High-Pressure Formation of Cobalt Polyhydrides: A First-Principle Study

Strong correlation between electronic bonding network and critical temperature in hydrogen-based superconductors

By analyzing structural and electronic properties of more than a hundred predicted hydrogen-based superconductors, we determine that the capacity of creating an electronic bonding network between localized units is key to enhance the critical temperature in hydrogen-based superconductors. We define a magnitude named as the networking value, which correlates with the predicted critical temperature better than any other descriptor analyzed thus far. By classifying the studied compounds according to their bonding nature, we observe that such correlation is bonding-type independent, showing a broad scope and generality. Furthermore, combining the networking value with the hydrogen fraction in the system and the hydrogen contribution to the density of states at the Fermi level, we can predict the critical temperature of hydrogen-based compounds with an accuracy of about 60 K. Such correlation is useful to screen new superconducting compounds and offers a deeper understating of the chemical and physical properties of hydrogen-based superconductors, while setting clear paths for chemically engineering their critical temperatures.

High-temperature superconductivity in transition metallic hydrides MH11 (M = Mo, W, Nb, and Ta) under high pressure

The discovery of H₃S and LaH₁₀ is an important step towards the development of room temperature superconductors which fuels the enthusiasm for finding promising superconductors among hydrides at high pressure. In the present study, three new and stable stoichiometric MoH₅, MoH₆ and MoH₁₁ compounds were found in the pressure range of 100-300 GPa. The highly hydrogen-rich phase of Cmmm-MoH₁₁ has a layered structure that contains various forms of hydrogen: H, H₂^- and H₃^- units. It is a high-T_c material with an estimated T_c value in the range of 165-182 K at 250 GPa. The same structures are also found in NbH₁₁, TaH₁₁, and WH₁₁, each material showing T_c ranging from 117 to 168 K. By combining the method of using two coupling constants λ_opt and λ_ac, and two characteristic frequencies (optical and acoustic) with first-principle calculations, we found that the high values of T_c are mainly caused by the presence of high frequency optical modes, but the acoustic modes also play a noticeable role.

Synthesis of Superconducting Cobalt Trihydride

The Co-H system has been investigated through high-pressure, high-temperature X-ray diffraction experiments combined with first-principles calculations. On compression of elemental cobalt in a hydrogen medium, we observe face-centered cubic cobalt hydride (CoH) and cobalt dihydride (CoH₂) above 33 GPa. Laser heating CoH₂ in a hydrogen matrix at 75 GPa to temperatures in excess of ∼800 K produces cobalt trihydride (CoH₃) which adopts a primitive structure. Density functional theory calculations support the stability of CoH₃. This phase is predicted to be thermodynamically stable at pressures above 18 GPa and to be a superconductor below 23 K. Theory predicts that this phase remains dynamically stable upon decompression above 11 GPa where it has a maximum T_c of 30 K.

Complex Hydrogen Substructure in Semimetallic RuH4

When compressed in a matrix of solid hydrogen, many metals form compounds with increasingly high hydrogen contents. At high density, hydrogenic sublattices can emerge, which may act as low-dimensional analogues of atomic hydrogen. We show that at high pressures and temperatures, ruthenium forms polyhydride species that exhibit intriguing hydrogen substructures with counterintuitive electronic properties. Ru₃H₈ is synthesized from RuH in H₂ at 50 GPa and at temperatures in excess of 1000 K, adopting a cubic structure with short H-H distances. When synthesis pressures are increased above 85 GPa, we observe RuH₄ which crystallizes in a remarkable structure containing corner-sharing H₆ octahedra. Calculations indicate this phase is semimetallic at 100 GPa.

Superconducting Zirconium Polyhydrides at Moderate Pressures

Highly compressed hydrides have been at the forefront of the search for high-T_c superconductivity. The recent discovery of record-high T_c's in H₃S and LaH_10±x under high pressure fuels the enthusiasm for finding good superconductors in similar hydride groups. Guided by first-principles structure prediction, we successfully synthesized ZrH₃ and Zr₄H₁₅ at modest pressures (30-50 GPa) in diamond anvil cells by two different reaction routes: ZrH₂ + H₂ at room temperature and Zr + H₂ at ∼1500 K by laser heating. From the synchrotron X-ray diffraction patterns, ZrH₃ is found to have a Pm3̅n structure corresponding to the familiar A15 structure, and Zr₄H₁₅ has an I4̅3d structure isostructural to Th₄H₁₅. Electrical resistance measurement and the dependence of T_c on the applied magnetic field of the sample showed the emergence of two superconducting transitions at 6.4 and 4.0 K at 40 GPa, which correspond to the two T_c's for ZrH₃ and Zr₄H₁₅.