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

The recent discovery of H₃S and LaH₁₀ superconductors with record high superconducting transition temperatures Tc at high pressure has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we introduce a new class of high Tc hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH₁₀, with remarkably high value of Tc (around 213-234 K) at 250 GPa. As concerns the novel structure, the H ions in HfH₁₀ 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 H₃S and clathratelike structure in LaH₁₀. The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H₁₀ structure is also found in ZrH₁₀, ScH₁₀, and LuH₁₀ at high pressure, each material showing a high Tc 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 Tc superconductors.