New superhard phases for three-dimensional C60-based fullerites

o-C240: a new sp3-dominated allotrope of carbon

We report a new allotrope of carbon predicted from first principles simulations. This allotrope is formed in a simulated conversion of two-dimensional polymeric C60precursor subjected to uniaxial compression at high temperature. The structure is made up of 240 carbon atoms in an orthorhombic unit cell (termed as o-C240) having a mixed sp2/sp3hybridization with the ratio of about 1:5. o-C240is stable at ambient condition and exhibits superior mechanical performance including optimum Vickers hardness (45 GPa) and fracture toughness (4.10 MPa m1/3), outperforming most of widely used hard ceramics. The electronic structure reveals semiconducting ground state with an indirect band gap of 1.72 eV. The simple reaction pathway could accelerate discovery of this allotrope in laboratory, and the simultaneous occurrence of high fracture toughness, superhardness and semiconductivity is expected to find applications for this material.

High-pressure studies with x-rays using diamond anvil cells

Pressure profoundly alters all states of matter. The symbiotic development of ultrahigh-pressure diamond anvil cells, to compress samples to sustainable multi-megabar pressures; and synchrotron x-ray techniques, to probe materials' properties in situ, has enabled the exploration of rich high-pressure (HP) science. In this article, we first introduce the essential concept of diamond anvil cell technology, together with recent developments and its integration with other extreme environments. We then provide an overview of the latest developments in HP synchrotron techniques, their applications, and current problems, followed by a discussion of HP scientific studies using x-rays in the key multidisciplinary fields. These HP studies include: HP x-ray emission spectroscopy, which provides information on the filled electronic states of HP samples; HP x-ray Raman spectroscopy, which probes the HP chemical bonding changes of light elements; HP electronic inelastic x-ray scattering spectroscopy, which accesses high energy electronic phenomena, including electronic band structure, Fermi surface, excitons, plasmons, and their dispersions; HP resonant inelastic x-ray scattering spectroscopy, which probes shallow core excitations, multiplet structures, and spin-resolved electronic structure; HP nuclear resonant x-ray spectroscopy, which provides phonon densities of state and time-resolved Mössbauer information; HP x-ray imaging, which provides information on hierarchical structures, dynamic processes, and internal strains; HP x-ray diffraction, which determines the fundamental structures and densities of single-crystal, polycrystalline, nanocrystalline, and non-crystalline materials; and HP radial x-ray diffraction, which yields deviatoric, elastic and rheological information. Integrating these tools with hydrostatic or uniaxial pressure media, laser and resistive heating, and cryogenic cooling, has enabled investigations of the structural, vibrational, electronic, and magnetic properties of materials over a wide range of pressure-temperature conditions.

Toward the Ultra-incompressible Carbon Materials. Computational Simulation and Experimental Observation

The common opinion that diamond is the stiffest material is disproved by a number of experimental studies where the fabrication of carbon materials based on polymerized fullerenes with outstanding mechanical stiffness was reported. Here we investigated the nature of this unusual effect. We present a model constituted of compressed polymerized fullerite clusters implemented in a diamond matrix with bulk modulus B0 much higher than that of diamond. The calculated B0 value depends on the sizes of both fullerite grain and diamond environment and shows close correspondence with measured data. Additionally, we provide results of experimental study of atomic structure and mechanical properties of ultrahard carbon material supported the presented model.

A first-principles study on three-dimensional covalently-bonded hexagonal boron nitride nanoribbons

We studied three-dimensional honeycomb-structure boron nitride (BN) allotrope using first-principles calculations and the tight-binding method. Interconnected by sp(3)-bonding at the vertices, hexagonal BN nanoribbons construct highly-porous, covalently-bonded hexagonal BN nanoribbons (CBBNs). We investigated the structural and mechanical properties of CBBNs with various sizes, compared with those of carbon and other BN allotropes. The mechanical and thermal stabilities are also checked. Our calculations show that, despite the high porosity and low mass density, CBBNs are stable and mechanically hard materials as cubic BN. Moreover, our calculated results suggest that CBBNs can be regarded as a binary alloy of sp(2)- and sp(3)-bonded BNs following the Vegard's rule in average bond lengths and bulk moduli. Calculated band structures show that the band gap of CBBNs has similar variation upon increasing size as BN nanoribbons and is also limited by the second-neighbor interaction between the pz states of sp(2)-bonded atoms in adjacent nanoribbons.

High pressure study of Li-doped fullerides, Li(x)C60 (x = 4,12), by x-ray diffraction and Raman spectroscopy

In this article we study the alkali metal-intercalated 2D polymeric Li4C60 and the monomeric Li12C60 under pressure up to 40 GPa at room temperature, using x-ray diffraction and Raman spectroscopy. Li4C60 undergoes several transitions in the studied pressure range. At pressures lower than 8 GPa, we observed changes in both diffraction patterns and Raman scattering spectra, probably due to the displacement of Li atoms. At 8 GPa another structural and electronic transition occurs. We observe an enhancement of background and a broadening of diffraction peaks. Raman modes weaken and broaden considerably. An important structural transition occurs at around 16 GPa, in which new Raman bands exhibit features similar to those of a reported 3D C60 polymeric structure. The XRD data shows a collapse in volume with the simultaneous formation of amorphous material. The cell parameters deviate from their early pressure evolution and become less compressible. The high pressure study of highly doped monomeric Li12C60 shows that its structural integrity is retained up to 13 GPa, with increasing pressure-induced structural distortion and disorder. Above 13 GPa, Li12C60 transforms to a highly disordered state.

Continuous transformations of C60 crystals: polymorphs, polymers, and the ideal strength of fullerites

Application of pressure is a versatile method to tailor the properties of organic semiconductors. For example, it is known that high pressure can transform C60 face-centred-cubic (FCC) crystals to polymer structures with inter-molecular bonds. Here we use first-principles calculations to describe continuous crystalline transformation paths that include the FCC and polymer structures as distinct local energy minima. In addition to analysing the atomic-scale details of polymerization, we obtain the ideal strength of FCC-C60, identify metastable C60 crystalline polymorphs, and characterize their electronic properties-all key features for the performance of C60 crystals in organic electronic devices.

Metastable C-centered orthorhombic Si8 and Ge8

A theoretical prediction on the structural stabilities, mechanical properties, and electronic properties of the C-centered orthorhombic (Cco) Si(8) and Ge(8) is presented, inspired by a recently proposed carbon allotrope structure, Cco-C(8). Energetically comparable with previously known metastable phases, Cco-Si(8) and Cco-Ge(8) may be obtained by decompressing the high-pressure β-Sn phases, or by compressing the corresponding nanotubes. The calculated bulk moduli of Cco-Si(8) and Cco-Ge(8) are close to those of the diamond phases. Further study of the electronic properties reveals that the band gaps of Cco-Si(8) and Cco-Ge(8) are tunable with variations in lattice parameters.

First-principles study of electron-conduction properties of C60 bridges

The electron-conduction properties of fullerene-based nanostructures suspended between electrodes are examined by first-principles calculations based on the density functional theory. The electron conductivity of the C60-dimer bridge is low owing to the constraint of the junction of the molecules. When the fullerenes are doped electrons by being inserted Li atoms into the cages, the unoccupied state around the junction is filled and the conductivity can be significantly improved.

Electron conductive three-dimensional polymer of cuboidal C60

Single crystals of three-dimensional (3D) C60 polymer were prepared by the topotactic conversion of two-dimensional (2D) C60 polymer single crystals at a pressure of 15 GPa at 600 degrees C. The x-ray single crystal study revealed that the 3D C60 polymer crystallized in a body centered orthorhombic space group Immm, and spherical C60 monomer units were substantially deformed to rectangular parallelepiped (cuboidal) shapes, each unit being bonded to eight cuboidal C60 neighbors via [3 + 3] cycloaddition. The 3D C60 polymer was electron conductive, in contrast with the nonconductive behavior of 2D polymers.

Exceptional ideal strength of carbon clathrates

We study by means of ab initio calculations the ideal tensile and shear strengths of the C-46 clathrate phase. While its bulk modulus and elastic constants are smaller than in diamond, its strength is found to be in all directions larger than the critical stresses associated with the diamond [111] planes of easy slip. This can be related to the frustration by the clathrate cage structure of the diamond to graphite instability under nonhydrostatic stress conditions [corrected] The criteria for designing strong materials are discussed.

Ultralow compressibility silicate without highly coordinated silicon

The bulk modulus of scheelite-structured ZrSiO(4) is 301.4+/-12.5 GPa, as derived from static compression experiments to 52.5 GPa. It is as stiff as the most incompressible known silicate, SiO(2) stishovite. This high incompressibility indicates that octahedrally coordinated silicon is not required to generate ultrastiff silicates: ZrSiO(4) scheelite is the most incompressible material containing SiO(4) tetrahedra. Its incompressibility is in accord with a semitheoretical relation we derive for the bulk modulus of scheelite-structured materials. Based upon correlations between incompressibility and hardness, scheelite-structured oxides may thus represent a new family of ultrahard materials.