Determination of antisite defects in double perovskite Sr2FeMoO6 by diffraction method: Simulation studies

The crystal structure of double perovskite Sr₂FeMoO₆ synthesized via solid-state reaction at 1280 °C under a reduction atmosphere is refined by Rietveld technique based on X-ray powder diffraction (XRD) data in 2θ range of 15-140°. An antisite content (AS), i.e. Fe on the Mo sites (=Mo on the Fe sites), of 12.1(1)% is derived. In reference to the refinement results, a series of X-ray and neutron powder diffraction (NPD) data with different antisite contents, ranging from 0 (completely ordered) to 50% (completely disordered), are generated with a Poisonian noise added and subjected to Rietveld refinements with the same initial values for the refinable parameters. The AS is reproduced satisfactorily from the refinement of XRD data and the combined refinement of XRD and NPD data with a relative deviation smaller than 4%, whereas the relative deviation of AS derived from the refinement of NPD data can be as large as 50%. However, the atomic occupancies and isotropic temperature factors of all the atoms can be reasonably reproduced from the refinement of NPD data and the combined refinement of XRD and NPD data, whereas these data can be reproduced only for cations (Sr, Fe, Mo) from the refinement of XRD data. The present simulation studies shed light on understanding the controversial statements derived from XRD and NPD work regarding the antisite defects in Sr₂FeMoO₆, which in turn is indispensable for understanding the mechanism of large room temperature low-field magnetoresistance of the compound.     

Formation of defect density superlattices in binary compounds under nuclear irradiation

Manifestation of antisite defects in crystals under irradiation is studied. Calculations show that the concentration of such defects can reach high levels under typical irradiation intensities and temperatures. It is shown that buildup of antisite defects and interaction between them can result in instability of the system during irradiation with respect to spatially nonuniform perturbations. The instability should give rise to a periodic modulation of the density of antisite defects. The region of instability as a function of crystal parameters and irradiation has been obtained.     

Defect studies of hydrogen-loaded thin Nb films

Hydrogen interaction with defects in thin niobium (Nb) films was investigated using slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thin Nb films on Si substrates were prepared using cathode beam sputtering at room temperature. Initially, the microstructure of the virgin (hydrogen-free) films was characterized. Subsequently, the films were step-by-step electrochemically charged with hydrogen and the evolution of the microstructure with increasing hydrogen concentration was monitored. Hydrogen loading leads to a significant lattice expansion which was measured by XRD. Contrary to free-standing bulk metals, thin films are highly anisotropic. The in-plane expansion is prevented because the films are clamped on the elastically hard substrate. On the other hand, the out-of-plane expansion is substantially higher than in the bulk samples. Moreover, an enhanced hydrogen solubility in the α-phase was found in nanocrystalline Nb films. It was found that most of positrons in the films are trapped at open-volume defects at grain boundaries (GBs). These defects represent trapping sites also for hydrogen atoms. Hydrogen trapping at vacancy-like defects like GBs leads to a local increase of the electron density and is reflected by a pronounced decrease of the S parameter in the hydrogen-loaded samples. In addition, it was found that new defects are introduced at higher concentrations of hydrogen due to the formation of NbH (β-phase) particles.     

Ga sublattice defects in (Ga,Mn)As: thermodynamical and kinetic trends

We have used positron annihilation spectroscopy and infrared absorption measurements to study the Ga sublattice defects in epitaxial Ga(1-x)MnxAs with Mn content varying from 0% to 5%. We show that the Ga vacancy concentration decreases and As antisite concentration increases with increasing Mn content. This is in agreement with thermodynamical considerations for the electronic part of the formation energy of the Ga sublattice point defects. However, the absolute defect concentrations imply that they are determined rather by the growth kinetics than by the thermodynamical equilibrium. The As antisite concentrations in the samples are large enough to be important for compensation and magnetic properites. In addition, the Ga vacancies are likely to be involved in the diffusion and clustering of Mn at low annealing temperatures.     

晶粒尺寸对掺氦纳米多晶铁机械性能影响的分子动力学模拟研究

采用分子动力学方法研究了晶粒尺寸对掺氦纳米多晶铁机械性能的影响.在拉伸形变过程中,纳米多晶铁将产生裂纹与晶格畸变,通过模拟XRD谱探索二者之间的联系.拉伸模拟结果显示,由于晶界氦原子的引入,峰值应力将显著减小.另外,在拉伸模拟中观察到,沿晶裂纹的产生与长大随着晶界氦原子的引入而增强.研究结果表明,晶界氦原子能够促进沿晶...     

The influence of changes in the framework on the diffusion in zeolites. Molecular dynamics simulations

Some less known methods are described for the analysis of trajectories of guest molecules in porous solids. Such trajectories can be calculated by Molecular Dynamics (MD) computer simulations in order to analyze the interrelations between the structure and the particle behaviour including collective phenomena. Some results obtained with these methods for diffusing methane in zeolites of type LTA are presented. An analytical potential model for LTA type zeolites is given that make extremely long runs or simulations of large lattice regions for systems with rigid lattice possible. Such runs are necessary e.g. to examine questions as the influence of extented lattice defects or the fractal behaviour of the partical trajectories.     

Vortex System Dynamics upon Pulsed Magnetization of a HTS

A numerical investigation of the response of a HTS with different numbers of defects to the action of rectangular magnetic field pulses with a varying amplitude is carried out. The time dependences of a magnetic field trapped by the superconductor are calculated at different pulse parameters. The values of the HTS response to pulsed and gradual magnetization and demagnetization of the sample are compared, which reveals that magnetic flux trapping is more efficient under the action of pulses. A feature in the behavior of the vortex lattice acted on by field pulses is established, which relates to correlation between the number of defects and pulse amplitude. Flux trapping is shown to have a maximum dependent on the concentration of defects at each pulse amplitude.     

硅单粒子位移损伤多尺度模拟研究

本文结合分子动力学方法和动力学蒙特卡罗方法, 研究了单个粒子入射硅引起的位移损伤缺陷的产生和演化过程; 基于Shockley-Read-Hall理论计算了单个粒子入射引起的位移损伤缺陷导致的泄漏电流增加及其演化过程, 比较了缺陷退火因子与泄漏电流退火因子之间的差异, 并将计算结果与实验值进行了对比. 结果表明, 计算泄漏电流时, 仅考虑一种缺陷的情况下缺陷退火因子与泄漏电流退火因子相同, 考虑两种缺陷类型情况下二者在数值上有所区别, 但缺陷退火因子仍能在一定程度上反映泄漏电流的退火行为. 分子动力学模拟中采用Stillinger-Weber势函数和Tersoff势函数时缺陷退火因子和泄漏电流退火因子与实验结果一致, 基于Stillinger-Weber势函数的计算结果与实验值更为接近.     

Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu₃Al₅O₁₂ (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of Lu_Al antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and Lu_Al antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb²⁺ ions. The structure of the luminescence centers, related to the background emission of impurity Pb²⁺ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al₂O₃, Y₂O₃) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O^? ions and electrons localized at excited levels of Lu³⁺ cations.     

Atomic-scale visualization of antisite defects in LiFePO4

We visualize the antisite exchange defects in LiFePO4 crystals with an ordered olivine structure by using annular dark-field scanning transmission electron microscopy (STEM). A recognizable bright contrast is observed in some of the Li columns of STEM images in a sample annealed at a lower temperature, which directly demonstrates the disordered occupations by Fe atoms. Furthermore, such exchange defects appear to be locally aggregated rather than homogeneously dispersed in the lattice, although their overall concentration is fairly low. The present study emphasizes the significance of atomic-level observations for the defect distribution that cannot be predicted by macroscopic analytical methods.     

Atomic simulation of AlGaN film deposition on AlN template

In this article, we study the deposition of AlGaN film on AlN template by molecular dynamics (MD) simulations. The effects of growth temperature and film thickness on the dislocation of deposited AlGaN film are simulated and studied. The atomic structure of deposited AlGaN film is also investigated. We find that the dislocations usually occur at the interface between AlN template and AlGaN film and then extend towards the growth direction. The dislocation density decreases with the increase of AlGaN film thickness, which indicates that increasing the thickness of deposited AlGaN film to a certain extent is beneficial to reducing dislocation. In addition, increasing the growth temperature can also effectively reduce the dislocation in deposited AlGaN film. Furthermore, the crystallinity of deposited AlGaN film could be improved by increasing the growth temperature. This is consistent with the dislocation discussion. The mobility of adatoms increases as the growth temperature increases. So it is easier for adatoms to find their ideal lattice points at higher temperature. Thus the dislocation and other defects can be effectively reduced and the crystal quality of deposited AlGaN film could be improved.     

Luminescence and scintillation properties of Y<Subscript>3</Subscript>A<Subscript>l5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

Luminescence and scintillation properties of Y₃A_l5O₁₂:Ce single crystals grown from the melt by the Czochralski and horizontal directed crystallization methods in various gas media and Y₃A_l5O₁₂:Ce single-crystal films grown by liquid-phase epitaxy from a melt solution based on a PbO-B₂O₃ flux have been comparatively analyzed. The strong dependence of scintillation properties of Y₃A_l5O₁₂:Ce single crystals on their growth conditions and concentrations of Y_Al antisite defects and vacancy defects has been established. Vacancy defects are involved in Ce³⁺ ion emission excitation as the centers of intrinsic UV luminescence and trapping centers. It has been shown that Y₃A_l5O₁₂:Ce single-crystal films are characterized by faster scintillation decay kinetics than single crystals and a lower content of slow components in Ce³⁺ ion luminescence decay during high-energy excitation due to the absence of Y_Al antisite defects in them and low concentration of vacancy defects. At the same time, the light yield of Y₃A_l5O₁₂:Ce single-crystal films is comparable to that of single crystals grown by directed crystallization due to the quenching effect of the Pb²⁺ ion impurity as a flux component and is slightly lower (∼25%) than the light yield of single crystals grown by the Czochralski method.     

X-Ray Luminescence of Defects in Spinel Single Crystals

The flareup of x-ray luminescence in spinel single crystals (MgAl₂O₄) depending on the time of x-ray irradiation and the decay of fluorescence depending on the time elapsed after the termination of irradiation have been investigated. These dependences were measured at different powers of the irradiation dose (power of the x-ray tube) and at different temperatures of the samples. The experimental results suggest the existence of large-size complexes of defects, which include antisite defects and impurity ions, the exchange of charge carriers between which during and after irradiation leads to luminescence of the impurity ions. Transfer of charge carriers between isolated elements of the pairs of antisite defects (not interacting with the impurity ions) leads to the formation of a competing channel of recombination luminescence in the UV region of the spectra. The decay of fluorescence attributable to transitions in the Mn²⁺ and Cr³⁺ ions depending on the time elapsed after the termination of x-ray irradiation points to the existence of various combinations of antisite defects in the surroundings of these ions.     

Energy formation of antisite defects in doped Sb2Te3 and Bi2Te3 crystals

The free carrier concentration of the Sb_2−xIn_xTe₃, Bi_2−xIn_xTe₃ and Bi₂Te_3−xS_x crystals has been determined from the values of the Hall constants and the free carrier concentration of the Sb_2−xTl_xTe₃ has been calculated from the plasma resonance frequency; with increasing value of x, the hole concentration decreases. As the incorporation of the elements In, Tl and S into the lattice Sb₂Te₃ or Bi₂Te₃, respectively, gives rise to the uncharged defects In^x_Sb, Tl^x_Sb, In^x_Bi and S^x_Te, the x causes the decrease of the antisite defects concentration. The proven effect is explained in the following way: the antisite defects can be created only in crystals whose atoms are bound by weakly polarized bonds. The incorporation of In, Tl and S atoms into the crystal lattice of Sb₂Te₃ or Bi₂Te₃ increases the bond polarity, the ionicity of ternary crystals increases. This unfavorably affects the increase of antisite defects whose concentration decreases. The change of the bond polarity is considered from the changes discovered in the formation energy of antisite defects of the above mentioned ternary crystals.     

嵌入原子法研究Ni3Al中点缺陷以及Re择优占位和集团化

基于纯金属元素Ni，Al和Re的基本物理性质，建立了一个Ni-Al-Re三元体系的分析型嵌入原子多体势.结合分子动力学计算了Ni₃Al的平衡晶格常数、弹性模量、结合能、空位形成能以及反位置缺陷形成能，并分析了Ni₃Al中点缺陷的存在形式.计算结果表明，当成分偏离理想化学配比时出现反位置缺陷.同时研究了Re在Ni₃Al中的择优占位以及Re在Ni₃Al和Ni中的集团化行为.计算结果表明，Re在Ni₃Al中优先置换Al的位置，且发现当Re原子团的尺寸接近于11?时，Re原子团的长大趋势变弱.计算结果与实验以及其他的理论计算结果相符合. 关键词： 嵌入原子势 3Al')" href="#">Ni₃Al Re 占位 集团化     

Lattice boltzmann versus molecular dynamics simulation of nanoscale hydrodynamic flows

A fluid flow in a simple dense liquid, passing an obstacle in a two-dimensional thin film geometry, is simulated by molecular dynamics (MD) computer simulation and compared to results of lattice Boltzmann (LB) simulations. By the appropriate mapping of length and time units from LB to MD, the velocity field as obtained from MD is quantitatively reproduced by LB. The implications of this finding for prospective LB-MD multiscale applications are discussed.     

Structural, thermal and deformation-induced point defects in PdIn

Perturbed angular correlation of gamma rays (PAC) was applied to measure concentrations of point defects in quenched and milled intermetallic compounds. Two systems with high ordering energies were studied, PdIn and NiAl, with principal results obtained for PdIn. The defects, lattice vacancies and antisite atoms, were detected by quadrupole interactions induced at nuclei of nearby In/Cd probe atoms. Measurements on annealed, quenched and milled samples are compared to identify quadrupole interaction signals with specific configurations of defects near probe atoms. Müller and Hahn had previously identified signals with a Pd-vacancy in the first near-neighbor shell of the In probe and a Pd-antisite atom in the second shell. Six additional signals are identified in this work, including two distinct configurations of two Pd-vacancies in the first shell, a Pd-vacancy in the fourth shell, an In-vacancy in the second or third shell, and an In-antisite atom in the first shell. After quenching, observation of both Pd and In-vacancies after quenching from 1100-1500 K demonstrates that the high temperature equilibrium defect is the Schottky vacancy pair. An activation energy of 0.91(18) eV was determined for formation of a Schottky pair. After milling, the site fraction of probes having one Pd-vacancy in the first shell increased rapidly and saturated at a value of about 20%. This implies a Pd-vacancy concentration of about 3.3 at.%, with a corresponding enthalpy of the order of 3 kJ?mol^?1, making a significant fraction of the excess enthalpy in the milled intermetallic. After milling NiAl, a Ni-vacancy concentration of order 1–10 at.% was observed. A comparison is made of defect combinations observed after mechanical milling of PdIn and NiAl by PAC and of other intermetallics by Mössbauer spectroscopy. Dominant defects produced by milling are the Schottky pair for PdIn, triple defect for NiAl and FeRh and antisite atom pair for FeAl.     

Manifestations of charged lattice defects in excitonic luminescence and control over properties of wide-band scintillation crystals

A fast energy transfer from the host crystal to impurity centres and charged lattice defects (e.g., vacancies in an ionic crystal) is due to the two-site exciton motion on high vibrational levels. At low temperatures this motion occurs in a coherent directional way with the sound velocity whereas a small separation between vibrational levels provides an easy exciton trapping by shallow potential wells near impurity centres. Charged defects have a much larger trapping section than isovalent impurity centres and manifest themselves in luminescence at a concentration smaller by two orders of magnitude.     

氘化对KH2PO4晶体微观缺陷影响的正电子湮没研究

采用传统降温法从不同程度氘化(x=0, 0.51, 0.85)的生长溶液中生长氘化KH₂PO₄(KDP) 晶体, 利用正电子湮没技术(正电子寿命谱和多普勒展宽谱)、结合X射线衍射谱(XRD) 结构分析, 对KDP晶体氘化生长的微观缺陷进行了研究, 讨论了氘化程度对晶体内部微观结构特性、缺陷类型和浓度的影响. XRD结果显示晶胞参数a, b值随氘含量的增加而增加, c值无明显变化; 正电子寿命谱结果发现随着氘化浓度的提高, KDP晶体内部中性填隙缺陷以及氧缺陷不断增加, 引起晶体晶格畸变; 氢空位、K空位、杂质替位缺陷不断发生缔合反应形成复合缺陷, 缺陷浓度不断减少; 团簇、微空洞等大尺寸缺陷也在不断发生聚合反应, 缺陷浓度表现为不断减少. 多普勒实验结果表明随着氘化程度的提升, 晶体内部各类缺陷表现为同步变化. 实验结果表明, KDP晶体在低浓度氘化生长(50%以内)下缺陷反应较弱, 而在高浓度氘化(50%以上)下的缺陷反应显著增强.