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Crystals, Volume 14, Issue 6 (June 2024) – 75 articles

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21 pages, 5316 KiB  
Article
Superlattice Symmetries Reveal Electronic Topological Transition in CaC6 with Pressure
by Bruce Wang, Antonio Bianconi, Ian D. R. Mackinnon and Jose A. Alarco
Crystals 2024, 14(6), 554; https://doi.org/10.3390/cryst14060554 - 14 Jun 2024
Viewed by 83
Abstract
The electronic properties of calcium-intercalated graphite (CaC6) as a function of pressure are revisited using density functional theory (DFT). The electronic band structures of CaC6, like many other layered superconducting materials, display cosine-shaped bands at or near the Fermi [...] Read more.
The electronic properties of calcium-intercalated graphite (CaC6) as a function of pressure are revisited using density functional theory (DFT). The electronic band structures of CaC6, like many other layered superconducting materials, display cosine-shaped bands at or near the Fermi level (FL). Such bands encompass bonding/antibonding information with a strong connection to superconducting properties. Using a hexagonal cell representation for CaC6, the construction of a double supercell in the c-direction effects six-folding in the reciprocal space of the full cosine function, explicitly revealing the bonding/antibonding relationship divide at the cosine midpoint. Similarly, folding of the Fermi surface (FS) reveals physical phenomena relevant to electronic topological transitions (ETTs) with the application of pressure. The ETT is characterised by a transition of open FS loops to closed loops as a function of pressure. As the highest transition temperature is reached with pressure, the dominant continuous, open FS loops shift to a different region of the FS. For CaC6, the peak value for the superconducting transition temperature, Tc, occurs at about 7.5 GPa, near the observed pressure of the calculated ETT. At this pressure, the radius of the nearly spherical Ca 4s-orbital FS coincides with three times the distance from the Γ centre point to the Brillouin zone (BZ) boundary of the 2c supercell. In addition, the ETT coincides with the alignment of the nonbonding (inflection) point of the cosine band with the FL. At other calculated pressure conditions, the Ca 4s-orbital FS undergoes topological changes that correspond and can be correlated with experimentally determined changes in Tc. The ETT is a key mechanism that circumscribes the known significant drop in Tc for CaC6 as a function of increasing pressure. Consistent calculated responses of the ETT to pressure match experimental measurements and validate the examination of superlattices as important criteria for understanding mechanisms driving superconductivity. Full article
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13 pages, 5877 KiB  
Article
Study of Leakage Current Transport Mechanisms in Pseudo-Vertical GaN-on-Silicon Schottky Diode Grown by Localized Epitaxy
by Mohammed El Amrani, Julien Buckley, Thomas Kaltsounis, David Plaza Arguello, Hala El Rammouz, Daniel Alquier and Matthew Charles
Crystals 2024, 14(6), 553; https://doi.org/10.3390/cryst14060553 - 14 Jun 2024
Viewed by 99
Abstract
In this work, a GaN-on-Si quasi-vertical Schottky diode was demonstrated on a locally grown n-GaN drift layer using Selective Area Growth (SAG). The diode achieved a current density of 2.5 kA/cm2, a specific on-resistance RON,sp of [...] Read more.
In this work, a GaN-on-Si quasi-vertical Schottky diode was demonstrated on a locally grown n-GaN drift layer using Selective Area Growth (SAG). The diode achieved a current density of 2.5 kA/cm2, a specific on-resistance RON,sp of 1.9 mΩ cm2 despite the current crowding effect in quasi-vertical structures, and an on/off current ratio (Ion/Ioff) of 1010. Temperature-dependent current–voltage characteristics were measured in the range of 313–433 K to investigate the mechanisms of leakage conduction in the device. At near-zero bias, thermionic emission (TE) was found to dominate. By increasing up to 10 V, electrons gained enough energy to excite into trap states, leading to the dominance of Frenkel–Poole emission (FPE). For a higher voltage range (−10 V to −40 V), the increased electric field facilitated the hopping of electrons along the continuum threading dislocations in the “bulk” GaN layers, and thus, variable range hopping became the main mechanism for the whole temperature range. This work provides an in-depth insight into the leakage conduction transport on pseudo-vertical GaN-on-Si Schottky barrier diodes (SBDs) grown by localized epitaxy. Full article
(This article belongs to the Special Issue Wide Bandgap Semiconductor: GaN and SiC Material and Device)
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21 pages, 11142 KiB  
Review
Recent Progress of Floating-Zone Techniques for Bulk Single-Crystal Growth
by Naoki Kikugawa
Crystals 2024, 14(6), 552; https://doi.org/10.3390/cryst14060552 - 14 Jun 2024
Viewed by 123
Abstract
This review describes the recent progress of floating-zone techniques for bulk single-crystal growth. The most crucial point of the crucible-free technique is to keep the molten zone stable. It has been investigated and reported to yield a steeper temperature gradient at the liquid–solid [...] Read more.
This review describes the recent progress of floating-zone techniques for bulk single-crystal growth. The most crucial point of the crucible-free technique is to keep the molten zone stable. It has been investigated and reported to yield a steeper temperature gradient at the liquid–solid interface along the growth direction and a homogeneous molten liquid along the rotation axis. This article overviews several recent achievements starting from the conventional setup, particularly for lamps equipped in horizontal or vertical configurations, tilting mirrors, shielding the irradiation, and filament sizes for the optical-lamp floating-zone furnaces. Also, the recently advancing laser-heated floating-zone furnaces are described. Throughout the article, the author emphasizes that the floating-zone technique has been a powerful tool for crystal growth since the 1950s with its roots in the zone-melting method, and it has still been advancing for further materials’ growth such as quantum materials with modern scientific concepts. Full article
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13 pages, 2173 KiB  
Article
Abundant Catalytic Edge Sites in Few-Layer Horizontally Aligned MoS2 Nanosheets Grown by Space-Confined Chemical Vapor Deposition
by Alin Velea, Angel-Theodor Buruiana, Claudia Mihai, Elena Matei, Teddy Tite and Florinel Sava
Crystals 2024, 14(6), 551; https://doi.org/10.3390/cryst14060551 - 14 Jun 2024
Viewed by 154
Abstract
Recently, a smart strategy for two-dimensional (2D) materials synthesis has emerged, namely space-confined chemical vapor deposition (CVD). Its extreme case is the microreactor method, in which the growth substrate is face-to-face stacked on the source substrate. In order to grow 2D transition metal [...] Read more.
Recently, a smart strategy for two-dimensional (2D) materials synthesis has emerged, namely space-confined chemical vapor deposition (CVD). Its extreme case is the microreactor method, in which the growth substrate is face-to-face stacked on the source substrate. In order to grow 2D transition metal dichalcogenides by this method, transition metal oxides, dispersed in very small amounts on the source substrate, are used as source materials in most of the published reports. In this paper, a colloidal dispersion of MoS2 in saline solution is used and MoS2 nanosheets with various shapes, sizes (between 5 and 60 μm) and thicknesses (2–4 layers) have been synthesized. Small MoS2 flakes (regular or defective) are present on the surface of the nanosheets. Catalytic sites, undercoordinated atoms located at the edges of MoS2 flakes and nanosheets, are produced in a high number by a layer-plus-island (Stranski–Krastanov) growth mechanism. Several double-resonance Raman bands (at 147, 177, 187, 225, 247, 375 cm−1) are assignable to single phonon processes in which the excited electron is elastically scattered on a defect. The narrow 247 cm−1 peak is identified as a topological defect-activated peak. These findings highlight the potential of defect engineering in material property optimization, particularly for solar water splitting applications. Full article
(This article belongs to the Special Issue Advanced Materials for Applications in Water Splitting)
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19 pages, 12474 KiB  
Article
Unveiling the Synthesis of Strontium Ferrites by Sol-Gel and Laser Floating Zone Methods for Energy Application
by Silvia Soreto Teixeira, Rafael Ferreira, João Carvalho and Nuno M. Ferreira
Crystals 2024, 14(6), 550; https://doi.org/10.3390/cryst14060550 - 13 Jun 2024
Viewed by 147
Abstract
This work proposes the synthesis of strontium ferrite by two different methods: sol-gel (SG), using powdered coconut water (PCW) as a precursor, and laser floating zone (LFZ). The SG samples were after treated at temperatures of 700, 1000, and 1200 °C, while the [...] Read more.
This work proposes the synthesis of strontium ferrite by two different methods: sol-gel (SG), using powdered coconut water (PCW) as a precursor, and laser floating zone (LFZ). The SG samples were after treated at temperatures of 700, 1000, and 1200 °C, while the samples obtained by LFZ were grown at pulling rates of 10, 50, and 100 mm/h. All samples studied were subjected to structural characterization techniques, as well as electrical (AC and DC) and magnetic characterization. Through X-ray diffraction, it was possible to observe that all the samples presented strontium ferrites, but none were single phase. The phases detected in XRD were confirmed by Raman spectroscopy. Scanning electron micrography allowed the observation of an increase in grain size with the temperature of SG samples and the reduction of the porosity with the decrease in growth rate for LFZ fibers. Through electrical analysis, it was observed that the most suitable samples for energy storage were the samples grown at 100 mm/h (εr = 430,712; εr = 11,577; tan δ = 0.84; σac = 0.0006 S/m, at 1 kHz). The remaining samples had high dielectric losses and can be applied in electromagnetic shielding. The SG 700 °C sample presented the highest magnetization (38.5 emu/g at T = 5 K). Full article
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9 pages, 9146 KiB  
Article
The Temperature-Dependent Thermal Conductivity of C- and O-Doped Si3N4: First-Principles Calculations
by Hongfei Shao, Jiahao Qiu, Xia Liu, Xuejun Hou and Jinyong Zhang
Crystals 2024, 14(6), 549; https://doi.org/10.3390/cryst14060549 - 13 Jun 2024
Viewed by 130
Abstract
Silicon nitride (Si3N4) possesses excellent mechanical properties and high thermal conductivity, which is an important feature in many applications. However, achieving the theoretically high thermal conductivity of Si3N4 in practice is challenging. In this study, we [...] Read more.
Silicon nitride (Si3N4) possesses excellent mechanical properties and high thermal conductivity, which is an important feature in many applications. However, achieving the theoretically high thermal conductivity of Si3N4 in practice is challenging. In this study, we adopted a first-principles calculation method to assess the effects of doping β-Si3N4 and γ-Si3N4 with carbon and oxygen atoms. Applying geometric structure optimization combined with calculation of the electronic phonon properties generated a stable doped structure. The results revealed that carbon and oxygen doping have little effect on the Si3N4 unit cell size, but that oxygen doping increases the unit cell volume. Energy band structure and state density calculation results showed that carbon doping reduces the nitride band gap width, whereas oxygen doping results in an n-type Si3N4 semiconductor. The findings from this study are significant in establishing a basis for targeted increase of the thermal conductivity of Si3N4. Full article
(This article belongs to the Section Materials for Energy Applications)
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13 pages, 999 KiB  
Article
Lactonization of α-Ferrocenyl Ketocarboxylic Acids via Nucleophilic Attack of Carbonyl Oxygen
by Uttam R. Pokharel, Brennan J. Curole, Autumn M. Andras, Brandon P. LeBlanc and Frank R. Fronczek
Crystals 2024, 14(6), 548; https://doi.org/10.3390/cryst14060548 - 12 Jun 2024
Viewed by 213
Abstract
The effects of the ferrocenyl moiety to enhance the nucleophilicity of the carbonyl group, situated at its adjacent position, have been explored in a series of α-ferrocenyl ketocarboxylic acids. In the presence of trifluoroacetic anhydride, 3-ferrocenoylpropionic acid and 4-ferrocenoylbutyric acid gave 5-ferrocenyl-4-trifluoroacetyl-2(3H [...] Read more.
The effects of the ferrocenyl moiety to enhance the nucleophilicity of the carbonyl group, situated at its adjacent position, have been explored in a series of α-ferrocenyl ketocarboxylic acids. In the presence of trifluoroacetic anhydride, 3-ferrocenoylpropionic acid and 4-ferrocenoylbutyric acid gave 5-ferrocenyl-4-trifluoroacetyl-2(3H)-furanone and 6-ferrocenyl-5-trifluoroacetyl-3,4-dihydropyran-2-one, respectively. Under similar reaction conditions, 2-ferrocenylcarbonylbenzoic acid, a keto carboxylic acid without a β-hydrogen, gave a dimerized lactone, 3,3′-diferrocenyl-3,3′-diphthalide, possibly due to radical coupling. The nucleophilic attack of carbonyl oxygen, activated by the ferrocenyl moiety, on the carboxylic carbon is assumed to be the crucial mechanistic step in forming these lactones. When the carbonyl group was reduced to an alcohol to break its conjugation with the ferrocenyl moiety, saturated lactones were isolated after the acidic workup. These results indicate that the α-ferrocenyl carbinols readily undergo solvolysis under acidic conditions, giving ferrocenylcarbenium ions, which are attacked by the carboxy oxygen to give lactones. Full article
(This article belongs to the Special Issue Coordination Complexes: Synthesis, Characterization and Application)
12 pages, 2070 KiB  
Article
Investigations on the Carrier Mobility of Cs2NaFeCl6 Double Perovskites
by Jiyuan Xing, Yiting Zhao, Wei-Yan Cong, Chengbo Guan, Zhongchen Wu, Dong Liu and Ying-Bo Lu
Crystals 2024, 14(6), 547; https://doi.org/10.3390/cryst14060547 - 12 Jun 2024
Viewed by 180
Abstract
Double perovskite materials have gradually become widely studied due to their potential applications in solar cells and other optoelectronic devices. We take Cs2NaFeCl6 as an example to investigate the carrier mobility with respect to the acoustic phonon and the optical [...] Read more.
Double perovskite materials have gradually become widely studied due to their potential applications in solar cells and other optoelectronic devices. We take Cs2NaFeCl6 as an example to investigate the carrier mobility with respect to the acoustic phonon and the optical phonon scattering mechanisms. By considering the deformation potential, carrier effective mass, and bulk modulus, the longitudinal acoustic (LA) phonon-determined mobilities for electrons and holes in Cs2NaFeCl6 are found to be μe = 2886.08 cm2 v−1 s−1 and μh = 39.09 cm2 v−1 s−1, respectively. The optical scattering mechanism involves calculating the Fröhlich coupling constant, dielectric constant, and polaron mass to determine the multiple polar optical (PO) phonon-scattering-determined mobilities, resulting in μe = 279.25 cm2 v−1 s−1 and μh = 21.29 cm2 v−1 s−1, respectively. By combining both interactions, the total electron mobility and hole mobility are determined to be 254.61 cm2 v−1 s−1 and 13.78 cm2 v−1 s−1, respectively. The findings suggest that the polarization of both electrons and ions, small coupling constant, and bulk modulus in Cs2NaFeCl6’s lattice make PO scattering a significant contribution to carrier mobility in this specific double perovskite, highlighting the importance of considering this in enhancing the optoelectronic properties of Cs2NaFeCl6 and other double perovskites. Full article
(This article belongs to the Section Inorganic Crystalline Materials)
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14 pages, 4037 KiB  
Article
The Annealing Kinetics of Defects in CVD Diamond Irradiated by Xe Ions
by Eugene A. Kotomin, Vladimir N. Kuzovkov, Aleksandr Lushchik, Anatoli I. Popov, Evgeni Shablonin, Theo Scherer and Evgeni Vasil’chenko
Crystals 2024, 14(6), 546; https://doi.org/10.3390/cryst14060546 - 12 Jun 2024
Viewed by 213
Abstract
The radiation-induced optical absorption at 1.5–5.5 eV (up to the beginning of fundamental absorption) has been analyzed in CVD diamond disks exposed to 231-MeV 132Xe ions with four fluences from 1012 to 3.8 × 1013 cm−2. The 5 [...] Read more.
The radiation-induced optical absorption at 1.5–5.5 eV (up to the beginning of fundamental absorption) has been analyzed in CVD diamond disks exposed to 231-MeV 132Xe ions with four fluences from 1012 to 3.8 × 1013 cm−2. The 5 mm diameter samples (thickness 0.4 mm) were prepared by Diamond Materials, Freiburg (Germany); the average grain size at growth site was around 80 μm; and the range of xenon ions was R = 11.5 μm. The intensity of several bands grows with ion fluence, thus confirming the radiation-induced origin of the defects responsible for these bands. The recovery of radiation damage has been investigated via isochronal (stepwise) thermal annealing procedure up to 650 °C, while all spectra were measured at room temperature. Based on these spectra, the annealing kinetics of several defects, in particular carbon vacancies (GR1 centers with a broad band ~2 eV) and complementary C-interstitial-related defects (~4 eV), as well as impurity-related complex defects (narrow bands around 2.5 eV) have been constructed. The experimental kinetics have also been analyzed in terms of the diffusion-controlled bimolecular reactions. The migration energies of tentatively interstitial atoms (mobile components in recombination process) are obtained, and their dependence on the irradiation fluences is discussed. Full article
(This article belongs to the Section Materials for Energy Applications)
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12 pages, 3818 KiB  
Article
Electro-Reactivity of Resorcinol on Pt(111) Single-Crystal Plane and Its Influence on the Kinetics of Underpotentially Deposited Hydrogen and Hydrogen Evolution Reaction Processes in 0.1 M NaOH Solution
by Bogusław Pierożyński, Mateusz Kuczyński and Tomasz Mikołajczyk
Crystals 2024, 14(6), 545; https://doi.org/10.3390/cryst14060545 - 12 Jun 2024
Viewed by 190
Abstract
This article primarily presents cyclic voltammetry, Tafel polarization and ac. impedance spectroscopy electrochemical examinations of resorcinol (RC) electro-reactivity on the Pt(111) surface and its influence on the kinetics of UPD H (underpotentially deposited hydrogen) and the HER (hydrogen evolution reaction) in a [...] Read more.
This article primarily presents cyclic voltammetry, Tafel polarization and ac. impedance spectroscopy electrochemical examinations of resorcinol (RC) electro-reactivity on the Pt(111) surface and its influence on the kinetics of UPD H (underpotentially deposited hydrogen) and the HER (hydrogen evolution reaction) in a 0.1 M NaOH supporting solution. The collected data provided evidence of the RC-ion’s surface adsorption and its further electroreduction in the presence of surface-adsorbed H radicals along with their primary beneficial role on the kinetics of the UPD H process. The above was elucidated through an evaluation of the associated charge-transfer resistance and capacitance parameters, and was carried out on the platinum (111) electrode plane, comparatively, for the RC-free and resorcinol-modified NaOH electrolyte. In addition, the recorded cathodic charge transients (obtained by injecting small amounts of RC-based 0.1 M NaOH solution to initially resorcinol-free electrolyte, carried out at the constant electrode potential characteristic to the UPD H potential zone) provided evidence that the RC species undergoes electrocatalytic reduction through the involvement of the Pt(111)-chemisorbed hydrogen radicals. Full article
(This article belongs to the Special Issue Materials for Applications in Water Splitting and Battery)
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4 pages, 220 KiB  
Editorial
Self–Assembled Complexes: “Love at First Sight”
by Ana M. García-Deibe and Jesús Sanmartín-Matalobos
Crystals 2024, 14(6), 544; https://doi.org/10.3390/cryst14060544 - 11 Jun 2024
Viewed by 266
Abstract
Self-assembly is a key process to obtain auto-organized species from disordered components [...] Full article
(This article belongs to the Special Issue Self-Assembled Complexes: “Love at First Sight”)
5 pages, 164 KiB  
Editorial
Recent Advances in Photonic Crystal and Optical Devices
by Muhammad A. Butt and Svetlana N. Khonina
Crystals 2024, 14(6), 543; https://doi.org/10.3390/cryst14060543 - 11 Jun 2024
Viewed by 246
Abstract
In recent years, photonic crystals (PhCs) have garnered significant attention due to their extraordinary ability to control and manipulate light at the nanoscale [...] Full article
(This article belongs to the Special Issue Recent Advances in Photonic Crystal and Optical Devices)
1 pages, 162 KiB  
Correction
Correction: Ghezzi, F.; Shmayda, W.T. Aging Effects in Zr(Fe0.5V0.5)2 Tritides. Crystals 2024, 14, 159
by Francesco Ghezzi and Walter Theodore Shmayda
Crystals 2024, 14(6), 542; https://doi.org/10.3390/cryst14060542 - 11 Jun 2024
Viewed by 126
Abstract
There was an error in the original publication [...] Full article
(This article belongs to the Special Issue Hydrogen Embrittlement of Metals)
9 pages, 3236 KiB  
Article
Change in Growth Mode of BGaN Layers Grown on GaN
by Jacek M. Baranowski, Kinga Kosciewicz, Ewelina B. Mozdzynska and Julita Smalc-Koziorowska
Crystals 2024, 14(6), 541; https://doi.org/10.3390/cryst14060541 - 11 Jun 2024
Viewed by 266
Abstract
A change in the growth mode from Stranski–Krastanov one, which is characteristic of MOCVD grown GaN, to the laterally grown BGaN in the Volmer–Weber growth mode is described. This change in growth is evidenced by scanning electron microscopy (SEM) and transmission electron microscopy [...] Read more.
A change in the growth mode from Stranski–Krastanov one, which is characteristic of MOCVD grown GaN, to the laterally grown BGaN in the Volmer–Weber growth mode is described. This change in growth is evidenced by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of BGaN grown on GaN at high temperatures. It is postulated on the basis of SIMS and XRD results that this change in growth is initiated by the transfer of boron atoms from gallium substitutional to interstitial. The proposed mechanism for the observed growth change is related to the generation of nitrogen interstitials and subsequent reactions with boron interstitials, which result in the formation of a BN layer at the growth front. The observed large change in the growth mode is due to a lattice mismatch between the grown BGaN and the atomic layer of BN and stays behind the change to the Volmer–Weber growth mode. The consequence of the Volmer–Weber growth mode is the textural layer of BGaN. The textural character of this material is associated with large voids between grown BGaN “plates”. These large voids are responsible for the termination of threading dislocations propagating in the c-direction. It is also postulated that the blocked threading dislocations from the GaN underlayer and laterally grown BGaN layers along the a-directions are responsible for the decrease in defect concentration within these layers. Full article
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11 pages, 3523 KiB  
Article
Multifunctional Experimental Studies of Sm-Ion-Influenced Pseudo-Cubic Morphotropic Phase Boundary Regional BiFeO3-xSrTiO3 Ceramics for High-Temperature Applications
by Ahmad Hussain, Nawishta Jabeen, Aasma Tabassum, Muhammad Usman Khan, Laiba Basharat and Islam H. El Azab
Crystals 2024, 14(6), 540; https://doi.org/10.3390/cryst14060540 - 9 Jun 2024
Viewed by 428
Abstract
In this manuscript, for the first time, the exploration of the microstructural, ferroelectric, piezoelectric, and dielectric performances are measured for Sm-ion-influenced pseudo-cubic, morphotropic phase boundary (MPB) regional 0.62BiFeO3−0.38SrTiO3:xwt%Sm2O3 (BFST:xSm) ceramics with x = 0–0.25. All the [...] Read more.
In this manuscript, for the first time, the exploration of the microstructural, ferroelectric, piezoelectric, and dielectric performances are measured for Sm-ion-influenced pseudo-cubic, morphotropic phase boundary (MPB) regional 0.62BiFeO3−0.38SrTiO3:xwt%Sm2O3 (BFST:xSm) ceramics with x = 0–0.25. All the compositions maintained their pseudo-cubic MPB structural stability. The composition of BFST:0.15Sm ceramics exhibited an excellent remnant polarization (Pr) of ~52.11 μC/cm2, an enhanced d33 of 101 pC/N, and the highest relative dielectric constant (ɛr) of ~1152, which are much improved as compared to that of pure BFST ceramics. BFST:0.15Sm ceramics demonstrated a Curie temperature (TC) of 378 °C. Moreover, the composition exhibited high thermal stability for d33 72 pC/N (only a 28% decrease), even at a high temperature of 300 °C. Such outstanding outcomes make BFST:0.15Sm ceramics an ideal applicant for high-temperature piezoelectric applications. Full article
(This article belongs to the Section Polycrystalline Ceramics)
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10 pages, 1831 KiB  
Article
The Wannier-Mott Exciton, Bound Exciton, and Optical Phonon Replicas of Single-Crystal GaSe
by Long V. Le, Tran Thi Thu Huong, Tien-Thanh Nguyen, Xuan Au Nguyen, Thi Huong Nguyen, Sunglae Cho, Young Dong Kim and Tae Jung Kim
Crystals 2024, 14(6), 539; https://doi.org/10.3390/cryst14060539 - 8 Jun 2024
Viewed by 292
Abstract
We report the absorption and photoluminescence spectra of GaSe single crystals in the near-edge region. The temperatures explored the range from 17 to 300 K. Specifically, at a temperature of 17 K, the photoluminescence spectrum reveals an interesting phenomenon: the Wannier-Mott exciton separates [...] Read more.
We report the absorption and photoluminescence spectra of GaSe single crystals in the near-edge region. The temperatures explored the range from 17 to 300 K. Specifically, at a temperature of 17 K, the photoluminescence spectrum reveals an interesting phenomenon: the Wannier-Mott exciton separates into two states. These states are a triplet state with an energy of 2.103 eV and a singlet state with an energy of 2.109 eV. The energy difference between these two states is 6 meV. Furthermore, the bound exciton (BX) can be localized at an energy of 2.093 eV. It is worth noting that its phonon replicas (BX-nLO) can be clearly distinguished up to the fourth order. Interestingly, the energy gaps between these replicas exhibit a consistent spacing of 7 ± 0.5 meV. This intriguing finding suggests a high-quality crystalline structure as well as a strong coupling between the phonon and BX-nLO. Additionally, at low temperatures, both the ground state (n = 1) at 2.11 eV and the excited state (n = 2) at 2.127 eV of free excitons can be observed. Full article
(This article belongs to the Topic Optoelectronic Materials, 2nd Volume)
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17 pages, 2690 KiB  
Article
Pressure-Driven Responses in Cd2SiO4 and Hg2GeO4 Minerals: A Comparative Study
by Jaspreet Singh, Daniel Errandonea, Venkatakrishnan Kanchana and Ganapathy Vaitheeswaran
Crystals 2024, 14(6), 538; https://doi.org/10.3390/cryst14060538 - 7 Jun 2024
Viewed by 182
Abstract
The structural, elastic, and electronic properties of orthorhombic Cd2SiO4 and Hg2GeO4 were examined under varying pressure conditions using first-principles calculations based on density functional theory employing the Projector Augmented Wave method. The obtained cell parameters at 0 [...] Read more.
The structural, elastic, and electronic properties of orthorhombic Cd2SiO4 and Hg2GeO4 were examined under varying pressure conditions using first-principles calculations based on density functional theory employing the Projector Augmented Wave method. The obtained cell parameters at 0 GPa were found to align well with existing experimental data. We delved into the pressure dependence of normalized lattice parameters and elastic constants. In Cd2SiO4, all lattice constants decreased as pressure increased, whereas, in Hg2GeO4, parameters a and b decreased while parameter c increased under pressure. Employing the Hill average method, we calculated the elastic moduli and Poisson’s ratio up to 10 GPa, noting an increase with pressure. Evaluation of ductility/brittleness under pressure indicated both compounds remained ductile throughout. We also estimated elastic anisotropy and Debye temperature under varying pressures. Cd2SiO4 and Hg2GeO4 were identified as indirect band gap insulators, with estimated band gaps of 3.34 eV and 2.09 eV, respectively. Interestingly, Cd2SiO4 exhibited a significant increase in band gap with increasing pressure, whereas the band gap of Hg2GeO4 decreased under pressure, revealing distinct structural and electronic responses despite their similar structures. Full article
(This article belongs to the Section Crystalline Metals and Alloys)
11 pages, 3305 KiB  
Article
Ferroelectric Domain Intrinsic Radiation Resistance of Lithium Niobate Ferroelectric Single−Crystal Film
by Jiahe Li, Jinlong He, Liya Niu, Hao Lu, Xiaojun Qiao, Bo Zhong, Mingzhu Xun, Xiujian Chou and Wenping Geng
Crystals 2024, 14(6), 537; https://doi.org/10.3390/cryst14060537 - 7 Jun 2024
Viewed by 237
Abstract
The study of the properties of ferroelectric materials against irradiation has a long history. However, anti−irradiation research on the ferroelectric domain has not been carried out. In this paper, the irradiation of switched domain structure is innovatively proposed. The switched domain of 700 [...] Read more.
The study of the properties of ferroelectric materials against irradiation has a long history. However, anti−irradiation research on the ferroelectric domain has not been carried out. In this paper, the irradiation of switched domain structure is innovatively proposed. The switched domain of 700 nm lithium niobate (LiNbO3, LN) thin film remains stable after gamma irradiation from 1 krad to 10 Mrad, which was prepared by piezoresponse force microscopy (PFM). In addition, the changing law of domain wall resistivity is explored through different sample voltages, and it is verified that the irradiated domain wall conductivity is still larger than the domain. This domain wall current (DWC) property can be applied to storage, logic, sensing, and other devices. Based on these, a ferroelectric domain irradiation resistance model is established, which explains the reason at an atomic level. The results open a possibility for exploiting ferroelectric materials as the foundation in the application of space and nuclear fields. Full article
20 pages, 3067 KiB  
Article
Properties of Z1 and Z2 Deep-Level Defects in n-Type Epitaxial and High-Purity Semi-Insulating 4H-SiC
by Paweł Kamiński, Roman Kozłowski, Jarosław Żelazko, Kinga Kościewicz and Tymoteusz Ciuk
Crystals 2024, 14(6), 536; https://doi.org/10.3390/cryst14060536 - 7 Jun 2024
Viewed by 236
Abstract
For the first time, the Z1 and Z2 defects with closely spaced energy levels having negative-U properties are revealed in high-purity semi-insulating (HPSI) 4H-SiC using Laplace-transform photoinduced transient spectroscopy (LPITS). In this material, after switching off the optical [...] Read more.
For the first time, the Z1 and Z2 defects with closely spaced energy levels having negative-U properties are revealed in high-purity semi-insulating (HPSI) 4H-SiC using Laplace-transform photoinduced transient spectroscopy (LPITS). In this material, after switching off the optical trap-filling pulse, either the one-electron or the two-electron thermally stimulated emission from these defects is observed at temperatures 300–400 K. It is found that the former corresponds to the Z10/+ and Z20/+ transitions with the activation energies of 514 and 432 meV, respectively, and the latter is associated with the Z1−/+ and Z2−/+ transitions with the activation energies of 592 meV and 650 meV, respectively. The Z1 and Z2 defect concentrations are found to increase from 2.1 × 1013 to 2.2 × 1014 cm−3 and from 1.2 × 1013 to 2.7 × 1014 cm−3, respectively, after the heat treatment of HPSI 4H-SiC samples at 1400 °C for 3 h in Ar ambience. Using the electrical trap-filling pulse, only the thermal two-electron emission from each defect was observed in the epitaxial 4H-SiC through Laplace-transform deep level transient spectroscopy (LDLTS). The activation energies for this process from the Z1 and Z2 defects are 587 and 645 meV, respectively, and the defect concentrations are found to be 6.03 × 1011 and 2.64 × 1012 cm−3, respectively. It is postulated that the Z1 and Z2 defects are the nearest-neighbor divacancies involving the carbon and silicon vacancies located at mixed, hexagonal (h), and quasi-cubic (k) lattice sites. Full article
(This article belongs to the Special Issue Wide Bandgap Semiconductor: GaN and SiC Material and Device)
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4 pages, 163 KiB  
Editorial
Organic–Inorganic Hybrids: A Class of Material with Infinite Opportunities
by Haoran Lin, Wei Liu and Xin Wu
Crystals 2024, 14(6), 535; https://doi.org/10.3390/cryst14060535 - 6 Jun 2024
Viewed by 245
Abstract
The continuous research interest in organic–inorganic hybrid materials can be attributed to the synergistic or complementary interactions between their organic and inorganic components, which, in turn, opens up a wide array of potential applications [...] Full article
(This article belongs to the Special Issue Organic-Inorganic Hybrids: Synthesis, Property and Application)
4 pages, 379 KiB  
Editorial
New Insights into the Assessment of Archaeological Crystalline Structures
by Claudia Scatigno, Giulia Festa and Maite Maguregui
Crystals 2024, 14(6), 534; https://doi.org/10.3390/cryst14060534 - 6 Jun 2024
Viewed by 248
Abstract
New analytical approaches and tools have become essential for safeguarding archaeological assets, given the accelerated degradation caused by diagenetic alteration or exposure to the atmosphere [...] Full article
(This article belongs to the Special Issue Archaeological Crystalline Materials)
10 pages, 14914 KiB  
Article
Modeling of Texture Development during Metal Forming Using Finite Element Visco-Plastic Self-Consistent Model
by Johannes Kronsteiner, Elias Theil, Alois Christian Ott, Aurel Ramon Arnoldt and Nikolaus Peter Papenberg
Crystals 2024, 14(6), 533; https://doi.org/10.3390/cryst14060533 - 5 Jun 2024
Viewed by 244
Abstract
In directional forming processes, such as rolling and extrusion, the grains can develop preferred crystal orientations. These preferred orientations—the texture—are the main cause for material anisotropy. This anisotropy leads to phenomena such as earing, which occur during further forming processes, e.g., during the [...] Read more.
In directional forming processes, such as rolling and extrusion, the grains can develop preferred crystal orientations. These preferred orientations—the texture—are the main cause for material anisotropy. This anisotropy leads to phenomena such as earing, which occur during further forming processes, e.g., during the deep drawing of sheet metal. Considering anisotropic properties in numerical simulations allows us to investigate the effects of texture-dependent defects in forming processes and the development of possible solutions. Purely phenomenological models for modeling anisotropy work by fitting material parameters or applying measured anisotropy properties to all elements of the part, which remain constant over the duration of the simulation. In contrast, crystal plasticity methods, such as the visco-plastic self-consistent (VPSC) model, provide a deeper insight into the development of the material microstructure. By experimentally measuring the initial texture and using it as an initial condition for the simulations, it is possible to predict the evolution of the microstructure and the resulting effect on the mechanical properties during forming operations. The results of the simulations with the VPSC model show a good agreement with corresponding compression tests and the earing phenomenon, which is typical for cup deep drawing. Full article
(This article belongs to the Special Issue Dislocations and Twinning in Metals and Alloys)
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17 pages, 3544 KiB  
Article
Helix Formation from Hydrogen Bond Kinetics in Alanine Homopeptides
by Krzysztof Kuczera, Gouri S. Jas and Robert Szoszkiewicz
Crystals 2024, 14(6), 532; https://doi.org/10.3390/cryst14060532 - 4 Jun 2024
Viewed by 290
Abstract
We present an analysis of α-helix folding in the coarse-grained coordinate of number of formed helical hydrogen bonds (NHBs) for four alanine peptides (ALA)n, with n = 5, 8, 15, and 21 residues. Starting with multi-microsecond all-atom molecular dynamics trajectories in aqueous solution, [...] Read more.
We present an analysis of α-helix folding in the coarse-grained coordinate of number of formed helical hydrogen bonds (NHBs) for four alanine peptides (ALA)n, with n = 5, 8, 15, and 21 residues. Starting with multi-microsecond all-atom molecular dynamics trajectories in aqueous solution, we represent the system dynamics in a space of between four (for ALA5) and twenty (for ALA21) hydrogen-bonding microstates. In all cases, transitions changing the hydrogen bond count by 1–2 dominate and the coil formation, NHB 1 → 0, is the fastest process. The calculation of global maximum weight paths shows that, when analyzed at a sufficiently long lag time, folding in the NHB coordinate is consecutive, with direct folding, 0 → 3, for ALA5 and bottlenecks at transitions 4 → 6 for ALA8, 0 → 5 for ALA15, and 0 → 9 for ALA21. Further coarse-graining to 2–4 dimensions was performed with the optimal dimensionality reduction method, allowing the identification of crucial folding intermediates and time scales of their formation in ALA8, ALA15, and ALA21. The detailed analysis of hydrogen bonding patterns revealed that folding is initiated preferentially at both peptide termini. The kinetic model was also used to estimate diffusion and friction coefficients for helix propagation. The description of the helix formation process in the hydrogen bonding coordinate NHB was in good general agreement with the experimental data and qualitatively similar to previous kinetic models of higher dimensions based on structural clustering. Use of the low-dimensional hydrogen bonding picture thus provides a different, complementary way of describing the complex and fascinating mechanism of helix formation. Full article
(This article belongs to the Section Crystal Engineering)
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19 pages, 11624 KiB  
Article
Effect of Ultrasonic Shot Peening on Microstructure and Corrosion Properties of GTA-Welded 304L Stainless Steel
by Hyunhak Cho, Young-Ran Yoo and Young-Sik Kim
Crystals 2024, 14(6), 531; https://doi.org/10.3390/cryst14060531 - 4 Jun 2024
Viewed by 205
Abstract
Austenitic stainless steels used in structural applications suffer from stress corrosion cracking due to residual stresses during welding. Much research is being conducted to prevent the stress corrosion cracking of austenitic steels by inducing compressive residual stresses. One method is ultrasonic shot peening [...] Read more.
Austenitic stainless steels used in structural applications suffer from stress corrosion cracking due to residual stresses during welding. Much research is being conducted to prevent the stress corrosion cracking of austenitic steels by inducing compressive residual stresses. One method is ultrasonic shot peening (USP), which is used to apply compressive stress by modifying the mechanical properties of the material’s surface. In this study, 304L stainless steel was butt-welded by gas tungsten arc welding (GTAW) and subsequently subjected to compressive residual stress to a depth of 1 mm from the surface by a USP treatment. The influence of USP on microstructural changes in the base metal, the HAZ and weldment, and the corrosion properties was analyzed. A microstructural analysis was conducted using SEM-EDS, XRD, and EBSD methods alongside residual stress measurements. The surface and cross-sectional corrosion behavior was evaluated and analyzed using a potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) measurements, a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test, and an ASTM A262 Pr. C test. The surface was deformed and roughened by the USP. The deformed areas formed crevices, and the inside of the crevices contained some cracks. The crevices and internal cracks caused pitting, which reduced the resistance of the passivation film. The cross-section was subjected to compressive residual stress to a depth of 1 mm from the surface, and the outermost area of the cross-section had fine grain refinement, forming a solid passivation film that improved the corrosion resistance. Full article
(This article belongs to the Special Issue Plastic Deformation and Welding on Metallic Materials)
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11 pages, 755 KiB  
Article
The Inability of Metal Coordination to Control the Regioselectivity of Dimerization of Trans-Cinnamic Acid Derivatives
by Guillaume Wery, Karol Pastucha, Koen Robeyns and Tom Leyssens
Crystals 2024, 14(6), 530; https://doi.org/10.3390/cryst14060530 - 3 Jun 2024
Viewed by 116
Abstract
Upon exposure to irradiation, trans-cinnamic acid can dimerize, producing truxinic and truxillic acids, regioisomers distinguished by the relative arrangement of acid and phenyl groups on the formed cyclobutane ring. Solid-state dimerization, governed by Schmidt’s specified conditions, hinges on the initial molecular setup. [...] Read more.
Upon exposure to irradiation, trans-cinnamic acid can dimerize, producing truxinic and truxillic acids, regioisomers distinguished by the relative arrangement of acid and phenyl groups on the formed cyclobutane ring. Solid-state dimerization, governed by Schmidt’s specified conditions, hinges on the initial molecular setup. This study endeavors to manipulate the reaction’s outcome in the solid state. To achieve this, the target molecule was paired with metals (Ag, Cu) to modify molecular orientation in the solid. Investigated derivatives included para-hydroxy-trans-cinnamic acid, ortho-methoxy-trans-cinnamic acid, ortho-ethoxy-trans-cinnamic acid, and ortho-chloro-trans-cinnamic acid. Despite easy synthesis of all complexes, only the complex between Ag and ortho-chloro-trans-cinnamic acid exhibits photoreactivity, mirroring the outcome of the metal-free derivative. Thus, while this approach has the potential to alter the photobehavior of cinnamic acid derivatives, obtaining the desired structure will require extensive screening to identify an appropriate metal complex. Full article
(This article belongs to the Special Issue Coordination Complexes: Synthesis, Characterization and Application)
17 pages, 1345 KiB  
Article
Evaluating the Effect of Hydrogen on the Tensile Properties of Cold-Finished Mild Steel
by Emmanuel Sey and Zoheir N. Farhat
Crystals 2024, 14(6), 529; https://doi.org/10.3390/cryst14060529 - 31 May 2024
Viewed by 226
Abstract
One of the major sources of catastrophic failures and deterioration of the mechanical properties of metals, such as ductility, toughness, and strength, in various engineering components during application is hydrogen embrittlement (HE). It occurs as a result of the adsorption, diffusion, and interaction [...] Read more.
One of the major sources of catastrophic failures and deterioration of the mechanical properties of metals, such as ductility, toughness, and strength, in various engineering components during application is hydrogen embrittlement (HE). It occurs as a result of the adsorption, diffusion, and interaction of hydrogen with various metal defects like dislocations, voids, grain boundaries, and oxide/matrix interfaces due to its small atomic size. Over the years, extensive effort has been dedicated to understanding hydrogen embrittlement sources, effects, and mechanisms. This study aimed at assessing the tensile properties, toughness, ductility, and susceptibility to hydrogen embrittlement of cold-finished mild steel. Steel coupons were subjected to electrochemical hydrogen charging in a carefully chosen alkaline solution over a particular time and at various charging current densities. Tensile property tests were conducted immediately after the charging process, and the results were compared with those of uncharged steel. The findings revealed a clear drop in toughness and ductility with increasing hydrogen content. Fracture surfaces were examined to determine the failure mechanisms. This evaluation has enabled the prediction of steel’s ability to withstand environments with elevated hydrogen concentrations during practical applications. Full article
(This article belongs to the Special Issue Hydrogen Embrittlement of Metals)
13 pages, 1629 KiB  
Article
A Comparative Study of Microstructural Characteristics and Mechanical Properties of High-Strength Low-Alloy Steel Fabricated by Wire-Fed Laser Versus Wire Arc Additive Manufacturing
by Dayue Zhang, Qian Fang, Binzhou Li, Yijia Wang, Shanshan Si, Yuanbo Jiang and Zhiping Hu
Crystals 2024, 14(6), 528; https://doi.org/10.3390/cryst14060528 - 31 May 2024
Viewed by 196
Abstract
This study evaluates the feasibility of producing high-strength low-alloy (HSLA) steel using advanced wire-fed laser additive manufacturing (LAM-W) and wire arc additive manufacturing (WAAM) technologies. Optimized parameters were independently developed for each heat source, utilizing a self-designed HSLA steel wire as the feedstock. [...] Read more.
This study evaluates the feasibility of producing high-strength low-alloy (HSLA) steel using advanced wire-fed laser additive manufacturing (LAM-W) and wire arc additive manufacturing (WAAM) technologies. Optimized parameters were independently developed for each heat source, utilizing a self-designed HSLA steel wire as the feedstock. Microstructural features and mechanical properties of the fabricated steels were characterized and compared, taking into account differences in heat input and cooling rates. LAM-W samples exhibited a finer columnar grain microstructure, while both LAM-W- and WAAM-produced steels predominantly showed lower bainite and granular bainite microstructures. LAM-W demonstrated higher strength and hardness, lower ductility, and comparable low-temperature toughness compared to WAAM. Both processes demonstrated an excellent balance between strength and ductility, with absorbed energy exceeding 100 J at −40 °C. The study confirms the feasibility of producing high-strength and tough HSLA steel parts using LAM-W and WAAM technologies, and compares the advantages and disadvantages of each method. These findings assist in selecting the most suitable wire-fed AM process for HSLA steel fabrication at high deposition rates. Full article
(This article belongs to the Section Crystalline Metals and Alloys)
3 pages, 206 KiB  
Editorial
Mineralogical Crystallography Volume III
by Vladislav V. Gurzhiy
Crystals 2024, 14(6), 527; https://doi.org/10.3390/cryst14060527 - 31 May 2024
Viewed by 130
Abstract
The United Nations and UNESCO designated 2014 as the International Year of Crystallography, in which the scientific community celebrated the centenary of the discovery of X-ray diffraction [...] Full article
(This article belongs to the Special Issue Mineralogical Crystallography (3rd Edition))
12 pages, 3468 KiB  
Article
Numerical Simulations of the Impact of CaO/Al2O3 on the Structure and Crystallization Behavior of Red Mud
by Lei Xing, Zhi-Hui Li, Pei-Pei Du and Yue Long
Crystals 2024, 14(6), 526; https://doi.org/10.3390/cryst14060526 - 31 May 2024
Viewed by 147
Abstract
The problem of large stockpiles of red mud needs to be solved, and the use of red mud to prepare inorganic fibers is a new way of applying red mud on a large scale. The role of CaO/Al2O3 in the [...] Read more.
The problem of large stockpiles of red mud needs to be solved, and the use of red mud to prepare inorganic fibers is a new way of applying red mud on a large scale. The role of CaO/Al2O3 in the melting point and melt structure of red mud was investigated by molecular dynamics simulations and thermodynamic calculations. Liquid phase line temperatures for different CaO/Al2O3 systems were calculated using the Factsage program. The radial distribution function and the type of oxygen bonding were used to characterize the effect of different CaO/Al2O3 on the structure of the red mud melt. The melting point of MgAl2O4 is lower than that of CaTiO3 due to the fact that the type of oxygen bonding in MgAl2O4 is predominantly bridging oxygen bonds. When the red mud system has a low SiO2 content and CaO/Al2O3 is between 0.3 and 3.9, the melting point temperature increases significantly, which is not conducive to the fibrillation of the red mud melt. Full article
(This article belongs to the Special Issue Crystallization Process and Simulation Calculation, Second Edition)
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14 pages, 12846 KiB  
Article
Molecular Dynamics Study of the Deformation Behavior and Strengthening Mechanisms of Cu/Graphene Composites under Nanoindentation
by Guangan Ren, Cong Zhou, Yongle Hu, Li Wang, Jingzhong Fang, Yejun Li, Yi Wang, Jian Liu, Mingjun Zhang and Yonggang Tong
Crystals 2024, 14(6), 525; https://doi.org/10.3390/cryst14060525 - 31 May 2024
Viewed by 195
Abstract
The mechanical performance of pure copper can be significantly strengthened by adding graphene without greatly sacrificing its electrical and thermal conductivity. However, it is difficult to observe the deformation behavior of Cu/graphene composites efficiently and optically using experiments due to the extremely small [...] Read more.
The mechanical performance of pure copper can be significantly strengthened by adding graphene without greatly sacrificing its electrical and thermal conductivity. However, it is difficult to observe the deformation behavior of Cu/graphene composites efficiently and optically using experiments due to the extremely small graphene size. Herein, Cu/graphene composites with different graphene positions and layers were built to investigate the effect of these factors on the mechanical performance of the composites and the deformation mechanisms using molecular dynamics simulations. The results showed that the maximum indentation force and hardness of the composites decreased significantly with an increase in the distance from graphene to the indentation surface. Graphene strengthened the mechanical properties of Cu/graphene composites by hindering the slip of dislocations. As the graphene layers increased, the strengthening effect became more pronounced. With more graphene layers, dislocations within the Cu matrix were required to overcome higher stress to be released towards the surface; thus, they had to store enough energy to allow more crystalline surfaces to slip, resulting in more dislocations being generated. Full article
(This article belongs to the Section Crystalline Metals and Alloys)
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