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Natural and synthetic diamonds mostly have a cubic lattice, whereas a rare hexagonal structure—known as hexagonal diamond (HD)—has been largely unexplored due to the low purity and minuscule size of most samples obtained. The synthesis of HD remains a challenge and even its existence remains controversial. Here we report the synthesis of well-crystallized, nearly pure HD by heating highly compressed graphite, which is applicable to both bulk and nanosized graphitic precursors. Experiments and theoretical analyses show that the formation of a post-graphite phase within compressed graphite and temperature gradients promote HD growth. Using this approach, a millimetre-sized, highly oriented HD block comprising stacked single-crystal-like HD nanolayers is obtained. This HD exhibits high thermal stability up to 1,100 °C and a very high hardness of 155 GPa. Our findings offer valuable insights regarding the graphite-to-diamond conversion under elevated pressure and temperature, providing opportunities for the fabrication and applications of this unique material.

The endohedral fullerene Lu3N@C80 was examined using in situ high-pressure measurements, which included electrical transport, Fourier-transform infrared spectroscopy, and Raman spectroscopy, in combination with theoretical calculations. Lu3N@C80 was found to undergo a reversible n- to p-type conversion at ∼8.9 GPa. This p-type semiconductor remains stable up to 25 GPa. The fullerene cage collapses at ∼29 GPa, resulting in an irreversible p- to n-type conversion. Raman, infrared, and X-ray absorption spectroscopy reveal that an anisotropic distortion of the carbon cage and a pyramidalization of the planar Lu3N clusters occur during compression. Density functional theory simulations indicate that the p orbitals of C atoms in the fullerene cage primarily contribute to the density of states (DOS). Pressure-induced deformation of the fullerene cage dominates the DOS changes in the conduction and valence bands close to the Fermi level. The findings elucidate the relationship between the conductivity and structural changes in the endohedral clusters.

One-dimensional (1D) van der Waals heterostructures (vdWHs) exhibit many properties, but many of these are difficult to understand because of their complicated structures and host-guest couplings that remain challenging to understand. Here, we observe a significant electron-phonon coupling (EPC) in 1D armchair graphene nanoribbons (AGNRs) confined in single-walled carbon nanotubes (SWNTs) via resonance Raman spectroscopy combined with theoretical calculation. A strong coupling between radial breathinglike mode (RBLM) phonons and the host nanotubes occurs due to the large vibration amplitude of RBLMs of the guest AGNRs. This results in unique deformation potential interactions in the heterostructure, contributing to the observed EPC enhancement. The EPC could be further modulated and strengthened by high pressure through tuning the RBLM-nanotube interactions. In this paper, we discover a mechanism governing the EPC in vdWHs and pave the way for manipulating the host-guest EPC for further control of the physical properties and potential device applications.

A novel piezo-activated luminescent material with wide range modulation of the luminescence wavelength and a giant intensity enhancement upon compression was prepared using a strategy of molecular doping. The doping of THT molecules into TCNB-perylene cocrystals results in the formation of a weak but pressure-enhanced emission center in the material at ambient pressure. Upon compression, the emissive band from the undoped component TCNB-perylene undergoes a normal red shift and emission quenching, while the weak emission center shows an anomalous blue shift from 615 nm to 574 nm and a giant luminescence enhancement up to 16 GPa. Further theoretical calculations show that doping by THT could modify intermolecular interactions, promote molecular deformation, and importantly, inject electrons into the host TCNB-perylene upon compression, which contributes to the novel piezochromic luminescence behavior. Based on this finding, we further propose a universal approach to design and regulate the piezo-activated luminescence of materials by using other similar dopants.

As an advanced amorphous material, sp³ amorphous carbon exhibits exceptional mechanical, thermal and optical properties, but it cannot be synthesized by using traditional processes such as fast cooling liquid carbon and an efficient strategy to tune its structure and properties is thus lacking. Here we show that the structures and physical properties of sp³ amorphous carbon can be modified by changing the concentration of carbon pentagons and hexagons in the fullerene precursor from the topological transition point of view. A highly transparent, nearly pure sp³−hybridized bulk amorphous carbon, which inherits more hexagonal-diamond structural feature, was synthesized from C₇₀ at high pressure and high temperature. This amorphous carbon shows more hexagonal-diamond-like clusters, stronger short/medium-range structural order, and significantly enhanced thermal conductivity (36.3 ± 2.2 W m−1 K−1) and higher hardness (109.8 ± 5.6 GPa) compared to that synthesized from C₆₀. Our work thus provides a valid strategy to modify the microstructure of amorphous solids for desirable properties.

The electrical resistivities of metals and alloys tend to saturate at some value in the range 150–250 μΩcm, corresponding to semi-classical mean free paths near a few interatomic distances. The origin of this effect is still under debate and no accepted theory exists. However, the saturation behavior can often be described quite accurately by algebraic functions derived from simple theory. Such functions may be used for analyzing resistivity data in experimental and applied physics. In this work, several functions recently proposed have been fitted to 184 sets of resistivity data from the literature, measured on 149 elements, alloys, and intermetallic compounds. The results have been evaluated in terms of both accuracy of fit and of the validity of fitted parameter values. Different functions give the best fit to data for different materials, suggesting that there is no universal functional dependence and thus probably no universal mechanism behind the saturation phenomenon.

Graphitic carbon nitride (C₂N and C₃N) with various p electron distributions on layers have been studied under pressure through acombined theoretical and experimental approach and a comparison with graphite. It is found that as these materials transform into lowcompressibility phases in the pressure range from 15 to 45 GPa, strong electrostatic repulsion between p electrons and in-plane sp2 electronsmay distort and soften the sp2 bonds, leading to anomalous pressure evolutions of the intralayer phonon vibrations, such as a plateau-likebehavior of E_2g mode (G-band) in C₂N and C₃N. This also causes a slow increase in the resistivity/resistance of C₂N and C₃N as pressureincreases, and the gradual interlayer bonding leads to an abrupt increase in resistance of the materials but with different pressure responsesdue to their different p electron distributions. Moreover, the intensity enhancement of the G band in both CN materials may be related totheir electronic structure changes. The results deepen our understanding of the effects of p electron distribution on the structural transitionof graphitic materials and may explain some unexplained in previous studies.

Carbon is an element with extremely versatile bonding properties and theoretical calculations have suggested the possible existence of several hundred structural allotropes. Many, or even most, of these are predicted to be formed under conditions of high pressure and temperature. On the other hand, experimental high pressure studies have identified surprisingly few structural allotropes. In this paper, physical properties and structural transformations observed in high pressure experiments, at and above room temperature, are reviewed for a large number of solid carbon allotropes. The materials discussed include bulk carbon such as graphite, diamond, glass-like and amorphous carbon, two-dimensional graphene, and molecular carbon in the form of one-dimensional carbon nanotubes and zero-dimensional fullerenes. Results from recent studies on twisted graphene, graphdiyne, graphyne, carbon dots and other interesting all-carbon allotropes are also briefly described. Observed similarities and differences between the high pressure behavior and evolution of carbon materials are discussed. In spite of the enormous volume of experimental work carried out on these materials, few new structural allotropes have been identified and most carbon materials studied convert into diamond at sufficiently high temperature and pressure. Further theoretical work thus seems to be needed to elucidate possible transformation processes and transition paths for the many undiscovered allotropes proposed from calculations. In particular, it is recommended that, for every new allotrope predicted by theory, suitable precursors and transformation conditions should also be investigated. Efficient creation of new structural allotropes or functional materials based on pure carbon by high pressure methods should ideally start from designed, preassembled precursor structures or composites for which transition paths can be theoretically predicted.

Resistivity saturation in Ti-Al alloys has been studied by fitting empirical functions of temperature, T, to 20 sets of literature data. By replacing the commonly used temperature-independent saturation term by a term in T−1/2, to model weak localization effects at high temperature, the quality of the fits improved significantly and physically acceptable values were obtained for the fitted parameters. A linear correlation was found between the fitted high-T weak localization parameters and experimental data for the corresponding low-T weak localization conduction terms. No such correlation could be detected when using constant saturation terms. The results suggest that weak localization is a possible cause for resistivity saturation in Ti-Al alloys.

The polymerization of three typical aromatic solvent-doped fullerene materials with similar hexagonal closest packed (hcp) structures (mesitylene/C60, m-dichlorobenzene/C60 and m-xylene/C60 solvates) is studied under high pressure and high temperature (HPHT, 1.5 GPa, 573 K and 2 GPa, 700 K, respectively). Raman and photoluminescence spectroscopies reveal that the intercalated aromatic solvents play a crucial role in tailoring the extent of polymerization of C60 molecules. In the solvates, the solvents confine formation of covalent bonds between C60 molecules to the 001 direction and the (001) plane of the hcp lattices, leading to the formation of mixed polymeric phases of monomers, dimers, one-dimensional (1D) chainlike oligomers, and two-dimensional (2D) tetragonal phase polymers under suitable HPHT conditions. The type and number of substituent groups of the aromatic solvents are found to have significant influence, determining the amounts and types of polymeric phases formed. Our studies enrich the understanding of the formation mechanisms for controllably fabricating polymeric fullerenes and facilitate targeted design and synthesis of unique fullerene-based carbon materials.