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

Computations on NaHn, n = 6-12, show that NaH9 is stable by P = 25 GPa. Cmc2(1)-NaH9 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 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 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 MHn with n > 1 become stable. The larger the radius of M, the greater the optimal value of n.