the stability of M(BH₄)n. The enthalpy change for the desorption reaction, ΔHdes, is estimated using the predicted ΔHboro and the reported data for decomposed product, ΔHhyd/boride. The estimated ΔHdes show a good correlation with the observed Td, indicating that the predicted stability of borohydride is experimentally supported. These results are useful for exploring M(BH₄)n with appropriate stability as hydrogen storage materials