Electronic Structure and Properties of Calcium and Iron Polyhydrides Under Pressure

The recent synthesis of the calcium and iron polyhydride phases under pressure has inspired theoretical investigations of their structures and electronic structures. First principles calculations have helped to characterize a newly synthesized C2/m symmetry Ca₂H₅ phase at 25 GPa and an I4/mmm symmetry CaH₄ phase at 120 GPa. Both of these phases contain atomic and molecular hydrogen. The H-H distances in the molecular units are elongated as compared to those in elemental H₂ at the same pressure. Electronic structure calculations illustrate that the bond-lengthening mechanism under pressure in the solid state resembles the one responsible for lengthening the H-H bond in Kubas like molecualr complexes. Crystal structure prediction techniques have also been employed to predict stable and metastable high hydrides of iron between 150-300 GPa that have not been discussed in other studies. Density functional theory calculations show that neither the I4/mmm nor the Cmca symmetry FeH₅ phases found to be stable are superconducting.