Lattice dynamics and thermodynamic properties of NaAlH4:: Density-functional calculations using a linear response theory

We present a first-principles investigation of the lattice dynamics and thermodynamical properties of a complex hydride NaAlH4, a promising material for hydrogen storage. The calculations are performed within the density-functional-theory framework and using a linear response theory. Calculations of the phonon spectrum, Born effective charges Z(*), and dielectric constants in high and low frequency limits are reported. The mode characters of the zone-center phonons, including the LO-TO splitting, are identified and compared to the experiment. The quasiharmonic approach is used to study thermal expansion as well as the mean square displacement of each atom as a function of temperature. A connection is established between the latter and the melting point. The inclusion of the zero-point motion leads to an expanded lattice compared to the static lattice, while the low frequency oscillations are found to play an important role in the melting and decomposition of NaAlH4.