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As an important phase-change material, GeTe has many high-pressure phases as well, but its phase transitions under pressure are still lack of clarity. It is challenging to identify high-pressure GeTe crystal structures owing to the phase coexistence in a wide pressure range and the reversibility of phase transitions. Hence, first-principles calculations are required to provide further information in addition to limited experimental characterizations. In this work, a new orthorhombic Cmca GeTe high-pressure phase has been predicted via the CALYPSO method as the most energetically favorable phase in the pressure range between ∼30 and ∼38.5 GPa, which would update the GeTe high-pressure phase transition sequence. The crystal structure of the Cmca phase is composed of alternate stacking puckered layers of Ge six-membered rings and Te four-membered rings along the b direction. The high density of states near the Fermi level and delocalization of electrons from the two-dimensional electron localization function indicate a strong metallic property of the Cmca phase. Electron–phonon coupling calculations indicate that the Cmca phase is superconductive below ∼4.2 K at 35 GPa. The simulated x-ray diffraction pattern of the Cmca phase implies that this phase might coexist with the Pnma-boat phase under high pressure. These results offer further understanding on the high-pressure structural evolution and physical properties in GeTe and other IV–VI semiconductors. [ABSTRACT FROM AUTHOR]

Carbon materials with full sp2-hybridized buckling is a major challenge pervading fundamental nanoscience and nanotechnology research. Carbon atoms that are sp2 hybridized prefer to form hexagonal rings, such as in carbon nanotubes and graphene, which are low-dimensional materials. The incorporation of heptagonal, octagonal, and/or larger rings into a hexagonal sp2 carbon meshwork has been identified as a strategy for assembling three-dimensional (3D) sp2 carbon crystals, and one of the typical representatives are Schwarzite carbons, which possess a negative surface Gaussian curvature as well as unique physical properties. Herein, a 3D Schwarzite carbon consisting of only sp2-buckled heptagonal carbon rings, which is referred to as Hepta-carbon, is proposed based on first-principles calculations. Hepta-carbon is mechanically and thermodynamically stable, and energetically more favourable than experimental graphdiyne, fullerene C20 and most Schwarzite carbons under ambient conditions. Molecular dynamics simulations indicate that Hepta-carbon exhibits high-temperature thermostability up to 1500 K. Band structure and mechanical property simulations indicate that Hepta-carbon is a semi-metallic material with electron conduction and exhibits impressive mechanical properties such as high strength with quasi-isotropy, high incompressibility similar to diamonds, elastic deformation behaviour under uniaxial stress, and high ductility. Hepta-carbon presents a porous network with a low mass density of 1.84 g cm−3 and connected channels with diameters of 3.3–6.1 Å. Theoretical simulations of gas adsorption energy demonstrate that Hepta-carbon can effectively adsorb and stabilize greenhouse gases, including N2O, CO2, CH4, and SF6. [ABSTRACT FROM AUTHOR]

Three novel hexagonal Si–C–N structures, namely SiC3N3, SiC7N6, and SiC13N14, were constructed on the basis of the α-Si3N4 crystal structure. The stability of the three structures is demonstrated by analyzing their elastic constants and phonon dispersion spectra and by calculating their formation energies. The calculated band structures and partial densities of states suggest that the SiC3N3 and SiC7N6 structures possess hole conductivity. The electron orbital analyses indicate that the SiC3N3 and SiC7N6 crystals possess three-dimensional and one-dimensional conductivity, respectively. SiC13N14 is a semiconductor with a wide bandgap of 4.39 eV. Based on two different hardness models and indentation shear stress calculations, the Vickers hardness values of SiC3N3, SiC7N6, and SiC13N14 are estimated to be 28.04/28.45/16.18 GPa, 31.17/34.19/20.24 GPa, and 40.60/41.59/36.40 GPa. This result indicates that SiC3N3 and SiC7N6 are conductive hard materials while SiC13N14 is a quasi superhard material. [ABSTRACT FROM AUTHOR]

Compared with traditional structure prediction methods, the purposeful bottom-up approach is better able to obtain structures with specified performance. In this study, we established two novel carbon phases in purely sp2-bonded networks, termed H61-carbon and H62-carbon, using a self-assembling approach. These carbyne-connected carbon allotropes had helix chains joined by cumulative double-bond chains. We certified the new carbon allotropes to be dynamically and mechanically stable. Both of these carbon allotropes exhibited excellent mechanical properties, and they had metallic and superconductive characteristics featuring superconducting transition temperatures of 10 K (H61-carbon) and 7.4 K (H62-carbon), respectively. These results provide an important strategy for the design of novel carbon allotropes with specified properties. [ABSTRACT FROM AUTHOR]

We report here the high‐pressure and high‐temperature (HPHT) synthesis of well‐sintered B6O–diamond composites from B6O and carbon black nanopowders. The carbon black was transformed into diamond nanograins at HPHT conditions, and simultaneously formed high‐strength B6O–diamond interfaces. The ultrafine B6O and diamond nanograins and the high‐strength B6O–diamond interfaces synergistically construct excellent mechanical properties for the synthesized composites. The B6O–diamond composites possess a hardness (avg. 52 GPa) comparable to that of polycrystalline diamond (40–60 GPa), whereas the fracture toughness (avg. 7.2 MPa m1/2) is increased several times compared to previously synthesized polycrystalline B6O ceramics (1.7–3.1 MPa m1/2) and B6O‐based composites (3–4 MPa m1/2). Fracture behavior analysis demonstrates that the main toughening mechanisms in this B6O–diamond composite are nanotwin toughening, crack deflection, and crack bridging. [ABSTRACT FROM AUTHOR]

High-density B–C–N ternary compounds have attracted considerable attention due to their potential and excellent properties combined with diamond and cubic boron nitride (cBN). However, the development of B–C–N is restricted due to the lack of an ideal precursor with a graphite-like structure. Recently, graphite/hBN superlattices have been synthesized, of which two specific atomic structures have been determined. Here, we simulated the phase transformation of these two graphite/hBN superlattices under high pressure. Results revealed that the puckering pattern of graphite-like layers (i.e., armchair or boat like) can be controlled by adjusting the pressurization process to achieve customized phase transition results, resulting in five different superhard BC2N compounds. Among these compounds, only R3m-BC2N, R3m-BC2N-2, and R3¯m-BC2N are diamond-like structures consisting of six-membered rings, while P2/m-BC2N and P2/m-BC2N-2 have different structures with additional 4+8 rings due to the varying bonding processes. Band structure analysis revealed that these structures are semiconductors with band gaps in the range of 2.6–4.6 eV. Their theoretical hardness of 70–78 GPa exceeds that of cBN. This simulation study provides insights for the controllable preparation of high-density phase BC2N compounds and demonstrates the great potential of variable speed pressurization techniques for the discovery of new materials. [ABSTRACT FROM AUTHOR]

Carbon is one of the most fascinating elements due to its structurally diverse allotropic forms stemming from its bonding varieties (sp , sp 2 and sp 3). Exploring new forms of carbon has been the eternal theme of scientific research. Herein, we report on amorphous (AM) carbon materials with a high fraction of sp 3 bonding recovered from compression of fullerene C60 under high pressure and high temperature, previously unexplored. Analysis of photoluminescence and absorption spectra demonstrates that they are semiconducting with a bandgap range of 1.5–2.2 eV, comparable to that of widely used AM silicon. Comprehensive mechanical tests demonstrate that synthesized AM-III carbon is the hardest and strongest AM material known to date, and can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance. [ABSTRACT FROM AUTHOR]

Compared with traditional structure prediction methods, the purposeful bottom-up approach is better able to obtain structures with specified performance. In this study, we established two novel carbon phases in purely sp 2 -bonded networks, termed H6 1 -carbon and H6 2 -carbon, using a self-assembling approach. These carbyne-connected carbon allotropes had helix chains joined by cumulative double-bond chains. We certified the new carbon allotropes to be dynamically and mechanically stable. Both of these carbon allotropes exhibited excellent mechanical properties, and they had metallic and superconductive characteristics featuring superconducting transition temperatures of 10 K (H6 1 -carbon) and 7.4 K (H6 2 -carbon), respectively. These results provide an important strategy for the design of novel carbon allotropes with specified properties.