Pressure-stabilized sodium polyhydrides: NaH(n) (n>1).

Computations on NaH(n), n=6-12, show that NaH(9) is stable by P=25  GPa. Cmc2(1)-NaH(9) containing both H(2) and H(-) units is metallic at P>250  GPa. Other phases with only H(2) units metallize at lower pressures as a result of the partial filling of the H(2) σ(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 H(-) 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 MH(n) with n>1 become stable. The larger the radius of M, the greater the optimal value of n.