Impurities In Crystals Research Articles

Impurity effects in Debye–Waller factors of bcc crystals based on anharmonic correlated Einstein model

Impurity effects in Debye–Waller factors (DWFs) describing thermodynamic properties of bcc impure crystals included in X-ray absorption fine structure (XAFS) have been studied based on the anharmonic correlated Einstein model. The impurity is obtained by replacing absorber of host element by an atom of doping element. Analytical expressions of DWFs presented in terms of cumulant expansion up to the third-order and thermal expansion coefficient of impure crystals have been derived. Anharmonic effective potential of impure crystal includes interactions of absorber and backscatterer atoms with their first shell near neighbors. Morse potential is assumed to describe single-pair atomic interaction. The obtained expressions for impure crystal can also be used for calculating the considered XAFS quantities of pure material based on replacing all data of the doping atoms by those of pure host element. The advantage of using the anharmonic effective potential is shown by its possibility of defining the difference of XAFS quantities between the two inverse doping processes, which cannot be obtained by using the single-pair potential. Numerical results are found to be in good agreement with experiment for the impure Fe doped by Mo or inversely for Mo doped by Fe, as well as for pure Fe and Mo.

Unusual Oxygen Partial Pressure Dependence of Electrical Transport of Single-Crystalline Metal Oxide Nanowires Grown by the Vapor-Liquid-Solid Process.

In general, the electrical conductivities of n-type semiconducting metal oxide nanostructures increase with the decrease in the oxygen partial pressure during crystal growth owing to the increased number of crystal imperfections including oxygen vacancies. In this paper, we report an unusual oxygen partial pressure dependence of the electrical conductivity of single-crystalline SnO2 nanowires grown by a vapor-liquid-solid (VLS) process. The electrical conductivity of a single SnO2 nanowire, measured using the four-probe method, substantially decreases by 2 orders of magnitude when the oxygen partial pressure for the crystal growth is reduced from 10-3 to 10-4 Pa. This contradicts the conventional trend of n-type SnO2 semiconductors. Spatially resolved single-nanowire electrical transport measurements, microstructure analysis, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations reveal that the observed unusual oxygen partial pressure dependence of the electrical transport is attributed to the intrinsic differences between the two crystal growth interfaces (LS and VS interfaces) in the critical nucleation of the crystal growth and impurity incorporation probability as a function of the oxygen partial pressure. The impurity incorporation probability at the LS interface is always lower than that at the VS interface, even under reduced oxygen partial pressures.

Specific Features of the Electron Spin Resonance of an Iron Impurity in HgSe Crystals

The electron-spin-resonance spectra of transition metal ions Fe in the HgSe matrix are analyzed. The spectra have an appearance typical of the Fe3+ ion, i.e., they consist of five fine-structure lines with a splitting that corresponds to a weak crystal field. The spectra are observed at temperatures of up to 50 K. The spectral lines are deformed and have a Dyson shape. An analysis of the temperature dependences of the line amplitudes shows that a transition from paramagnetic ordering to ferromagnetic ordering occurs with decreasing temperature, with the Curie temperature T ≈ 7 K.

Electric Polarization in ErCrO3 Induced by Restricted Polar Domains

Electric polarization in ErCrO3 single crystals has been investigated in the temperature range of 5_370 K. Ferroelectric ordering has not been found in any of the directions. However, electric polarization induced by restricted polar domains of structural origin has been observed. These domains are formed in the crystal matrix near impurity Bi^{3+} ions partially substituting Er^{3+} ions during the growth of single crystals by the method of spontaneous crystallization using solvent Bi_2O_3. The restricted polar domains form the superparaelectric state. Hysteresis loops with remanent polarization, both along the c axis and in the [110] directions, have been observed below some temperatures T_fr (in the frozen superparaelectric state). The polarization exists up to certain temperatures, which depend on the applied electric field orientation with respect to the crystal axes and exceed significantly temperature T_N of magnetic ordering.

Reduced ionic effect and accelerated electro-optic response in a 2D hexagonal boron nitride planar-alignment agent based liquid crystal device

The presence of excess free-ion impurities in liquid crystals (LCs) gives rise to a number of problems in the electro-optical liquid crystal displays (LCDs), e.g., slow electro-optical responses and image sticking effects. Here we experimentally present that the two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheet can serve as a planar-alignment agent and as an ion-capturing agent at the same time in an electro-optic LC device. The 2D h-BN nanosheet is employed as a planar-alignment agent on one side of an LC cell, where the standard planar-aligning polyimide (PI) layer is used on the other side of the cell. The LC exhibits uniform planar-alignment in this h-BN/PI hybrid device. It is found that the free-ion impurities in the LC are significantly suppressed in this h-BN/PI hybrid cell compared to that in a standard PI/PI LC cell. The free-ion density is reduced in the hybrid cell due to the 2D h-BN nanosheet’s ion-capturing process. The reduction of ionic impurities results in an accelerated electro-optic response of the LC in the h-BN based hybrid cell—which may have potential application for faster electro-optic devices.

Crystal Structure and Thermal Expansion Coefficient of Cordierite Honeycomb Ceramics

Cordierite honeycomb ceramics are sintered under a range of sintering atmosphere and temperature conditions. The linear thermal expansion coefficient (CTE) is measured along the extrusion direction. The samples sintered in an enclosed crucible have a much lower CTE than those sintered in open air. The CTE decreases with increasing sintering temperature up to 1450 °C. The polymorphic phases, texture, amorphous phase, impurity crystals, and porosity are analyzed using scanning electron microscopy, HR‐TEM, porosimeter, and Rietveld refinement method. The sintered samples consist of two polymorphic phases: indialite (hexagonal symmetry) and cordierite (orthorhombic symmetry). Both the volume % of the cordierite phase and the crystal texture increase with increasing sintering temperature. The orthorhombic distortion of cordierite from the hexagonal lattice (indialite) increases with increasing sintering temperature. The characteristic six‐membered ring of the (Si,Al)‐O4 tetrahedra in the cordierite structure shows substantial changes in the bond length due to Al‐Si ordering. Amorphous phase and impurity crystal phases such as silimanite and spinel remain at temperatures as high as 1450 °C. The effect of the polymorphic phases, the degree of texture, amorphous phase, impurity crystal phases, and the orthorhombic distortion is discussed in relation to the CTE characteristics of honeycomb.

Развитие метода локальной симметрии в модели суперъячейки для кристалла с примесью

The symmetry aspects of the supercell (extended primitive unit cell) model of the crystal with point defect are considered: Wyckoff positions splitting and the change of the one-electron states classification over k-vector (Brillouin zone folding). The supercell choice has to be made in a such a way that the one-electron states at the valence band top and the conduction band bottom for the perfect crystal are reproduced. Wyckoff positions splitting in the supercell model allows one to consider the defective crystal using different site symmetry point groups and also without use of the point symmetry at all. The site symmetry method allows one to predict the real symmetry of the defective crystal. This is demonstrated in the calculations of Cu impurity in LiCl crystal (the cubic symmetry of the perfect crystal is maintained for defective crystal), Fe impurity in SrTiO3 crystal (the point defect lowers the symmetry from cubic to tetragonal) and interstitial iodine atom in CsPbI3 crystal (the experimentally found I_2^- dumbell structure is confirmed only in the calculations without point symmetry restriction).

Электрическая поляризация в ErCrO-=SUB=-3-=/SUB=-, индуцированная локальными полярными областями

AbstractElectric polarization in ErCrO_3 single crystals has been investigated in the temperature range of 5‒370 K. Ferroelectric ordering has not been found in any of the directions. However, electric polarization induced by restricted polar domains of structural origin has been observed. These domains are formed in the crystal matrix near impurity Bi^3+ ions partially substituting Er^3+ ions during the growth of single crystals by the method of spontaneous crystallization using solvent Bi_2O_3. The restricted polar domains form the superparaelectric state. Hysteresis loops with remanent polarization, both along the c axis and in the [110] directions, have been observed below some temperatures T _fr (in the frozen superparaelectric state). The polarization exists up to certain temperatures, which depend on the applied electric field orientation with respect to the crystal axes and exceed significantly temperature T _N of magnetic ordering. These temperatures correspond to the condition kT _fr ≈ E _A for activation barriers at the boundaries of the restricted polar domains.

A thick frame decreases the fracture toughness of veneering ceramics used for zirconia-based all-ceramic restorations

PurposeTo investigate the influence of firing condition and thickness of a yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) frame on the fracture toughness of veneering ceramics. MethodsLayering and pressable ceramics without frames or on 0.8- and 1.2-mm-thick Y-TZP frames were fired with 5 schedules (normal and 4 customized conditions). The fracture toughness of the ceramics was measured at 0.5–2.5mm from the frame by the indentation fracture method. Crystal structures at the measurement points were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). ResultsUnder customized firing conditions, the fracture toughness of the layering ceramic significantly decreased, particularly near the frame. Use of a 1.2-mm-thick frame significantly decreased the fracture toughness of both layering and pressable ceramics compared to ceramics without the frame. The decreased fracture toughness only occurred close (0.5 and 1.0mm) to the frame. XRD analysis showed identical diffraction patterns between points near and far from the frame, suggesting that the decreased fracture toughness was not due to crystal defects or impurities. SEM revealed many microcracks and large crystals in layering ceramic near the 1.2-mm-thick frame, possibly resulting from the thermal properties of the zirconia frame, such as large heat capacity and low thermal conductivity. ConclusionsModification of the firing conditions from the manufacturer’s instructions and inclusion of a thick Y-TZP frame decreased the fracture toughness of veneering ceramics at the interface, suggesting that a thick frame would pose a potent fracture risk in veneering ceramics used for zirconia-based all-ceramic restorations.

Nematic to smectic-A phase transition in mixture of liquid crystal and nonmesogenic impurities: existence of tricritical point

ABSTRACTExperiments on the mixture of liquid crystals and nonmesogenic impurities showed the significant role of nonmesogenic impurities on the nematic–smectic-A phase transition. Using both Flory–Huggins theory of isotropic mixing and Landau–de Gennes theory, we present a phenomenological theory that discusses the role of such impurities on the nematic–smectic-A phase transition in a mixture of smectic liquid crystal and nonmesogenic impurities. We discuss the impact of nonmesogenic impurities on the order parameters, Frank elastic constants (splay and bend) and transition temperature of the nematic–smectic-A phase transition. Our theoretical results show that there exists a tricritical point for which the second-order nematic–smectic-A phase transition becomes first order at a tricritical point by increasing the concentration of nonmesogenic solute. We find a remarkable agreement between theoretical and experimental results.

Investigation of oxygen and other impurities and their effect on the transparency of a Na flux grown GaN crystal

In this study, we investigate several impurities in bulk GaN crystals grown by the sodium flux technique. In our system, black GaN is in general associated with oxygen, sodium, and molybdenum concentrations in excess of 1019 atoms/cm3. Here, we report on the correlation of high levels of oxygen with opaque crystals. Samples that contain oxygen concentrations above 1019 atoms/cm3 are highly absorbing regardless of the presence of other impurities. Experiments with partially submerged seeds indicated material grown mainly above the nominal liquid surface was more transparent and had a generally lower impurity concentration, especially oxygen. In optically transparent regions, concentrations of oxygen, sodium, and carbon were as low as 7 × 1016, 2 × 1016, and 5 × 1016 atoms/cm3, respectively, while the concentration of molybdenum was below the detection limit.

Mechanisms and Control of Impurities in Continuous Crystallization: A Review

Crystallization has been applied to a broad range of industries such as bulk and fine chemicals and the pharmaceutical and food industries. It is important to strategically control the in situ purification process during crystallization to meet the regulatory and functional specifications of the crystals. While the control of the crystallization–purification process has been widely discussed for batch crystallizers, there has been little focus with the literature on controlling purification for continuous crystallizers. Continuous crystallization is a more intensified approach to crystallization, with lower capital footprint and potentially offering more consistent quality control. This review paper provides an in-depth discussion of the strategies and scientific understanding in controlling the crystallization–purification process in continuous crystallization. In particular, it describes how scientific understanding in the purification process, generated so far for batch crystallization, can be translat...

Transition levels of acceptor impurities in ZnO crystals by DFT-LCAO calculations

Large scale ab-initio calculations are carried out to study the charge state transition levels of nitrogen and phosphorus impurity defects in zinc oxide crystals using the DFT-LCAO approximation as implemented into the CRYSTAL computer code. It is shown that at a high concentration of defects (close location of defects) their formation energy is underestimated due to a significant delocalization of the charge within the supercell. After inclusion the energy offset correction and defect-defective interaction, the formation energy is improved, in a comparison with that calculated in a large supercell. The optical transition levels obtained by a direct calculation confirm the experimental observation: nitrogen and phosphorus impurities are deep acceptor centers with large formation energy in a charged state and, therefore, cannot serve as the effective source of hole charge. The obtained results are in good agreement with the previous theoretical work, in which other calculation methods were used, and are capable of qualitatively describing the energy characteristics of the charged defects.

Influence of Crystal Structure and 3d Impurities on the Electronic Structure of the Topological Material Cd3As2

This paper presents a theoretical study of the effects of crystal structure and Mn and Co substitutions for Cd on the electronic structure of the topological material Cd3As2. We have carried out density functional theory calculations of the band structure and density of states in tetragonal and cubic Cd3As2, as well as in Cd3 –xMnxAs2 and Cd3 –xCoxAs2 crystals. The results indicate that the band structure of the cubic Cd3As2 phase differs significantly from that of a Dirac semimetal, characteristic of the tetragonal phase. It has also been shown that, after Co substitution for 1/24 of the Cd atoms, the structure of the density of 3d electron states is similar to that of the density of states in the magnetic semiconductor Cd3 –xMnxAs2, with a characteristic minimum at the Fermi energy. At the same time, in the case of analogous Mn substitution for Cd, the density of d-electron states has no such minimum.

Deformation twinning in vanadium single crystals tested in compression at 77 K

Vanadium is the least studied non-magnetic body centered cubic metal whose mechanism of plastic deformation is still a subject of controversy. Depending on the level of purity and spatial distribution of interstitial impurities, it has been shown to deform by anomalous slip on the low-stressed (01̅1) plane or by twinning along {112} planes in the 〈111〉 direction. In order to resolve this controversy, we test high-purity vanadium single crystals of four orientations at 77 K under a low strain rate to 1–2% of the plastic strain. All our samples contain spatially homogeneous distribution of impurities and deform predominantly by twinning on two {112}〈111〉 systems in which the {112} planes are sheared in the twinning sense. The secondary surface markings correspond to slip on {110}〈111〉 systems with the highest Schmid factors, whereas weak (01̅1) slip is found mostly in the vicinity of twins. These results support the argument made previously by Thompson and Carlson (1965) that a spatially homogenenous distribution of interstitial impurities in high purity vanadium single crystals leads to predominant twinning.

One-Dimensional Atomic Segregation at Semiconductor–Metal Interfaces of Polymorphic Transition Metal Dichalcogenide Monolayers

Interface segregation is a powerful approach to tailor properties of bulk materials by interface engineering. Nevertheless, little is known about the chemical inhomogeneity at interfaces of polymorphic two-dimensional transition metal dichalcogenides (TMDs) and its influence on the properties of the 2D materials. Here we report one-dimensional monatomic segregation at coherent semiconductor-metal 1H/1T interfaces of Mo-doped WS2 monolayers. The monatomic interface segregation takes place at an intact transition metal plane and is associated with the topological defects caused by reflectionsymmetry breaking at the 1T/1H interfaces and the weak difference in bonding strength between Mo-S and W-S. This finding enriches our understanding of the interaction between topological defects and impurities in 2D crystals and enlightens a potential approach to manipulate the properties of 2D TMDs by local chemical modification and interface engineering for applications in 2D TMD electronic devices.

Time-Resolved Photoluminescence Microscopy Combined with X-ray Analyses and Raman Spectroscopy Sheds Light on the Imperfect Synthesis of Historical Cadmium Pigments.

Recent studies have shown that modern pigments produced after the Second Industrial Revolution are complex systems characterized by a high level of heterogeneities. Therefore, it is fundamental to adopt a multianalytical approach and highly sensitive methods to characterize the impurities present within pigments. In this work we propose time-resolved and spectrally resolved photoluminescence (PL) microscopy for the mapping of luminescent crystal defects and impurities in historical cadmium-based pigments. PL analysis is complemented by X-ray diffraction, X-ray fluorescenceand Raman spectroscopies, andbyscanning electron microscopy to determine the chemical composition and crystal structure of samples. The study highlights the heterogeneous and complex nature of historical samples that can be associated with the imperfect manufacturing processes tested during the period between the 1850s and 1950s. The results also allow us to speculate on a range of synthesis processes. Since it is recognized that the stability of paints can be related to pigments synthesis, this research paves the way to a wider study on the relationship between synthesis methods and deterioration of cadmium pigments and paints. This rapid and immediate approach using PL can be applied to other semiconductor pigments and real case studies.

Distribution of light-element impurities in Si crystals grown by seed-casting method

We investigated the distributions of interstitial oxygen (Oi) and substitutional carbon (Cs) in high-performance (HP) multicrystalline Si (mc-Si) and monocrystalline-like Si (mono-like Si) and compared them with those in conventional mc-Si, grown using the same furnace. The Oi concentration in mono-like Si grown using a Czochralski (Cz) silicon seed was the highest among the three crystals. On the other hand, the Oi and Cs concentrations in HP mc-Si grown using Siemens Si incubation seeds were the same as those in conventional mc-Si. Therefore, it is considered that Oi incorporated into the growing Si crystal originates not only from the quartz crucible wall but also from the seed. Additionally, Oi and Cs in HP mc-Si grown on the incubation seeds with adequately low Oi and Cs concentrations are distributed similarly to those in conventional mc-Si grown under the same conditions. We believe that it is important to consider the Oi and Cs concentrations in the feed stock materials both for the seed and whole ingots in the seed-casting method.

Nonlinear Thermodiffusion in Gases at Moderate Temperatures

Abstract The paper discusses the properties of the nonlinear thermodiffusion equation corresponding to the diffusion processes, which occur with a finite velocity. In the previous papers, A. J. Janavičius proposed the nonlinear diffusion equation with the diffusion coefficient directly proportional to the concentration of impurities. This equation provides a more exact description of the profiles of impurities in Si crystals. The heat transfer in gases carries a greater average kinetic energy based on nonlinear diffusion of gas molecules from hot regions to the coldest ones with a finite velocity by random Brownian motions. In this case, the heat transfer in gases can be described by using nonlinear thermodiffusion equation with heat transfer and thermodiffusion coefficients directly proportional to temperature T. The obtained approximate analytical solutions are successfully applied in defining temperature profiles and heat transfer coefficients in gases as well as providing opportunities for practical applications. It has been concluded that heat spreading in gases depends on temperature differences and pressure in gases.

Induced Photonic Response of ZnO Nanorods Grown on Oxygen Plasma-Treated Seed Crystallites.

We examined the influence of O2 plasma treatment for the ZnO seed layer (SL) crystallites on the material characteristics of ZnO nanorods (NRs) synthesized by the hydrothermal method. Diode photocurrent and photo-response transient characteristics of the p-Si/n-ZnO-NR heterojunction-based ultraviolet (UV) photodetectors were also examined according to the plasma treatment for the SLs. The superior optical properties of NRs were measured from the photoluminescence by exhibiting 4.6 times greater near-band edge emission when grown on the O2-plasma-treated SL. The degree of (002) orientation of the NR crystals was improved from 0.67 to 0.95, as revealed by X-ray diffraction analysis, and a higher NR surface density of ~80 rods/μm2 with a smaller mean diameter of 65 nm were also observed by the SL modification using plasma-treatment. It was shown by X-ray photo-electron spectroscopy that this improvement of NR crystalline quality was due to the recovery of stoichiometric oxygen with significant reduction of oxygenated impurities in the SL crystals and the subsequent low-energy growth mode for the NRs. UV PDs fabricated by the proposed SL plasma treatment technique showed significantly enhanced UV-to-dark current ratio from 2.0 to 83.7 at a forward bias of +5 V and faster photo-response characteristics showing the reduction in recovery time from 16 s to 9 s.