Impurities In Crystals Research Articles

Braking indices of young radio pulsars: theoretical perspective

ABSTRACT Recently, Parthsarathy et al. analysed long-term timing observations of 85 young radio pulsars. They found that 15 objects have absolute values of braking indices ranging ∼10–3000, far from the classical value n = 3. They also noted a mild correlation between measured value of n and characteristic age of a radio pulsar. In this article, we systematically analyse possible physical origin of large braking indices. We find that a small fraction of these measurements could be caused by gravitational acceleration from an unseen ultra-wide companion of a pulsar or by precession. Remaining braking indices cannot be explained neither by pulsar obliquity angle evolution, nor by complex high-order multipole structure of the poloidal magnetic field. The most plausible explanation is a decay of the poloidal dipole magnetic field which operates on a time-scale ∼104−105 yr in some young objects, but has significantly longer time-scale in other radio pulsars. This decay can explain both amplitude of measured n and some correlation between n and characteristic age. The decay can be caused by either enhanced crystal impurities in the crust of some isolated radio pulsars or, more likely, by enhanced resistivity related to electron scattering off phonons due to slow cooling of low-mass neutron stars. If this effect is indeed the main cause of the rapid magnetic field decay manifesting as large braking indices, we predict that pulsars with large braking indices are hotter in comparison to those with n ≈ 3.

Sol-Gel Synthesis of Spherical Mesoporous High-Entropy Oxides.

High-entropy oxides (HEOs) have attracted increasing interest owing to their unique structures and fascinating physicochemical properties. Spherical mesoporous HEOs further inherit the advantages of spherical mesoporous materials including high surface area and tunable pore size. However, it is still a huge challenge to construct HEOs with uniform spheres and a mesoporous framework. Herein, a wet-chemistry sol-gel strategy is demonstrated for the synthesis of spherical mesoporous HEOs (e.g., Ni-Co-Cr-Fe-Mn oxide) with high specific surface area (42-143 m2/g), large pore size (5.5-8.3 nm), unique spherical morphology (∼55 nm), and spinel structure without any impure crystal phase using polyphenol as a polymerizable ligand. The metal/polyphenol-formaldehyde resin colloidal spheres are first synthesized via a sol-gel process. Because of their abundant catechol groups and strong chelating ability with different metal species, polyphenols can not only accommodate five different metal ions in their networks but also be well polymerized by formaldehyde to form colloidal spheres. After calcination, the metal species aggregate together to form HEOs, while the organic resin is fully decomposed to produce mesopores. Because of the open framework with accessible mesopores, they could be used as a peroxymonosulfate catalyst for degradation of organic pollutants and a nanoplatform for efficient detection of DNA. This work demonstrates a straightforward sol-gel strategy for design and synthesis of spherical mesoporous high-entropy materials, which would promote the exploration of new properties of high-entropy materials and extend their application.

Quantification of Hydrogen in Natural Diamond by Secondary Ion Mass Spectrometry (SIMS)

The volumetric concentration of hydrogen in two Brazilian diamonds is determined using secondary ion mass spectrometry and implantation of hydrogen into an external standard sample (with a dose of 1 × 16 at/cm2 and energy of 120 KeV). The diamonds studied differ noticeably in their intensities of IR-active hydrogen from 0 to 1.5 cm–1 according to the analyses of their IR spectra. The results demonstrate that for both samples studied, the volumetric concentration of hydrogen does not exceed the reached detectable level of (1–2) × 1018 at/cm3 or 1.7–3.3 at. ppm; i.e., it is lower by an order of magnitude than in the early chemical analysis and by 2–3 orders of magnitude lower than the results of the ion-beam spectrochemical, nuclear-physical, and ERDA analyses. Only a part of the hydrogen forms optically active impurities in diamond crystals and can be determined by spectral methods.

Effect of Structural Defects and Impurities on the Excited State Dynamics of 2D BA2PbI4 Perovskite

AbstractIn this work, we show that the quality of the precursor and the thin film preparation strongly affect the optoelectronic properties of the 2D perovskite BA2PbI4. 2D perovskites with alkylammonium organic cations such as butylammonium (BA) are relatively soft structures that exhibit large dynamic disorder and phase variations. Here we show, by a variety of spectroscopy techniques (steady state absorption, photoluminescence and ultrafast transient absorption), that at temperatures below the phase transition (253 K) the material exhibits excitonic features from the room temperature phase (due to incomplete structural transition) and a broadband emission at 560–600 nm (due to self‐trapped excitons) with varied relative intensities depending on the precursors and processing conditions. This suggests that the processing conditions have a large influence on the crystallization and introduction of extrinsic defect impurities directly affecting the optoelectronic properties. Making absolute statements about the properties of BA2PbI4 requires improved control over the materials thin film deposition and a better understanding of the role of the lattice vibrational dynamics and extrinsic defects on the exciton dynamics.

Spin echo studies on Fe3+ ions in GaN: Spin-phonon relaxation and ligand hyperfine interactions

The electron spin echo detected inversion recovery technique at the Q-band frequency was used to characterize spin diffusion effects in spin-lattice relaxation of compensating Fe3+ impurities in n-type doped GaN crystals. It was found that the selective saturation can be achieved in the GaN:Fe3+ system due to magnetization transfer based on the spin flip-flop cross-relaxation processes. The temperature dependence of 1/T1 can be explained by direct spin-phonon processes (∼T) below 25 K and by Raman two-phonon processes (∼T9) at higher temperatures. Spin diffusion in this system is characterized by an additional cross-relaxation rate which is weakly temperature-dependent below 25 K. The transferred hyperfine interactions of Fe3+ centers with gallium and nitrogen neighbor nuclei were resolved using pulsed-electron nuclear double resonance. A comparative analysis of quadrupole interactions indicates the essential increase in the electric field gradients on the nearest nitrogen and gallium shells.

Electronic and Transport Properties in Defective MoS2: Impact of Sulfur Vacancies

Crystal impurities, such as atomic vacancies, are known to modulate the charge transport characteristics of two-dimensional (2D) materials. Here, we apply a first-principles-enriched tight-binding ...

Ge incorporation in gallium oxide nanostructures grown by thermal treatment

Effective doping of semiconductor oxide nanostructures is needed to control n-type conductivity; however, out-diffusion of impurities in high quality crystals is still a bottleneck in nanotechnology. Here, germanium-doped gallium oxide (β-Ga2O3) nanowires have been grown by a thermal evaporation method. Two temperatures, close to Ge melting point, were chosen for the dynamic annealing of metallic gallium and germanium powders. At 840 °C, Ga2O3 nanowires and nanobelts grow along the b or c direction, leaving gallium germanate particles at the tip in both cases. Wider structures were produced at 940 °C, decorated with a Ge nanocrystal at the end. Transmission electron microscopy has been used to establish the vapor–liquid–solid-like growth process for the formation of these structures. The influence of Ge incorporation in the nanostructures has been assessed by cathodoluminescence and micro-Raman spectroscopy. The presence of Ge in β-Ga2O3 nanostructures is essential for electronic and optoelectronic applications since Ge is potentially the most effective n-type dopant for β-Ga2O3.

Charge stability of nitrogen-vacancy color centers in organic nanodiamonds

The negatively-charged nitrogen-vacancy (NV) color center in diamond undergoes stochastic charge state transitions between the negatively charged state (NV−) and the neutral charged state (NV o ) upon optical illumination. While the negative charge state is normally preferred for magnetic sensing, optically-controlled switching between charges states is often desirable, for example in super-resolution imaging. The concentration of electron donor impurities in (bulk/nano) diamond crystals determine how much optical control can be exercised over the NV− and NV o charge states. Here we report how the growth speed of nanodiamonds (NDs) can control the concentration of substitutional nitrogen (P1) donors, ranging from highly pure to highly doped diamond. Hence by growth temperature, it is possible to tune the stability of the NV charge state to optimally match the intended application. This work has many promising bio-sensing applications, especially for super-resolution magnetic-sensing with the NV color center.

Convective heat and mass transfer modeling under crystal growth by vertical Bridgman method

This paper presents the results of numerical modeling by the finite element method (FEM) of convective heat and mass transfer during the growth of single crystals by the vertical Bridgman method with a submerged heater. Numerical calculations were performed using the implicit matrixless finite element method based on the iterative process of conjugate gradients and significantly reducing the requirements for RAM and computer speed. The effects of gravity, rotation, crystallization rate and vibration on heat and mass transfer in the melt, the geometry of the crystallization front, and the thickness of the boundary layers were studied. It is shown that the above effects can be effectively used to control the distribution of impurities in crystals grown by the vertical Bridgman method.

Numerical analysis of thermohydraulic behavior in a directional solidification furnace

In this study, the crystallization process in a directional solidification furnace is studied by using the transient numerical simulation. The flow field in melt silicon in conjunction with different solidification stages is investigated. A separated side-heater design is proposed, showing a pronounced improvement on temperature gradient in axial direction normal to the solidification interface at late period. Result shows that the Voronkov ratio which can reflect the crystal quality is reduced by 16.5–24.8% and the energy consumption can be saved by 6% simultaneously. On the other hand, the airflow management for argon gas in the crucible chamber, which contains advantage to reduce chemical impurity in crystal during solidification process, is also numerically studied. By installing a guiding plate beneath the graphite cover can appreciably lower the oxygen mass fraction in contact with the cover plate and reduce the potential chemical reaction accordingly. The plate diameter and the spacing between the plate and graphite cover are optimized. Compared to the original design, the oxygen mass fraction at the graphite cover can be pronouncedly reduced by 10.6–33.3%, which can be further improved by combining a corner outlet configuration. Meanwhile, under the same volumetric flow rate of argon gas, yet the argon gas could be saved by almost half (e.g., from 40 SLM to 20 SLM) for the same chemical concentration with the optimized configuration.

Material Impurity Distribution of Lattice-Incorporated Impurities in Salicylic Acid

The mechanisms of purging structurally similar impurities in solution crystallization have been evaluated using the model compound salicylic acid. Of the 11 added impurities, 3 showed appreciable e...

Bistable behavior of the nitrogen impurity in SiC nanoclusters.

Nitrogen doped SiC nanoclusters of various shapes and polytype structures were modeled in the framework of density functional theory using the PBE0 hybrid functional. While in the bulk crystal NC impurity may be introduced in a large concentration and shows features of the conventional shallow effective-mass donor, in the nanocluster this defect can exist in two forms with sharply different electronic structures. The first one resembles the shallow donor confined in the nanocluster with a small and nearly isotropic lattice relaxation, and spin density spread over distant shells. The second form resembles a small bound polaron structure with a large and anisotropic lattice relaxation, which leads to a strong localization of the spin density on one of the nearest silicon atoms. Relative energies of these two states depend on the location of nitrogen in the lattice as well as on the shape and polytype structure of the nanocluster. The calculations show that the position of the impurity in the vicinity of the hexagonal double layers and cluster surface facilitates the formation of small-bound-polaron-like states. As follows from the calculations, the two impurity states can be experimentally distinguished by the values of the hyperfine and quadrupole coupling parameters that can be measured in magnetic resonance experiments. They are also characterized by rather different electronic absorption spectra. We have demonstrated that for negatively charged SiC nanoclusters the polaron-like impurity states can be formed even if they do not exist in neutral clusters. This provides a reason to believe that the nitrogen impurity occupying particular sites in SiC nanoclusters is an analog of the DX centers in the bulk crystal. In addition, the formation of the polaron-like states depends on the method of the surface passivation. In particular, the substitution of the Si-H surface bond with the Si-NH3 one facilitates a bistable behavior of the NC impurity located in the central part of the nanocluster. We have also shown that the effect of the self-purification manifests itself in different ways depending on the nanocluster shape and polytype structure.

Enhancing impurities removal from Si by controlling crystal growth in directional solidification refining with Al–Si alloy

In this paper, the principle of “plain materials” was attempted to advancing silicon (Si) purity, i.e., controlling solid-liquid interface and Si crystal growth pattern to enhance impurities removal from Si in directional solidification refining with Al–Si alloy. The relationship between Si crystal growth and impurity removal was established, and it was found that the removal ratio of metal impurities (less than 95% to more than 99%) gradually decreased with the order evolution of Si crystal from bulk Si, porous Si, acicular Si to Al–Si eutectic alloy. More than 99.5% of metallic impurities could be removed in the case of bulk silicon. However, the removal of boron (B) and phosphorus (P) showed the opposite trend. According to the principle of constitutional supercooling, the crystal Si growth and interface morphology were evaluated based on the conservation of mass of silicon atoms at the solidification interface, which was also used to explain the mechanism of impurities removal. Control of silicon crystal growth was achieved by changing the temperature gradient (lowering rate), Si alloy composition (Si content) and the heat transfer mode of solid-liquid interface that was expressed in terms of the effective heat transfer area ratio between wall and bottom of crucible (LA). A more stable and flat liquid-solid interface and 45% bulk Si was obtained at a lowering rate of 0.05 mm/min when the value of LA was 0.1, which can greatly enhance the removal of impurities.

Preparation and electrochemical properties of LiFePO4/C-Li4Ti5O12 composites

LiFePO4/C-Li4Ti5O12 composite electrode materials were synthesized by blending LiFePO4/C and Li4Ti5O12 materials then calcining at 500 °C for 14 h. The effects of different composite ratios on the phase structure and electrochemical properties of LiFePO4/C-Li4Ti5O12 composite cathode materials were investigated. The results showed that the structure of LiFePO4/C and Li4Ti5O12 in the composite prepared by the simple blending method did not change. XRD shows only the characteristic peaks of these two substances and no characteristic peaks of other crystal impurities. When the recombination ratio was 8:2 (LPF-8), the obtained composite material had fine particles and uniform size. The initial discharge capacity at 0.1C rate is 151.56 mAh g−1 at the voltage of 1.0–4.2 V. After 100 cycles, the capacity retention rate was 97.3% which exhibited more outstanding electrochemical performance than samples with other composite ratios.

Effects of Na2S treatment and post-annealing on Sn-rich Cu2ZnSnS4-based thin film solar cells

In this study, chemical etching by Na2S and annealing under nitrogen atmosphere were combined to remove the secondary phases, which precipitated in the CZTS films produced by sulfurizing the metal-alloy precursors. Both treatments indicate positive outcome for removing SnS2 impurity crystals precipitated on the surface of the CZTS thin films. In addition, both treatments were found effective in dissolution of Sn-based secondary phases mainly from Sn-rich CZTS. By Na2S etching, spectral response at short wavelength of the thin film solar cell is enhanced, while that of the wavelength region of 500–700 nm is degraded. On the other hand, post-annealing improved the spectral response in this wavelength region, and subsequent etching did not degrade the spectral response. By applying an annealing treatment followed by Na2S etching, the spectral response was improved in both short and long wavelength regions and the photovoltaic conversion efficiency over 4% was realized. This treatment process was especially effective in the Sn-rich CZTS absorption layer, and hence open circuit voltage, short circuit current, and fill factor were greatly enhanced compared with the other treatments procedure.

Synthesis of least defect NaY membranes: Defining optimum heating and cooling rate of hydrothermal treatment (HT) and application of zeta potential measurement via different seeding methods

Zeolite membrane is one type of microporous inorganic membrane which has gained so much interests among researchers as an alternative especially for gas separation. Among major challenges to produce high quality zeolite membranes with less defects are to have a good seeding and hydrothermal treatment (HT) method. On this matter, a study on the colloidal stability of the zeolite NaY seeds in the seeds solution in order to obtain homogenous seeds layer on top of the alumina support via several seeding methods and pHs of the seeds solutions were investigated. Then, the seeded samples were taken into HT process in order to form zeolite NaY membranes. During this stage, heating and cooling rates ( o C/hr) were varied in order to study their effects towards formation of zeolite membrane with least defects. Characterizations were performed by using X-Ray Diffraction (XRD) in order to identify degree of crystallization and amount of impurities while Scanning Electron Microscopy (SEM) was used in order to study surface morphology of the samples. From this study, it was found that densification effect from different seeding methods played important factor in forming zeolite membranes with least defects. Accordingly, suitable heating and cooling rates were required in order to optimize the growths of zeolite NaY seeds to form zeolite layer on top of the alumina support. In this case, due to thermal mismatch between the substrate and zeolite material, it could form some cracks on top of the alumina support. Thus, by implementing optimum heating/cooling rate, a zeolite NaY membrane with least amount of defects i.e. cracks and pinholes could be achieved with less degree of impurities i.e. zeolite NaP during HT process.

Peculiarities of the Dielectric Spectra of Triglycine Sulfate Crystals with a Profile Distribution of Chromium

The temperature evolution of the dielectric spectra of TGS–TGS + Cr crystals (TGS stands for triglycine sulfate) with a periodic distribution of chromium impurity (Cr3+) has been studied in comparison with nominally pure TGS crystals in the frequency range from 10 to 107 Hz upon heating and cooling. It is shown that impurity crystals are characterized by the absence of dielectric dispersion in both measurement modes. An approach is proposed to explain the peculiarities of the dielectric dispersion of crystals with a periodic distribution of chromium impurity from the standpoint of mobile activity of certain elements of the TGS crystal structure.

Optimization of reaction parameters affecting crystal phase growth and purity of BaCeO3 and BaCe1-xYbxO3-δ nanopowders and investigating high protonic conductivity of sulfonated poly(ether ether ketone) – BaCe0.85Yb0.15O3-δ composite membrane

Pure and ytterbium-doped BaCeO3 nanostructures were synthesized by solid-state reaction with the mixtures of Ba(NO3)2, BaCO3, (NH4)2Ce(NO3)6, and Yb2O3 at 800 °C for 10 and 24 h. Doping of ytterbium ions in the BaCeO3 host matrix has been studied and confirmed using powder X-ray diffraction. The results from the Rietveld analysis indicated that the sample has a main BaCeO3 structure with the space group of [Formula: see text]. Through intensive experiments and analysis, optimum reaction conditions for the synthesis of doped nanoparticles including the crystal phase impurity and reaction time are proposed. The results of the study showed that for the reaction time of 24 h, BaCO3 reacted more effectively with (NH4)2Ce(NO3)6 than Ba(NO3)2 did. On the other hand, the purity values of 97% and 95% were obtained for pure and Yb3+ doped BaCeO3 samples, respectively. Field emission scanning electron microscope images revealed that the synthesized BaCeO3 nanomaterials have mono-shaped sphere morphology. Furthermore, ytterbium-doped nanoparticles were incorporated into the matrix of sulfonated poly(ether ether ketone) (SPEEK) membranes (SPYb) with the aim of enhancing proton conductivity. The prepared SPYb nanocomposite membrane containing 1.7 wt.% of BaCe0.85Yb0.15O3-δ nanoparticles exhibited a high proton conductivity (67 mS/cm) at 80 °C.

Analysis of electron and phonon heat capacities of La2−xSrxCuO4 cuprate superconductor

Adopting the many body quantum dynamical Green's function (GF) theory via a versatile Hamiltonian that incorporates the contribution due to harmonic electron and phonon fields and their interactions, anharmonicities and point impurities; the expressions for phononic heat capacity (PHC) and electronic heat capacity (EHC) have been obtained using the method of density of states of phonons and electrons (PDOS and EDOS). The theory is applied to the high temperature superconductor (HTS) La2−xSrxCuO4 for successful numerical estimation. The expressions of PDOS and PHC can be separated into diagonal and non-diagonal components. The non-diagonal contribution emerges only in the impure crystals and becomes extinct in pure crystals. The presence of electron-phonon (ep) coupling constant in each term of EHC is an additional and unique feature of present formulation. The results are compared with the experimental data of heat capacity for La2−xSrxCuO4 superconductor and results are found in good agreement. The electronic contribution is quite small in the comparison to the phononic contribution to total heat capacity. The doping concentration and ep coupling constant dependence of the PHC and EHC independently are investigated for La2−xSrxCuO4 crystal and observed that orderly increases with doping concentration and ep coupling constant.

Synthesis of CO[formula omitted]-adsorbing ZSM-5 zeolite from rice husk ash via the colloidal pretreatment method

The novel ZSM-5 zeolite was successfully prepared by the colloidal pretreatment method and using rice husk ash (RHA) and pseudo-boehmite as raw materials of silicon and aluminum. A series of samples (ZSM-5-X) with various colloidal silica usage quantity were characterized by XRD, FTIR, SEM and nitrogen adsorption–desorption methods, and the results indicated that the method can reduce the formation of impurity crystals and increase the micropore volume. The CO2 adsorption measurement showed that the maximum adsorption amount of the samples was 55.01 cm3/g (ZSM-5-25) at 0 °C and 23.08 cm3/g (ZSM-5-75) at 90 °C. The Langmuir, Freundlich and Sips isotherm models were used to fit gas adsorption experimental data and analyze adsorption mechanisms. The three-parameter Sips model with a heterogeneous coefficient was suitable for simulating CO2 adsorption, and the correlation coefficient of the Langmuir model was close to 1 for fitting N2 and O2 adsorption. Higher CO2/N2 and CO2/O2 adsorption selectivity and stable adsorption–desorption regeneration performance have been demonstrated, and it indicated that the adsorbent had a promising application in the adsorption separation of CO2 from industrial flue gas.