| Hydrogen is an kind of abundant and pollution free energy. The key problem of widely application of hydrogen is the storage of hydrogen. While complex hydride is an kind of compound contaning large amount of hydrogen. However, hydrogen release from complex hydride need very high temperature. Morover, the reaction is ranther slow and irreversiable. The situation doesn't change until1997. In experiment, it is found that doping of NaAlH4with Ti-base catalyst can bring the reaction faster and reversibale.In the last decade, many effort are made in searching suitable catalyst and optimizing the reaction. Another research interest is to find better compound other than NaAlH4. Boron based complex hydride contains more hydrogen than NaAlH4does, so it attract large interest and more researchers. So, we studied NaBH4with first principles method.First, we studied the physical properties of NaBH4, including electronic strcture. bonding charactristic, mechanical properties etc. We found that the behaviour is similar to that of NaAlH4. The basic composer is Na cation and BH4anion. The chemical bonding between Na and BH4is ionic, while bonding within BH4is covalent. The stability of NaBH4is influenced by charge transfer from Na cation to BH4anion. Based on this chemical bonding character, we studied the influence of catalyst Ti on stability of NaBH4. We found that when Na replace Na. the crystal structure around Ti is not affected largely. While the situation has a large change when Na replace B. The Na cation around the Ti in B site is repeled and Ti-H bond length is larger indicating the weaker binding of hydrogen in this compound. In the view of charge transfer, it is found that Ti replacing Na does not affect the charge transfer. So, the stability of the compound is almost unchanged. When Na replace B. Ti-H bonding is rather different from B-H bonding. The bonding between Ti-H is more like ionic while not covalent. In one word, Ti doping not only influences the crystal structure around the Ti, it also affect the binding of hydrogen in the compound.Beside the electronic structure, we also inversitigate the mechnical and thermodynanic properties of NaBEH4and related compound, such as KBH4. RbBH4, and CsBH4. We found that the bulk moduli are small indicating large volume collapse with not large pressure. It is found that these compounds have phase transition within several GPa. so. it should be careful when storage hydrogen using these compound. Because different phases may have different hydrogen storage behaviour. Meanwhile, we also inverstigate the elastic behaviour under pressure and anharmonicity of elastic constant. We found that the elastic constants have a linear relationship with pressure within9GPa. This phenomenon indicates the phase transition can not be reflected by elastic constants. In fact, the phase transition of NaBH4under high pressure is order-disorder phase transition and it is related the the orientational disorder of BH4anion. So, it seems very important to consider the pressure effect on rotational barriers when study order-disorder phase transition. Through high order elastic constant, we found the anharmonicity of elastic constant is not large. In fact, considering of high order factors did not give very different second order elastic constant. And when using experimental lattice constants, the calculated elastic constants are very close to the values of experimental ones. This suggest that the optimized lattice constants is very sensitive to the predictions of elastic constants.Using strain-energy relationship, we studied the stability of β-LiBH4. We found that when two different strains are imposed on β-LiBH4, the structure becomes unstable within the used order model. And the more stable structure is Cmc21and Cc. Together previous theoretical study, we found that β-LiBH4with P63mc space group is unstable not only relating to BH4rotation, but also relating to the lattice structure. As previous theoretical results are got with order model, the unstability may also comes from this order model. Because BH4is orientational disorder in β-LiBBH4structure. So, whether our results is reliable or not is depend on the entropy in different strained structure. Moreover, as the entropy is related to the orientational disorder of BH4, so, if rotaional barriers in different strained strcuture is similar, then the entropy contribute the same value in different strained structure. If it is indeed this situation, then our model predictions is correct.Biside the studies out effect of Ti catalyst, we also investigate the different catalytic effect of different catalyst. We found that almost all catalysts have abvious catalystic effect except Ag. The catalytic effect can be explained using Pt example. When Pt replace Na. the Pt-BH4local structure becomes Pt-H-BH2structure when one hydrogen atom is released. Then one B-H bond in BH3anion is weaker. This may be the key factors influencing the catalytic effect of different catalyst. When Pt replace B. PtH3will attract one H atom from nearby BH4after one hydrogen in PtH4is released. PtH3-H is almost the planar structure while not maitains tetrahedron structure. This planar structure has a weaker Pt-H bond and will be earsier to release H atom.
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