Crystal Lattice Defects Research Articles

Hydrogen Trapping in bcc Iron.

Fundamental understanding of H localization in steel is an important step towards theoretical descriptions of hydrogen embrittlement mechanisms at the atomic level. In this paper, we investigate the interaction between atomic H and defects in ferromagnetic body-centered cubic (bcc) iron using density functional theory (DFT) calculations. Hydrogen trapping profiles in the bulk lattice, at vacancies, dislocations and grain boundaries (GBs) are calculated and used to evaluate the concentrations of H at these defects as a function of temperature. The results on H-trapping at GBs enable further investigating H-enhanced decohesion at GBs in Fe. A hierarchy map of trapping energies associated with the most common crystal lattice defects is presented and the most attractive H-trapping sites are identified.

GRANULATED LOW-MODULUS ZEOLITES FOR EXTRACTION OF CO CATIONS

The paper gives a characteristic of LTA and SOD zeolite pellets, which were synthesized using preliminary ultrasonic processing. By X-ray diffraction and IR spectroscopy methods, it was established that the investigated samples contained about 100% of the LTA or SOD phase. It was shown that zeolite LTA particles have a dimension of coherent scattering region of 780 nm and root mean square microdeformations of 0.05%, while SOD zeolite particles have this parameter about 460 nm and the defectiveness of crystal lattice is 0.15%. The values of the specific surface area of zeolite pellets were determined, the values of which are 148.8 and 115.6 m2/g for LTA and SOD, respectively. It was noted that the SOD zeolite pellets have virtually no micropores. Equilibrium curves were obtained for the saturation of zeolite pellets with cobalt cations depending on the concentration of Co2+ in an aqueous solution of cobalt chloride. It was shown that in the entire investigated range of Co2+ concentrations, the LTA zeolite capacity is 30% higher than that of the SOD zeolite which can be explained by two main reasons. Firstly, this is a higher specific surface area of the LTA zeolite. Secondly, these are structural features of the zeolite frameworks, namely, LTA zeolite has α-cavities which are more accessible for Co2+. It is also shown that with increasing temperature, the zeolites capacity on cobalt cations grows. It was discovered that a new absorption band with a wave number at 1390 cm–1 appears on the IR spectra of both zeolites after saturation. In addition, an increase in the size of the unit cell and a growth in the defectiveness of particles were noted in cobalt-saturated zeolites. Taken together, these facts point to the cation exchange. It was shown that 1 g of LTA zeolite provides almost complete purification of 50 ml of the solution from 57Co cations in a dynamic mode.

The Important Questions of Statistical Theory of the Crystals' Thermal and Kinetics Properties

In this paper, the short algorithmic formulas for computations of the crystals thermal and kinetic properties are given. These formulas are fulfilled the case of isotropic crystals for any one of dispersion laws of current carriers, where these carriers can be scattered by all kinds of the crystal lattice defects. In the paper, these algorithmic formulas were used to calculations the important properties of crystals with the nonparabolic Kane’s dispersion law of current carriers. Here, the passing from these non-parabolic dispersion law crystals to the parabolic dispersion law crystals was also described.

Lattice defects and micro-strains in V60Ti25Cr3Fe12 alloy and influence on the ab/desorption of hydrogen

Particle refinement is expected to be beneficial for the hydrogen storage properties of V60Ti25Cr3Fe12 alloy. However, crystal lattice defects and micro-strains are easy to appear and accumulate in the alloy during the refinement process, leading to a loss of hydrogen storage capacity and a decay of cycle performance. Thereinto, annealing at high temperature is an efficient method to reduce the lattice defects and micro-strains in the crystals. As we have known, the mechanisms for the generation and evolution of micro-strains and lattice defects in the alloy as well as the influences on the hydrogen storage performance are still elusive. In this work, the effects of annealing temperatures on the quantity of crystal lattice defects and micro-strains are investigated. Furthermore, the consequent impact of lattice defects and micro-strains on the hydrogen ab/desorption abilities is deeply explored. The results indicate that the micro-strains and relative defect concentrations decrease with the temperatures raising, endowing the alloy a better integrality. Thus, the phase transformation during de/hydrogenation from monohydride to dihydride becomes easier, leading to an improvement of the hydrogen absorption and desorption capacities. Compared with heat treated at 673K, the heat-treatment of the alloys results in a 41% enhancement of hydrogen desorption ability at 1273K. Furthermore, two functions are proposed to explain the influences of micro-strains and relative defect concentrations on phase transformation.

Optical and luminescent properties of ceria nanoparticles produced by gas phase method

The study on luminescent and optical properties of ceria nanoparticles allow one to indirectly assess the defectiveness of the material crystal lattice. The decomposition of the luminescence spectrum in the Gaussian bands shows the presence of Ce3+ ions in the CeO2 crystal lattice, as well as optical active centers, probably associated with oxygen vacancies. These characteristics permit us to estimate the relative amount of Ce3+ ions in the crystal lattice and could be considered as an important prerequisite for the use of nanoparticles as redox agents in industry and medicine.

Hydrogen sorption capacity of crystal lattice defects and low Miller index surfaces of copper

The effect of hydrogen on the physical–chemical properties of copper is directly dependent on the types of chemical bonding between H and lattice defects in Cu. In this work, we performed a systematic study of the bonding of H-atoms with crystal lattice defects of copper. This included three types of symmetric tilt grain boundaries (GBs), Σ3, Σ5 and Σ11, and the low Miller index surfaces, (111), (110) and (100). A comparison with literature data for the bonding of H-atoms with point defects such as vacancies was done. From the defects investigated and analyzed, we conclude that the bond strength with H-atoms varies in the decreasing order: surfaces [(111), (110) and (100)] > vacancy > Σ5 GB > Σ11 GB > bulk ≈ Σ3 GB. A study on the effects of the fcc lattice expansion on the binding energies of H-atoms shows that the main driving force behind the segregation of H-atoms at some GBs is the larger volume at those interstitial GB sites when compared to the interstitial bulk sites.

Insights into the CaSO4–H2O System: A Raman-Spectroscopic Study

Even though being the subject of natural scientific research for many decades, the system CaSO4–H2O, consisting of the five crystalline phases gypsum, bassanite, and the anhydrites III, II, and I, has left many open questions for research. Raman spectroscopy was used because of its structural sensitivity and in situ measurement capability to obtain further insight by studying phase transitions in both ex situ and in situ experiments. The findings include significant contributions to the completeness and understanding of Raman spectroscopic data of the system. The dehydration path gypsum–bassanite–anhydrite III was shown to have strong parallels to a physical drying process, which depends on many parameters beyond the burning temperature. Raman band width determination was demonstrated to enable the quantitative discrimination of α-bassanite and β-bassanite as well as the postulated three sub-forms of anhydrite II (AII), which are all based on differences in crystallinity. In the latter case, the observed continuous structural variations over increasing burning temperatures were elucidated as a combination of decreasing surface areas and healing of crystal lattice defects. We propose an only two-fold sub-division of AII into reactive “disordered AII” and much less reactive “crystalline AII” with a transition temperature of 650 °C ± 50 K.

Design of photonic crystal horn antenna for transverse electric modes

In this paper, by modifying defects in a photonic crystal lattice, a two-dimensional photonic crystal horn antenna is designed. The photonic crystal used for this purpose is composed of a hexagonal lattice of circular holes in a dielectric slab. The results of this paper allow us to design a photonic crystal antenna capable of separating TE and TM modes. The designed structure has a very simple design that allows low cost fabrication. The structure is numerically simulated using a finite-difference time-domain (FDTD) method. Its wide bandwidth, its low loss and the ability to transmit waves at a terahertz frequency range are the antenna’s main advantages. The return loss for the frequency range of 180 to 215 THz is from –6.63 to –28.3 dB. Moreover, a 35 THz bandwidth is obtained for this structure.

Фотокаталитические свойства композиции alpha-Bi-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-/Bi в видимой области света в зависимости от концентрации металлического висмута и степени дефектности кристаллической решетки оксида висмута

The photocatalytic activity of the α-Bi2O3/Bi heterostructure compositions obtained by the method of low-temperature pyrolysis of bismuth-sorbitol complexes was investigated. The effectiveness of catalysts is estimated by the rate of decomposition of methylene blue (MS) upon irradiation with light of the visible spectrum. The catalysts obtained by varying the amount of sorbitol in the composition of the precursor organo-mineral complex of bismuth and by varying the temperature of synthesis, differ in different contents of metallic Bi and have different degrees of defectiveness of the α-Bi2O3 crystal lattice. It is established that isothermal annealing of α Bi2O3/Bi compositions in air in the range of 400–600°C leads to relaxation of defects and an increase in the volume of the α-Bi2O3 crystal lattice. An increase in the annealing temperature to 550°С is accompanied by an increase in the catalytic activity of α-Bi2O3/Bi. However, at higher temperatures, degradation of the heterostructure occurs as a result of the oxidation of metallic bismuth, and the catalytic activity of the composition decreases.

Colossal thermoelectric enhancement in Cu2+xZn1−xSnS4 solid solution by local disordering of crystal lattice and multi-scale defect engineering

Synergistic effects of Cu self-doping and hot forging of Cu2ZnSnS4 realize the decoupling of thermal and electric transport. And hot forging produces defects on different length scales and effectively optimize the electric and thermal conductivities.

Structural and electroresistive properties of layered compounds based on the 1-2-3 HTSC system and transition metal dichalcogenides under extreme external influences (review)

The problem of the influence of extreme external influences (high pressure, sharp temperature drops, structural relaxation, and strong magnetic fields) on various mechanisms of electric transport of HTSC compounds Re1Ba2Cu3O7-δ (Re = Y or another rare-earth ion) and dichalcogenides of transition metals are considered. The features of the crystal structure and the effect of structural defects of various morphologies on the electrical conductivity of these compounds in the normal, pseudogap, and superconducting states are discussed. A review of the experimental data obtained in the study of the effect of high hydrostatic pressure and other extreme effects on various mechanisms of electric transport of Re1Ba2Cu3O7-δ compounds of various compositions and transition metal dichalcogenides of various technological backgrounds is carried out. Various theoretical models devoted to the effect of high pressure on the electrical conductivity of HTSC compounds of the 1-2-3 system and transition metal dichalcogenides are discussed, and a comprehensive comparative analysis of their magnetoresistive characteristics under extreme external influences is performed. In particular, it was shown, that the relatively weak effect of pressure on the Tc value of optimally doped samples can be explained within the framework of a model assuming the presence of a Van Hove singularity in the spectrum of charge carriers which is characteristic of strongly coupled lattices. This is confirmed by the observation similar features of the behavior of the baric derivatives dTc/dP depending on the change composition in NbSe2 single crystals, which also belong to systems of two-dimensional lattices and have a similar anisotropy parameter. Nevertheless, it is still possible to formulate a number of questions that have not yet found a final experimental and theoretical solution. Namely, what is the role of the crystal lattice and structural defects and, in particular, twinning planes? What is the reason for the broadening of the resistive transitions of HTSC compounds into the superconducting state under pressure, and what is the relationship between this broadening and charge transfer and the nature of the redistribution of the vacancy subsystem? What is the role of phase separation in the implementation of different modes of longitudinal and transverse transport? Obviously, more research, both experimental and theoretical, is needed to answer these questions.

Research progress of hydrogen/helium effects in metal materials by positron annihilation spectroscopy

An important feature of the irradiation process in nuclear system is the formation of large displacement cascades, in which primary knock-on atoms and secondary particles formed by nuclear reactions generate a considerable number of defects such as dislocations, vacancies and transmutation gases. Predicting and mitigating the adverse effects of damage defect and transmutation hydrogen/helium produced by high-dose neutron irradiation on the mechanical properties of structural materials is the most significant challenge facing the current development of nuclear energy. To solve this problem, understanding the interaction mechanism between hydrogen/helium atoms and micro-defects is a very important breakthrough. Precursors of helium/ hydrogen bubble, small helium/hydrogen-filled vacancy complexes, may play an important role in realizing bubble nucleation, and the formation of these complexes is affected by many factors. However, only a little information about helium/hydrogen-vacancy clusters’ behavior has been obtained in metal/alloy materials. This is mainly limited by the characterization methods, such as the limited resolution of transmission electron microscope (TEM). Helium/hydrogen-vacancy clusters cannot be observed by TEM before the formation of helium bubbles. Applications of positron annihilation to the study of crystal lattice defects started around 1970s, when it was realized that positron annihilation is particularly sensitive to vacancy-type defects and that annihilation properties manifest the nature of each specific type of defect. In recent years, with the continuous development of slow positron beam and the improvement of various experimental testing methods based on slow positron beam, the application of positron annihilation technology has been extended to the research field of hydrogen/helium behavior in metal materials, which plays an important role in studying the hydrogen/helium radiation damage to metal materials. In this review, the basic principles of positron annihilation spectroscopy are briefly discussed and the three most important measurement methods used for hydrogen/helium effect studies are described (i.e. positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS), coincidence Doppler broadening spectroscopy (CDBS)). In this paper, the application of positron annihilation spectroscopy to the study of hydrogen/helium behavior in metal materials is reviewed in combination with the reported relevant developments (including our research group’s achieve-ments). The advantages of three commonly used measurement methods in the following specific studies are highlighted: 1) The estimation of bubble size and concentration; 2) irradiation damage induced by hydrogen/helium; 3) the evolution behavior of irradiation-induced defects in the heat treatment process; 4) sy-nergistic effect of hydrogen and helium.

The comparison of Pr3+:LaF3 and Pr3+:LiYF4 luminescent nano- and microthermometer performances

In the present work, we make a comparison of Pr3+:LaF3 and Pr3+:LiYF4 luminescent nano- and microthermometer performances. We studied Pr3+:LaF3 nanoparticles, synthesized via co-precipitation method (further Pr3+:LaF3 (co-precipitation)), Pr3+:LaF3 nanoparticles, synthesized via hydrothermal method (further Pr3+:LaF3 (hydrothermal)), and Pr3+:LaF3 microparticles as well as Pr3+:LiYF4 nanoparticles, synthesized via hydrothermal method (further Pr3+:LiYF4 nanoparticles) and Pr3+:LaF3 microparticles. According to the X-ray diffraction, Pr3+:LaF3 (co-precipitation) and Pr3+:LaF3 (hydrothermal) nanoparticles are hexagonal-structured nanocrystals. Pr3+:LiYF4 nanoparticles are tetragonal-structured nanocrystals. The average diameters of Pr3+:LaF3 (co-precipitation), Pr3+:LaF3 (hydrothermal), and Pr3+:LiYF4 nanoparticles are 13.9, 19.4, and 33.3 nm, respectively. The Pr3+:LaF3 (co-precipitation) and Pr3+:LaF3 (hydrothermal) nanoparticles demonstrate broadband luminescence caused by crystal lattice defects (luminescence background). This luminescence background notably decreases the temperature sensitivity of these samples. The luminescent background removing procedure significantly complicates the signal processing procedure. Pr3+:LaF3 microparticles, Pr3+:LiYF4 nanoparticles, and Pr3+:LaF3 microparticles do not demonstrate this undesirable phenomenon. The absolute temperature sensitivity Sa of Pr3+:LiYF4 nanoparticles, Pr3+:LiYF4 microparticles, and Pr3+:LaF3 microparticles at 300 K are 0.0117 ± 0.0010, 0.0106 ± 0.0010, and 0.0102 ± 0.0012 K−1, respectively. Although the values of Sa are very close for these samples, the nanosized dimensionality of Pr3+:LiYF4 nanoparticles allows achieving high spatial resolution and expanding the fields of application of Pr3+:LiYF4 nanoparticles.

Enhanced Photocatalytic Activity of ZnS:Mn2+ Quantum Dots

The photocatalytic activity of Mn2+ doped ZnS quantum dots stabilized by glutathione capping agent in aqueous media was studied in situ by photoluminescence spectroscopy. The methylene blue was chosen as model organic dye for photodecomposition under excitation in near UV-range. It is shown that the rate of photodegradation of the methylene blue depends strictly on the concentration of the manganese activator ions and the presence of its own intrinsic defects in the host ZnS crystal lattice. It is found that the most effective photocatalyst are ZnS quantum dots doped with 0.25 at % of Mn2+ providing the total dye decolorization within 5 minutes.

Face–centered cubic p–type NiO films room–temperature prepared via direct-current reactive magnetron sputtering–Influence of sputtering power on microstructure, optical and electrical behaviors

High–quality p–type NiO films are crucial for NiO–based heterojunction solar cells. Herein, a series of face-centered cubic, p–type NiO films was prepared at room temperature through DC reactive magnetron sputtering. The increased sputtering power (SP) improves the film crystallization. The film's lattice strain is increased and a microstress transition from compressive to tensile stress occurs with increased SP. The film's lattice strain reduces the LO and 2LO phonon frequency. The blueshifted optical absorption edge with increased SP is caused by the improved film crystallization. The film's free hole concentration is slightly increased and subsequently reduced with increased SP. The former is mainly due to the increased number of the nickel vacancies, whereas the latter to the improved film crystallization that reduces the content of the nickel vacancies and interstitial oxygen. The hole mobility is strongly affected by the scatterings by the crystal lattice and point defects.

Dramatically improved electron transport performance by a deep triangular potential well in organic field-effect transistors

Overcoming the intrinsic properties of organic molecules to achieve high device performance, particularly for electron transport, is a challenge in the field of organic semiconductor device physics. Energy-band engineering in modern inorganic electronics has been introduced in organic electronics. Here, we utilized Al2O3 and PbO metal-oxide layers as dielectric modification layers and fabricated high-quality organic heterojunction films with large band bending and sharp interfaces. The large band bending indicated a deeper triangular potential well, which can confine more electrons and form a high carrier density organic two-dimensional electron gas. A sharp heterojunction interface can reduce interface carrier transport scattering and crystal lattice defects and can retain uniform interfacial electronic structures. These two features of organic heterojunctions are the key factors to realize energy-band engineering in organic electronics. By using these heterojunction films as active layers in organic field-effect transistors, the transistors exhibited a high electron mobility of approximately 8 cm2 V−1 s−1. Therefore, we demonstrated the feasibility and solutions of energy-band engineering in organic electronics, and provided a novel strategy to improve device performance.

Comparative Analysis of the Luminescence of Ge:Sb Layers Grown on Ge(001) and Si(001) Substrates

Comparative studies of the luminescence properties of Sb-doped Ge layers grown on Ge(001) and Si(001) substrates are carried out. It is shown that, in contrast to the case of Ge:Sb layers grown on Si, a considerable contribution to the photoluminescence signal from Ge:Sb/Ge(001) layers is made by indirect optical transitions. This fact is attributed to the longer charge-carrier lifetime in Ge:Sb/Ge homoepitaxial structures because of the lack of crystal-lattice defects related to the relaxation of elastic strains in such structures. It is shown that the experimentally observed significant increase in the contribution of direct optical transitions to the total photoluminescence signal with increasing doping level of Ge:Sb/Ge(001) layers results from an increase in the population of electron states in the Γ valley. At Sb concentrations much higher than the equilibrium solubility of Sb in Ge, partial electrical activation of the impurity is observed, and in this case, a profound effect on the emission properties of Ge:Sb layers grown on different substrates is produced by nonradiative-recombination centers, whose role can be played by clusters of impurity atoms.

Interactions of intrinsic defects formed in ZnO and their contribution to electric conductivity

The formation process of lattice defects in single crystals of ZnO and their contribution to electric conduction were investigated by means of electric conductivity measurements and positron annihilation lifetime spectroscopy (PALS). For the ZnO samples once heated at high temperatures between 1073 K and 1273 K, we observed defect-induced n-type conductivity by sample warming or light irradiation to the sample, suggesting that interstitial zinc atoms (Zni) and oxygen vacancies (VO) as shallow and deep donors, respectively, were formed by the high-temperature heating. After heat treatment at 1373 K, however, the conductivity arises only from thermal excitation by sample warming, demonstrating that only Zni contributes to the conductivity but VO does not. Based on the above observation and the results of PALS measurements for the samples heated at high temperature, we propose a mechanism of the n-type conduction in ZnO that carrier electrons are provided by interstitial zinc atoms as a result of electron-hole cancellation through interaction between zinc vacancies (VZn) and VO.

In-situ observation of hardmetal deformation processes by transmission electron microscopy. Part II: Deformation caused by tensile loads

In the 1st part of this article, hardmetal deformation processes caused by bending loads were examined in-situ by transmission electron microscopy. The major objective of this work is to examine hardmetal deformation processes in special thin hardmetal samples as a result of applying tensile loads in-situ directly in a transmission electron microscope with the aid of the push-to-pull method. Applying tensile loads to the samples results in the plastic deformation of the Co-based binder phase leading to the formation of different crystal lattice defects in the binder. Force-time and displacement-time curves recorded when loading the samples and maintaining the loads provide evidence for continuous processes of the formation and movement of crystal lattice defects, presumably dislocations, in the WC phase and Co-based binder leading to a high rate of the binder plastic deformation. After increasing the tensile loads up to a certain level leading to the severe plastic deformation of the binder phase, the samples suddenly fail as a result of the crack initiation and propagation at WC-Co interfaces. Presence of cobalt on the WC surface after the cracking suggests that the cracks propagate through the binder region adjacent to the interface rather than through the interface itself.

In-situ observation of hardmetal deformation processes by transmission electron microscopy. Part I: Deformation caused by bending loads

WC-based hardmetals have a unique combination of different properties including high hardness, fracture toughness, abrasion resistance, strength, fatigue resistance, etc. Such properties are achieved due to extraordinary features of tungsten carbide, which is characterized by a certain degree of plastic deformation before failure when loading. The major objective of this work is to examine hardmetal deformation processes leading to the formation and movement of crystallographic defects in a hardmetal lamella as a result of its in-situ bending directly in a transmission electron microscope. The deformation is found to result in the formation of different crystal lattice defects in the Co-based binder and WC grains. Mainly dislocations form in the tungsten carbide grains. The dislocations start moving when increasing the applied bending load. The deformation of the binder phase is found to result in the formation of mainly lamellae and stacking faults in the Co crystal lattice. As a result of the formation, interaction and movement of the crystal lattice defects in the WC phase and binder phase a significant rate of plastic deformation of the hardmetal lamella was achieved under bending loads without its breakage.