6 research outputs found
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 âbottom-upâ
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 filmsâ 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 âbottom-upâ
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 filmsâ 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 CrâB, CrâC, and
CrâN 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 Cr<sub>23</sub>C<sub>6</sub> with its large unit cell) and
discover a novel stable phase <i>Pmn</i>2<sub>1</sub>-Cr<sub>2</sub>C. We resolve the structure of Cr<sub>2</sub>N and find it
to be of anti-CaCl<sub>2</sub> type (space group <i>Pnnm</i>). Many of these phases possess remarkable hardness, but only CrB<sub>4</sub> 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 Cr<sub>2</sub>N, a WC-type phase of CrN, and a new compound CrN<sub>4</sub>. Nitrogen-rich chromium nitride CrN<sub>4</sub> 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; CrN<sub>4</sub> becomes stable at âŒ15 GPa (cf. 110
GPa for synthesis of pure polymeric nitrogen)
Actinium Hydrides AcH<sub>10</sub>, AcH<sub>12</sub>, and AcH<sub>16</sub> as High-Temperature Conventional Superconductors
The
stability of numerous unexpected actinium hydrides was predicted
via the evolutionary algorithm USPEX. The electronâphonon interaction
was investigated for the hydrogen-richest and most symmetric phases: <i>R</i>3Ì
<i>m</i>-AcH<sub>10</sub>, <i>I</i>4/<i>mmm</i>-AcH<sub>12</sub>, and <i>P</i>6Ì
<i>m</i>2-AcH<sub>16</sub>. Predicted structures of actinium hydrides
are consistent with all previously studied AcâH phases and
demonstrate phonon-mediated high-temperature superconductivity with <i>T</i><sub>C</sub> in the range of 204â251 K for <i>R</i>3Ì
<i>m</i>-AcH<sub>10</sub> at 200 GPa
and 199â241 K for <i>P</i>6Ì
<i>m</i>2-AcH<sub>16</sub> at 150 GPa, which was estimated by directly solving
the Eliashberg equation. Actinium belongs to the series of d<sup>1</sup> elements (ScâYâLaâAc) that form high-<i>T</i><sub>C</sub> superconducting (HTSC) hydrides. Combining
this observation with previous predictions of p<sup>0</sup>-HTSC hydrides
(MgH<sub>6</sub> and CaH<sub>6</sub>), we propose that p<sup>0</sup> and d<sup>1</sup> metals with low-lying empty orbitals tend to form
phonon-mediated HTSC metal polyhydrides
Actinium Hydrides AcH<sub>10</sub>, AcH<sub>12</sub>, and AcH<sub>16</sub> as High-Temperature Conventional Superconductors
The
stability of numerous unexpected actinium hydrides was predicted
via the evolutionary algorithm USPEX. The electronâphonon interaction
was investigated for the hydrogen-richest and most symmetric phases: <i>R</i>3Ì
<i>m</i>-AcH<sub>10</sub>, <i>I</i>4/<i>mmm</i>-AcH<sub>12</sub>, and <i>P</i>6Ì
<i>m</i>2-AcH<sub>16</sub>. Predicted structures of actinium hydrides
are consistent with all previously studied AcâH phases and
demonstrate phonon-mediated high-temperature superconductivity with <i>T</i><sub>C</sub> in the range of 204â251 K for <i>R</i>3Ì
<i>m</i>-AcH<sub>10</sub> at 200 GPa
and 199â241 K for <i>P</i>6Ì
<i>m</i>2-AcH<sub>16</sub> at 150 GPa, which was estimated by directly solving
the Eliashberg equation. Actinium belongs to the series of d<sup>1</sup> elements (ScâYâLaâAc) that form high-<i>T</i><sub>C</sub> superconducting (HTSC) hydrides. Combining
this observation with previous predictions of p<sup>0</sup>-HTSC hydrides
(MgH<sub>6</sub> and CaH<sub>6</sub>), we propose that p<sup>0</sup> and d<sup>1</sup> 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 âŒ2 to âŒ10 nm and exhibit {111} spacing of 2.1 Ă
. 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