Effect of Mg Doping in NaAlH$_4$

First principles calculations based on density functional theory have been carried out to search for suitable catalysts that can lower the hydrogen desorption temperature from sodium-alanate (NaAlH$_{4})$. We focus here on the possibilities of doping Mg in bulk sodium alanate. The result shows that Mg prefers to occupy the Na site and weakens the covalent bond between Al and H. The energy needed to remove a hydrogen atom from (Mg,Na)AlH$_{4}$ is found to be significantly lower than that from NaAlH$_{4}$. The effect is similar to Ti doping which is supposedly the best catalyst found to date for hydrogen desorption from sodium alanate. Furthermore, the Mg doping is shown to promote the formation of Na vacancy which in turn plays an important role in the hydrogen desorption process.