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In 1967, a diamond polymorph was reported from hard, diamond-like grains of the Canyon Diablo iron meteorite and named lonsdaleite. This mineral was defined and identified by powder X-ray diffraction (XRD) features that were indexed with a hexagonal unit cell. Since 1967, several natural and synthetic diamond-like materials with XRD data matching lonsdaleite have been reported and the name lonsdaleite was used interchangeably with hexagonal diamond. Its hexagonal structure was speculated to lead to physical properties superior to cubic diamond, and as such has stimulated attempts to synthesize lonsdaleite. Despite numerous reports, several recent studies have provided alternative explanations for the XRD, transmission electron microscopy and Raman data used to identify lonsdaleite. Here, we show that lonsdaleite from the Canyon Diablo diamond-like grains are a nanocomposite material dominated by subnanometre-scale cubic/hexagonal stacking disordered diamond and diaphite domains. These nanostructured elements are intimately intergrown, giving rise to structural features erroneously associated with h diamond. Our data suggest that the diffuse scattering in XRD and the hexagonal features in transmission electron microscopy images reported from various natural and laboratory-prepared samples that were previously used for lonsdaleite identification, in fact arise from cubic/hexagonal stacking disordered diamond and diaphite domains. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'. [ABSTRACT FROM AUTHOR]

Abstract Hexagonal diamond (h-C) and wurtzite boron nitride (w-BN) are two superhard materials recently identified to be comparable to or even harder than their cubic counterparts, cubic diamond (c-C) and cubic boron nitride (c-BN). To understand the effect of lattice structure on thermal transport in these materials, we conduct first-principles calculations to investigate their harmonic and anharmonic lattice properties. Owing to the strong C-C or B-N bonds, h-C and w-BN are found to have a high lattice thermal conductivity (κ L) exceeding the overall thermal conductivity of metals, albeit lower than that of their cubic counterparts. By analyzing the phonon band structure and volume of the 3-phonon scattering phase space, we attribute the lower κ L of the hexagonal phases to their larger volume of 3-phonon scattering phase space than the cubic ones. Moreover, we reveal that a high pressure of 125 GPa leads to a two-to three-fold increase in the κ L of these materials, because the pressure enlarges the optical-acoustic phonon bandgap and thus reduces the volume of the 3-phonon scattering phase space. This work uncovers the significant effect of lattice structure and pressure on phonon scattering and transport, which is crucial for the application of superhard materials. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Chemical mapping imaged by electron energy-loss spectroscopy based on scanning transmission electron microscopy was conducted on a compressed graphite specimen containing different carbon allotropes (hexagonal diamond, cubic diamond, and graphite phases). This imaging process allows visualization of the complex spatial distribution of different diamond phases, and their coexistence was confirmed using dark field (DF) imaging. The chemical mapping images showed spatial distribution of local bonding state for hexagonal and cubic diamond phases in the whole specimen, while the DF images showed only a part of crystalline segments with long-range order. Thus, the chemical mapping method has an advantage for the purpose of observing locally the existence of individual carbon allotropes in the whole specimen. The size distribution of the hexagonal diamond phase is approximately 10–100 nm. These findings indicate that the compressing method can potentially synthesize ~ 100 nm large diamond phases. [ABSTRACT FROM AUTHOR]

In a reactive direct current (DC) magnetron sputtering (MS) system, converting the cathode (target) magnetic field configuration from a balanced magnetron (BM) to an unbalanced magnetron (UBM) is an impressive strategy to enforce the energetic ion bombardment, providing sufficient activation energy for deposition of high-quality nanostructured films. In the present work, the nanostructured (grain size ˂ 100 nm) BM and UBM sputtered Niobium-doped amorphous Carbon (Nb: a-Carbon) films' microstructural , mechanical, and tribological characteristics changes were thoroughly investigated and compared. From the XRD and Raman analysis outcomes both graphite-like Carbon (GLC) and Niobium carbide (NbC) phases were detected in the BM sputtered film. However, novel cubic and hexagonal crystalline Diamond (CD) phases were successfully constructed from the UBM sputtered films besides the GLC and NbC phases. Records displayed that the change over the cathode magnetic field configuration from BM to UBM is a responsible factor for the reduction of the surface roughness (from 32 nm to 25 nm) and increment of the crystallite size (from 14 nm to 15.5 nm), hardness (from 18 GPa to 27 GPa), and elastic modulus (from 210 GPa to 425 GPa) values. On the contrary, via changing BM to UBM sputtering the films' tribological properties were negatively affected, leading to the increase of the coefficient of friction (COF) from 0.09 to 0.11 and the wear rate (W R) from 5.25 × 10−5 mm3/N.m to 9 × 10−5 mm3/N.m. The achieved results confirmed that the UBM configuration could be selected as a critical facility over the MS system that gives enough energy to arrange Carbon species in the crystalline Diamond phase at a relatively low temperature. One step further, the Ar/CH 4 ratio variation was assessed as an influential factor that leaves considerable changes over the UBM sputtered film characteristics. The results indicated that by doubling the Ar/CH 4 ratio, the nanocrystalline Diamond phases are still observable. However, this change led to the increase in surface roughness (from 25 nm to 28 nm) and crystallite sizes (NbC, from 15.5 nm to 18.6 nm, cubic Diamond, from 25.8 nm to 36 nm, and hexagonal Diamond from 18.9 nm to 21.6 nm). While the variations adversely affected the UBM sputtered films' hardness (from 27 GPa to 21 GPa) and modulus of elasticity (from 425 GPa to 330 GPa). Simultaneously, the COF and W R endured a slight increment, from 0.11 to 0.14 and 9 × 10−5 mm3/N.m to 9.8 × 10−5 mm3/N.m, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Synthesis of hexagonal diamond on Al/Ni coated thermally oxidized SiO2 covered Si wafer substrate with photo-enhanced chemical vapor deposition (CVD) method at 450°C is reported here. The Raman spectroscopy on grown samples shows a 1322cm−1 peak with 75.4cm−1 full width at half maximum (FWHM). The X-ray diffraction (XRD) data shows hexagonal diamond peaks. Various stages of hexagonal diamond formation are observed in the scanning electron microscope (SEM) images. Based on these images, a mechanism of hexagonal diamond nucleation and growth is proposed. [Copyright &y& Elsevier]

Abstract: Electron energy-loss spectroscopy measurements of a hexagonal diamond (h-DIA) were performed using a transmission electron microscope equipped with a monochromator. From the measurements, the dielectric function of h-DIA was derived using Kramers–Kronig analysis (KKA) for the first time. The results of KKA showed that the band gap energy of h-DIA was 4.2eV, which is 1.3eV smaller than that of a cubic diamond (c-DIA). The peak energies that appeared in the imaginary part of the dielectric function were assigned to the interband transition energies predicted by theoretical calculations. The K-shell excitation spectrum of h-DIA, which was apparently different from that of c-DIA, also showed good agreement with the reported theoretical prediction. [Copyright &y& Elsevier]

Abstract: We studied the structure of diamond nanoparticles grown by chemical vapor deposition. SEM images show that the material contains cubic, hexagonal, and possibly icosahedral structures ranging in size from 10 to 200nm. Raman spectroscopy shows bands, which are characteristic of crystalline diamond, E 2g mode of hexagonal diamond, a-C, and graphite. [Copyright &y& Elsevier]

Abstract: This paper reports for the first time the synthesis of hexagonal diamond thin films on high-speed steel substrates by multi-mode microwave plasma enhanced chemical vapor deposition. Before deposition of the films, the substrate surface was treated by scratching with diamond powder. The deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy. The XRD patterns of (100) and (101) planes and the Raman peaks at ~1317–1322 cm−1 were observed, confirming the formation of hexagonal diamond phase in the prepared films. The effects of voltage bias on the phase formation, microstructure and hardness of the films were also studied by setting the voltage to 0, −70, −150 and −190 V. The highest hardness of 23.8 GPa was found in the film having clusters of size about 550 nm deposited under a bias voltage of −150 V. These clusters were built up of grains of size about 14 nm. [Copyright &y& Elsevier]

At the nanoscale, the proportion of atoms at the surface of solids becomes significant, which may change the equilibrium of atomic edifices. In the case of silicon, experimental observations have evidenced indeed the presence of the hexagonal diamond (HD) metastable structure in as-grown nanowires of a few nm in diameter, as if that phase could become the stable one in such small objects. We present ab initio calculations that demonstrate the existence of stable domain for the HD structure in silicon nanowires. Surfaces of HD Si are first studied without and with hydrogen, including possible relaxations, and compared to CD Si surfaces, with a globally favourable energy ratio for HD surfaces. The energies of several plausible HD and CD Si NWs of different thicknesses are then calculated and compared to estimate their relative phase stabilities, again in favour of HD NWs, without and with hydrogen at surfaces. Analytically extrapolating the ab initio results as functions of bulk, surface and edge energy contributions, the main result is that HD Si NWs are intrinsically stable with respect to CD Si NWs for effective diameters up to only about 15 nm, for pure Si NWs as well as for surface hydrogenated NWs. Thicker HD NWs can thus only be metastable. This may explain why Si HD NWs are so difficult to grow. The diameter size limit for germanium, silicon's big brother, is three times smaller, making HD Ge NWs much less likely, in agreement with recent experimental attempts to grow Ge NWs. [ABSTRACT FROM AUTHOR]

Diamond nanoparticles have been synthesized using various methods. Nanodiamonds generated in the gas phase were captured on the membrane of a transmission electron microscope grid during a hot filament chemical vapor deposition (HFCVD) diamond process. In total, ~600 nanoparticles, which were captured for 10 s in six conditions of the capture temperatures of 900 °C, 600 °C and 300 °C and the gas mixtures of 1% CH4-99% H2 and 3% CH4-97% H2, were analyzed for phase identification using high-resolution transmission electron microscopy and fast Fourier transformation. Hexagonal diamond, i-carbon, n-diamond, and cubic diamond were identified. The observation of two or more carbon allotropes captured on the same membrane suggested their coexistence in the gas phase during HFCVD. The crystal structure of carbon allotropes was related to the size of the nanodiamond. The crystal structure of the nanoparticles affected the crystal structure of diamond deposited for 8 h. Confirmation of various carbon allotropes provides new insight into the nanodiamond synthesis in the gas phase and the growth mechanism of HFCVD diamond. [ABSTRACT FROM AUTHOR]