Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides

Tetrahydrides crystallizing in the ThCr₂Si₂ structure type have been predicted to become stable
for a plethora of metals under pressure, and some have recently been synthesized. Herein, we
analyze the bonding and electronic structures of these phases as a function of the metal atom, and
determine the factors important for superconductivity. Hydrides with: (i) divalent metal atoms can
be superconducting only if the distance between H^- units is small enough so these bands disperse
above the Fermi level (e.g. MgH₄); (ii) trivalent metal atoms will generally be superconducting,
but only the lighter metals will afford high T_cs (e.g. ScH₄); (iii) tetravalent metal atoms can only
be superconducting if pressure induces a change in the oxidation state, and the metal ions are
not too large so that H₂ molecules with stretched bonds can form (e.g. ZrH₄). T_c increases with
decreasing pressure, and the phonon modes soften, particularly at the M-point.