Lonsdaleite Films with Nanometer Thickness

We investigate the properties of potentially the stiffest quasi-2-D films with lonsdaleite structure. Using a combination of ab initio and empirical potential approaches, we analyze the elastic properties of lonsdaleite films in both elastic and inelastic regimes and compare them with graphene and diamond films. We review possible fabrication methods of lonsdaleite films using the pure nanoscale “bottom-up” paradigm: by connecting carbon layers in multilayered graphene. We propose the realization of this method in two ways: by applying direct pressure and by using the recently proposed chemically induced phase transition. For both cases, we construct the phase diagrams depending on temperature, pressure, and film thickness. Finally, we consider the electronic properties of lonsdaleite films and establish the nonlinear dependence of the band gap on the films’ thicknesses and their lower effective masses in comparison with bulk crystal

Lonsdaleite Films with Nanometer Thickness

We investigate the properties of potentially the stiffest quasi-2-D films with lonsdaleite structure. Using a combination of ab initio and empirical potential approaches, we analyze the elastic properties of lonsdaleite films in both elastic and inelastic regimes and compare them with graphene and diamond films. We review possible fabrication methods of lonsdaleite films using the pure nanoscale “bottom-up” paradigm: by connecting carbon layers in multilayered graphene. We propose the realization of this method in two ways: by applying direct pressure and by using the recently proposed chemically induced phase transition. For both cases, we construct the phase diagrams depending on temperature, pressure, and film thickness. Finally, we consider the electronic properties of lonsdaleite films and establish the nonlinear dependence of the band gap on the films’ thicknesses and their lower effective masses in comparison with bulk crystal

Computational Search for Novel Hard Chromium-Based Materials

Nitrides, carbides, and borides of transition metals are an attractive class of hard materials. Our recent preliminary explorations of the binary chemical compounds indicated that chromium-based materials are among the hardest transition metal compounds. Motivated by this, here we explore in detail the binary Cr–B, Cr–C, and Cr–N systems using global optimization techniques. Calculated enthalpy of formation and hardness of predicted materials were used for Pareto optimization to define the hardest materials with the lowest energy. Our calculations recover all numerous known stable compounds (except Cr₂₃C₆ with its large unit cell) and discover a novel stable phase <i>Pmn</i>2₁-Cr₂C. We resolve the structure of Cr₂N and find it to be of anti-CaCl₂ type (space group <i>Pnnm</i>). Many of these phases possess remarkable hardness, but only CrB₄ is superhard (Vickers hardness 48 GPa). Among chromium compounds, borides generally possess the highest hardnesses and greatest stability. Under pressure, we predict stabilization of a layered TMDC-like phase of Cr₂N, a WC-type phase of CrN, and a new compound CrN₄. Nitrogen-rich chromium nitride CrN₄ is a high-energy-density material featuring polymeric nitrogen chains. In the presence of metal atoms (e.g., Cr), polymerization of nitrogen takes place at much lower pressures; CrN₄ becomes stable at ∼15 GPa (cf. 110 GPa for synthesis of pure polymeric nitrogen)

Actinium Hydrides AcH₁₀, AcH₁₂, and AcH₁₆ as High-Temperature Conventional Superconductors

The stability of numerous unexpected actinium hydrides was predicted via the evolutionary algorithm USPEX. The electron–phonon interaction was investigated for the hydrogen-richest and most symmetric phases: <i>R</i>3̅<i>m</i>-AcH₁₀, <i>I</i>4/<i>mmm</i>-AcH₁₂, and <i>P</i>6̅<i>m</i>2-AcH₁₆. Predicted structures of actinium hydrides are consistent with all previously studied Ac–H phases and demonstrate phonon-mediated high-temperature superconductivity with <i>T</i>_C in the range of 204–251 K for <i>R</i>3̅<i>m</i>-AcH₁₀ at 200 GPa and 199–241 K for <i>P</i>6̅<i>m</i>2-AcH₁₆ at 150 GPa, which was estimated by directly solving the Eliashberg equation. Actinium belongs to the series of d¹ elements (Sc–Y–La–Ac) that form high-<i>T</i>_C superconducting (HTSC) hydrides. Combining this observation with previous predictions of p⁰-HTSC hydrides (MgH₆ and CaH₆), we propose that p⁰ and d¹ metals with low-lying empty orbitals tend to form phonon-mediated HTSC metal polyhydrides

Actinium Hydrides AcH₁₀, AcH₁₂, and AcH₁₆ as High-Temperature Conventional Superconductors

The stability of numerous unexpected actinium hydrides was predicted via the evolutionary algorithm USPEX. The electron–phonon interaction was investigated for the hydrogen-richest and most symmetric phases: <i>R</i>3̅<i>m</i>-AcH₁₀, <i>I</i>4/<i>mmm</i>-AcH₁₂, and <i>P</i>6̅<i>m</i>2-AcH₁₆. Predicted structures of actinium hydrides are consistent with all previously studied Ac–H phases and demonstrate phonon-mediated high-temperature superconductivity with <i>T</i>_C in the range of 204–251 K for <i>R</i>3̅<i>m</i>-AcH₁₀ at 200 GPa and 199–241 K for <i>P</i>6̅<i>m</i>2-AcH₁₆ at 150 GPa, which was estimated by directly solving the Eliashberg equation. Actinium belongs to the series of d¹ elements (Sc–Y–La–Ac) that form high-<i>T</i>_C superconducting (HTSC) hydrides. Combining this observation with previous predictions of p⁰-HTSC hydrides (MgH₆ and CaH₆), we propose that p⁰ and d¹ metals with low-lying empty orbitals tend to form phonon-mediated HTSC metal polyhydrides

Radiation-Induced Nucleation of Diamond from Amorphous Carbon: Effect of Hydrogen

Electron irradiation of anthracite functionalized by dodecyl groups leads to recrystallization of the carbon network into diamonds. The diamonds range in size from ∼2 to ∼10 nm and exhibit {111} spacing of 2.1 Å. A bulk process consistent with bias-enhanced nucleation is proposed in which the dodecyl group provides hydrogen during electron irradiation. Recrystallization into diamond occurs in the hydrogenated graphitic subsurface layers. Unfunctionalized anthracite could not be converted into diamond during electron irradiation. The dependence of the phase transition pressure on cluster size was estimated, and it was found that diamond particles with a radius up to 20 nm could be formed