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Register a new userSuperstructures of Cubic and Hexagonal Diamonds Comprising a Family of Novel $sp^3$ Superhard Carbon Allotropes
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Superstructures of cubic and hexagonal diamonds (h- and c-diamond) comprising a family of stable diamond-like $sp^3$ hybridized novel carbon allotropes are proposed, which are referred to as U$_n$-carbon where $n geq 2$ denotes the number of structural layers in a unit cell. The conventional h- and c-diamond are included in this family as members with $n=2$ and 3, respectively. U$_n$-carbon ($n=4-6$), which are unveiled energetically and thermodynamically more stable than h-diamond and possess remarkable kinetic stabilities, are shown to be insulators with indirect gaps of $5.6 sim 5.8$ eV, densities of $ 3.5 sim 3.6$ g/cm$^3$, bulk modulus of $4.3 sim 4.4 times 10^{2}$ GPa, and Vickers hardness of $92.9 sim 97.5$ GPa even harder than h- and c-diamond. The simulated x-ray diffraction and Raman spectra are presented for experimental characterization. These new structures of carbon would have a compelling impact in physics, chemistry, materials science and geophysics.
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We identify by ab initio calculations a new type of three-dimensional carbon allotropes constructed by inserting acetylenic or diacetylenic bonds into a body-centered cubic C$_8$ lattice. The resulting $sp+sp^3$-hybridized cubane-yne and cubane-diyne structures consisting of C$_8$ cubes can be characterized as a cubic crystalline modification of linear carbon chains, but energetically more favorable than the simplest linear carbyne chain and the cubic tetrahedral diamond and yne-diamond consisting of C$_4$ tetrahedrons. Electronic band calculations indicate that these new carbon allotropes are semiconductors with an indirect band gap of 3.08 eV for cubane-yne and 2.53 eV for cubane-diyne. The present results establish a new type of carbon phases consisting of C$_8$ cubes and offer insights into their outstanding structural and electronic properties.
The phonon properties, electronic structures and optical properties of novel carbon allotropes, such as monolayer penta-graphene (PG), double-layer PG and T12-carbon, were explored by means of first-principles calculations. Results of phonon calculations demonstrate that these exotic carbon allotropes are dynamically stable. In addition, the bulk T12 phase is an indirect-gap semiconductor having a bandgap of ~4.89 eV. Whereas the bulk material transforms to a 2D phase, the monolayer and double-layer PG become quasi-direct gap semiconductors with smaller band gaps of ~2.64 eV and ~3.27eV, respectively. Furthermore, the partial charge density analysis indicates that the 2D phases retain part of the electronic characteristics of the T12 phase. The linear photon energy-dependent dielectric functions and related optical properties including refractive index, extinction coefficient, absorption spectrum, reflectivity, and energy loss spectrum were also computed and discussed. The structural estimation obtained as well as other findings are in agreement with existing theoretical data. The calculated results are beneficial to the practical applications of these exotic carbon allotropes in optoelectronics and electronics.
Traditionally, all superhard carbon phases including diamond are electric insulators and all conductive carbon phases including graphite are mechanically soft. Based on first-principles calculation results, we report a superhard but conductive carbon phase C21-sc which can be obtained through increasing the sp3 bonds in the previously proposed soft and conductive phase C20-sc (Phys. Rev. B 74, 172101 2006). We also show that further increase of sp3 bonds in C21-sc results in a superhard and insulating phase C22-sc with sp3 bonds only. With C20-sc, C21-sc, C22-sc and graphite, the X-ray diffraction peaks from the unidentified carbon material synthesized by compressing the mixture of tetracyanoethylene and carbon black (Carbon, 41, 1309, 2003) can be understood. In view of its positive stability, superhard and conductive features, and the strong possibility of existence in previous experiments, C21-sc is a promising multi-functional material with potential applications in extreme conditions.
At least four two- or quasi-one- dimensional allotropes and a mixture of them were theoretically predicted or experimentally observed for low-dimensional Te, namely the {alpha}, b{eta}, {gamma}, {delta} and chiral-{alpha}+{delta} phases. Among them the {gamma} and {alpha} phases were found the most stable phases for monolayer and thicker layers, respectively. Here, we found two novel low-dimensional phases, namely the {epsilon} and {zeta} phases. The {zeta} phase is over 29 meV/Te more stable than and the {epsilon} phase shows comparable stability with the most stable monolayer {gamma} phase. The energetic difference between the {zeta} and {alpha} phases reduces with respect to the increased layer thickness and vanishes at the four-layer (12-sublayer) thickness, while this thickness increases under change doping. Both {epsilon} and {zeta} phases are metallic chains and layers, respectively. The {zeta} phase, with very strong interlayer coupling, shows quantum well states in its layer-dependent bandstructures. These results provide significantly insight into the understanding of polytypism in Te few-layers and may boost tremendous studies on properties of various few-layer phases.
We describe three previously unreported superconductors, BaPb3, Ba0.89Sr0.11Pb3 and Ba0.5Sr0.5Pb3. These three materials, together with SrPb3, form a distinctive isoelectronic family of intermetallic superconductors based on the stacking of Pb planes, with crystal structures that display a hexagonal to cubic perovskite-like progression, as rarely seen in metals. The superconducting transition temperatures (Tc) are similar for all - 2.2 K for BaPb3, 2.7 K for Ba0.89Sr0.11Pb3 and 2.6 K for Ba0.5Sr0.5Pb3, and the previously reported Tc of SrPb3, ~ 2 K, is confirmed. The materials are moderate coupling superconductors, and calculations show that the electronic densities of states at the Fermi energy are primarily contributed by Pb. The observations suggest that the Pb-stacking variation has only a minor effect on the superconductivity.
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