First-principles calculations of structural, electronic, and thermodynamic properties of Na2BeH4

The plane-wave ultrasoft pseudopotential method based on the density-functional theory has been used to calculate the electronic structures of Na2BeH4. These calculations reveal that at ambient conditions, the crystal structure of alpha-Na2BeH4 at ground state is alpha-K2ZnBr4-type monoclinic (space group P2(1)/m). With increasing pressure, the structural transition from alpha- to beta-Na2BeH4 (alpha-Cs2MgH4 type, space group Pnma) occurs at 1.1 GPa accompanied with a volume reduction of 8.7%. The density of states and electron localization function analyses show that a stronger covalent-bonding interaction prevails in BeH4 subunits and the crystals exhibit a nonmetallic feature. The relative thermal stability of alpha- to beta-Na2BeH4 was further investigated by performing phonon calculations based on the density-functional perturbation theory. The calculated Helmholtz free energies and vibrational entropies show that alpha-Na2BeH4 is always kept more stable than beta-Na2BeH4 with increasing temperature and is impossible to be transformed to the beta phase.