Nonmetallic FeH₆ under High Pressure

Abstract

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.

Crystal Structures and Properties of Iron Hydrides at High Pressure

“…The geometries, electronic structures and propensity for superconductivity of stable and metastable phases has been investigated. Our work adds the following contributions to the already published theoretical studies of hydrides of iron under pressure: [21][22][23]25,36 • in disagreement with the results of Majumdar et al 25 and Kvashnin et al, 23 but in agreement with Heil and co-workers 26 we do not find superconductivity in the recently synthesized I4/mmm FeH 5 structure, 2 nor in the structurally similar and isoenthalpic Cmca FeH 5 phase.…”

“…At 200 GPa the H-H bond lengths in the three phases are nearly equidistant, measuring 0.729Å, 0.730Å and 0.733Å in Cmmm, P mma and C2/c, respectively. Phonon calculations revealed that Cmmm FeH 6 is dynamically stable only at stable at 150 GPa and 200 GPa.Above 200 GPa P 2/c FeH 6 becomes unstable with respect to the C2/c phase that was predicted via the PSO technique 36. This structure does not possess any molecular hydrogen units, with the shortest H-H distance measuring 1.156Å at 300 GPa.…”

A perspective on conventional high-temperature superconductors at high pressure: Methods and materials

“…This package is extremely popular and has been interfaced to a number of local structural optimization codes (VASP, QE, GULP, SIESTA, CP2K CASTEP) varying from highly accurate DFT methods to fast semiempirical approaches that can deal with large systems. Notably this PSObased algorithm [286,287,288,289] combined with a fingerprint and matrix bond analysis has been successfully used in solving numerous structural problems [290,291,292,293,294], including prediction of new high-pressure superconducting hydrides [295,182,296,297].…”

Computational Design of Novel Hydrogen-Rich YS–H Compounds

“…23−26 Several H–S compounds (e.g., H 2 S, HS 2 , H 4 S 3 , H 5 S 2 , and H 5 S 8 ) 1,13,17,19 have subsequently been identified under high pressure as well. Beside the above-mentioned H–S compounds, abroad range of hydrogen-containing materials have also attracted considerable attention under high pressure, e.g., iron hydrides, 27,28 LaH 10 , YH 10 , 4,5 etc. Remarkably, unique sodalite-like hydrogen cages are interpreted to play a key role in improving the superconductivity of YH 6 (H 24 cages) and YH 10 (H 32 cages), as estimated with T c of 250–320 K at high pressure, which have much higher T c than fcc-YH 3 (∼40 K) is found at lower pressure.…”