Mechanism of the formation of a diamond nanocomposite during transformations of C60 fullerite at high pressure

The results of an investigation of the transformation of C₆₀ fullerite to diamond under pressure through intermediate three-dimensionally polymerized and amorphous phases are reported. It is found that treatment of fullerite C₆₀ at pressures 12–14 GPa and temperatures ∼1400°C produces a nanocrystalline graphite-diamond composite with a concentration of the diamond component exceeding 50%. At lower temperatures (700–1200°C) nanocomposites consisting of diamondlike (sp ³) and graphitic (sp ²) amorphous phases are formed. The nanocomposites obtained have extremely high mechanical characteristics: hardness comparable to that of best diamond single crystals and fracture resistance two times greater than that of diamond. Mechanisms leading to the transformation of C₆₀ fullerite into diamond-based nanocomposites and the reasons for the high mechanical characteristics of these nanocomposites are discussed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 822–827 (10 June 1999)     

Allotropic forms of carbon in the Invar Fe–Ni–C alloy before and after plastic deformation by upsetting

The effect of cold plastic deformation by upsetting (e = 1.13) on structure and hybridised bonds of carbon in the fcc Invar Fe-30.9%Ni-1.23% C alloy was studied by means of X-ray phase analysis and X-ray photoelectron spectroscopy. Carbon precipitates along grain boundaries and inside of grains in the alloy after annealing and plastic deformation were revealed. The presence of mainly sp²- and sp³-hybridised C–C bonds attributing to graphite and amorphous carbon as well as the carbon bonds with impurity atoms and metallic Fe and Ni atoms in austenitic phase were revealed in the annealed and deformed alloy. It was shown for the first time that plastic deformation of the alloy results in partial destruction of the graphite crystal structure, increasing the relative part of amorphous carbon, and redistribution of carbon between structural elements as well as in a solid solution of austenitic phase.     

Amorphous silicon hexaboride at high pressure

ABSTRACT

We investigate the pressure-induced structural phase transformation of amorphous silicon hexaboride (a-SiB₆) using a constant pressure first principles approach. a-SiB₆ is found to undergo a gradual phase transformation to a high-density amorphous phase (HDA) in which the average coordination number of both B and Si atoms is about 6. The HDA phase consists of differently coordinated motifs ranging from 4 to 8. B₁₂ icosahedra are found to persist during compression of a-SiB₆ and the structural modifications primarily occur around Si atoms and in the regions linking pentagonal pyramid-like configurations to each other. Upon pressure release, an amorphous structure, similar to the uncompressed one, is recovered, indicating a reversible amorphous-to-amorphous phase change in a-SiB_6. When the electronic structure is considered, the HDA phase is perceived to have a wider forbidden band gap than the uncompressed one.     

类金刚石a-C:H膜的热释氢和红外吸收研究

本文利用热释氢和红外吸收研究了H在非晶态碳(a-C:H)膜中的含量和组态。实验结果表明,随着样品制备时衬底温度的增大:1)H在a-C:H膜中的组态从两相结构过渡为单相结构;2)H在a-C:H膜中的含量单调减少;3)a-C:H膜中sP³/sP²键合比例单调增大。 关键词：     

Raman spectroscopy of melamine at high pressures up to 60 GPa

In this work, the Raman scattering of melamine was studied under high pressure up to 60 GPa. The behavior of the most intensive peaks of the Raman spectrum of melamine, 677 cm^?1 and 985 cm^?1 modes, and their line widths do not show any phase transition or indication of formation of sp ³ bonds. Comparing the behavior of the line width of the Raman peaks of graphite under pressure and that of melamine leads us to conclude that the s-triasine (C–N) ring is more rigid than the C–C graphite ring. High pressure results with melamine suggest that the direct phase transition g-C₃N₄ to dense C₃N₄ phase should occur above 60 GPa.     

Effect of nickel on the structure and bond type in amorphous diamond-like carbon films

Experimental data are presented from studies of the structure and bond type of carbon atoms in amorphous carbon-nickel films deposited from pulsed vacuum-arc discharge plasma sources. X-ray photoelectron spectroscopy was used. The characteristics of the plasmon loss spectra depend significantly on the deposition parameters. Carbon exists in a mixed sp²+sp³ hybridized state in the carbon–nickel films. The ratio of sp³/sp² carbon bonds increases when the nickel content is reduced (from 5.5 to 1.0 atomic %) and the deposition angle is increased. The structure closest to that of diamond was with a substrate bias voltage of –80 to –100 V and a deposition angle of 90°.     

高压Ni77P23非晶合金自由体积变化的同步辐射研究

利用同步辐射高能X光散射的方法，研究了室温下非晶合金Ni₇₇P₂₃的自由体积的变化所引起的压缩行为的变化规律，通过傅里叶变换得到不同压力下的径向分布函数，并由此获得了不同压力下，该非晶合金的配位数、近邻原子间距等原子构型的结构信息. 研究表明，至直30.5GPa压力，Ni₇₇P₂₃合金仍保持稳定的非晶结构,根据Bridgman方程通过拟合数据，得到状态方程为-ΔV/V₀=0.08606P-3.2×10^-4P²+5.7×10^-6P³. 关键词： 非晶合金 自由体积 同步辐射     

Raman spectroscopy of the BC3 phase obtained under high pressure and high temperature

A direct transformation of the g‐BC₃ phase to a new diamond‐like d‐BC₃ phase was observed in a diamond‐anvil cell (DAC) at high temperature, 2033 ± 241 K, and high pressure, 50 GPa. Analysis of the peak positions of the d‐BC₃, B₄C, α‐boron, and the boron‐doped diamond leads to the conclusion that the positions of the peaks of the d‐BC₃ are more similar to the peak pattern of the boron‐doped diamond rather than that of boron carbide, α‐boron. Copyright © 2007 John Wiley & Sons, Ltd.     

Mechanism and kinetics of the reversible transformation lda-hda of amorphous ice under pressure

An ultrasonic investigation of pressure-induced direct and reverse transformations lda-hda between the amorphous phases of low-and high-density ice is performed and the change in the shear modulus is determined. It is found that elastic softening of the amorphous network precedes both transformations, and the behavior of lda phase is similar to that of hexagonal 1h ice before amorphization. At the same time a number of features of the lda-hda transformation are due to the topological disordering of the amorphous phases, and the kinetics of the transformation is different from the standard behavior at a first-order phase transition and also from the compression kinetics of “ classic” glasses of the type a-SiO₂ and a-GeO2. It is shown that the difference in the behavior of the lda and hda amorphous phases under pressure can be attributed to characteristic features of their spectra of low-energy two-level states. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 653–658 (10 May 1999)     

Raman studies on nanocomposite silicon carbonitride thin film deposited by r.f. magnetron sputtering at different substrate temperatures

Raman studies of nanocomposite SiCN thin film by sputtering showed that with increase of substrate temperature from room temperature to 500 °C, a transition from mostly sp² graphitic phase to sp³ carbon took place, which was observed from the variation of I_D/I_G ratio and the peak shifts. This process resulted in the growth of C₃N₄ and Si₃N₄ crystallites in the amorphous matrix, which led to increase in hardness (H) and modulus (E) obtained through nanoindentation. However, at a higher temperature of 600 °C, again an increase of sp² C concentration in the film was observed but the H and E values showed a decrease due to increased growth of the graphitic carbon phase. The whole process got reflected in a modified four‐stage Ferrari–Robertson model of Raman spectroscopy. Copyright © 2010 John Wiley & Sons, Ltd.     

Ferromagnetism in amorphous MgO

Abstract

We report, for the first time, the atomic structure of amorphous MgO based on ab initio molecular dynamics simulations. We find that its main building blocks are four-fold and five-fold coordinated configurations, similar to those formed in the liquid state. Its average coordination is estimated to be about 4.36. The amorphous form having a perfect stoichiometry has a band gap energy of 2.4 eV. On the other hand, Mg vacancies induce an insulator to metal transition and ferromagnetism in amorphous MgO whilst O vacancies do not cause such a transition, implying that the magnetism in amorphous MgO is related to the non-stoichiometry and Mg vacancies. With the application of pressure, the stoichiometric and non-stoichiometric (Mg vacancies) models undergo a phase transformation into a rocksalt state, suggesting that the electronic structure of the initial configurations has no influence on the resulting high-pressure phase in amorphous MgO.     

Nanotube nanoscience: A molecular-dynamics study

Carbon nanotubes, fullerenes, and other nanostructured carbon materials are now the most important material phases in the field of nanoscience and nanotechnology. We study the structural stabilities and the interconversion of carbon nanotubes and various other carbon nanostructured phases at elevated temperatures as well as under high pressure using the molecular dynamics method combined with a newly parametrized transferable tight-binding model. The model can deal with not only sp² and sp³ covalent bonds but also the interaction between sp² layers, which plays an important role in the structural and electronic properties of carbon nanostructured materials. It is found that, during a thermal transformation process of carbon nanotubes with C₆₀ fullerenes trapped inside into double-walled carbon nanotubes, the outer carbon-nanotube wall is chemically active and forms covalent bonds with inner carbon atoms, and that most vacancies on the initially imperfect outer tube wall are eventually filled with atoms migrated from inner fullerenes. It is also found that external pressure of about 20 GPa induces a variety of structural transformations in carbon nanostructures. On the other hand, pressure of 30 GPa or higher usually results in sp³-rich amorphous carbon materials. Finally, the rotational interlayer friction force in double-walled carbon nanotubes is studied for the system of (4,4)@(9,9), and the torque of the friction force per unit area acting on each nanotube of the system is found to be as small as . This small value indicates the importance of carbon nanostuctured materials not only for nanoelectronics but also for nanometer-scale machines in the future.     

Pressure effect on the Raman and photoluminescence spectra of Eu3+-doped Na2Ti6O13 nanorods

Eu³⁺-doped Na₂Ti₆O₁₃ (Na₂Ti₆O₁₃:Eu) nanorods with diameters of 30 nm and lengths 400 nm were synthesized by hydrothermal and heat treatment methods. Raman spectra at ambient conditions indicated a pure monoclinic phase (space group C2/m) of the nanorods. The relations between structural and optical properties of Na₂Ti₆O₁₃:Eu nanorods under high pressures were obtained by photoluminescence and Raman spectra. Two structural transition points at 1.39 and 15.48 GPa were observed when the samples were pressurized. The first transition point was attributed to the crystalline structural distortion. The later transition point was the result of pressure-induced amorphization, and the high-density amorphous (HDA) phase formed after 15.48 GPa was structurally related to the monoclinic baddeleyite structured TiO₂ (P2₁/c). However, the site symmetry of the local environment around the Eu³⁺ ions in Na₂Ti₆O₁₃ increased with the rising pressure. These above results indicate the occurrence of short-range order for the local asymmetry around the Eu³⁺ ions and long-range disorder for the crystalline structure of Na₂Ti₆O₁₃:Eu nanorods by applying pressure. After releasing the pressure from 22.74 GPa, the HDA phase is transformed to low-density amorphous form, which is attributed to be structurally related to the α-PbO₂-type TiO₂.     

A structural comparison of supercooled water and intermediate density amorphous ices

New data are presented on neutron diffraction in ultrapure bulk supercooled heavy water measured down to 262?K. The data are analysed in terms of the trends observed in the first sharp diffraction peak (FSDP) parameters, the feature which dominates the measured neutron spectra. The neutron FSDP position, height and width are compared to literature data for supercooled water, water under pressure and to the same parameters obtained for recently discovered intermediate density amorphous ices. It is found that the FSDP parameters in supercooled water and the amorphous ices generally exhibit a similar behaviour, suggesting a new structural regime may occur in deeply supercooled water below Q ₀?～?1.83?Å^?1 (T?～?251?K) associated with increased intermediate range ordering. It is argued that this structural regime may be linked to a similar trend in the density which appears when the density is plotted as a function of FSDP position. A detailed comparison of the neutron and X-ray structure factors for supercooled water and intermediate density amorphous ices with the same FSDP positions is also made. The diffraction data show that although the overall general structures are qualitatively very similar, the amorphous ice correlations are considerably sharper and extend to much higher radial distances.     

Anomalies of the baric and temperature dependences of the elastic characteristics of ice during solid-phase amorphization and the phase transition in the amorphous state

The elastic characteristics of ice up to pressures of 1.7 GPa are determined for the first time at a temperature of 77 K, along with features of their variation associated with the phase transformation of hexagonal ice Ih into high-density amorphous ice hda. The elastic instability of the ice lattice before solid-phase amorphization is experimentally confirmed. Elastic instability during a transition from one amorphous state to another amorphous state was also observed for the first time; this took place when hda ice was warmed at p=0.05 GPa from T=77 K. Zh. éksp. Teor. Fiz. 112, 200–208 (July 1997)     

Structural transformations of the cumulene form of amorphous carbyne at high pressure

Structural transformations of the cumulene form of amorphous carbyne which are induced by heating at high pressure (7.7 GPa) are investigated. These can be described by the sequence amorphous phase — crystal — amorphous phase — disordered graphite. Raman scattering shows that predominately the chain structure of carbyne remains at the first three stages. It was found that the intermediate crystalline phase is an unknown modification of carbon whose structure is identified as cubic (a=3.145 Å). A mechanism of structural transformations in carbyne that involves the formation of new covalent bonds between chains is discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 237–242 (25 August 1997)     

Effect of ECR-assisted microwave plasma nitriding treatment on the microstructure characteristics of FCVA deposited ultra-thin ta-C films for high-density magnetic storage applications

There are higher technical requirements for protecting layer of magnetic heads and disks used in future high-density storage fields. In this paper, ultra-thin (2 nm thickness) tetrahedral amorphous carbon (ta-C) films were firstly prepared by filtered cathodic vacuum arc (FCVA) method, then a series of nitriding treatments were performed with nitrogen plasma generated using electron cyclotron resonance (ECR) microwave source. Here it highlighted the influence of nitrogen flow and applied substrate bias voltage on the structural characteristics of ta-C films during the plasma nitriding process. The chemical compositions, element depth distribution profiles, physical structures and bonding configurations of plasma-nitrided ta-C films were investigated by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and UV-vis Raman spectroscopy. The experimental results show that the carbon nitride compounds (CN_x) are formed in nitrogenated ta-C films in which the N content and its depth distribution depends on bias voltage to large extent rather than N₂ flow. The N content of nitrogenated ta-C films can reach 16 at.% for a substrate bias of −300 V and a N₂ flow of 90 sccm. With increasing nitrogen content, there is less G peak dispersion and more ordering of structure. Furthermore, appropriate nitriding treatment (substrate bias: −100 V, N₂ flow: 150 sccm) can greatly increase the fraction of sp³ and sp³C-N bonds, but the values begin to fall when the N content is above 9.8 at.%. All these indicate that suitable ECR-assisted microwave plasma nitriding is a potential modification method to obtain ultra-thin ta-C films with higher sp³ and sp³C-N fractions for high-density magnetic storage applications.     

The structure of thin carbon films deposited at 13.5 nm EUV irradiation

In this work, the structure and chemistry of thin nm-thick carbon films deposited on a substrate using strong 13.5 nm EUV irradiation under a strong vacuum were studied. The film structure was studied by Raman spectroscopy in comparison with the Raman spectra of well-known carbon phases: diamond, single-wall nanotubes, nano- and micro-crystalline graphite and amorphous carbon. As well, FTIR spectroscopy was used to study possible IR-active chemical bonds, primarily, hydrogen bonds. It was shown that films deposited on a surface under EUV irradiation consists of amorphous sp ²-carbon. The mechanisms of deposition are discussed briefly. Knowledge about the structure and chemistry of such carbon films is very important for EUV lithography.     

NMR study of rapidly quenched crystalline and amorphous Fe-B alloys

Amorphous and microcrystalline Fe-B alloys (4–25 at % B) obtained by rapid quenching of the melt were studied using the pulsed nuclear magnetic resonance (NMR) of ¹¹B nuclei at 4.2 K. Alloy samples were prepared from both a natural isotope mixture and a mixture of the ⁵⁶Fe and ¹¹B isotopes. The NMR spectra were measured as a function of the boron content. The maximum hyperfine fields at the ¹¹B nuclei sites are 25–29 kOe and overlap the values of the hyperfine fields at the ¹¹B nuclei sites in the tetragonal and orthorhombic Fe₃B phases and also in the α-Fe phase containing boron as a substitutional impurity. The short-range order and local atomic structure of the amorphous Fe-B alloys were determined. The amorphous alloys are found to consist of microregions (clusters) with a short-range order similar to that in the tetragonal or orthorhombic Fe₃B phase or the α-Fe phase.     

Phase transitions in the CN<Subscript>x</Subscript>-Co system induced by changes in the film growth temperature

The structures of amorphous CN_x-Co films grown at different temperatures (T _s = 200–365°C) are studied by X-ray diffraction. As the growth temperature increases above T _s = 200°C, a concentration phase transition is found to occur in the amorphous state; this transition is related to a change in the major portion of carbon or cobalt in the structure of the cluster films. At T _s = 365°C, a disorder-order phase transition, which is accompanied by the transition from the amorphous to crystalline state, occurs in the films.