High pressure induces unexpected chemical and physical properties in materials. For example, hydrogen-rich compounds under pressure have recently gained much attention as potential room-temperature superconductors, and iron hydrides have also gained significant interest as potential candidates for being the main constituents of the Earth’s core. It is well-known that pressure induces insulator-to-metal transitions, whereas pressure-induced metal-to-insulator transitions are rare, especially for transition metal hydrides. In this article, we have extensively explored the structural phase diagram of iron hydrides by using ab initio particle swarm optimization. We have 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. This is in sharp contrast with P2₁/m FeH₄. 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.

中文翻译：

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高压非金属FeH ₆

高压会导致材料产生意想不到的化学和物理特性。例如，最近在压力下富含氢的化合物作为潜在的室温超导体受到了广泛关注，而氢化铁作为潜在的地核主要成分也引起了极大的兴趣。众所周知，压力会引起绝缘体到金属的转变，而压力引起的金属到绝缘体的转变却很少，尤其是对于过渡金属氢化物而言。在本文中，我们使用了从头算粒子群优化技术来广泛探索氢化铁的结构相图。我们发现了一种新的稳定化学计量FeH ₆，高于213.7 GPa且具有C 2 / c对称性。有趣的是，C 2 / c FeH ₆表现出意外的非金属性，并且其带隙随着压力的增加而变大。这与P 2 ₁ / m FeH ₄形成鲜明对比。C 2 / c FeH ₆的非金属性主要源自Fe和H轨道之间的压力诱导杂化。这种新化合物显示出独特的结构，在螺旋铁骨架中具有未键合氢原子的混合物。Fe-Fe的强相互作用和离子Fe-H键是其结构稳定性的原因。此外，我们还发现了具有相同结构的更稳定的四方FeH ₂结构I 4 / mmm对称性与先前提出的对称，其X射线衍射图与实验完全吻合。