Crystal Structures and Properties of Iron Hydrides at High Pressure

Evolutionary algorithms and the particle swarm optimization method have been used to predict stable and metastable high hydrides of iron between 150 and 300 GPa that have not been discussed in previous studies. Cmca FeH₅, Pmma FeH₆, and P2/c FeH₆ contain hydrogenic lattices that result from slight distortions of the previously predicted I4/mmm FeH₅ and Cmmm FeH₆ structures. Density functional theory calculations show that neither the I4/mmm nor the Cmca symmetry FeH₅ phase is superconducting. A P1 symmetry FeH₇ phase, which is found to be dynamically stable at 200 and 300 GPa, adds another member to the set of predicted nonmetallic transition metal hydrides under pressure. Two metastable phases of FeH₈ are found, and the preferred structure at 300 GPa contains a unique 1-dimensional hydrogenic lattice.