Structural stability and theoretical strength of the single crystal Ag under uniaxial loading

The structural stability and theoretical strength of FCC crystal Ag under uniaxial loading have been investigated by combining MAEAM with Wang modified Born stability criteria. The results reveal that, under sufficient compression, there exists a stress-free BCC phase, which is unstable and slips spontaneously to a stress-free mBCT phase by consuming internal energy. The stable region ranges from −2.076 to 4.390 GPa in the theoretical strength or from −7.972% to 8.721% in the strain correspondingly. The calculated structural energy difference between the BCC and FCC phase is in good agreement with the experimental value.     

The stability of FCC crystal Ni under uniaxial loading

Taking Ni as an example, the structural stability and theoretical strength of FCC crystals under uniaxial loading were investigated by combining the modified analytical embedded atom method (MAEAM) with modified Born stability criteria. The results revealed that, under sufficient compression, there existed a stress-free instable BCC phase and then a stress-free metastable BCT phase corresponding to local maximum and minimum internal energy, respectively. The stable region ranged for the theoretical strength from −5.143 eV/nm³ with a corresponding strain of −9.94% under compression to 10.396 eV/nm³ with a corresponding strain of 9.66% under tension.     

THEORETICAL STRENGTH AND PHASE STABILITY OF Cu AND Ni UNDER [100] UNIAXIAL LOADING

Based on Born's criteria we studied phase stability and theoretical strength of fcc crystals of copper and nickel under [100] uniaxial loading. The calculation was carried out using a simple and completely analytical embedded atom method(EAM) potential proposed by the present authors. For Cu, the calculated value of its theoretical strength (0.33×10¹¹ dyn·cm^-2) agrees well with the experimental value (0.30×10¹¹ dyn·cm^-2), while the calculated strain (9.76%) is somewhat larger than the experimental one (2.8%). For Ni, its theoretical strength and strain predicted using the EAM potential are found smaller than those predicted using a pair potential. It is worthy to note that unlike previous calculations, in which pair potentials were used and three unstressed fcc, bcc, and fct structures included (for Ni only fcc state is found stable, while for Cu both fcc and bcc states are predicted stable), in present calculations using EAM potential the [100] primary loading path passes through only two zeroes (a stable unstressed fcc structure and an unstable stress-free bcc structure) either for Cu or for Ni.     

Effect of pressure and mechanical stress on the electronic properties of AlN and GaN

The fundamental properties of the AlN and GaN compounds with a wurtzite structure under external hydrostatic pressure, uniaxial mechanical stress σ_⊥ along the hexagonal axis, and biaxial mechanical stress σ_⊥ in the basal plane of the unit cell have been considered in terms of first-principles calculations in the frame-work of the density functional theory. The pressures of the phase transitions from the structures of wurtzite and zinc blende to the structure of rock salt have been obtained. The behavior of the structural parameters, interband transitions, and positions of the charge neutrality level has been investigated. The calculated pressure coefficients of the band gap are as follows: ∂E _g /∂p = 40.9 meV/GPa, −∂E _g /∂σ _| = −4.2 meV/GPa, and −∂E _g /∂σ _⊥ = 45.2 meV/GPa for AlN and ∂E _g /∂p = 33.0 meV/GPa, −∂E _g /∂σ _| = 23.6 meV/GPa, and −∂E _g /∂σ _⊥ = 9.6 meV/GPa for GaN. The pressure coefficients of the charge neutrality level in almost all cases are substantially smaller than the corresponding values obtained for the band gap E _g .     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Spectroscopic study ofβ-Ni(OH)2 under pressure

Infrared absorption and Raman study ofβ-Ni(OH)₂ has been carried out up to 25 GPa and 33 GPa, respectively. The frequency ofA _2u internal antisymmetric stretching O-H mode decreases linearly with pressure at a rate of −0.7 cm⁻1/GPa. The FWHM of this mode increases continuously with pressure and reaches a value of ∼ 120 cm⁻¹ around 25 GPa. There was no discernible change observed in the frequency and width of the symmetric stretchingA _1g O-H Raman mode up to 33 GPa. The constancy of the Raman mode is taken as a signature of the repulsion produced by H-H contacts in this material under pressure. Lack of any discontinuity in these modes suggests that there is no phase transition in this material in the measured pressure range.     

Kinetics of the initial stage in a first-order phase transformation in thin films

The nucleation and growth of islands of a new phase on the surface of solids has been studied both experimentally and theoretically for the particular case of the transition from the pyrochlore to perovskite phase in a thin film of a lead zirconate-titanate ferroelectric. This transformation was chosen because the new-phase islands have a stable circular shape in this case, a relatively large size (10⁻⁵–10⁻⁴ m) permitting their observation with an optical microscope, and a low growth rate (10⁻⁸–10⁻⁹ m/s). A theoretical analysis of the process, based on the kinetic theory of first-order phase transitions proposed earlier, has been carried out and the behavior in time of all main characteristics of a phase transformation, namely, nucleation rate, concentration of the new-phase islands, their size distribution, and relative overheating, has been calculated. The same characteristics have been measured experimentally, thus permitting one for the first time to make a thorough comparison of the theoretical with experimental data on the kinetics of first-order phase transitions. They have been found to be in a good agreement. Fiz. Tverd. Tela (St. Petersburg) 39, 121–126 (January 1997)     

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C₇₀ at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Grüneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the d ω/dP value for certain phonon modes abruptly changes at pressures of P ₁≈2 GPa and P ₂≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C₇₀ crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C₇₀, which is about 2.1±0.1 eV. Zh. éksp. Teor. Fiz. 111, 262–273 (January 1997)     

Stability of polymer films as a 2D system

According to classical theory of phase transition, fluctuations in systems with low dimensions are so violent that the phase boundary between unstable and metastable states would be smeared. In this experiment, we measure the growth of surface fluctuations on an unstable polymer film with a thickness, h₀ = 5.1 nm, which is much less than the spinodal thickness, h _sp (= 243 nm) thereby the film is in the very deep unstable region. We find the film to show rupturing behavior markedly different from that of an unstable film. Specifically, nucleation of holes – a characteristic rupturing feature of metastable films – is prominent, which is surprising for a film in the very deep unstable region even provision is given to thin films being two-dimensional and hence are susceptible to broadening of the phase boundary by fluctuations. Monte Carlo simulation shows that the nucleated holes can be caused by stochastic thermal fluctuations. Our result thus confirms the broadening of the phase boundary in thin films by fluctuations to be extremely large. As a consequence, the phase behavior of thin films cannot be predicted by the mean-field calculated phase boundary, which however has been the general practice so far.     

Behavior of aluminum near an ultimate theoretical strength in experiments with femtosecond laser pulses

The dynamics of the motion of the free surface of micron and submicron films under the action of a compression pulse excited in the process of femtosecond laser heating of the surface layer of a target has been investigated by femtosecond interferometric microscopy. The relation between the velocity of the shock wave and the particle velocity behind its front indicates the shock compression to 9–13 GPa is elastic in this duration range. This is also confirmed by the small (≤1 ps) time of an increase in the parameters in the shock wave. Shear stresses reached in this process are close to their estimated ultimate values for aluminum. The spall strength determined at a strain rate of 10⁹ s⁻¹ and a spall thickness of 250–300 nm is larger than half the ultimate strength of aluminum.     

Ab initio calculations of yttrium nitride: structural and electronic properties

Using first principles total energy calculations within the full-potential linearized augmented plane wave method, we have studied the structural and electronic properties of yttrium nitride (YN) in the three phases, namely wurtzite, caesium chloride and rocksalt structures. The calculations are performed at zero and under hydrostatic pressure. In agreement with previous findings, it is found that the favored phase for YN is the rocksalt-like structure. We predict that at zero pressure YN in the rocksalt structure is a semiconductor with an indirect bandgap of 0.8 eV. A phase transition from a rocksalt to a caesium chloride structure is found to occur at ∼134 GPa. Besides, a transition from an indirect (Γ−X) bandgap semiconductor to a direct (X−X) one is predicted at pressure of ∼84 GPa. For the electron effective mass of rocksalt YN, these are the first results, to our knowledge. The information derived from the present study may be useful for the use of YN as an active layer in electronic devices such as diodes and transistors.     

Effect of biaxial strain on the band gap of wurtzite AlxGa1?xN

First-principles calculations are applied to investigate the effect of biaxial strain on the band gap of wurtzite Al _x Ga_1−x N. The band gap and band gap bowing parameter increase with compressive strain and decrease with tensile strain. The strain-induced changes in the band gap of Al _x Ga_1−x N are linear in the strain range of about −1% to 1% while the linearity is invalid out of the range. The linear coefficient B(x), characterizing the relationship between the band gap and the biaxial stress, with a quadratic form is obtained. The value of the band gap bowing parameter decreases from 1.0 eV for −2% strain to 0.91 eV for unstrained and to 0.67 eV for 2% strain.     

Shift and broadening of surface polaritons of sapphire upon deposition of a quasicrystalline film

The influence of a quasicrystalline Al-Pd-Re film on the shift and broadening of surface polaritons of a substrate (sapphire) has been studied. Measurements have been performed both on a sample containing only the quasicrystalline phase and on a sample which, in addition to the quasicrystalline phase, contains the crystalline (metallic) phase. The complex dielectric function of the films in the mid-IR region (650–800 cm⁻¹) has been estimated.     

Effect of pressure on Raman spectra of SnS2 single crystals

The 2H polytype of a SnS₂ layered crystal has been studied using Raman spectroscopy at pressures of up to 5 GPa in a diamond anvil cell. The Raman frequency of the intralayer mode increases linearly with increasing pressure at baric coefficients of 5.2 cm⁻¹/GPa for P<3 GPa and 3.4 cm⁻¹/GPa for P>3 GPa. This change in the baric coefficient for Raman scattering and the available data on X-ray measurements of the compressibility of 2H-SnS₂up to 10 GPa suggest that the crystal structure undergoes a transformation at about 3 GPa.     

Electrical properties of thermal donors formed in silicon under elastic tensile stress

The electrical properties of thermal donors formed in the bulk and near-surface regions in silicon samples with (3–9) × 10¹⁷ cm⁻³ oxygen concentrations under elastic tensile stress σ of about 1 GPa have been studied. The original method allowing us to control an introduced elastic tensile stress during the thermal donor’s formation at T = 450°C by a double-crystal X-ray diffractometer has been used. The formation of thermal donors in silicon with a high oxygen concentration of 9.3 × 10¹⁷ cm⁻³ under tensile stress has been found to be less effective than in silicon with a low oxygen concentration of (3–5) × 10¹⁷ cm⁻³. Single-charged donors are formed in silicon with a low oxygen concentration under tensile stress while double-charged donors are formed in silicon with a high oxygen concentration.     

Superconductivity and possible overdoping in Lu1−x Ca x Ba2Cu3O7−δ

Structural and superconducting properties of the system Lu_1−x Ca _x Ba₂ Cu₃O_7−δ, both in bulk as well as thin film form, have been investigated. Presence of large Ca-ions at the Lu-sites is responsible for phase stability (in 1–2–3 phase) in bulk. In argon annealed tetragonal samples (δ:1) superconductivity is reinstated as in these samples, presence of Ca-ions lead to the generation of holes in the Cu−O planes. In oxygen annealed bulk samples and thin films, there is a large depression ofT _c due to divalent Ca-ions present in concentrationx<0.2. Our data indicate that this depression ofT _c is likely due to overdoping effect.     

Magnetostriction constants of epitaxial La,Ga: YIG films measured by microwave resonance

The magnetostriction constants of Y_2.85La_0.15Fe_3.75Ga_1.25O₁₂ epilayers have been measured by observing the shift of the resonance line of a thin film which is stressed by three-point bending. The result is λ₁₁₁=−(0.75±0.15) × 10⁻⁶ and λ₁₀₀=−(0.4±0.1) × 10⁻⁶ which is in agreement with the measurements on bulk single crystals. This result indicates that there is no growth induced contribution to the magnetostriction in thin films of this garnet grown by liquid phase epitaxy.     

铀的结构相变及力学性能的第一性原理计算

基于密度泛函理论, 分别计算了α, γ铀的晶格常数、平衡态体积、体弹模量及其导数等, 与实验和其他第一性原理计算结果符合较好; 并根据焓-压强曲线得到了两相的相变压强～111GPa. 通过体心立方结构理想拉伸强度的计算, 分析其在极端加载条件下的结构行为. 另外, 计算了小应变情况下U-Nb (6.25at.%) 的能量-应变关系, 发现对应于剪切模量c’的应变会使得该结构的能量降低, 揭示了该结构的力学不稳定性. 关键词： 铀 相变 理想强度 结构稳定性     

Properties of shock-compressed liquid krypton at pressures of up to 90 GPa

The following quantities of shock-compressed liquid krypton are measured behind a plane shock front at pressures up to 90 GPa: compressibility up to densities of 7 g/cm³, brightness (color) temperatures of 6000–24000 K, and electrical conductivities of 40–60000 (Ω·m)⁻¹. X-t diagram methods are used to estimate sound speeds of up to 5.5 km/s at pressures of 30–75 GPa. The optical absorption coefficients in the violet and red (30–300 cm⁻¹) are measured at pressures of 20–90 GPa from the rise in brightness of the shock front luminosity. The optical reflection coefficient of the shock front (∼13%) at a pressure of 76.1 GPa is measured for the first time. Zh. éksp. Teor. Fiz. 116, 551–562 (August 1999)     

Nanocomposite and nanostructured superhard Ti-Si-B-N coatings

Electron microscopy, x-ray diffraction analysis, and micro-and nanohardness measurements were used to investigate the interrelations between the fine structure and the variations in strength properties of nanostructured and nanocomposite Ti-Si-B-N coatings with high oxygen and carbon contents. It has been shown that under the conditions of low-temperature (T = 200°C) coating deposition, a two-level grain structure forms with {200} texture and grains 0.1–0.3 μm in size fragmented into subgrains 15–20 nm in size. As the silicon content is increased, textureless coatings with the crystal phase grain size less than 15 nm and high amorphous component or coatings of amorphous-crystalline structure are produced. At coating deposition temperatures of 400–450°C, a nanocomposite structure with a grain size d = 10–15 nm and no texture is observed. For all test compositions and conditions of coating production, a Ti _1−x Si _x N crystal phase with the lattice parameter a = (0.416–0.420) ± 0.001 nm has been detected. For optimum coating compositions and synthesis conditions, the hardness is over 40–50 GPa. It has been supposed that superhardness can be attained with multiphase grain-boundary interlayers of thickness more than 1 nm. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 13–23, October, 2007.