The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

490 / 2019-03-18 20:17:06

Nonmetallic FeH6 under High Pressure

High pressure,Iron hydride,Metal-nonmetal transtion,Structural prediction

Plenary track > High Pressure Physics and Materials Science

It is well-known that pressure induces insulator-to-metal transitions, but, even more interestingly, counterintuitive and rare pressure-driven metal-to-insulator transitions have been predicted and experimentally observed in very few systems, such as alkali and alkaline earth metals (e.g., Li, Na, and Ca) and their compounds (e.g., Li4C8 and Ca2N9). To better understand the pressure-induced nonmetallic transitions, it is really interesting to explore this unusual phenomenon in other compounds as well.
We have extensively explored the structural phase diagram of iron hydrides by using ab initio particle swarm optimization, and found a new stable stoichiometry, FeH6, above 213.7 GPa with C2/c symmetry. Interestingly, C2/c FeH6 presents an unexpected nonmetallicity, and its band gap becomes larger with increasing pressure. The nonmetallicity of C2/c FeH6 mainly originates from the pressure-induced hybridization between the Fe and H orbitals. This new compound shows a unique structure with a mixture of nonbonded hydrogen atoms in a helical iron framework. The strong Fe-Fe interaction and ionic Fe-H bonds are responsible for its structural stability. In addition, we have also found a more stable tetragonal FeH2 structure with the same I4/mmm symmetry as the previously proposed one, the X-ray diffraction pattern of which perfectly agrees with that of the experiment. Our study provides an opportunity for understanding pressure induced nonmetallic transitions in transition metal hydrides.