Local structure of Gd3+ impurity center at cubic sites in fluorites

A computer controlled ENDOR spectrometer and special software are used to determine the coordinates of¹⁹F nuclei in four anionic spheres of cubic MeF₂:Gd³⁺ (Me = Ca, Sr, Ba, Pb) centers. The computer simulation of Gd³⁺ cubic impurity center in crystals MeF₂ (Me = Ca, Sr, Ba) has been also performed. The analysis of lattice relaxation near the impurity center carried out on the base of ENDOR experiments data has allowed us to obtain potential of interaction Gd³⁺-F^?. For the first time not only radial displacements but also angular displacements of F^? ions of the third sphere have been taken into account and it has been shown that it influences the determination of lattice relaxation around the impurity center essentially. The influence of hydrostatic pressure (up to 30 kbar) on the local structure of the impurity center has also been investigated using computer simulation. The comparison of the experimental and calculated displacements of distant ions gives a reliable test of the validity of theoretical lattice relaxation model and accuracy of calculations of impurity-directed shifts of ligands.     

面心立方体金属中间隙原子内耗理论

在面心立方体金属中间隙原子一般不发生内耗。只有在合金元素(杂质)或空穴周围的间隙原子才会发生微扩散的内耗。因为空穴或合金原子的存在破坏了邻近间隙位置的对称性,在这样位置上的间隙原子才有可能在往复应力作用下发生应力感生微扩散内耗。本文按以上所述两种情况,利用作者之一所作的内耗热力学理论作计算,发现由于合金原子存在而引起的内耗弛豫强度应该与x_A(1—x_A)·C成正比,x_A是合金原子浓度,C是间隙原子浓度。由于空穴存在而引起的内耗强度应与N·1/(B/C²+1/C)成正比,此处N是空穴的浓度,C是间隙原子浓度,B是一常数。在葛庭隧、钱知强两氏对面心立方系锰钢的内耗强度的实验中,内耗强度的数值基本上与合金原子浓度无关。因此,在高锰合金钢中像是空穴所引起内耗的那种机构。同时,可以估计出碳原子落入空穴放出的能量约为0.14eV。但目前实验数据有限,实际上面心合金钢中是何种机构在起主要作用,尚待进一步研究。     

Influence of impurity hydrogen on the structure and properties of bulk Li and pressure effects

The structure and properties of a 16-atom body-centered cubic lithium cell with an interstitial hydrogen atom are studied using a pseudopotential-plane-wave method within the density functional theory at 0 K and high pressures. The host lattice is dramatically distorted by the introduction of H. Although the hydrogen atom is stable at the tetragonal site in perfect bcc host lattice, it favors the octahedral site formed by six non-equivalent Li atoms after full relaxation of the cell, showing P4/mmm symmetry within the pressures ranging from 0 to 6 GPa. The lattice ratio (a/c) changes irregularly with external pressure at about 3 GPa. The hydrogen band lies in the bottom of the valence band, separated by a gap from the metallic bands, illustrating the electronegativity of hydrogen. High reflectivity in the low frequency area induced by the impurity hydrogen is observed when only interband transitions are taken account of. A dip in reflectivity due to parallel band transitions is observed at ∼0.4 eV. Another dip at ∼4.3 eV appears when external pressure increases over 4 GPa.     

Atomic and electronic structure of oxygen vacancies and Nb-impurity in SrTiO<Subscript>3</Subscript>

We present the results of a first-principle full-potential linearized augmented plane wave (FLAPW) method to study the effect of defects on the electronic structure of SrTiO₃. In addition, the relaxation of nearest neighbor atoms around those defects were calculated self-consistently. The calculations were performed using the local (spin) density approximations (L(S)DA), for the exchange-correlation potential. SrTiO₃ was found to experience an insulator-to-metal transition upon the formation of oxygen vacancies or the substitution of Nb at the Ti site. The formation of oxygen divacancy disclosed additional states below the conduction band edge. The crystalline lattice relaxation showed displacements of atoms in rather large defective region. The magnitudes of atomic movements, however, were not large, normally not exceeding 0.15 Å. Our results were compared to the available experimental observations.     

Ba0.5Sr0.5TiO3有序构型的第一性原理研究

采用第一性原理计算了Ba_0.5Sr_0.5TiO₃三种有序构型的晶格结构和对应的电子结构,晶格结构的详细分析结果表明BST{100}有序构型为四方相,Ti-O八面体中Ti原子和Ba-Sr平面上的O原子沿[100]方向分别偏心位移0.040 Å和0.065 Å,八面体畸变导致反平行自发极化出现,构型处于反铁电态. BST{110}构型也是四方相,并且(110)和(1 关键词： 钛酸锶钡 第一性原理 有序结构 铁电性     

Dynamic collective displacements of atoms in metals and their role in the vacancy mechanism of diffusion

The phenomenon of dynamic collective displacements of atoms in face-centered cubic crystals has been revealed using molecular dynamics method. This phenomenon plays an important role in the vacancy mechanism of diffusion. The vacancy mechanism is provided by the collision of two regions of collective atomic displacements that move a migrating atom and a vacancy toward each other. The collective thermal atomic displacements from crystal lattice sites occur as a result of the nonuniform momentum distribution of atoms according to the Maxwellian distribution. Owing to their statistical nature, the degree of correlation of the atomic displacements depends neither on the temperature nor on the interatomic interaction potential.     

Changes in the structure of the Rb2NaYF6: Yb3+ crystal upon transition from the cubic to tetragonal phase

The pattern of lattice distortions occurring in the vicinity of Yb³⁺ ions during the transition of the Rb₂NaYF₆: Yb³⁺ crystal from the cubic to tetragonal phase has been revealed using all the parameters of the empirically found crystal fields for paramagnetic centers of the Yb³⁺ ions with cubic and tetragonal symmetry. It has been shown that the YbF₆ octahedra are rotated about the fourfold axis through an angle approximately equal to 1.2°. Moreover, the octahedra themselves are deformed so that the F^? ions symmetrically located in the plane perpendicular to the axis of rotation come close to the impurity ion at a distance of 0.0004 nm. The fluoride ions located on the axis of rotation, conversely, move away from the Yb³⁺ ion at a distance of 0.0005 nm. Based on the obtained results, it has been concluded that the total condensate of order parameters of the studied phase transition involves not only the critical rotations of octahedral groups but also the noncritical displacements of atoms in the rotated octahedra.     

Hydrogen binding to fixed interstitial impurities in metals

We have calculated the binding energy of hydrogen to the inert impurity atoms He, Ne, Ar, Kr, and Xe and also to the chemically active impurities C, O, and N in fixed interstitial configurations in Ni using quasiatom (effective medium) theory including lattice relaxation. We find that hydrogen is only weakly bound to any of these defects, that the chemically active defects do not produce a chemical bond with the hydrogen, and that the (～0.1 to 0.6 eV) binding is due to the strain field surrounding the defect.     

炸药爆轰合成纳米金刚石的拉曼光谱和红外光谱研究

用负氧平衡炸药爆轰法合成纳米金刚石,并用粉末X射线衍射(XRD)仪、激光Raman光谱仪和红外光谱仪等分析仪器对其结构进行表征。XRD结果表明,纳米金刚石为立方结构,由于其内部结构的高密度缺陷、杂质原子的夹杂使谱线偏离,晶格常数比静压合成的大颗粒金刚石大0.72%。由于金刚石晶粒细小,Raman光谱特征峰产生宽化,并且向小波数方向偏移了3 cm-1,此外在纳米金刚石中还含有极少量的石墨。红外光谱测试结果中,3 422 cm-1吸收峰为O—H伸缩振动峰;在1 634 cm-1出现了H₂O的弯曲振动峰,表明在纳米金刚石样品粉末中含有水分;2 930和2 857 cm-1是CH₂的反对称和对称伸缩振动吸收峰;2 971 cm-1是CH₃的反对称伸缩振动吸收峰,说明样品中存在极少量的碳氢化合物;1 788 cm-1吸收峰为CO伸缩振动吸收峰。文章从纳米金刚石的生成机理上分析了产生这些峰位的原因,结果表明纳米金刚石属于Ⅰ型金刚石,在它之中含有ⅠaA型和Ⅰb型金刚石,ⅠaA型金刚石的含量比Ⅰb型金刚石多。     

Quantum–chemical study of photo–excited states in tetragonal SrTiO3 lattice

The nature of photo-excited states in SrTiO₃ has been a topic of intense debate during the last few decades. Using a quantum-chemical method developed for crystals, we present a theoretical interpretation of the structural and electronic properties of triplet excitons in the tetragonal SrTiO₃ crystal. Our study demonstrates that the defect structure may correspond to the Mott–Wannier-type exciton having a considerable distance, 2.14Å, between the hole and the electron parts of the defect. The geometry optimization leads to an extensive defective region containing up to five atoms. However, the obtained magnitudes of atomic movements do not exceed 0.15Å. It is also observed that the self-trapped exciton polarizes the lattice around it. Using the so-called (ΔSCF method, the luminescence energy due to the exciton is found to be equal to 1.13 eV. As it corresponds to the infrared part of the spectrum, the experimentally detected green luminescence due to photo-excited states should be attributed to the singlet excitons.     

Self-correlation functions for impurity diffusion in hexagonal lattices

Long-range diffusion of the substitutional impurity in the otherwise monoatomic hcp-like lattices (H1 type) via the monovacancy mechanism has been considered within the encounter approximation. A five-frequency model (valid for cubic lattices) extends here naturally into a sixteen-frequency model, leaving fourteen normalized frequencies, the latter describing an anisotropic part of the self-correlation function. Two simplified cases can be distinguished: a self-diffusion being described by a single normalized frequency and a true hcp lattice, where only three normalized frequencies remain independent of each other. A general formalism is discussed and a few examples are evaluated numerically.     

Multiscale structural changes of atomic order in severely deformed industrial aluminum

The regularities of multiscale structural changes in the atomic order of the aluminum alloy AD-1 after a severe cold plastic deformation by conventional rolling in smooth rolls or in rolls with relief recesses favorable for shear deformation have been investigated. It has been found that there are four types of structural fractions that differ in scale and perfection of atomic order: crystallographic planes with a long-range order; nanoscale fragments of the planes (D = 100–300 Å) with an incipient long-range order; smaller groups of atoms (D = 20–30 Å) of amorphized structure; and the least ordered structural fraction of intercluster medium, keeping only a short-range atomic order (2–3 interatomic distances, 10 Å). The presence of diffuse halo bands in the region of intense Debye lines indicates phase transitions of the order → disorder type with the formation of one to three groups of amorphous clusters with the dominance, in the nanometer scale, of the atomic order characteristic of the family of planes (111), (220), and (311) of crystalline aluminum. We have found a dynamic phase transition with the changing crystallographic order of aluminum, with the matrix structure of a face-centered cubic (FCC) lattice, in the form of nanosized local groups of atoms, that is, the deformation clusters of aluminum with a simple cubic K6 lattice. In the case of conventional rolling, the development of large clusters 50–500 Å in size is observed; however, in the use of rolls with relief recesses, the difference in the sizes of the clusters is one half as much: 50–250 Å. Based on the analysis of the integrated intensity of incoherent X-ray scattering by the samples, we have elucidated the nature of the lowest measured density for the sample subjected to conventional rolling, which consists in the volume concentration of disorderly arranged atoms, the highest of the compared structures, which indicates the formation therein of the greatest amount of fluctuation “voids.”     

Hydrostatic pressure effects on oxygen-related irradiation-produced defects in silicon

Abstract

Hydrostatic pressure has been used as a variable to investigate the E_c-0.164 eV acceptor level for the oxygen-vacancy (O—V) defect in γ-ray irradiated Si, and the annealing/formation of oxygen-related defects in neutron-irradiated Si. The acceptor level is found by deep level transient spectroscopy to move closer to the conduction band and away from the valence band at rates of 3.9 meV/kbar and 2.4 meV/kbar, respectively, i.e., the level moves higher in the gap. There is also a relatively large inward (outward) breathing mode lattice relaxation (4.6±1.2 ų/electron) accompanying electron emission (capture) from this level. Both results reflect the antibonding nature of the level and are qualitatively consistent with the Watkins—Corbett model for the O—V defect. The annealing rate was found by infrared absorption to increase with pressure for the O—V defect at 350°C with a derived activation volume of ?4.5 ų/defect, where the negative sign implies inward relaxation (contraction) on annealing. Pressure has relatively little effect on annealing of the C—Si—O C(3) defect which is interstitial in nature, but strongly favors the formation of the dioxygen (2 oxygen atoms per vacancy, i.e., O₂—V) defect. The intensity of the O₂—V band after annealing at 20 kbar is 5 times higher than that following similar annealing at 0 kbar. Additionally, this intensity at 20 kbar is higher than that achievable by any isochronal or isothermal annealing steps at 0 kbar. These annealing/formation results are discussed qualitatively in terms of models for the various defects.     

Argentite–Acanthite Transition in Silver Sulfide as a Two-Sublattice Ordering

We propose a model of phase transformation cubic argentite–monoclinic acanthite in silver sulfide Ag2S (AgS0.5) as ordering in two argentite sublattices. We have determined the channel of the disorder–order transition including four nonequivalent superstructure vectors of stars {k9} and {k4}. For monoclinic acanthite α-Ag2S, we have calculated the distribution function for silver atoms occupying b positions in argentite, as well as the distribution function for sulfur atoms. Ordering in both sublattices is complicated by static atomic displacements. The displacement of S atoms distort the body-centered cubic (bcc) nonmetallic argentite sublattice, forming a monoclinic lattice in which silver atoms are at large distances from one another and occupy their crystallographic positions with unit probability. We have determined the range of admissible values of long-range order parameters η9 and η4 for the model monoclinic ordered α-Ag2S phase.     

立方相Ag3PO4(111)面原子几何及弛豫结构的第一性原理计算

采用平面波超软赝势方法研究了立方相Ag3PO4(111)面的表面能和表面原子弛豫结构.首先对Ag3PO4(111)面的八种不同原子终止结构的体系总能量进行计算,结果表明B种表面模型被证实为最稳定的(111)面原子几何结构.针对该表面结构,探讨了表面能和原子弛豫与模型中原子层数和真空厚度的关系,当原子层数为24层,真空厚度为0.6 nm时,表面能收敛于1.41 J/m2(LDA-CAPZ)和1.39 J/m2(GGA-PBE).表面原子弛豫后,表面两个三配位的Ag原子均向里移动,超过0.06 nm,而表面次层的O原子则均向外移动约0.0042 nm,导致弛豫后暴露在最表面的是O原子,同时表面原子的核外电子向表面内部发生转移,结构趋于稳定.这些结果为进一步深入研究Ag3PO4表面的光催化活性起源提供理论支持.     

立方相Ag_3PO_4(111)面原子几何及弛豫结构的第一性原理计算

采用平面波超软赝势方法研究了立方相Ag_3PO_4(111)面的表面能和表面原子弛豫结构.首先对Ag_3PO_4(111)面的八种不同原子终止结构的体系总能量进行计算,结果表明B种表面模型被证实为最稳定的(111)面原子几何结构.针对该表面结构,探讨了表面能和原子弛豫与模型中原子层数和真空厚度的关系,当原子层数为24层,真空厚度为0.6 nm时,表面能收敛于1.41 J/m2(LDA-CAPZ)和1.39 J/m2(GGA-PBE).表面原子弛豫后,表面两个三配位的Ag原子均向里移动,超过0.06 nm,而表面次层的O原子则均向外移动约0.0042 nm,导致弛豫后暴露在最表面的是O原子,同时表面原子的核外电子向表面内部发生转移,结构趋于稳定.这些结果为进一步深入研究Ag_3PO_4表面的光催化活性起源提供理论支持.     

Fe中刃型位错上扭折及掺杂体系的电子结构

利用离散变分方法和DMol方法,研究了体心立方Fe中1/2［111］（110）刃型位错上扭折及掺杂（N,O）体系的电子结构.能量（杂质偏聚能及格位能）计算结果表明,杂质元素N,O进入扭折芯区的偏聚趋势,这与位错扭折引起的晶格畸变有关.同时,在杂质元素周围有一些电荷聚集,导致扭折上电荷的不均匀分布,杂质原子得到电子,其周围Fe原子失去电子.由于N原子的2p轨道与近邻Fe原子的3d4s4p轨道之间杂化,使N原子与近邻Fe原子间有较强的相互作用,不利于扭折的迁移,使位错运动受阻,有利于材料强度的提高;而O与最近邻Fe原子之间的相互作用较弱.杂质-扭折复合体的局域效应明显影响体系的电子结构、能量及性能. 关键词： 电子结构 刃型位错 扭折 杂质元素     

First-principle study of nanostructures of functionalized graphene

We present first-principle calculations of 2D nanostructures of graphene functionalized with hydrogen and fluorine, respectively, in chair conformation. The partial density of states, band structure, binding energy and transverse displacement of C atoms due to functionalization (buckling) have been calculated within the framework of density functional theory as implemented in the SIESTA code. The variation in band gap and binding energy per add atom have been plotted against the number of add atoms, as the number of add atoms are incremented one by one. In all, 37 nanostructures with 18C atoms, 3 × 3 × 1 (i.e., the unit cell is repeated three times along x-axis and three times along y-axis) supercell, have been studied. The variation in C–C, C–H and C–F bond lengths and transverse displacement of C atoms (due to increase in add atoms) have been tabulated. A large amount of buckling is observed in the carbon lattice, 0.0053–0.7487 Å, due to hydrogenation and 0.0002–0.5379 Å, due to fluorination. As the number of add atoms (hydrogen or fluorine) is increased, a variation in the band gap is observed around the Fermi energy, resulting in change in behaviour of nanostructure from conductor to semiconductor/insulator. The binding energy per add atom increases with the increase in the number of add atoms. The nanostructures with 18C+18H and 18C+18F have maximum band gap of 4.98 eV and 3.64 eV, respectively, and binding energy per add atom –3.7562 eV and –3.3507 eV, respectively. Thus, these nanostructures are stable and are wide band-gap semiconductors, whereas the nanostructures with 18C+2H, 18C+4H, 18C+4F, 18C+8F, 18C+10F and 18C+10H atoms are small band-gap semiconductors with the band gap lying between 0.14 eV and 1.72 eV. Fluorine being more electronegative than hydrogen, the impact of electronegativity on band gap, binding energy and bond length is visible. It is also clear that it is possible to tune the electronic properties of functionalized graphene, which makes it a suitable material in microelectronics.     

Correlated recovery kinetics

Diffusion controlled correlated recovery kinetics are compared for the Continuum and discrete lartice treatments. Specific results are given for the simple cubic, face-centered cubic, body-centered cubic and diamond lattices. It is concluded that samples with anisotropic damage distributions created by irradiation in different crystallographic directions will not exhibit substantial differences in their correlated recovery, but it is argued that for small capture volumes and for the treatment of the early recovery times, the discrete lattice treatment is required.     

Effect of the fermi level position in silicon on ion-induced displacement of impurities

Abstract

With the channeling technique the lattice location of both As and B is studied in single As-or B-doped and in doubly As-and B-doped silicon single crystals. The influence of the position of the Fermi-level on the displacement of impurity atoms off substitutional lattice sites is investigated by changing the crystals from n- to p?type or vice versa by choosing implant conditions and annealing termperatures for the doubly doped crystals in an appropriate way. Big changes were found in displacement cross sections for As and B after conversion of the crystals from n- to p-type. The results can be explained by assuming that the interaction between primary defects and impurity atoms causing the displacement of the impurity atom is controlled by Coulomb attraction between charged point defects and the impurity atoms.