Charge Transfer Induced Band Gap Closure: Trend in Rare Earth Tetrahydrides Discovered under Pressure

The recent success of the high-pressure synthesis of CaH₄ inspired this analysis of a plethora of previously predicted high-pressure rare-earth metal tetrahydrdes: MH₄ (M = Ca, Sr, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu). This type of structure, assuming the space group of I4/mmm and resembling the ThCr₂Si₂ structure type, contains layers of edge-sharing tetrahedra in the xy plane comprised by the negatively charged atomistic hydrogen layers and molecular H₂ layers. When this set of structures are optimized to 0 GPa, while the pressure-induced band broadening is eliminated, the H-H distances within the H₂ units are inversely associated with the charges transferred to it, which leads to bandgap closure. The existence of quasi-molecular H₂ is stabilized by the electron donation from H₂ σ bonding state to the vacant metal d states and the back donation from occupied metal d states to the H₂ σ* anti-bonding states. The analysis of the electronic properties and stabilization mechanisms of these high-pressure rare earth metal tetrahydrides would potentially benefit the understanding of their exotic physical properties: i.e. superconductivity novel, magnetic behavior.