Hydrogen Pentagraphenelike Structure Stabilized by Hafnium: A High-Temperature Conventional Superconductor

Abstract

The recent discovery of H 3 S and LaH 10 superconductors with record high superconducting transition temperatures T c at high pressure has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we introduce a new class of high T c hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH 10 , with remarkably high value of T c (around 213-234 K) at 250 GPa. As concerns the novel structure, the H ions in HfH 10 are arranged in clusters to form a planar "pentagraphenelike" sublattice. The layered arrangement of these planar units is entirely different from the covalent sixfold cubic structure in H 3 S and clathratelike structure in LaH 10 . The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H 10 structure is also found in ZrH 10 , ScH 10 , and LuH 10 at high pressure, each material showing a high T c ranging from 134 to 220 K. Our study of dense superhydrides with pentagraphenelike layered structures opens the door to the exploration of a new class of high T c superconductors.

Materials under high pressure: A chemical perspective

“…[18] The Sc-H family of high-pressure phases displays another strange structural motif: H 5 pentagons. [225,274] These curious units appear joined together in strips in the ScH 9 and ScH 12 structures, where plots of the Electron Localization Function (ELF) confirms the presence of localized bonding. [225] In ScH 10 at ∼200 GPa two nearly isoenthalpic phases with Immm and P 6 3 /mmc symmetry feature H 5 pentagons fused to one another in sets of three in a "pentagraphene-like" motif.…”

“…[225] In ScH 10 at ∼200 GPa two nearly isoenthalpic phases with Immm and P 6 3 /mmc symmetry feature H 5 pentagons fused to one another in sets of three in a "pentagraphene-like" motif. [274] As with many other metal superhydride phases, the electrons donated from the metal atom (investigated for a hafnium analogue HfH 10 ) go into antibonding orbitals of the hydrogen framework, weakening the H-H bonding and reducing H 2 molecular character.…”

Tuning Chemical Precompression: Theoretical Design and Crystal Chemistry of Novel Hydrides in the Quest for Warm and Light Superconductivity at Ambient Pressures

“…Layers of the fused pentagons -where bonding interactions are again revealed via plots of the ELF -alternate with layers of Sc atoms. Essentially isoenthalpic with this structure is another ScH 10 geometry 41,43 -also based on triads of fused H 5 pentagons, but this time with P 6 3 /mmc symmetry (Figure 4b). In this structure, the fused "pentagraphene-like" units are stacked along the c axis and rotated relative to one another by 60 • .…”

Materials by design at high pressures

“…Subsequent theoretical attempts have further conrmed hydrogen cages as common species at high pressure. 6,7,[82][83][84][85][86][87][88][89][90][91][92][93][94] [96][97][98][99][100][101][102] However, currently, no general mechanism can accurately predict the T c of hydrides. For example, H 32 cage-structured PrH 10 has a low T c of 1.4 K, 89 demonstrating that not all hydrogen cage-structured hydrides possess high superconductivity.…”