Pressure-induced structural phase transition inNaBH4

Abstract

We present a combined experimental and theoretical study of the technologically important ${\mathrm{NaBH}}_{4}$ compound under high pressure. Using Raman spectroscopy at room temperature, we have found that ${\mathrm{NaBH}}_{4}$ undergoes a structural phase transformation starting at 10.0 GPa with the pure high-pressure phase being established above 15.0 GPa. In order to compare the Raman data recorded under high pressure with the low-temperature tetragonal phase of ${\mathrm{NaBH}}_{4}$, we have also performed a cooling experiment. The known order-disorder transition from the fcc to the tetragonal structure was then observed. However, the new high pressure phase does not correspond to this low-temperature structure. Using first-principle calculations based on the density functional theory, we show that the high-pressure phase corresponds to the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{LiAlH}}_{4}$--type structure. We have found a good agreement between the measured and calculated transition pressures. Additionally, we present the electronic structure of both the fcc and the high-pressure phases.