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We searched for the structure of calcium in phases IV and V by a metadynamics simulation based on ab initio calculations, and found two structures. One is a tetragonal lattice which consists of two helical chains along the c axis. The other is an orthorhombic lattice of four zigzag chains. We have calculated the x-ray diffraction patterns and enthalpies of the two structures discovered by our simulation. From comparisons of the patterns with the experimental x-ray patterns of the phases IV and V and from the pressure dependence of the enthalpies, we conclude that the structure with a helical pattern corresponds to phase IV and the structure with a zigzag pattern to phase V.

We searched for the structure of calcium in phases IV and V by a metadynamics simulation based on ab initio calculations, and found two structures. One is a tetragonal lattice which consists of two helical chains along the c axis. The other is an orthorhombic lattice of four zigzag chains. We have calculated the x-ray diffraction patterns and enthalpies of the two structures discovered by our simulation. From comparisons of the patterns with the experimental x-ray patterns of the phases IV and V and from the pressure dependence of the enthalpies, we conclude that the structure with a helical pattern corresponds to phase IV and the structure with a zigzag pattern to phase V.

Enriquez J.I.G., Halim H.H., Yamasaki T., et al. Origin of the surface facet dependence in the thermal degradation of the diamond (111) and (100) surfaces in vacuum investigated by machine learning molecular dynamics simulations. Carbon 226, 119223 (2024); https://doi.org/10.1016/j.carbon.2024.119223., We perform machine learning molecular dynamics simulations to gain an atomic-level understanding of the dependence of the graphitization and thermal degradation behavior of diamond to the (111) and (100) surface facets. The interatomic potential is constructed using graph neural network model, trained using energies and forces from spin-polarized van der Waals-corrected density functional theory calculations. Our results show that the C(111) surface is more susceptible to thermal degradation, which occurs from 2850 K through synchronized bilayer exfoliation mechanism. In comparison, the C(100) surface thermally degrade from a higher temperature of 3680 K through the formation of sp1 carbon chains and amorphous sp2-sp3 carbon network. Due to the dangling bonds at the step edges, the stepped surfaces are more susceptible to thermal degradation compared to the corresponding flat surfaces, with the stepped C(111) and C(100) surfaces thermally degrading from 1810 K to 3070 K, respectively. We propose potential applications of this study in diamond tool wear suppression, diamond polishing, and production of graphene directly from the diamond surface.

Enriquez J.I.G., Halim H.H., Yamasaki T., et al. Origin of the surface facet dependence in the thermal degradation of the diamond (111) and (100) surfaces in vacuum investigated by machine learning molecular dynamics simulations. Carbon 226, 119223 (2024); https://doi.org/10.1016/j.carbon.2024.119223., We perform machine learning molecular dynamics simulations to gain an atomic-level understanding of the dependence of the graphitization and thermal degradation behavior of diamond to the (111) and (100) surface facets. The interatomic potential is constructed using graph neural network model, trained using energies and forces from spin-polarized van der Waals-corrected density functional theory calculations. Our results show that the C(111) surface is more susceptible to thermal degradation, which occurs from 2850 K through synchronized bilayer exfoliation mechanism. In comparison, the C(100) surface thermally degrade from a higher temperature of 3680 K through the formation of sp1 carbon chains and amorphous sp2-sp3 carbon network. Due to the dangling bonds at the step edges, the stepped surfaces are more susceptible to thermal degradation compared to the corresponding flat surfaces, with the stepped C(111) and C(100) surfaces thermally degrading from 1810 K to 3070 K, respectively. We propose potential applications of this study in diamond tool wear suppression, diamond polishing, and production of graphene directly from the diamond surface.

John Isaac Enriquez, Fahdzi Muttaqien, Masato Michiuchi, Kouji Inagaki, Masaaki Geshi, Ikutaro Hamada, Yoshitada Morikawa, Oxidative etching mechanism of the diamond (100) surface, Carbon, Volume 174, 2021, Pages 36-51, https://doi.org/10.1016/j.carbon.2020.11.057., We performed density functional theory calculations with van der Waals corrections to elucidate diamond oxidation mechanism on the atomic-level which could lead to insights that will advance the improvement of nascent nanofabrication technologies. We developed a comprehensive theory of oxidative etching of the diamond (100) surface, from the adsorption of gas phase O2, including details of metastable adsorption states, intersystem crossing, and induced surface dereconstruction, to the desorption of CO and CO2, complete etching of the top surface layer and its subsequent stabilization. Oxygen adsorption induce C-dimer bond breaking through the formation of carbonyl structures at low surface coverage. At high surface coverage, adjacent carbonyls can transform into the more stable ether chain with small energy barrier requirement. A mix of carbonyl and ether will form in surfaces with defects, and several models of possible structures have been presented. CO desorption creates a point defect that serves as nucleation site for the preferred etching direction along [011], perpendicular to the top layer C-dimer bonds. We obtained a wide range of desorption activation energies, which depend on the existence of point defects and reconstruction of surfaces. C-dimer formation and oxygen adsorption stabilize vacancies and single-atomic-layer-deep trough.

John Isaac Enriquez, Fahdzi Muttaqien, Masato Michiuchi, Kouji Inagaki, Masaaki Geshi, Ikutaro Hamada, Yoshitada Morikawa, Oxidative etching mechanism of the diamond (100) surface, Carbon, Volume 174, 2021, Pages 36-51, https://doi.org/10.1016/j.carbon.2020.11.057., We performed density functional theory calculations with van der Waals corrections to elucidate diamond oxidation mechanism on the atomic-level which could lead to insights that will advance the improvement of nascent nanofabrication technologies. We developed a comprehensive theory of oxidative etching of the diamond (100) surface, from the adsorption of gas phase O2, including details of metastable adsorption states, intersystem crossing, and induced surface dereconstruction, to the desorption of CO and CO2, complete etching of the top surface layer and its subsequent stabilization. Oxygen adsorption induce C-dimer bond breaking through the formation of carbonyl structures at low surface coverage. At high surface coverage, adjacent carbonyls can transform into the more stable ether chain with small energy barrier requirement. A mix of carbonyl and ether will form in surfaces with defects, and several models of possible structures have been presented. CO desorption creates a point defect that serves as nucleation site for the preferred etching direction along [011], perpendicular to the top layer C-dimer bonds. We obtained a wide range of desorption activation energies, which depend on the existence of point defects and reconstruction of surfaces. C-dimer formation and oxygen adsorption stabilize vacancies and single-atomic-layer-deep trough.