Crystal Imperfections Research Articles

Impact of the host lattice on crystallographic, Raman, optical, and luminescent properties on the terbium ion

AbstractA comparative study of the Gd2O3:Tb and Y2O3:Tb nanoparticles (NPs) was presented to examine the influence of the host lattices on the crystallographic phase, surface characteristics, optical properties, Raman effect, and photoluminescence properties. Cubic shapes Gd2O3:Tb and Y2O3:Tb NPs with an estimated crystalline size of 17 and 11 nm were recorded from the X‐ray diffraction pattern, respectively. Comparative absorption spectra of both oxides NPs were presented to examine the dispersibility, colloidal stability, and optical behavior of the NPs. Energy bandgap values were estimated to monitor the optical characteristics of the metal oxides in the UV/Vis region. Fourier transform infrared was recorded to confirm the surface‐attached water and organic moieties. The Raman spectra of metal oxides were recorded to examine the symmetrical dis‐order and polarizability of the vibrational bands. The ionic radius of the atoms distorts the bond structure resulting in it greatly influenced the vibrational bands of the materials. The emission spectra certified the effective doping of the luminescent Tb3+‐ion in the metal oxide host lattices. Raman spectra and emission spectra validated the crystal symmetry imperfections; subsequently, the efficiency of the Raman active modes and emission transitions was greatly influenced. In a comparative analysis, Y2O3:Tb NPs exhibited higher luminescence intensity in comparison with the Gd2O3:Tb NPs.

Origin of Au-Ag Mineralization in Sphalerite Ores from Data on Sphalerite Co-Crystallization with Ag and Au in Model Hydrothermal Systems

Abstract ––Au-Ag mineralization occurrences in sphalerite ores of hydrothermal genesis are paradoxical in view of the incompatibility of these elements in sphalerite. The formation of sphalerite with Au and Ag impurities under hydrothermal crystallization of ZnS at 450 °C and 1 kbar pressure was studied experimentally. Sn impurity was taken as a source of point defects in crystals modelling the interaction of Au and Ag with vacancies. The Ag solubility in low-Fe sphalerite is estimated as 3.8 ± 0.7 μg/g, Au ̶ ≤ 0.6 μg/g. The main forms of Ag and Au occurrence in sphalerite are the inclusions of (Ag, Au)xS phases with x varies mainly from 1.8 to 2.0, and Au varies from 0.01 to 0.75 a.p.f.u. The primary forms of the elements in ores might be microinclusions (Ag, Au) 1.8-2.1S or close to (Ag, Au)S at higher fS2. In presence of Sn, solubilities of Au and Ag become higher. The behavior of Au corresponds to the substitution reaction Sn4+ + Au+ + v‒ ↔ 2Zn2+ in the presence of two types of vacancy defects (v–) ‒ the “inherent” vacancies dependent on the crystallization conditions and the vacancies accompanying Sn4+ incorporation. Ag entrance is seemingly more dependent on fS2 conditions and does not correlate with Sn. The extra vacancies arise because of metastable crystallization under the conditions of oversaturation of growth medium. This is supported by the spherulite morphology of growth products and the admixture of wurtzite ZnS form. The distribution and cocrystallization coefficients show an increasing trend for both precious metals (PM), due to which Au changes from incompatible to the category of highly compatible elements in sphalerite. The geochemical environments favorable for the formation of imperfect mineral crystals are considered. Such crystals are capable to uptake PMs and other incompatible in “ideal” crystal elements because of their interaction with vacancies, both constitutional (inherent to the substance) and non-equilibrium defects, and surficial nano-sized formations (nonautonomous phases). The evolution of these initially “invisible” forms of PM under metamorphic processes and remobilization of ore substance may result in Au and Ag escape and aggregation into microparticles.

Comparison between carrier transport property and crystal quality of TlBr semiconductors

Thallium bromide (TlBr) semiconductor detectors are being developed as promising candidates for high-detection-efficiency, high-energy-resolution, and room-temperature gamma-ray spectrometers. This study presents methods for evaluating TlBr crystal quality and carrier transport characteristics using neutron Bragg-dip imaging and the time-of-flight method for pulsed-laser-induced carriers, respectively. Neutron Bragg-dip imaging effectively determines the crystal orientation distribution, revealing crystal imperfections and grain boundaries. Time-of-flight measurements provide a spatial distribution of carrier mobility. In this study, two samples obtained from both the upstream and downstream region in the crystal ingot were evaluated. Although both samples show similar crystal quality, the upstream sample showed high carrier mobility across all areas, whereas the downstream sample exhibits low mobility in some areas. These findings suggest that, at least within the range of carrier mobility currently obtained, the effect of crystal integrity on carrier mobility is less significant than that of impurities. In conclusion, combining neutron Bragg-dip imaging with carrier mobility measurements offers a comprehensive approach to evaluating and improving TlBr detectors.

Carbon nanotube manipulates crystallization kinetics and viscoelastic behavior of PP/EBC blends

AbstractPolypropylene (PP) offers numerous benefits to industrial applications due to its excellent processability and cost‐effectiveness. However, the widespread utilization of this commodity thermoplastic is constrained by its inherently low‐impact resistance. Herein, we suggest a ternary system with the addition of ethylene‐butene copolymer (EBC) and multiwalled carbon nanotubes (MWCNTs) to improve the overall performance of PP. We unveil the role of MWCNTs on the crystalline behavior of the PP/EBC blends when various ratios are considered. The calculated half‐time crystallization from the Avrami equation indicates that 1.5 wt% MWCNTs accelerate the crystallization of the PP/EBC blend nearly threefold. The formation of a semisolid microstructure is also reported using the rheological measurements. An upturn in complex viscosity, along with a frequency‐independent storage modulus, indicates strong interactions between MWCNTs and polymer chains. We confirm such robust interactions by employing temperature ramp sweep on the prepared nanocomposites as the decomposition temperature of the materials shifts to higher values. The dynamic mechanical analysis further interprets the role of MWCNTs as compatibilizers by bridging the gap between the glassy temperature of PP and EBC components.Highlights Theoretical prediction of the MWCNTs localization in the PP/EBC blends. Depicting the solid‐like behavior and viscosity upturn of PP/EBC/MWCNTs. Decelerating polymer chain relaxation led by nucleation role of MWCNTs. Formation of larger and more imperfect crystals upon the addition of MWCNTs. Faster crystallization kinetics in the PP/EBC/MWCNTs nanocomposites.

Tailoring the Lattice Thermal Conductivity of Al‐Incorporated Ag2Se for Near Room Temperature Waste Heat Recovery

AbstractAg2Se is categorized as a typical ′phonon‐liquid‐electron‐crystal (PLEC)′material, exhibiting an extremely low . In this study, Al substituted Ag2Se samples were synthesized by mechanical alloying process followed by hot press densification technique. The Al substituted sample shows an exceptionally high mobility of 2360 cm2 V−1 S−1 at room temperature and a maximum power factor of 1442 μWm−1 K−2 at 383 K. The phonon scattering induced by various crystal imperfections due to Al substitution helped to obtain a very low lattice thermal conductivity value of 0.3 W/mK. A high figure of merit (zT) of 0.4 at 383 K was obtained as a result of the simultaneous effect of boosting the electrical transport properties and decreasing the thermal conductivity. This present work summarizes that the Al substitution is highly significant in improving the thermoelectric properties of Ag2Se.

Ta-ions doping impact on structural modifications and bandgap tuning of NiCo2O4 hexagonal nanosheets for high rated pseudocapacitor electrodes

Getting higher activity, lower cost and especially better stability at the same time is still one of the biggest problems in the supercapacitors research. Morphology tailoring and crystal imperfections induce d due to doping based defects in ternary transition metal oxides at the nano-scale level is crucial for enhancing the performance of supercapacitors. This research investigates of the utilization of Ta-doped NiCo2O4 (Ta-NCO) hexagonal nano-sheets for supercapacitor electrodes. The structural and morphological study of pristine and Ta-doped samples has been determined using XRD, SEM, TEM and HRTEM. The enhanced inherent conductivity and diffusion at the interface are also confirmed using electrochemical measurements study. The NCO-5 material exhibits a notable enhancement in capacitance and cycling retention. The NCO-5 electrode demonstrates a high specific capacitance (Cs) of 2445.7 F/g at 1 A/g. The pseudocapacitive behavior by power law (b = 0.67) and an increase in capacitive contribution with the scan rate reaching a value of 47.2 % at 100 mV/s by Dunn method exhibits it suitability of pseudocapacitors. The low Rct value signifies it improved capacitive performance as deduced from EIS measurements. Crucially, it maintains around 95.3 % of its capacitance after undergoing 10,000 GCD cycles. The high specific capacitance, remarkable capacitive characteristics and high stability of NCO-5 make it an excellent contestant for supercapacitor applications. The fabricated asymmetric supercapacitor (NCO-5//AC) shows exceptional energy density (ED) of 96.35 Wh/kg with power density (PD) of 825.03 W/kg.

Reorganization of Poly(Butylene Succinate) Containing Crystals of Low Stability.

Poly(butylene succinate) (PBS) forms small and imperfect crystals of low melting temperature at high supercooling of the melt. Slow heating allows reorganization of the obtained semicrystalline structure with the changes of the crystallinity and of the size and perfection of crystals analyzed by differential scanning calorimetry (DSC) and temperature-resolved X-ray scattering techniques. Crystals generated at 20 °C begin to melt and reorganize at a few K higher temperature with their initial imperfection and thickness unchanged upon heating to 70-80 °C. Slow heating to temperatures higher than 70-80 °C yields a distinct exothermic peak in the DSC scan, paralleled by detection of crystals of larger size/higher perfection, beginning to melt at ≈100 °C. These observations suggest that below 70-80 °C, reorganization of the semicrystalline morphology is constrained such that only minor and local improvement of the structure of crystals are possible. The formation of both perfect and thicker crystal lamellae at higher temperature proceeds via melting of imperfect crystals followed by melt-recrystallization as for PBS solid-state thickening is impossible. The study shows the limit of low-temperature reorganization processes when not involving both complete melting of crystals and rearrangement of the lamellar-stack structure.

3D Nanocrystallography and the Imperfect Molecular Lattice.

Crystallographic analysis relies on the scattering of quanta from arrays of atoms that populate a repeating lattice. While large crystals built of lattices that appear ideal are sought after by crystallographers, imperfections are the norm for molecular crystals. Additionally, advanced X-ray and electron diffraction techniques, used for crystallography, have opened the possibility of interrogating micro- and nanoscale crystals, with edges only millions or even thousands of molecules long. These crystals exist in a size regime that approximates the lower bounds for traditional models of crystal nonuniformity and imperfection. Accordingly, data generated by diffraction from both X-rays and electrons show increased complexity and are more challenging to conventionally model. New approaches in serial crystallography and spatially resolved electron diffraction mapping are changing this paradigm by better accounting for variability within and between crystals. The intersection of these methods presents an opportunity for a more comprehensive understanding of the structure and properties of nanocrystalline materials.

Creating Ferroelectricity in Monoclinic (HfO_{2})_{1}/(CeO_{2})_{1} Superlattices.

Ferroelectricity in CMOS-compatible hafnia (HfO_{2}) is crucial for the fabrication of high-integration nonvolatile memory devices. However, the capture of ferroelectricity in HfO_{2} requires the stabilization of thermodynamically metastable orthorhombic or rhombohedral phases, which entails the introduction of defects (e.g., dopants and vacancies) and pays the price of crystal imperfections, causing unpleasant wake-up and fatigue effects. Here, we report a theoretical strategy on the realization of robust ferroelectricity in HfO_{2}-based ferroelectrics by designing a series of epitaxial (HfO_{2})_{1}/(CeO_{2})_{1} superlattices. The designed ferroelectric superlattices are defects free, and most importantly, on the base of the thermodynamically stable monoclinic phase of HfO_{2}. Consequently, this allows the creation of superior ferroelectric properties with an electric polarization >25 μC/cm^{2} and an ultralow polarization-switching energy barrier at ∼2.5 meV/atom. Our work may open an avenue toward the fabrication of high-performance HfO_{2}-based ferroelectric devices.

Development of an innovative diffraction scattering theory of X-rays and electrons in imperfect crystals.

Fundamental equations describing the X-ray and electron diffraction scattering in imperfect crystals have been derived in the form of the matrix Fredholm-Volterra integral equation of the second kind. A theoretical approach has been developed using the perfect-crystal Green function formalism. In contrast, another approach utilizes the wavefield eigenfunctions related to the diagonalized matrix propagators of the conventional Takagi-Taupin and Howie-Whelan equations. Using the Liouville-Neumann-type series formalism for building up the matrix Fredholm-Volterra integral equation solutions, the general resolvent function solutions of the X-ray and electron diffraction boundary-valued Cauchy problems have been obtained. Based on the resolvent-type solutions, the aim is to reveal the features of the diffraction scattering onto the crystal lattice defects, including the mechanisms of intra- and interbranch wave scattering in the strongly deformed regions in the vicinity of crystal lattice defect cores. Using the two-stage resolvent solution of the second order, this approach has been supported by straightforward calculation of the electron bright- and dark-field contrasts of an edge dislocation in a thick foil. The results obtained for the bright- and dark-field profiles of the edge dislocation are discussed and compared with analogous ones numerically calculated by Howie & Whelan [Proc. R. Soc. A (1962), 267, 206].

Imperfect phononic crystals work too: The effect of translational and mid-plane symmetry breaking on hypersound propagation

Translationally symmetric nanostructures, termed phononic crystals (PnCs), offer control over the propagation of acoustic phonons in the gigahertz (GHz) range for signal-processing applications and thermal management at sub-Kelvin temperatures. In this work, we utilize Brillouin light scattering to investigate the impact of symmetry breaking on GHz phonon propagation in PnCs made of holey silicon nanomembranes. We show that the lattice of thimble-like holes leads to broken mid-plane symmetry and, hence, to anticrossing acoustic band gaps. With the rising level of uncorrelated translational disorder, the phononic effects are gradually suppressed, starting at higher frequencies. Strikingly, the low-frequency partial Bragg bandgap remains robust up to the highest level of disorder.

Local defect and mid-gap state analysis of GaN using monochromated EELS combined with nanodiffraction and atomic-resolution imaging

Defects in semiconductor materials significantly impact their inherent properties, making the evaluation of local defects and their energy levels crucial for controlling device performance. With advancements in monochromators, electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM) has emerged as a promising approach for assessing crystal and band structures of semiconductor materials at the nanoscale. However, there is limited experimental research investigating the relationship between actual defective structures in local regions and mid-gap states. In this study, we conducted high-energy resolution EELS (HR-EELS) measurements with an energy resolution of less than 100 meV to detect the mid-gap states of GaN. Various defects were intentionally induced through Ga-ion implantation, and the defect concentration varied in the depth direction. To understand the origin of the mid-gap states, we performed 4D-STEM analysis and atomic-resolution STEM observations. The HR-EELS measurements provided insights into the depth-dependent valence-loss spectra, revealing that the intensities corresponding to mid-gap states gradually increased toward the surface, whereas the slope at the onsets corresponding to interband transition decreased. Furthermore, local structural analysis unveiled the presence of structural disorder and defective structures, indicating the existence of extended defects such as stacking faults and domain boundaries. Observably, these defective structures were abundant near the surface and less pronounced in deeper regions. Based on these experimental findings, we concluded that the variations in valence-loss spectra can be utilized to qualitatively evaluate the crystal imperfections at the nanoscale.

The Franz–Keldysh effect in the optical absorption spectrum of a TlGaSe2 layered semiconductor caused by charged native defects

The Franz–Keldysh effect in the optical absorption edge of a bulk TlGaSe2 layered semiconductor poled under an external electric field was investigated in the present work. The Franz–Keldysh shift below the optical bandgap absorption region, as well as the quasi-periodic oscillations above the fundamental bandgap of TlGaSe2, were observed. The measured changes in optical light absorption of the TlGaSe2 sample were revealed after poling processing. The poling technique is used to produce the built-in internal electric field within the TlGaSe2 semiconductor. The frozen-in internal electric field in TlGaSe2 was experimentally monitored through changes in the lineshape of the absorption spectra at the fundamental band edge. The observed results are accurately fitted with the theoretical lineshape function of the Franz–Keldysh absorption tail below the bandgap of TlGaSe2 and quasi-periodic oscillations above the bandgap. A good agreement between the theoretical and experimental results was observed. The present study demonstrated that the Franz–Keldysh effect can be used to identify and characterize the localized internal electric fields originating from electrically active native imperfections in the TlGaSe2 crystals.

Piezoelectric Applications of Low-Dimensional Composites and Porous Materials.

Low-dimensional (LD) materials, with atomically thin anisotropic structures, exhibit remarkable physical and chemical properties, prominently featuring piezoelectricity resulting from the absence of centrosymmetry. This characteristic has led to diverse applications, including sensors, actuators, and micro- and nanoelectromechanical systems. While piezoelectric effects are observed across zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) LD materials, challenges such as effective charge separation and crystal structure imperfections limit their full potential. Addressing these issues requires innovative solutions, with the integration of LD materials with polymers, ceramics, metals, and other porous materials proving a key strategy to significantly enhance piezoelectric properties. This review comprehensively covers recent advances in synthesizing and characterizing piezoelectric composites based on LD materials and porous materials. The synergistic combination of LD materials with other substances, especially porous materials, demonstrates notable performance improvements, addressing inherent challenges. The review also explores future directions and challenges in developing these composite materials, highlighting potential applications across various technological domains.

Shell-Driven Localized Oxide Nanoparticles Determine the Thermal Stability of Microencapsulated Phase Change Material.

Not all encapsulation techniques are universally apt for every type of phase change material (PCM), highlighting the imperative for methodological precision. This study addresses the challenges of microencapsulated PCM (MEPCM) arising from the immiscible pairing of α-Al2O3 nanoparticles with Sn microparticles. The high-speed impact blending (HIB) dry synthesis technique is employed, facilitating large-volume production of Sn@α-Al2O3 MEPCMs. The resulting MEPCMs not only seamlessly endure 100 cycles of melting-solidification but also, with the strategic incorporation of a glass frit, exhibit remarkable thermal durability, withstanding up to 1000 melting-solidification cycles. Even under ultrafast thermal fluctuations, the α-Al2O3 shell remained resilient through 100 cycles. A marked reduction in supercooling is observed, which is attributed to the formation of SnO and SnO2 nanoparticles within the α-Al2O3 crystal lattice. The atomically resolved interface dynamics between SnO2 and α-Al2O3 play a pivotal role, lowering the energy barrier for Sn nuclei formation during solidification. This affects the accelerated Sn nucleation rate, effectively suppressing supercooling. Such insights offer a deeper understanding of the interplay between nanoscale crystal lattice imperfections and their implications for energy storage applications.

Исследование процесса вторичной конденсации LiF в ходе реактивного ионного травления тонкопленочного ниобата лития во фторсодержащей плазме

This paper investigates LiF redeposition during reactive ion etching (RIE) of X-cut thin-film lithium niobate (TFLN) with Ar/SF6 ICP/CCP plasma. RIE of TFLN is a topochemical reaction (TCR), described by the Kolmogorov-Avrami-Erofeev equation, during which there occurs LiF redeposition from the supersaturated vapor phase onto the sample surface. It is shown that the redeposition process is characterized by an induction period, which depends on the etch rate and the TFLN crystal structure imperfections. To study the LiF redeposition process, we used spectroscopic reflectometry, energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The dislocation density of TFLN was investigated with the help of etch pit method and X-ray diffraction. This study is the first to show that the duration of the induction period of TCR on the TFLN surface is inversely related to the etch rate. The RIE process is characterized by critical etch rates, above which a thin LiF film grows according to the two-dimensional Volmer-Weber mode. It was experimentally shown that the critical etch rates decrease with increasing the process pressure. For the first time, the dislocation density of TFLN was experimentally evaluated by means of the etch pit method. An experimental comparison of the induction periods of TCR on the surface of bulk and TFLN was carried out. It has been revealed that the increased dislocation density of TFLN significantly reduces the duration of the induction period. The impact of the modification of the TFLN structure with proton exchange on the kinetics of RIE was studied. It was demonstrated that the replacement of Li+ with H+ leads to decrease in the degree of LiF vapor saturation above the TFLN surface and, as a consequence, to a significant increase in the induction period of TCR and an increase in the etch rates by 5–9 times compared to unmodified TFLN.

Investigating the role of processing temperature on the microstructure evolution, mechanical properties, and corrosion behaviour of equal channel angular pressed AZ31 Mg alloy

AZ31 magnesium (Mg) alloy was subjected to equal channel angular pressing (ECAP) through Bc route at three different temperatures (200, 250 and 300 °C) to investigate the role of processing temperature on the microstructure evolution. All the sample were processed for one complete cycle which includes 4 consecutive pressings. Workpiece processed at 200 °C has shown relatively higher grain refinement (2.1 ± 1.4 μm) compared with the samples processed at 250 °C (5.6 ± 2.7 μm) and 300 °C (7 ± 5.5 μm) from a starting size of 36 ± 4.1 μm. Increased hardness was measured for the ECAPed samples at 200 °C due to the higher level of grain refinement. Workpiece produced at 200 °C exhibited higher tensile strength (270.9 ± 6.3 MPa) compared with the other processed samples and base alloy. Surface energy measurements from the contact angles show the positive effect of grain refinement on increasing the hydrophilic nature of AZ31 alloy. Corrosion experiments from the potentiodynamic polarization studies demonstrate lower corrosion current for the base alloy compared with all of the processed alloys. The deteriorated corrosion performance in the ECAPed alloys can be claimed to the increased crystal imperfections due to ECAP. From the results, it is understood that the lower processing temperatures are preferable to achieve higher level of grain refinement in ECAP of AZ31 Mg alloy. Excellent mechanical properties can be achieved in AZ31 alloy by ECAP and the decreased corrosion resistance is a concern to pay more attention.

Inhomogeneous entropy production in active crystals with point imperfections

The presence of defects in solids formed by active particles breaks their discrete translational symmetry. As a consequence, many of their properties become space-dependent and different from those characterizing perfectly ordered structures. Motivated by recent numerical investigations concerning the nonuniform distribution of entropy production and its relation to the configurational properties of active systems, we study theoretically and numerically the spatial profile of the entropy production rate (EPR) when an active solid contains an isotopic mass defect. The theoretical study of such an imperfect active crystal is conducted by employing a perturbative analysis that considers the perfectly ordered harmonic solid as a reference system. The perturbation theory predicts a nonuniform profile of the entropy production extending over large distances from the position of the impurity. The EPR decays exponentially to its bulk value with a typical healing length that coincides with the correlation length of the spatial velocity correlations characterizing the perfect active solids in the absence of impurities. The theory is validated against numerical simulations of an active Brownian particle crystal in two dimensions with Weeks–Chandler–Andersen repulsive interparticle potential.

Use of Special Devices for the X-Ray Interferometric Investigation of Structural Imperfections in Single Crystals

Use of Special Devices for the X-Ray Interferometric Investigation of Structural Imperfections in Single Crystals

X-ray dynamical diffraction by quasi-monolayer graphene

We study the processes of dynamical diffraction of the plane X-ray waves on the graphene film/SiC substrate system in the case of the Bragg diffraction geometry. The statistical dynamical theory of X-ray diffraction in imperfect crystals is applied to the case of real quasi-two-dimensional systems. The necessity of the taking into account of the variability of the lattice parameter of multilayer graphene, as well as the influence of thickness on the thermal Debye–Waller factor at the calculation of the complex structural factors and Fourier components of polarizability, is demonstrated. It is shown that the change of the structural characteristics of the 3-layer graphene/substrate system, as well as its strained state, leads to a significant change in the diffraction profiles, which makes it possible to determine the characteristics by the X-ray diffraction method.