The Carbon Allotrope Hexagonite and Its Potential Synthesis from Cold Compression of Carbon Nanotubes

Abstract

In a previous report, the approximate crystalline structure and electronic structure of a novel, hypothetical hexagonal carbon allotrope has been disclosed. Employing the approximate extended Hückel method, this C structure was determined to be a semiconducting structure. In contrast, a state-of-the-art density functional theory (DFT) optimization reveals the hexagonal structure to be metallic in band profile. It is built upon a bicyclo[2.2.2]-2,5,7-octatriene (barrelene) generating fragment molecule and is a Catalan network, with the Wells point symbol (6(6))2(6(3))3 and the corresponding Schläfli symbol (6, 3.4). As the network is entirely composed of hexagons and, in addition, possesses hexagonal symmetry, lying in space group P6/mmm (space group #191), it has been given the name hexagonite. The present report describes a density functional theory (DFT) optimization of the lattice parameters of the parent hexagonite structure, with the result giving the optimized lattice parameters of a = 0.477 nm and c = 0.412 nm. A calculation is then reported of a simple diffraction pattern of hexagonite from these optimized lattice parameters, with Bragg spacings enumerated for the lattice out to fourth order. Results of a synchrotron diffraction study of carbon nanotubes which underwent cold compression in a diamond anvil cell (DAC) to 100 GPa, in which the carbon nanotubes have evidently collapsed into a hitherto unknown hexagonal C polymorph, are then compared to the calculated diffraction pattern for the DFT optimized hexagonite structure. It is seen that a close fit is obtained to the experimental data, with a standard deviation over the 5 matched reflections being given by σx = 0.003107 nm/reflection.

ON THE STRUCTURE OF i-CARBON

“…The high pressure transformation of graphite to diamond [1] involves the preservation of C hexagons in going from a 2D van der Waals layering, with a topology of (6, 3) [2] and at a density of 2.27 g/cm 3 , see Figure 1, to the densest possible sphere packing of C in 3D as the diamond lattice, with a topology of (6, 4) [2] and at a density of 3.56 g/cm 3 , see Figure 2. The transformation of nanotubes with a topology of (5 (x/(x+y)) , 3) [2,3]; an example of which is shown in Figure 3, to the hexagonite lattice with a topology of (6, 3 2/5 ) [2,3], as shown in Figure 4, also involves the preservation of C hexagons, in going from a van der Waals cylinder packing of nanotubes, which then collapses into the densest possible C cylinder packing in the hexagonite lattice (see Figure 4). Evidently, at a pressure of about 1 Mbar [3], the densest possible nanotube cyclinder packing, collapses into the densest possible C cylinder packing, as the hexagonite lattice is created in the form of nanocrystals with a density of 2.45 g/cm 3 , see Figure 4.…”

“…The transformation of nanotubes with a topology of (5 (x/(x+y)) , 3) [2,3]; an example of which is shown in Figure 3, to the hexagonite lattice with a topology of (6, 3 2/5 ) [2,3], as shown in Figure 4, also involves the preservation of C hexagons, in going from a van der Waals cylinder packing of nanotubes, which then collapses into the densest possible C cylinder packing in the hexagonite lattice (see Figure 4). Evidently, at a pressure of about 1 Mbar [3], the densest possible nanotube cyclinder packing, collapses into the densest possible C cylinder packing, as the hexagonite lattice is created in the form of nanocrystals with a density of 2.45 g/cm 3 , see Figure 4. Therefore, the transformation of powdered nanotubes to nanocrystalline hexagonite [3], is perfectly analogous to the transformation of graphite into diamond [1], as C hexagons are preserved in each respective transformation to the denser structure.…”

“…Evidently, at a pressure of about 1 Mbar [3], the densest possible nanotube cyclinder packing, collapses into the densest possible C cylinder packing, as the hexagonite lattice is created in the form of nanocrystals with a density of 2.45 g/cm 3 , see Figure 4. Therefore, the transformation of powdered nanotubes to nanocrystalline hexagonite [3], is perfectly analogous to the transformation of graphite into diamond [1], as C hexagons are preserved in each respective transformation to the denser structure.…”

Equilibrium diamond‐like carbon nanostructures with cubic anisotropy: Elastic properties

“…Physical properties of these structures are strongly dependent on the / ratio between the concentrations of the two types of atoms, which can be controlled during the synthesis. Several materials with mixed hybridizations are well known to date: carbon foam, various honeycomb structures, fullerene‐based polymers to name a few . Ratio between the number of atoms with and hybridization during synthesis can be changed by edge‐orientation, width and length of structure, or by changing of initial structure itself (for example, from graphene layers to fullerenes or nanotubes) ().…”

Multiporous carbon allotropes transformed from symmetry-matched carbon nanotubes

“…Compressing CNTs yields more interesting but perplexing phases owing to the structural complexity of CNTs, such as chirality, diameter, single-wall or multi-wall. 20,[38][39][40][41][42][43] However, with the development and improvement of experimental techniques, small and homogeneous CNTs might be achieved and used as fundamental building block for other structures or devices. Actually, many studies have been performed to achieve this goal.…”