Molecular dynamics study of anisotropic growth of silicon

Based on the Tersoff potential, molecular dynamics simulations have been performed to investigate the kinetic coefficients and growth velocities of Si(100),(110),(111), and(112) planes. The sequences of the kinetic coefficients and growth velocities are μ_((100)) μ_((110)) μ_((112)) μ_((111))and v_((100)) v_((110)) v_((112)) v_((111)), respectively, which are not consistent with the sequences of the interface energies, interplanar spacings, and melting points of the four planes. However,they agree well with the sequences of the distributions and diffusion coefficients of the melting atoms near the solid–liquid interfaces. It indicates that the atomic distributions and diffusion coefficients affected by the crystal orientations determine the anisotropic growth of silicon. The formation of stacking fault structure will further decrease the growth velocity of the Si(111) plane.     

Thermodynamic properties of the clathrate-type silver-oxide Ag6O8AgHF2

We report normal-state and superconducting properties of the clathrate-type silver-oxide Ag₆O₈AgHF₂. We present electrical resistivity, DC- and AC-susceptibility and specific-heat measurements of single crystalline Ag₆O₈AgHF₂. In the normal state, Ag₆O₈AgHF₂ exhibits metallic conductivity and a phase transition near 110 K, possibly a structural phase transition as observed in the related compound Ag₆O₈AgNO₃. The onset of superconductivity of our samples is observed around 1.2–1.5 K, and the H–T phase diagram is determined for the first time. The upper critical field H_c2(0) is estimated to be about 2000–2200 Oe and the coherence length ξ_GL(0) to be 40 nm.     

Magnetic properties of Lu2Co17−xSix single crystals

The Co-sublattice anisotropy in Lu₂Co₁₇ consists of four competitive contributions from Co atoms at crystallographically different sites in the Th₂Ni₁₇-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu₂Co_17−xSi_x (x=0−3.4) grown by the Czochralski method. The SRT in Lu₂Co₁₇ was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T_SR1≈680 K and “easy-cone”–“easy-axis” at T_SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu₂Co₁₆Si (T_SR1≈75 K and T_SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu₂Co_17−xSi_x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.     

Frequency and temperature dependence of a.c. conductivity of vitreous silver phosphate electrolytes

The a.c. conductivity behaviour in the 20–300 K temperature range has been investigated for (Ag₂S)_x(AgPO₃)_1−x and (Ag₂SO₄)_x(AgPO₃)_1−x glasses at various salt contents (x). The temperature dependence at selected frequencies in the radioand micro-wave region displays several relaxational contributions which are indistinct in the frequency domain. The low temperature experimental data are discussed and a proposed ‘new universality’ has been examined.     

单晶LaB_6阴极材料典型晶面的电子结构和发射性能研究

单晶LaB_6是一种理想的热发射和场发射阴极材料,其不同晶面表现出不同的发射性能.采用基于密度泛函理论的第一性原理计算分析了LaB_6单晶的(100),(110),(111),(210),(211)和(310)典型晶面的差分电子密度、能带结构和态密度,并对光学区熔法制备的高质量单晶LaB_6的上述典型晶面的热发射性能进行了测试.理论计算结果表明LaB_6各晶面结构的不同和电子结构的差异导致LaB_6发射性能具有各向异性,晶面内La原子的密度越大、费米能级进入导带越深、费米能级附近态密度越大及其在导带区域的分布宽度越宽、导带在费米能级附近分布越多,晶面的逸出功越低,发射性能越好.热发射测试结果表明,当阴极测试温度为1773 K,测试电压为1 k V时,(100),(110),(111),(210),(211)和(310)晶面的最大发射电流密度分别为42.4,36.4,18.4,32.5,30.5和32.2 A/cm~2,其中(100)晶面具有最佳的发射性能.     

Ag(111)和Au(111)上铋的初始生长行为

半金属铋(Bi)的表面合金具有的Rashba效应,和其具体结构性质有重要关联.本文结合扫描隧道显微镜(STM)和密度泛函理论(DFT),系统地研究了Bi原子在Ag(111)和Au(111)上的不同初始生长行为.在室温Ag(111)上,连续的Ag2Bi合金薄膜会优先在Ag台阶边缘形成;在570 K Ag(111)上,随着...     

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO2 dissociation on Pt(111)

We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO₂ dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ_o) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO₂ on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO₂ is N-bonded, 6° off-normal beams are observed. When the substrate is precovered with θ_o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.     

A study on the nonstoichiometry of tin oxides by coulometric titration

In this investigation, nonstoichiometries and defect structures of tin oxides were studied between 694 and 990 K by coulometric titration using solid state electrolyte (YSZ) cells. The relationship between nonstoichiometry of the oxide (x) and equilibrium oxygen partial pressure (P_o2) was expressed by the proportionality: xP_O2^−1/6. An intermediate oxide phase, Sn₃O₄ between Sn and SnO₂ was observed in the temperature range of 696–732 K. The standard Gibbs energy of formation of Sn₃O₄ via the reaction; was found to be ΔG^o_Sn3O4 = −1163960+417.36 T (J/mol). The standard Gibbs energy change for the defect formation reaction in SnO_2−x was calculated to be ΔG^o_SnO2−x = 3.05×10⁵−38.97 T (J/mol)).     

Aliovalent ionic substitution as a probe of conduction mechanism

Silver and sodium have qualitatively different diffraction-determined ionic distributions in the conduction planes of a β-alumina host. That this can imply different conduction mechanisms in the two cases is probed by partially exchanging Cd²⁺ ions into both compounds, and measuring the resulting mixed-ion distributions. The systems Ag_{1.22 − y}Cd_y/2Al₁₁O_17.11 (for y = 0, 0.10, 0.41 and 1.11) and Na_{1.22 − y}Cd_y/2Al₁₁O_17.11 for (y = 0, 0.22, 0.45 and 1.22) were investigated by single-crystal X-ray diffraction. The problematical use of diffraction techniques for highly disordered systems of this type is discussed; it is also shown how some of these difficulties can be reduced through access to additional physical and chemical information. Qualitatively different mechanisms are deduced in the two cases, suggesting that aliovalent substitution can indeed be a useful general tool in studying ionic conduction mechanisms in crystalline systems.     

W doping-dependent structural and ferroelectric properties of SrBi2Nb2O9 ferroelectric ceramics

The structural and ferroelectric characteristics of SrBi₂(Nb_1−xW_x)₂O₉ (x=0–0.12) ferroelectric ceramics were investigated. SrBi₂(Nb_1−xW_x)₂O₉ ceramics consisted of a single-phase layered perovskite structure when x was less than 0.06. Uniform microstructure and grain size reduction were observed after the introduction of W. The maximum remanent polarization of 16 μC/cm² appeared at x=0.03, and the coercive field decreased with increasing concentration of W. The ferroelectric behavior of SrBi₂(Nb_1−xW_x)₂O₉ ceramics is interpreted based on the Raman measurement.     

New red Y0.85Bi0.1Eu0.05V1−yMyO4 (M=Nb, P) phosphors for light-emitting diodes

The Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) as new near-ultraviolet excited phosphors were synthesized and their luminescence properties under 365 nm excitation were investigated in detail. It indicated that by doping small amount of P⁵⁺ into V⁵⁺ sites, the excitation intensity of charge transfer (CT) band of Bi–O (330–400 nm) was greatly improved. By substituting Nb⁵⁺ for V⁵⁺, both the CT bands of Bi–O and Eu–O (240–320 nm) were significantly enhanced. As a result, the emission intensity of Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) could be improved about 90% by doping 5 mol% P⁵⁺ and 110% by doping 5 mol% Nb⁵⁺. Comparing with the commercial Y₂O₂S:Eu³⁺ phosphors, the Y_0.85Bi_0.1Eu_0.05V_0.95M_0.05O₄ (M=Nb, P) phosphors exhibited excellent color purity and much higher brightness. The results showed that these Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) phosphors could be considered as promising red phosphors for application in LED.     

Phase composition-dependent physical and mechanical properties of YbxZr1−xO2−x/2 solid solutions

The paper represents a detailed insight into the correlation between changes of the phase composition of crystalline Yb_xZr_1−xO_2−x/2 solid solutions and their structural, electrical, mechanical and optical properties. Particularly, the effect of the crystal growth conditions and stabilizer amount in the range of 1.5–13.8 mol% of Yb₂O₃ are studied in terms of Rietveld analysis of powder X-ray diffraction data, electrical conductivity measured by impedance spectroscopy, absorption coefficient and refractive index measurements, Vickers microhardness (classical technique) as well as the plastic microhardness and effective elastic modulus (DSI—depth-sensing indentation technique). Potential applications of the investigated systems are discussed in view of the results obtained.     

UO2 晶体中低密勒指数晶面表面能的分子动力学模拟

采用基于刚性离子势的分子动力学模拟方法初步计算了UO₂晶体中(100), (110)和(111) 3种低密勒指数晶面在300–1500 K范围内的表面能大小. 结果表明, 3种晶面的表面能大小随温度的升高而降低, 与实验结果趋势一致; 原子排列最紧密的(111)晶面具有最低的表面能, 3种晶面的表面能大小从高到低依次为(100), (110)和(111)晶面; 达到平衡状态下的表面层原子相对于体内原子层在表面的法线方向上发生了明显的压缩并且表面层原子的对称性也降低了, 表面原子的弛豫效应一直影响到了第5层. 计算研究结果将有助于深入认识UO₂燃料中裂变气体气泡的聚集长大以及燃料的辐照肿胀开裂行为. 关键词： 分子动力学 2')" href="#">UO₂ 低密勒指数晶面 表面能     

Improvement in superconductivity of TlBa2CaCu2Oy system by introduction of oxygen loss

The T_c and oxygen content of TlBa₂CaCu₂O_y have been investigated by quenching experiments, in which the heat-treated samples were dropped into liquid nitrogen. The oxygen loss, v, of TlBa₂CaCu₂O_{yo − v} was determined by thermogravimetry in a nitrogen atmosphere using an oxygen-annealed specimen of TlBa₂CaCu₂O_yo and also by measurements of the weight differences before and after quenching. T_c increased from 80 K of the oxygen-annealed specimen up to about 110 K with increasing oxygen loss up to v = 0.035 by annealing at 500°C in a nitrogen atmosphere. Judging from the high T_c above 110 K achieved by a small oxygen loss about v = 0.035, the as-sintered oxygen-annealed TlBa₂CaCu₂O_yo specimen was in the over-doping state and probably has an oxygen vacancy of 7 − y_o0.     

Synthesis and ionic conductivity of new layered perovskite compound, Ag2La2Ti3O10

A new layered perovskite compound, Ag₂La₂Ti₃O₁₀, was synthesized by an ion-exchange reaction of M₂La₂Ti₃O₁₀ (M = Na,K) with a AgNO₃ molten salt. The crystal structure and the ionic conductivity of the ion-exchanged compound were investigated. The ionic conductivities attributed to the interlayer silver ions were observed at high temperatures. The ionic conductivity of Ag₂La₂Ti₃O₁₀ was much higher than that of Na₂La₂Ti₃O₁₀, while the interlayer sodium ions in Na₂La₂Ti₃O₁₀ and silver ions in Ag₂La₂Ti₃O₁₀ have almost the same rock-salt type coordination. The higher conductivity of Ag₂La₂Ti₃O₁₀ is probably due to the higher polarizability of silver ions.     

Phase relationships in an Al3 (Ti,V) based in-situ composite

The phases and the crystallographic relationships between phases observed in an Al₇₀Ti₁₀V₂₀ alloy following chill casting and post-solidification heat treatment have been characterized using analytical transmission electron microscopy and electron diffraction. The chill-cast microstructure comprised a mixture of primary dendrites of δ-Al₃ (Ti,V) and interdendritic eutectic zones in which the continuous phase was δ-Al₃ (Ti,V) and the rode-like discontinuous phase was ζ-Al₈V₅. Following isothermal heat treatments in the temperature range 1073–1273 K, a uniform distribution of rod-like particles was observed to develop throughout the δ matrix and a phase transformation was detected within the dispersed particles, as the intermetallic ζ phase was replaced by b.c.c.β-(Ti,V) metallic solid solution. This transformation was typically complete within 100 hr at 1073 K, leading to the formation of an in-situ composite consisting of a uniform dispersion of metallic β-phase in a brittle Al₃(Ti,V) intermetallic matrix. The orientation relationship between the discontinuous β-(Ti,V) particles and the DO₂₂δ-Al₃(Ti,V) matrix involved parallelism of the closest-packed planes and close-packed directions in those planes, i.e. (110)_β//(112)_δ(DO22) and [ 11]_β//[ 10]_δ(DO22).     

The adsorption of carbon monoxide on TiO2(110) supported palladium

Palladium overlayers deposited on TiO₂(110) by metal vapour deposition have been investigated using LEED, XPS and FT-RAIRS of adsorbed CO. Low coverages of palladium (<3 ML) deposited at 300 K adsorb CO exclusively in a bridged configuration with a band (B₁ at 1990 cm⁻¹) characteristic of CO adsorption on Pd(110) and Pd(100) surfaces. When annealed to 500 K, XPS and LEED indicate the nucleation of Pd particles on which CO adsorbs predominantly as a strongly bound linear species which we associate with edge sites on the Pd particles (L* band at 2085 cm⁻¹). Both bridged and linear CO bands are exhibited as increases in reflectivity at the resonant frequency, indicating the retention of small particle size during the annealing process. Palladium overlayers of intermediate coverages (10–20 ML) deposited at 300 K undergo some nucleation during growth, and adsorbed CO exhibits both absorption and transmission bands in the B₁ (1990 cm⁻¹) and B₂ (1940 cm⁻¹) regions. The latter is associated with the formation of Pd(111) facets. Highly dispersed Pd particles are produced on annealing at 500 K. This is evidenced by the dominance of transmission bands for adsorbed CO and a significant concentration of edge sites, which accommodate the strongly bound linear species at 300 K. Adsorption of CO at low temperature also allows the identification of the constituent faces of Pd and the conversion of Pd(110)/(100) facets to Pd(111) facets during the annealing process. High coverages of palladium (100 ML) produce only absorption bands in FT-RAIRS of adsorbed CO associated with the Pd facets, but annealing these surfaces also shows a conversion to Pd(111) facets. LEED indicates that at coverages above 10 ML, the palladium particles exhibit (111) facets parallel to the substrate and aligned with the TiO₂(110) unit cell, and that this ordering in the particles is enhanced by annealing.     

Phase formation and magnetic properties of Nd2Fe14B-type Nd16Co76B8−xCx alloys and their hydrides

An attempt is made to synthesize Nd₂Co₁₄C compound by mechanical alloying Nd₁₆Co₇₆B_8−xC_x (0x8) alloys and subsequent annealing. Phase formation and magnetic properties of Nd₂Fe₁₄B-type Nd₁₆Co₇₆B_8−xC_x alloys and their hydrides are investigated. The Nd₂Co₁₄(B,C) phase with Nd₂Fe₁₄B-type structure is formed for Nd₁₆Co₇₆B_8−xC_x (0x7) alloys, while NdCo₇C_δ phase with TbCu₇-type structure is observed in Nd₁₆Co₇₆C₈ alloy. The lattice parameter c of the Nd₂Co₁₄(B,C) phase decreases with increasing the carbon content. A limit volume of the unit cell to form the Nd₂Fe₁₄B-type structure is estimated to be 0.870 nm³. The spin-reorientation temperature T_SR increases with increasing the carbon content, due to an enhancement of magnetocrystalline anisotropy caused by carbon substitution for boron. After hydrogenation, the lattice expansion is observed for Nd₁₆Co₇₆B_8−xC_x (0x7) alloys. The spin-reorientation temperature of Nd₁₆Co₇₆B_8−xC_xH_y (0x7) is much lower than that of the host alloys. Some structural and magnetic properties of hypothetic Nd₂Co₁₄C and Nd₂Co₁₄CH_y compounds are estimated by extrapolation.     

Simple theory of the density-of-states function in heavily doped non-linear optical and optoelectronic materials

We study theoretically the dispersion relation of the conduction electrons and the corresponding density-of-state (DOS) function in heavily doped non-linear optical, tetragonal, III–V, ternary and quaternary materials forming band tails. It has been found, taking CdGeAs₂, Cd₃As₂, InAs, In_1−xGa_xAs_yP_1−y lattice matched to InP and Hg_1−xCd_xTe as examples of the aforementioned materials that the complex nature of the energy spectrum, the oscillatory DOS for the negative values of the energy and the creation of a new forbidden zone is due to the presence of the crystal field splitting constant, the anisotropic effective electron mass and the anisotropic spin–orbit splitting for the tetragonal and non-linear optical materials and because of the interaction of the impurity atoms in the tails with the spin–orbit splitting of the valence bands for the other compounds. Analytically, the presence of non-removable poles in the dispersion relation of the undoped material creates the complex energy spectrum for the corresponding heavily doped sample and the solution of the Boltzmann transport equation will introduce new physical ideas and new experimental findings under different external conditions. The results of the perturbed III–V semiconductors whose unperturbed energy band structures are defined by the two band model of Kane and that of parabolic energy bands are special cases of our generalized analysis and no oscillations in the DOS are found in the aforementioned cases. The well-known results of the DOS for all materials in the absence of doping have also been obtained from the theoretical analysis under certain limiting conditions. This compatibility is the indirect test of our generalized formalism.     

Effect of γ-radiation on the phase transition temperature of Li0.5(NH4)0.5SO4

Polycrystalline lithium-ammonium sulphate samples were subjected to X-ray diffraction analysis for determining the lattice parameters of the prepared mixed crystals. The compositions of the samples were determined using X-ray microanalysis.

The Li_0.5(NH₄)_0.5SO₄ samples were irradiated with different doses of γ-radiation in order to investigate the effect of this ionizing radiation on the phase transition temperatures T_c1 and T_c2. The temperature dependence of the d.c. resistivity ρ_d.c. and dielectric constant of irradiated samples showed (i) a shift of T_c1 to higher temperatures; (ii) a shift of T_c2 to lower temperatures; and (iii) the appearance of a new anomaly near 128°C at moderately high γ-doses. The mechanisms giving rise to this behaviour are discussed.