Modeling of Phase Transitions of Graphites to Diamond-Like Phases

The method of the density functional theory is used to study structural transformations between graphites and diamond-like phases. The calculations have been carried out in two approximations: a local density approximation and a generalized gradient approximation. It is found that the phase transitions of hexagonal graphene layers to a cubic diamond and diamond-like phases must occur at uniaxial compressions of ~57–71 GPa, whereas some diamond-like phases can be obtained from tetragonal graphene layers at significantly lower pressures of 32–52 GPa. The X-ray diffraction patterns have been calculated for the phase transition of graphite I4₁/amd to tetragonal LA10 phase that takes place at the minimum pressure that can be used for experimental identification of these compounds.     

Structure,properties, and possible mechanisms of formation of diamond-like phases

An analysis was performed for relations between the structural parameters and the properties of 36 carbon diamond-like phases consisting of atoms occupying crystallographically equivalent positions. It was found that the crystal lattices of these phases were in stressed states with respect to the cubic diamond lattice. The density of diamond-like phases, their sublimation energies, bulk moduli, hardnesses, and band gaps depend on the deformation parameters Def and Str. The most stable phases must be phases with minimal parameters Def and Str and also with ring parameter Rng that is most close to the corresponding parameter of cubic diamond. The structures and energy characteristics of fullerites, nanotube bundles, and graphene layers of which diamond-like phases can be obtained as a result of polymerization at high pressures have been calculated.     

Deposition of fullerene films from the ablation plasma generated by high-power ion beams on graphite targets

A possibility of deposing carbon films with a high content of C₆₀ and C₇₀ fullerenes from an ablation plasma generated as a result of irradiation of graphite targets by pulsed high-power ion beams is shown. The relative contents of the crystalline diamond-like carbon phase, crystalline fullerene phase, and amorphous carbon phase have been determined by X-ray diffraction analysis for different deposition conditions. The nanohardness and Young’s modulus of the deposited films and their adhesion to the single-crystal silicon substrate have been measured.     

Dimensionally,morphologically, and thermally induced phase transformations in boron-nitrogen nanowires

Structural, thermal, electronic, and energetic properties of cubic boron nitride (BN) nanowires are studied using the density-functional tight-binding method. The effect of the total or partial rearrangement of the cubic structure of nanowires into the hexagonal one depending on the size, morphology, and thermal treatment of the starting wire has been revealed. As distinct from the known homogeneous carbon diamond-like nanowires, stable BN nanowires are two-phase systems whose “shell” has a hexagonal structure and “core” has a cubic structure. The changes in the electronic properties of BN nanowires induced by their structural transformations are discussed. It is shown that boron-nitrogen nanowires can exhibit both semiconducting and metallic properties.     

Phase transformations of carbon under extreme energy action

Experiments are performed on a high-current MIG generator (current amplitude 2.5 MA, current rise time 100 ns) to synthesize diamond-like carbon forms. Magnetohydrodynamic calculations carried out before the experiments show that the conditions required for synthesizing diamond-like structures appear during the compression of copper tubes 2–4 mm in diameter filled with graphite. The explosion products are analyzed on an EM-125 diffraction transmission electron microscope. During an explosion, the entire graphite is found to transform into cubic carbon: crystals with the simple cubic lattice and a lattice parameter a = 0.5545 nm and with the face-centered cubic lattice and a = 0.3694 nm are detected. The crystallite size is 5–25 nm.     

Nonmetallic crystals with high thermal conductivity

Nonmetallic crystals transport heat primarily by phonons at room temperature and below. There are only a few nonmetallic crystals which can be classed as high thermal conductivity solids, in the sense of having a thermal conductivity of > 1 W/cmK at 300K. Thermal conductivity measurements on natural and synthetic diamond, cubic BN, BP and AIN confirm that all of them are high thermal conductivity solids. Studies have been made of the effect on the thermal conductivity of nitrogen impurities in diamond, and oxygen impurities in AIN. The nitrogen impurities scatter phonons mostly from the strain field, the oxygen impurities scatter phonons mostly from the mass defects caused by aluminum vacancies. Pure A1N as well as pure SiC, BeO, BP and BeS conduct heat almost as well as does copper at room temperature, while pure natural and synthetic diamonds conduct heat five times better than copper.All of the nonmetallic solids that are known to possess high thermal conductivity have either the diamond-like, boron carbide, or graphite crystal structure. There are twelve different diamond-like crystals, a few boron carbide-type crystals, and two graphite structure crystals that have high thermal conductivity. Analyses of the rock-salt, fluorite, quartz, corundum and other structures show no candidates for this class. The four rules for finding crystals with high thermal conductivity are that the crystal should have (1) low atomic mass, (2) strong bonding, (3) simple crystal structure, and (4) low anharmonicity. The prime example of such a solid is diamond, which has the highest known thermal conductivity at 300K.     

Structural phase transition and elastic properties of Curium and Uranium monobismuthides under pressure effect

A density functional (DFT) calculations of the structural, elastic and high pressure properties of the cubic XBi (X=U,Cm) compounds, has been reported using the full potential linear muffin-tin orbital (FP-LMTO) method. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus and its pressure derivatives. The pressure transitions at which these compounds undergo structural phase transition from NaCl-type (B1) to CsCl-type (B2) phase were found to be in good agreement with the available theoretical results. We have determined the elastic constants C₁₁, C₁₂, C₄₄ and their pressure dependence which have not been established experimentally or theoretically.     

宽带隙薄膜材料场电子发射研究的背景、现状和问题

介绍了以金刚石为代表的宽带隙薄膜材料场电子发射研究背景和现状,对金刚石、类金刚石（ＤＬＣ）、立方氮化硼（ｃ－ＢＮ）、氮化铝（Ａ１Ｎ）和碳化硅（ＳｉＣ）薄膜场电子发射研究的进展进行了评述,着重介绍了发射性能与薄膜的结构特征、杂质含量和处理方法间的关系,并讨论了研究中存在的问题。     

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.     

Structural deformations in metastable cubic compounds Ni(1 − x)Zn x O (0.60 ≤ x ≤ 0.99)

X-ray diffraction has been used for studying the fine crystal structure of metastable cubic oxide compounds Ni_{(1 ? x)}Zn _x O (0.60 ≤ x ≤ 0.99) obtained from the initial hexagonal phase by quenching of the samples at a high temperature and under external hydrostatic pressure. It has been found that the diffraction patterns of these compounds include a system of diffuse superstructure maxima, whose number and intensity essentially depend on the composition. The origin of this superstructure has been discussed.     

Electronic structure and structural phase stability of CuAlX2 (X=S, Se, Te) under pressure

The electronic and structural properties of chalcopyrite compounds CuAlX₂ (X=S, Se, Te) have been studied using the first principle self-consistent Tight Binding Linear Muffin-Tin Orbital (TBLMTO) method within the local density approximation. The present study deals with the ground state properties, structural phase transition, equations of state and pressure dependence of band gap of CuAlX₂ (S, Se, Te) compounds.Electronic structure and hence total energies of these compounds have been computed as a function of reduced volume. The calculated lattice parameters are in good agreement with the available experimental results. At high pressures, structural phase transition from bct structure (chalcopyrite) to cubic structure (rock salt) is observed. The pressure induced structural phase transitions for CuAlS₂, CuAlSe₂, and CuAlTe₂ are observed at 18.01, 14.4 and 8.29 GPa, respectively. Band structures at normal as well as for high-pressure phases have been calculated. The energy band gaps for the above compounds have been calculated as a function of pressure, which indicates the metallic character of these compounds at high-pressure fcc phase. There is a large downshift in band gaps due to hybridatization of the noble-metal d levels with p levels of the other atoms.     

Phase transformation study of Cu2S

The phase transformations of crystalline Cu₂S in the temperature range 50?C to 600?C have been studied through electric quadrupole interactions of¹¹¹Cd impurities observed by time-differential perturbed angular correlations. The Cd is subject to no discernable quadrupolar interaction in the cubic phase above 430?C. In the hexagonal phase between approximately 103?C and 430?C, a single-site, weakly temperature dependent, axially symmetric quadrupolar interaction is found. At lower temperature, the structure is complex with a number of different sites. These data were taken with a microcomputer-controlled TDPAC spectrometer with logical steering that allows a standard 4-detector spectrometer to collect simultaneously 12 channels of data.     

Fine structure and magnetism of the cubic oxide compound Ni<Subscript>0.3</Subscript>Zn<Subscript>0.7</Subscript>O

The fine structure and spin system of the cubic oxide Ni_0.3Zn_0.7O compound prepared from the initial hexagonal phase by quenching a sample with a high temperature and applying an external hydrostatic pressure to it have been studied using magnetic measurements, synchrotron and X-ray diffraction. It has been revealed that the diffraction patterns of this compound contain a system of weak diffuse maxima with the wave vectors q = (1/6 1/6 1/6)2π/a and (1/3 1/3 1/3)2π/a, along with strong Bragg peaks of the cubic phase. It has been shown that the origin of the diffuse peaks is due to longitudinal and transverse displacements of ions with respect to symmetric crystallographic directions of the {111} type. The reasons for the ion displacement and specific features of the structure of the spin system of the strongly correlated oxide Ni_0.3Zn_0.7O compound have been briefly discussed.     

Cubic ZnO films obtained at low pressure by molecular beam epitaxy

A zinc oxide thin film in cubic crystalline phase, which is usually prepared under high pressure, has been grown on the Mg O(001) substrate by a three-step growth using plasma-assisted molecular beam epitaxy. The cubic structure is confirmed by in-situ reflection high energy electron diffraction measurements and simulations. The x-ray photoelectron spectroscopy reveals that the outer-layer surface of the film(less than 5 nm thick) is of ZnO phase while the buffer layer above the substrate is of ZnMgO phase, which is further confirmed by the band edge transmissions at the wavelengths of about 390 nm and 280 nm, respectively. The x-ray diffraction exhibits no peaks related to wurtzite ZnO phase in the film. The cubic ZnO film is presumably considered to be of the rock-salt phase. This work suggests that the metastable cubic ZnO films, which are of applicational interest for p-type doping, can be epitaxially grown on the rock-salt substrates without the usually needed high pressure conditions.     

Investigations of physical aspects of spinel ABi2O4 (A=Zn,Cd) oxides via ab-initio calculations

This present study aims to unravel physical properties of spinel oxides in cubic phase represented by general empirical formula of ABi₂O₄ (A=Zn, Cd) using full potential lineralized augmented plane plus local orbital (LAPW+lo) method. The structural aspect of spinel oxides in cubic phase has been investigated by Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol-GGA). The thermodynamic stability of ABi₂O₄ is established by computing the cohesive energies. On the other hand, born stability standard which is based on Chapin's tensor analysis method revealed that the mechanical stability of the oxide compounds under considerations gives a stable cubic phase. For a better prediction of electronic and thermoelectric properties of these compounds, the modified Tran-Bhala Becke and Johnson (TB-mBJ) potential is used which revels very precise band gap. Other than these investigations, thermoelectric characteristics i.e., Wiedman-Tranz constant, Seebeck coefficient and power factor are calculated and it provides essential data for fabrication of thermoelectric devices.     

Bonding in the superionic phase of water

The predicted superionic phase of water is investigated via ab initio molecular dynamics at densities of 2.0--3.0 g/cc (34-115 GPa) along the 2000 K isotherm. We find that extremely rapid (superionic) diffusion of protons occurs in a fluid phase at pressures between 34 and 58 GPa. A transition to a stable body-centered cubic O lattice with superionic proton conductivity is observed between 70 and 75 GPa, a much higher pressure than suggested in prior work. We find that all molecular species at pressures greater than 75 GPa are too short lived to be classified as bound states. Up to 95 GPa, we find a solid superionic phase characterized by covalent O-H bonding. Above 95 GPa, a transient network phase is found characterized by symmetric O-H hydrogen bonding with nearly 50% covalent character. In addition, we describe a metastable superionic phase with quenched O disorder.     

脉冲激光沉积无氢钨掺杂类金刚石膜的摩擦与机械性能

采用脉冲激光沉积技术制备出无氢钨掺杂非晶态类金刚石膜.膜中的钨含量与靶材中的钨含量保持稳定的线性关系,显示了脉冲激光沉积在难熔金属掺杂技术方面的亮点.由于碳-钨结构的形成和表面粗糙度影响,膜层的干摩擦系数随着钨含量的增加显现出先减后增的趋势,钨含量为9.67 at.%时达到最低值0.091.钨含量的增大降低了类金刚石膜纳米硬度和杨氏模量,但最佳的膜层耐磨性参数并非表现在硬度最大(52.2 GPa)的纯类金刚石膜中,而是出现在低掺杂含量(6.28 at.%)的类金刚石膜中.研究为脉冲激光沉积技术制备低摩擦、高硬度无氢钨掺杂类金刚石膜的应用提供了技术实践.     

相位连续可调诱导下双艾里光束形成的可控方形光瓶(英文)

提出一种利用相位连续可调诱导产生的双艾里光束形成方形光瓶的方法。该方法首先利用二值化后的相位调制出双艾里光束,为了能够实现光瓶能量分布,一个可调控的线性因子被引入到相位调制函数中形成新改进的相位,该相位能够灵活地调节光瓶的大小。数值模拟结果表明高斯光束通过该改进相位调制,能够形成光瓶能量分布的光束。该方形光瓶光束可应用于光镊、原子捕获与操纵。     

Structural disorder in the paraelectric phase of the Fe<Subscript>3</Subscript>B<Subscript>7</Subscript>O<Subscript>13</Subscript>Br boracite

A structural model of the cubic paraelectric phase of a Fe₃B₇O₁₃Br crystal belonging to the boracite family has been developed using the data obtained by single-crystal X-ray diffraction with due regard for the results of extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the best agreement between the data obtained by these two methods is achieved within a model assuming a disorder in the arrangement of both the Fe and Br atoms and a high degree of correlation of their displacements. It has been found that, during the phase transition from the rhombohedral ferroelectric phase to the cubic paraelectric phase, no significant transformation of the structure is observed on a local level. In this case, a change in the macroscopic symmetry occurs predominantly as a result of the variation in the set of possible spatial orientations of stable structural fragments, which is characteristic of order-disorder phase transitions.     

Single laser exposure fabrication of diamond-like 3-dimensional photonic crystal microstructures using circularly polarized light

Phase tunable multi-level diffractive optical elements define an attractive approach for single laser exposure fabrication of 3-dimensional photonic crystal microstructures. The significant advantage of these multi-level diffractive optical elements over two-level diffractive optical elements is the flexibility of fabricating a wide range of 3-dimensional periodic structures by manipulating the relative phase of different diffracted beams. Here, phase tuning was applied to demonstrate fabrication of a hybrid 3-dimensional structure intermediate between previously reported diamond-like Woodpile-type structure of tetragonal symmetry and structure having body-centered-tetragonal lattice symmetry. Circularly polarized light was applied for the first time to balance the diffraction order efficiencies and improve the structural uniformity. Design guidelines are presented for generating diamond-like photonic crystal template that possesses complete photonic bandgap when inverted with high refractive index materials.