脉冲激光诱导液-固界面反应合成金刚石纳米晶中的结构相变模型

 探讨了脉冲激光诱导液-固界面反应法（PLIR: Pulsed-Iaser Induced Liquid-Solid Interface Reaction ）制备金刚石纳米晶的物理化学机制,提出了金刚石纳米晶的成核机理,即由激光诱导石墨六方结构原子团过渡到石墨菱方结构、然后转变成立方金刚石晶核,以及由石墨六方结构直接转变成六方金刚石结构的相变模型,并讨论了基于液-固界面反应的纳米晶生长动力学,较好地从动力学上解释了合成金刚石纳米晶的物理化学机制。     

Shock metamorphism of the graphite quasimonocrystal

Abstract

Micro structure examination of graphite quasimonocrystal recovered after dynamic loading to pressure of 35-45 GPa was carried out. Only a small amount of cubic diamond and recrystalized graphite was detected. Most of the graphite (～80 vol.%) remained in initial high orientation, but transformed to fine, grained phase with crystalite size 0.1-1 microns. Relaxation time of the transformation (～ 10 ns) and the degree of the transformation (～ 70-80 vol.%) were determined by means of measurements of the electrical resistivity during loading up to 26 GPa and following computer simulation of the results. We proposed that two simultaneous processes take place at pressures higher than 20 GPa: i) relatively slow diffusive graphite to diamond transformation localized in the zones with defect structure: ii) highly oriented graphite transforms to a diamond like phase with density of about 3.2 g/cm³ at zero pressure. This, transformation has fast, martensitic kinetics and is reversible.     

Investigation on the formation of lonsdaleite from graphite

Structural stability and the possible pathways to experimental formation of lonsdaleite—a hexagonal 2H polytype of diamond—have been studied in the framework of the density functional theory (DFT). It is established that the structural transformation of orthorhombic Cmmm graphite to 2H polytype of diamond must take place at a pressure of 61 GPa, while the formation of lonsdaleite from hexagonal P6/mmm graphite must take place at 56 GPa. The minimum potential barrier height separating the 2H polytype state from graphite is only 0.003 eV/atom smaller than that for the cubic diamond. The high potential barrier is indicative of the possibility of stable existence of the hexagonal diamond under normal conditions. In this work, we have also analyzed the X-ray diffraction and electron-microscopic data available for nanodiamonds found in meteorite impact craters in search for the presence of hexagonal diamond. Results of this analysis showed that pure 3C and 2H polytypes are not contained in the carbon materials of impact origin, the structure of nanocrystals found representing diamonds with randomly packed layers. The term “lonsdaleite,” used to denote carbon materials found in meteorite impact craters and diamond crystals with 2H polytype structure, is rather ambiguous, since no pure hexagonal diamond has been identified in carbon phases found at meteorite fall sites.     

High-pressure synthesis of a novel form of endohedral Li diamond from Li graphite intercalation compound

A novel form of hexagonal diamond containing Li atoms in the open rooms surrounded by sp³-bonded carbon atoms was successfully synthesized from a Li graphite intercalation compound under high pressure, as had been predicted by theoretical studies. High-pressure experiments with LiC₆ were performed in the pressure range from 0.1 MPa to 43 GPa using a diamond-anvil cell. In situ X-ray diffractometry and optical microscopy revealed that LiC₆ was transformed to a hexagonal-diamond form without losing Li atoms. The c-axis of the hexagonal-diamond form was considerably longer than that of the hexagonal diamond transformed from pure graphite, which was consistent with the predicted structure of the endohedral Li diamond. The observed high-pressure form exhibited a golden metallic gloss, which was also consistent with the calculated metallic band structure.     

On the imaging mechanism of monatomic steps in graphite

The shape and the atomic arrangement of monolayer steps of graphite have been characterized by STM. The origin of the appearance of the imaged features along the steps is discussed, addressing for the first time both morphological and electronic considerations. Extended Hückel theoretical calculations of nanotubes are used to identify the contribution of the electronic structure to the STM image of monolayer steps. We show that mechanical tip–sample interactions dominate the imaging process of graphite, leading to step deformation during scanning and negative STM contrast of the atom positions in the hexagonal unit cell. Received: 11 April 2000 / Accepted: 18 April 2000 / Published online: 23 August 2000     

On the yellow-band emission in CdS films

CdS polycrystalline thin films were prepared by the chemical bath deposition (CBD) method on glass substrates. X-ray diffraction (XRD) studies show that the films grow in the cubic zinc-blende crystalline phase. Upon thermal annealing (TA) in Ar+S₂ flux at normal pressure in the temperature range 240–510 °C, the evolution of the transformation into the hexagonal wurtzite phase is observed. This hexagonal crystalline structure is the stable phase. From XRD diagrams the phase transition can be appreciated to occur upon TA at approximately 300 °C. Photoluminescence (PL) data prove that the green-emission band is present for well-defined phases – cubic or hexagonal ones. A second band located at 2.2 eV appears for samples near the transition region. This band at 2.2 eV, called the yellow band, has already been reported to be associated with interstitial Cd atoms. A model for this yellow-band-mechanism formation, arising during the phase transformation, has been proposed based on Frenkel-pair creation. Received: 27 June 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001     

Characterization of a growth-front interface in a HPHT-grown diamond crystal by transmission electron microscopy

The growth-front interface of a diamond single crystal, which was grown from the Fe-Ni-C system under high pressure and high temperature (HPHT), has been directly observed by transmission electron microscopy (TEM) for the first time. The presence of a cellular interface suggests that the diamond is grown from solution and there exists a narrow supercooling zone in front of the solid–liquid interface. Diamond-growth parallel layers were also found, which indicates that the diamond grows from solution layer by layer. It provides direct evidence that the diamond is synthesized through graphite dissolution and transformation to subcritical diamond particles in a molten catalyst, diamond subcritical particle connection to form diamond clusters, diffusion of the diamond clusters to the growing diamond, and unification of the diamond clusters on the growing diamond crystal. Received: 17 July 2000 / Accepted: 27 October 2000 / Published online: 10 January 2001     

Effect of a rhombohedral phase on lithium intercalation capacity in graphite

Five natural graphites with different rhombohedral phase (3R phase) content have been investigated as anode materials for rechargeable lithium-ion batteries. The reversible capacity varies from 250 mA h/g to 350 mA h/g for the same intercalation conditions depending on the content of the 3R-phase. With increasing rhombohedral phase in the graphite, the intercalation capacity will be high. A peak at about 10 mV in the cyclic voltammograms (CV) of a sample with a large 3R-phase content is caused by lithium ions occupying boundaries between the rhombohedral (3R) phase and the hexagonal (2H) phase and is responsible for a capacity exceeding the theoretical value.     

Structure Formation of Hexagonal Diamond: Ab Initio Calculations

Physics of the Solid State - The formation of structure of hexagonal diamond from graphite and cubic diamond is simulated with the density functional theory. Orthorhombic AB graphite transforms...     

Pressure-induced structural phase transition in rare-earth trihydrides. Part I. (GdH3, HoH3, LuH3)

High-pressure X-ray diffraction studies of gadolinum, holmium and lutetium trihydrides have been carried out in a diamond anvil cell up to 30 GPa at room temperature. A reversible structural phase transformation from the hexagonal to cubic phase has been observed for all the hydrides investigated. These results confirm our first discovery of the hexagonal to cubic phase transition in erbium trihydride published recently [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219. [1]]. The lattice parameters of the new cubic phases and the volume changes at transition points were determined for SmH₃, GdH₃, and HoH₃. The parameters of the equation of state for all the hexagonal and cubic phases of the investigated compounds have been determined.     

Disorder-order phase transition induced by size effect in bulk La2/3Sr1/3MnO3

Bulk La_2/3Sr_1/3MnO₃ ceramic samples prepared by thermal decomposition are investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). An abnormal phenomenon, where three kinds of La_2/3Sr_1/3MnO₃ phases with different structures and the same composition coexist in the same grain, has been observed. Besides the stable rhombohedral majority phase, the two other phases are a simple cubic structure with a=0.389 nm and a new hexagonal structure with a=0.544 nm, c=0.668 nm. The simple cubic phase is a residual phase of high-temperature due to the size effect and bondage of twin boundary. Image simulations have suggested that the new hexagonal phase is the La-Sr ordered structure with space group , which is converted from the disordered simple cubic phase. The formation mechanism of the ordered phase is explained from volume energy and interface energy considerations.     

Energetics and kinetics of direct phase conversion from graphite to diamond

The atomistic mechanism for direct conversion of graphite to diamond is a long-standing problem in condensed matter physics. The newly identified cold-compressed graphite phases of M, W and O carbon provide a crucial link to understand the graphite-to-diamond phase transformation. We demonstrate by ab initio calculations that pressure has a dual role in lowering the conversion barrier and enhancing the production stability during the first-stage cold-compressed phase conversion of graphite toward the intermediate metastable M, W and O carbon phases. However, it has little effect on the relative enthalpy and high conversion barrier during the second-stage conversion process toward the diamond polytypes, showing a temperature dominated conversion process. These results may give explanation regarding the necessity of high pressure and high temperature during the graphite-todiamond reaction.     

Electronic topological transition and phase transition in rhodium and iridium based inter metallic compounds

Of the Rh₃Y, Rh₃La, Ir₃Y and Ir₃La inter-metallic compounds, the compound Rh₃Y exists in hexagonal structure, Ir₃Y and Ir₃La exist in rhombohedral structure, whereas the compound Rh₃La exists in both hexagonal and rhombohedral structures. Based on our tight binding-linear muffin tin orbital (TB-LMTO) study of other rhodium and iridium-based Rh₃X and Ir₃X (where X=Ti, Zr, Hf, V, Nb, Ta and Sc) inter-metallic compounds of AuCu₃ type cubic structure, an attempt is made to examine whether the compounds Rh₃Y, Rh₃La, Ir₃Y and Ir₃La will undergo a structural phase transition to cubic structure from their experimentally reported structures. From our study, it is observed that the compounds Rh₃Y and Rh₃La undergo a structural phase transition to cubic phase at 4.5 and 10.1 GPa, respectively, from their experimentally reported hexagonal and rhombohedral phases. Further it is predicted that both the compounds Ir₃Y and Ir₃La can exist in the cubic phase itself at ambient condition, in contrary to the experimental observation. From the band structure outputs that have been plotted for the compounds under compression, it is observed that the compounds Rh₃La, Ir₃Y and Ir₃La undergo the Lifshitz type of transition which may change the Fermi surface topology and hence the physical properties of these compounds.     

Effect of the structural state of graphite on the parameters and kinetics of transformation into diamond under shock compression

The parameters of the transformation into diamond or a diamond-like high-pressure phase under shock compression were measured for a pressed natural OSCh-T1 grade graphite, a highly oriented monochromator-grade graphite, and a UPV-1 pyrolitic carbon. In the experiments, the compression-wave structure was measured in the transformation region using a VISAR laser Doppler velocimeter. It was found that the transformation pressure increases and the transformation rate decreases as the degree of three-dimensional ordering of graphite decreases.     

Formation and crystal structure of metallic inclusions in a HPHT as-grown diamond single crystal

One of the most important characteristics associated with crystal growth technology is the entrapment of inclusions by the growing crystal. Diamond single crystals prepared under high temperature-high pressure (HPHT) usually contain metallic inclusions. In the present paper, metallic inclusions in a diamond grown from a Fe-Ni-C system using the HPHT method have been, for the first time, systematically examined by transmission electron microscopy (TEM). Energy dispersive X-ray spectrometry (EDS) , combined with selected area electron diffraction (SAD) patterns, has been used to identify the chemical composition and crystal structure of the metallic inclusions. The metallic inclusions were found to be composed mainly of cubic γ-(FeNi), face-centered cubic (FeNi)₂₃C₆, ortho-rhombic Fe₃C and hexagonal Ni₃C, which may have been formed through the entrapment of molten catalyst by the growth front or through reaction of the trapped melt with contaminants in the diamond. Received: 19 June 2000 / Accepted: 21 June 2000 / Published online: 16 August 2000     

Diamond Structure BeO, Designable Super-Hard Materials and Semiconductor Be-Diamond

It is possible for Beryllium oxide （BeO） to have a cubic diamond structure although it normally has a hexagonal structure under ambient conditions. As the solution of cubic BN and diamond, the solid solution of cubic BeO- diamond or BeO-cBN-diamond can potentially be a kind of super-hard materials with designable hardness; and this solution has also been confirmed based on our preliminary first principles calculations. In addition, the nonstoichiometry of BeO could create a mobile carrier in the cubic BeO-C or BeO BN-C system and it might lead to a new type of semiconductor Be-diamond.     

Molecular dynamics simulations of diffusion of carbon into iron

Molecular dynamics (MD) simulations of diffusion couple tests were conducted between carbon (diamond/graphite) and iron at three different temperatures (300, 800 and 1600 K) and contact times (0, 40 and 80 ps) to investigate the chemical interaction between carbon and iron. Two different carbon structures, namely, diamond (cubic) and graphite (hexagonal), were considered. Diffusion of carbon into iron was observed only when a graphite interlayer was added to the diamond surface. When diamond alone was used, no diffusion was observed. This result provides corroborating evidence that diamond tool wear in the machining of iron occurs via a mechanism that involves an initial graphitization of diamond followed by diffusion of the newly formed graphite into the iron workpiece.     

Structure of diamond nanoparticles grown by chemical vapor deposition

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 200 nm. Raman spectroscopy shows bands, which are characteristic of crystalline diamond, E_2g mode of hexagonal diamond, a-C, and graphite.     

Orientation effect on the parameters of the polymorphic transformation of graphite under shock compression

A large effect of the shock compression direction with respect to the crystallographic planes of graphite on the detected pressure and rate of the transformation of graphite to a diamond or diamond-like high-pressure phase has been revealed. The effect is pronounced in more ordered graphite. The results indicate that shifts in basal planes complicate high-rate graphite-diamond transformation.     

Relative stability of cubic and different hexagonal forms of boron nitride

Boron nitride in the cubic form has a hardness approaching that of diamond. It is synthesized from a hexagonal graphitic phase. The physical behavior of the hexagonal phase depends upon the nature of layer stacking. This stacking is investigated using density functional approaches with the local density approach being far more successful than the generalized gradient approach. Various forms of the AaAa… stacking are predicted to be stable suggesting the existence of slightly different phases of the material. At the same time the energy differences between the stacking geometries is small and associated with small changes in the inter-layer spacing. This could have implications for the h-BN to c-BN transformation.