Pressure-Stabilized Sodium Polyhydrides:NaHn(n>1)

Computations on ${\mathrm{NaH}}_{n}$, $n=6--12$, show that ${\mathrm{NaH}}_{9}$ is stable by $P=25\text{ }\text{ }\mathrm{GPa}$. $Cmc{2}_{1}\mathrm{\text{\ensuremath{-}}}{\mathrm{NaH}}_{9}$ containing both ${\mathrm{H}}_{2}$ and ${\mathrm{H}}^{\ensuremath{-}}$ units is metallic at $P>250\text{ }\text{ }\mathrm{GPa}$. Other phases with only ${\mathrm{H}}_{2}$ units metallize at lower pressures as a result of the partial filling of the ${\mathrm{H}}_{2}$ ${\ensuremath{\sigma}}_{u}^{*}$ bands by the Na $3s$ electrons. Pressure induced overlap of the Na $2p$ cores forestalls closure of the band gap in the odd phases with ${\mathrm{H}}^{\ensuremath{-}}$ atoms, but the even phases remain good metals up to 300 GPa. The lower the IP of the metal, the lower the pressure at which $M{\mathrm{H}}_{n}$ with $n>1$ become stable. The larger the radius of $M$, the greater the optimal value of $n$.