First-principles study on the structural and electronic properties of metallic HfH2 under pressure

Prediction of potential high-temperature superconductivity in ternary Y-Hf-H compounds under high pressure

Compressed ternary alloy superhydrides are currently considered to be the most promising competitors for high-temperature superconducting materials. Here, the stable stoichiometries in the Y-Hf-H ternary system under pressure are comprehensively explored in theory and four fresh phases are predicted: Pmna-YHfH6 and P4/mmm-YHfH7 at 200 GPa, P4/mmm-YHfH8 at 300 GPa and P-6m2-YHfH18 at 400 GPa. The four Y-Hf-H ternary phases are thermodynamically and dynamically stable at corresponding pressure. In addition, structural features, bonding characteristics, electronic properties, and superconductivity of the four ternary Y-Hf-H phases are systematically calculated and discussed. As the hydrogen content and the density of states of H atoms at the Fermi level increase, the superconducting transition temperatures (Tc) of Y-Hf-H system are significantly enhanced. The P-6m2-YHfH18 with high hydrogen content exhibits a high calculated Tc value of 130 K at 400 GPa.

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.

Pressure induced topochemical polymerization of solid acrylamide facilitated by anisotropic response of the hydrogen bond network

The pressure induced polymerization of molecular solids is an appealing route to obtain pure, crystalline polymers without the need for radical initiators. Here, we report a detailed density functional theory (DFT) study of the structural and chemical changes that occur in defect free solid acrylamide, a hydrogen bonded crystal, when it is subjected to hydrostatic pressures. While our calculations are able to reproduce experimentally measured pressure dependent spectroscopic features in the 0-20 GPa range, our atomistic analysis predicts polymerization in acrylamide at a pressure of ∼23 GPa at 0 K albeit through large enthalpy barriers. Interestingly, we find that the two-dimensional hydrogen bond network in acrylamide templates topochemical polymerization by aligning the atoms through an anisotropic response at low pressures. This results not only in conventional C-C, but also unusual C-O polymeric linkages, as well as a new hydrogen bonded framework, with both N-HO and C-HO bonds. Using a simple model for thermal effects, we also show that at 300 K, higher pressures significantly accelerate the transformation into polymers by lowering the barrier. Thus, application of pressure offers an alternative route for topochemical polymerization when higher temperatures are undesirable.

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

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.

Structural, mechanical and electronic properties and hardness of ionic vanadium dihydrides under pressure from first-principles computations

Based on a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we explore the crystal structures of VH2 under the pressure range of 0-300 GPa. The cubic Fm-3m phase with regular VH8 cubes is predicted to transform into orthorhombic Pnma structure with fascinating distorted VH9 tetrakaidecahedrons at 47.36 GPa. Both the Fm-3m phase at 0 GPa and the Pnma phase at 100 GPa are mechanically and dynamically stable, as verified with the calculations of elastic constants and phonon dispersions, respectively. Moreover, the calculated electronic band structure and density of states indicate both stable phases are metallic. Remarkably, the analyses of the Poisson's ratio, electron localization function (ELF) and Bader charge substantiate that both stable phases are ionic crystals on account of effective charges transferring from V atom to H. On the basis of the microscopic hardness model, the Fm-3m and Pnma crystals of VH2 are potentially incompressible and hard materials with the hardness values of 17.83 and 17.68 GPa, respectively.

Pressure dependence of electronic structure and superconductivity of the MnX (X = N, P, As, Sb)

A recently experimental discovered (Cheng et al., Phys. Rev. Lett. 114, 117001 (2015)) of superconductivity on the border of long-range magnetic order in the itinerant-electron helimagnet MnP via the application of high pressure makes MnP the first Mn-based superconductor. In this paper, we carry out first-principles calculations on MnX (X = N, P, As, Sb) and find superconducting critical temperature TC of MnP sharply increases near the critical pressure PC ≈ 8 GPa, which is in good agreement with the experiments. Electron-phonon coupling constant λ and electronic density of states at the Fermi level N (EF) are found to increase with pressure for MnP, which lead to the increase of TC of MnP. Moreover, we also find that the TC of MnAs and MnSb are higher than MnP, implying that the MnAs and MnSb may be the more potential Mn-based superconducting materials.

Stability and properties of the Ru–H system at high pressure

The calculated formation enthalpies of RuH_n (n = 1–8) with respect to Ru and H at different pressures.