Prediction of train movement is required for traffic control. The readiness of systems to process incoming train flow depends on the quality of planning the situation on railroad. But predicting is influenced by various factors, and that is why the output parameters of train flows do not correspond to the planned parameters. The article considers the reliability factor of technical means of infrastructure, and compares its effect on train flows with various technical equipment of railway sections: double-track mountain-pass, single-track, double-track. As a result, the most unstable to failures of technical means section (in comparison with others) was determined and the reliability of its technical equipment was studied in more detail.

Local distortions in perovskite-like A-site-deficient (Sr,La)TiO3 solid solutions have been determined by refining large-scale atomic configurations against neutron/X-ray total-scattering and extended-X-ray-absorption-fine-structure data. Structural relaxations in this system are driven by the competing bonding requirements of Sr, La, and the undercoordinated oxygen atoms that surround vacant A-sites, which form upon substitution of La for Sr. La cations exhibit significant, disordered off-center displacements within their oversized oxygen cages required by the larger Sr cations. The resulting split-site probability density distributions of La vary with the Sr/La ratio and the state of the A-site ordering, which together modify the structure's ability to relieve the tensile bond strain around La through octahedral rotation and displacements of oxygens surrounding the vacancies. The displacive disorder of La can provide a hitherto overlooked mechanism for reducing the thermal conductivity, which is relevant to thermoelectric properties of this system. A comparison of the local structural behaviors in (Sr,La)TiO3 and the previously studied (Na,Bi)NbO3 solid solutions permits generalizations about A-site deficient perovskites. We find that A-site vacancies provide the nearest-neighbor oxygens with a degree of freedom to mediate the strain in the system, and their effects on local structural relaxations are determined by cation chemistry and stoichiometry.

Reverse Monte Carlo (RMC) model is a powerful tool based on supercell approach, targeting at the structure model that explains comprehensive experimental datasets.Typically, the RMCProfile package can incorporate neutron/X-ray total scattering, Bragg and extended X-ray absorption fine structure (EXAFS) data.For practical implementation, apart from theoretical pattern calculation and structure model adjustment based on metropolis algorithm, there are various effects under certain circumstances that one needs to take into account to avoid artificial effects.Here we are going to introduce several different types of correction that we recently developed and implemented, in the framework of RMCProfile, namely, 1) the correction for nano-size effect concerning total scattering modelling for nano-systems from 0D nanoparticles to 2D nanosheets [1].2) the implementation of arbitrary Bragg peak profile in a tabulated manner, through interacting with Topas software [2].3) the correction for finite instrument resolution effect going beyond the conventionally used analytical approach based on Gaussian assumption for peak shape [2].Through such development and implementation, we hope to extend the scope of application of RMCProfile package for solving structural problems from local perspective.Typically, the implementation of resolution correction enables the modelling to an otherwise-unreachable super-large length scale, e.g., 100 Å, following the supercell approach.Figure 1.(a) Demo for nano-size effect correction concerning total scattering pattern calculation for 1D nanorod.(b) Comparison of atomic displacement distribution for structure configuration obtained through fitting against data measured at different diffractometers, showing the robustness of our resolution correction routine.

Reported here are the development and application of new capabilities in the RMCProfile software for structural refinements using the reverse Monte Carlo (RMC) method. An algorithm has been implemented to enable the use of arbitrary peak-shape functions in the modeling of Bragg diffraction patterns and instrumental resolution effects on total-scattering data. This capability eliminates the dependence of RMCProfile on preset functions, which are inadequate for data produced by some total-scattering instruments, e.g. NOMAD at the Spallation Neutron Source (SNS) at Oak Ridge, Tennessee, USA. The recently developed procedure for the instrument-resolution correction has been modified to improve its accuracy, which is critical for recovering nanoscale structure. The ability to measure fine details of local and nanoscale structures with high fidelity is required because such features are increasingly exploited in the design of materials with enhanced functional properties. The new methodology has been tested via RMC refinements of large-scale atomic configurations (distances up to 8nm) for SrTiO3 using neutron total-scattering data collected on the Polaris and NOMAD time-of-flight powder diffractometers at the ISIS facility (Didcot, Oxfordshire, UK) and SNS, respectively. While the Polaris instrument is known to provide the high-quality data needed for RMC analysis, the similar and sound atomic configurations obtained from both instruments confirmed that the NOMAD data are also suitable for RMC refinements over a broad distance range.

The development of useful structure-function relationships for materials that exhibit correlated nanoscale disorder requires adequately large atomistic models which today are obtained mainly via theoretical simulations. Here, we exploit our recent advances in structure-refinement methodology to demonstrate how such models can be derived directly from simultaneous fitting of 3D diffuse- and total-scattering data, and we use this approach to elucidate the complex nanoscale atomic correlations in the classical relaxor ferroelectric PbMg 1/3 Nb 2/3 O 3 (PMN). Our results uncover details of ordering of Mg and Nb and reveal a hierarchical structure of polar nanoregions associated with the Pb and Nb displacements. The magnitudes of these displacements and their alignment vary smoothly across the nanoregion boundaries. No spatial correlations were found between the chemical ordering and the polar nanoregions. This work highlights a broadly applicable nanoscale structure-refinement method and provides insights into the structure of PMN that require rethinking its existing contentious models.

K0.5Bi0.5TiO3 (KBT)-one of the few perovskite-like ferroelectric compounds with room-temperature tetragonal symmetry differs from other members of its family (BaTiO3 and PbTiO3) by the presence of a disordered mixture of K and Bi on cuboctahedral sites. This disorder is expected to affect local atomic displacements and their response to an applied electric field. We have derived nanoscale atomistic models of KBT by refining atomic coordinates to simultaneously fit neutron/X-ray total scattering and extended X-ray absorption fine-structure data. Both Bi and Ti ions were found to be offset relative to their respective oxygen cages in the high-temperature cubic phase; in contrast, the coordination environment of K remained relatively undistorted. In the cubic structure, Bi displacements prefer the (100) directions and the probability density distribution of Bi features six well-separated sites; a similar preference exists for the much smaller Ti displacements, although the split sites for Ti could not be resolved. The cation displacements are correlated, yielding polar nanoregions, whereas on average, the structure appears as cubic. The cubic <-> tetragonal phase transition involves both order/disorder and displacive mechanisms. A qualitative change in the form of the Bi probability density distribution occurs in the tetragonal phase on cooling to room temperature because Bi displacements "branch off" to (111) directions. This change, which preserves the average symmetry, is accompanied by the development of nanoscale polar heterogeneities that exhibit significant deviations of their polarization vectors from the average polar axis.

Perovskite potassium sodium niobates, K 1−x Na x NbO 3 , are promising lead-free piezoelectrics. Their dielectric and piezoelectric characteristics peak near x = 0.5, but the reasons for such property enhancement remain unclear. We addressed this uncertainty by analyzing changes in the local and average structures across the x = 0.5 composition, which have been determined using simultaneous Reverse Monte Carlo fitting of neutron and X-ray total-scattering data, potassium EXAFS, and diffuse-scattering patterns in electron diffraction. Within the A-sites, Na cations are found to be strongly off-centered along the polar axis as a result of oversized cube-octahedral cages determined by the larger K ions. These Na displacements promote off-centering of the neighboring Nb ions, so that the Curie temperature and spontaneous polarization remain largely unchanged with increasing x , despite the shrinking octahedral volumes. The enhancement of the properties near x = 0.5 is attributed to an abrupt increase in the magnitude and probability of the short-range ordered octahedral rotations, which resembles the pre-transition behavior. These rotations reduce the bond tension around Na and effectively soften the short Na-O bond along the polar axis – an effect that is proposed to facilitate reorientation of the polarization as external electric field is applied.

Over the past decade, the RMCProfile software package has evolved into a powerful computational framework for atomistic structural refinements using a reverse Monte Carlo (RMC) algorithm and multiple types of experimental data. However, realizing the full potential of this method, which can provide a consistent description of atomic arrangements over several length scales, requires a computational speed much higher than that permitted by the current software. This problem has been addressed via substantial optimization and development of RMCProfile, including the introduction of the new parallelchains RMC algorithm. The computing speed of this software has been increased by nearly two orders of magnitude, as demonstrated using the refinements of a simulated structure with two distinct correlation lengths for the atomic displacements. The new developments provide a path for achieving even faster performance as more advanced computing hardware becomes available. This version of RMCProfile permits refinements of atomic configurations of the order of 500 000 atoms (compared to the current limit of 20 000), which sample interatomic distances up to 10 nm (versus 3 nm currently). Accurate, computationally efficient corrections of the calculated X-ray and neutron total scattering data have been developed to account for the effects of instrumental resolution. These corrections are applied in both reciprocal and real spaces, thereby enabling RMC fitting of an atomic pair distribution function, which is obtained as the Fourier transform of the total-scattering intensity, over the entire nanoscale distance range accessible experimentally.

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.

The use of Ag impurity in Er-doped ZnO films deposited by AC magnetron sputtering with a low growth rate has increased the emission intensity at λ = 1535–1540 nm. An increase in the deposition rate and in the temperature of substrates, as well as the use of Li and N+ impurities, led to a considerable increase in the intensity of the line with λ = 376–379 nm in the case of doping with rare-earth ions (Er, Tm), which makes it possible to use this semiconductor for creation of devices for the short-wavelength spectral region. Introduction of additional impurities in Er-doped ZnO films deposited on bulk ZnO crystals with increasing deposition rate and temperature caused an increase in the intensity of the line with λ = 1535–1540 nm. The photoluminescence spectra of ZnO films doped with Tm (ZnO) exhibited intense emission of lines with λmax = 377 nm.

We have studied PL and PLE spectra of two samples (A and B) of MBE grown CdSe/ZnSe asymmetric double quantum wells with different amount of deposited CdSe layers separated by 14 nm ZnSe barrier. It has been found that PLE spectra of the states forming short wavelength side of the PL spectra of both deep and shallow QWs of the sample A as well as that of deep QW of the sample B demonstrate oscillating structure in the spectral ranges corresponding to exciton states of self-assembled quantum dots only. Meanwhile PLE spectra of the short wavelength states of shallow QW the sample B revealed pronounced oscillating structure with energy period of ZnSe LO phonon under excitation with photons in a wide energy range both in the regions of quantum-dot states and in that of free states in the ZnSe barrier. In these spectra creating of excitons with kinetic energies more than 0.3 eV was observed which considerably exceed the exciton binding energy as well as LO phonon energy (both appr. 0.03 eV). It has been concluded that oscillating structure of the PLE spectra arises due to cascade relaxation of hot excitons. We discuss the model which explains these experimental findings.

The effect of the Er3+-ion excitation type on the photoluminescence spectra of crystalline ZnO(ZnO〈Ce, Yb, Er〉) films is determined in the cases of resonant (λ = 532 nm, Er3+-ion transition from 4S3/2, 2H11/2 levels to 4I15/2) and non-resonant (λ = 325 nm, in the region near the ZnO band-edge emission) excitation. It is shown that resonant excitation gives rise to lines with various emission intensities, characteristic of the Er3+-ion intracenter 4f transition with λ = 1535 nm when doping crystalline ZnO films with three rare-earth ions (REIs, Ce, Yb, Er) or with two impurities (Ce, Er) or (Er, Yb), independently of the measurement temperature (T = 83 and 300 K). The doping of crystalline ZnO films with rare-earth impurities (Ce, Yb, Er) leads to the efficient transfer of energy to REIs, a consequence of which is the intense emission of an Er3+ ion in the IR spectral region at λmax = 1535 nm. The kick-out diffusion mechanism is used upon the sequential introduction of impurities into semiconductor matrices and during the postgrowth annealing of the ZnO films under study. The crystalline ZnO films doped with Ce, Yb, Er also exhibit intense emission in the visible spectral region at room temperature, which makes them promising materials for optoelectronics.

Photoluminescence (PL) and PL excitation (PLE) spectra of structures containing MBE-grown CdSe layers with a nominal thickness t(CdSe) of 1.4-2.7 monolayers (MLs) in a ZnSe matrix have been studied. It is shown that the main features of the PLE spectra in structures with different amounts of deposited CdSe can be described in terms of two different models: in structures with t(CdSe) exceeding approximately 1.7 ML, the properties of electronic states correspond to the frequently used model of the quantum well formed by an inhomogeneous ZnCdSe solid solution with nanosize inclusions constituted by planar ZnCdSe islands whose composition is strongly enriched with CdSe, whereas the properties of electronic states in structures with t(CdSe) < 1.6 ML can be described in the model of isolated uncoupled ZnCdSe nanoislands. It was found that the main channel by which emitting states are occupied under excitation with photons in a wide energy range both in the region of quantum-dot states and in that of free states in the ZnSe barrier is the cascade relaxation of hot excitons, with emission of longitudinal optical phonons. Other properties of electronic states in nano-objects of this kind are discussed. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The effects of the parameters of ZnO-film deposition onto different substrates using the method of ac magnetron sputtering in a gas mixture of argon and oxygen hare studied. The phenomenon of self-organization is observed, which leads to invariability of the surface morphology of the ZnO films upon a variation in the substrate materials and deposition parameters. The parameters of the macro- and micro-photoluminescence spectra of the films differ insignificantly from the parameters of the photoluminescence spectra of bulk ZnO crystals obtained by the method of hydrothermal growth. The presence of intense emission with a narrow full-width at half-maximum (FWHM) in different regions of the spectrum allows ZnO films obtained by magnetron sputtering doped with rare-earth metal impurities (REIs) to be considered as a promising material for the creation of optoelectronic devices working in a broad spectral range. The possibility of the implementation of magnetic ordering upon legierung with REIs significantly broadens the functional possibilities of ZnO films. The parameters of the photoluminescence spectra of ZnO nanorods are determined by their geometrical parameters and by the concentration and type of the impurities introduced.

In the luminescence study of double quantum wells formed by depositing two CdSe layers with different nominal thicknesses into a ZnSe matrix, a heavy dependence of the photoluminescence spectrum on the thickness of the ZnSe barrier separating the quantum wells, the excitation photon energy, and temperature is observed. The photoluminescence spectra are studied at barrier widths of 34, 50, and 63 monolayers, excitation photon energies of 3.06, 2.71, and 2.54 eV, and temperatures T in the range of 5–200 K. Upon above- (3.06 eV) and below-barrier (2.71 eV) excitation, the photoluminescence spectrum exhibits two bands, I 1(T) and I 2(T), corresponding to the annihilation of excitons localized in the quantum dots of the shallow and deep quantum wells. An increase in temperature to ∼50 K yields only a slight decrease in the total integrated emission intensity of both bands I PL(T) and the intensities of each of the two bands, I 1(T) and I 2(T). A further increase in temperature results in substantial redistribution of the photoluminescence intensity between the two wells, which is attributed to the tunneling of excitons from the QD (quantum-dot) states of the shallow well to states of the deep well. This process is of the activation character and manifests itself as a sharp decrease in the integrated emission intensity related to the shallow quantum well, I 1(T), and a simultaneous increase in the integrated emission intensity of quantum dots of the deep quantum well, I 2(T). The experimentally detected effect is most profound in the range of temperatures T = 110–130 K and in the samples with a barrier thickness of 50 monolayers. It is most likely that the tunneling is of a resonance nature. This inference follows from the fact that the barrier width is much larger than the well widths for both wells, which predetermines only slight penetration of the wave functions into the neighboring well, and the effect of tunneling itself is only slightly supressed, as the barrier thickness is increased. At the same time, the activation energy is at least three time higher that the optical phonon energy, which cannot be explained on the basis of existing theory.

The influence of nanoparticles and thin layers of Au, Eu phthalocyanine, and Er nanoparticles on the formation of luminescence spectra of InGaN/GaN quantum-well structures has been investigated. It has been shown that the influence of localized plasmons on the generation of charge carriers is determined by the size of Au nanoparticles under the assumption that the interaction of plasmons with surface states of the structures plays a dominant role. The influence of Au nanoparticles on the formation of luminescence spectra of multiple quantum-well structures based on InGaN/GaN, unlike the case of Au layers, is determined by the indium concentration. The influence of Eu phthalocyanine films, which are deposited onto the surface of the studied structures, on their photoluminescence spectra is similar to the influence of doping of these structures with europium.

The influence of annealing in the medium of an ionized nitrogen, additionally introduced impurities, and regimes of post-growth annealing on photoluminescence spectra of intracenter 4 f transitions of rare-earth metal dopants (Ce, Eu, Sm, Er, Tm, Yb) in crystalline ZnO films has been investigated. The films have been prepared using molecular-beam epitaxial growth and magnetron sputtering. According to the X-ray diffraction analysis, the films have a single-crystal structure. It has been shown that the annealing in the medium of an ionized nitrogen, regardless of the method used for producing the ZnO films, leads to significant changes in the photoluminescence spectrum, i.e., to a decrease in the emission intensity and a shift in the position of the emission maximum toward the long-wavelength range of the spectrum. The nitrogen concentration has been determined by the nuclear reaction method. It has been revealed that the spectra contain intense emission lines due to the intracenter 4 f transitions of rare-earth elements (Sm, Er, Tm, Yb) in the crystalline ZnO films prepared by magnetron sputtering, and the intensity of the emission lines increases upon introduction of codopants, namely, Ce and Er.

It is shown that the intensity of emission from intracenter 4 f -transitions in amorphous a -Si:H films and crystalline (GaN, ZnO) films doped with rare-earth ions is governed by the local environment of doping impurity ions. In the case of a -Si:H, a pseudo-octahedron with the C 4 V point group is present due to nanocrystallites, which provides a local environment for rare-earth ions. In the case of a hexagonal crystal lattice in crystalline GaN and ZnO films, the local symmetry of rare-earth ions introduced into the semiconductor matrix by diffusion, with a pseudo-octahedron with the C 4 V point group, is formed by stresses due to rare-earth ion-oxygen complexes with a radius exceeding that of host ions incorporated at crystal lattice sites. In contrast to GaN films, ZnO films exhibit, on being doped with Tm, Sm, and Yb, both high-intensity emission in the long-wavelength spectral region, characteristic of intracenter 4 f transitions in rare-earth ions, and a substantial increase in intensity in the short-wavelength spectral region (λ = 368–370 nm). GaN films doped with rare-earth ions exhibit in this spectral range only an inhomogeneously broadened emission spectrum due to the presence of an emission band characteristic of donor-acceptor recombination.

On the basis of the results of complex investigations (photoluminescence, x-ray fluorescence, and infrared spectroscopy), the features of emission-spectra formation are shown under the change in the type (Fe, Cu, and Si) and concentration of background impurities appearing during both growth and treatment of bulk crystals by grinding and polishing. Special attention is given to the concentration and types of bonds with hydrogen-the basic impurity preventing the formation of crystals with the p-type conductivity.