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Crystals, Volume 8, Issue 8 (August 2018)

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Cover Story (view full-size image) Metal–organic frameworks (MOFs) are a class of highly ordered crystalline materials with great [...] Read more.
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Open AccessArticle The DN-6 Neutron Diffractometer for High-Pressure Research at Half a Megabar Scale
Crystals 2018, 8(8), 331; https://doi.org/10.3390/cryst8080331
Received: 18 July 2018 / Revised: 15 August 2018 / Accepted: 18 August 2018 / Published: 20 August 2018
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Abstract
A neutron diffractometer DN-6 at the IBR-2 high-flux reactor is used for the studies of crystal and magnetic structure of powder materials under high pressure in a wide temperature range. The high neutron flux on the sample due to a parabolic focusing section
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A neutron diffractometer DN-6 at the IBR-2 high-flux reactor is used for the studies of crystal and magnetic structure of powder materials under high pressure in a wide temperature range. The high neutron flux on the sample due to a parabolic focusing section of a neutron guide and wide solid angle of the detector system enables neutron diffraction experiments with extraordinarily small volumes (about 0.01 mm3) of studied samples. The diffractometer is equipped with high-pressure cells with sapphire and diamond anvils, which allow pressures of up to 50 GPa to be reached. The technical design, main parameters and current capabilities of the diffractometer are described. A brief overview of recently obtained results is given. Full article
(This article belongs to the Special Issue Neutron Diffractometers for Single Crystals and Powders)
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Open AccessArticle Self-Assembled Structures of Diblock Copolymer/Homopolymer Blends through Multiple Complementary Hydrogen Bonds
Crystals 2018, 8(8), 330; https://doi.org/10.3390/cryst8080330
Received: 13 August 2018 / Revised: 17 August 2018 / Accepted: 17 August 2018 / Published: 19 August 2018
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Abstract
A poly(styrene-b-vinylbenzyl triazolylmethyl methyladenine) (PS-b-PVBA) diblock copolymer and a poly(vinylbenzyl triazolylmethyl methylthymine) (PVBT) homopolymer were prepared through a combination of nitroxide-mediated radical polymerizations and click reactions. Strong multiple hydrogen bonding interactions of the A···T binary pairs occurred
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A poly(styrene-b-vinylbenzyl triazolylmethyl methyladenine) (PS-b-PVBA) diblock copolymer and a poly(vinylbenzyl triazolylmethyl methylthymine) (PVBT) homopolymer were prepared through a combination of nitroxide-mediated radical polymerizations and click reactions. Strong multiple hydrogen bonding interactions of the A···T binary pairs occurred in the PVBA/PVBT miscible domain of the PS-b-PVBA/PVPT diblock copolymer/homopolymer blend, as evidenced in Fourier transform infrared and 1H nuclear magnetic resonance spectra. The self-assembled lamellar structure of the pure PS-b-PVBA diblock copolymer after thermal annealing was transformed to a cylinder structure after blending with PVBT at lower concentrations and then to a disordered micelle or macrophase structure at higher PVBT concentrations, as revealed by small-angle X-ray scattering and transmission electron microscopy. Full article
(This article belongs to the Special Issue Self-Assembled Supramolecular Polymers via Strong H Bonding)
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Open AccessArticle The Effect of Different Atomic Substitution at Mn Site on Magnetocaloric Effect in Ni50Mn35Co2Sn13 Alloy
Crystals 2018, 8(8), 329; https://doi.org/10.3390/cryst8080329
Received: 23 June 2018 / Revised: 13 August 2018 / Accepted: 15 August 2018 / Published: 18 August 2018
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Abstract
The effect of different atomic substitutions at Mn sites on the magnetic and magnetocaloric properties in Ni50Mn35Co2Sn13 alloy has been studied in detail. The substitution of Ni or Co for Mn atoms might lower the Mn
[...] Read more.
The effect of different atomic substitutions at Mn sites on the magnetic and magnetocaloric properties in Ni50Mn35Co2Sn13 alloy has been studied in detail. The substitution of Ni or Co for Mn atoms might lower the Mn content at Sn sites, which would reduce the d-d hybridization between Ni 3d eg states and the 3d states of excess Mn atoms at Sn sites, thus leading to the decrease of martensitic transformation temperature TM in Ni51Mn34Co2Sn13 and Ni50Mn34Co3Sn13 alloys. On the other hand, the substitution of Sn for Mn atoms in Ni50Mn34Co2Sn14 would enhance the p-d covalent hybridization between the main group element (Sn) and the transition metal element (Mn or Ni) due to the increase of Sn content, thus also reducing the TM by stabilizing the parent phase. Due to the reduction of TM, a magnetostructural martensitic transition from FM austenite to weak-magnetic martensite is realized in Ni51Mn34Co2Sn13 and Ni50Mn34Co2Sn14, resulting in a large magnetocaloric effect around room temperature. For a low field change of 3 T, the maximum ∆SM reaches as high as 30.9 J/kg K for Ni50Mn34Co2Sn14. A linear dependence of ΔSM upon μ0H has been found in Ni50Mn34Co2Sn14, and the origin of this linear relationship has been discussed by numerical analysis of Maxwell’s relation. Full article
(This article belongs to the Special Issue Advances in Caloric Materials)
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Open AccessArticle Structural Identification of Binary Tetrahydrofuran + O2 and 3-Hydroxytetrahydrofuran + O2 Clathrate Hydrates by Rietveld Analysis with Direct Space Method
Crystals 2018, 8(8), 328; https://doi.org/10.3390/cryst8080328
Received: 24 July 2018 / Revised: 14 August 2018 / Accepted: 14 August 2018 / Published: 18 August 2018
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Abstract
The structural determination of clathrate hydrates, nonstoichiometric crystalline host-guest materials, is challenging because of the dynamical disorder and partial cage occupancies of the guest molecules. The application of direct space methods with Rietveld analysis can determine the powder X-ray diffraction (PXRD) patterns of
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The structural determination of clathrate hydrates, nonstoichiometric crystalline host-guest materials, is challenging because of the dynamical disorder and partial cage occupancies of the guest molecules. The application of direct space methods with Rietveld analysis can determine the powder X-ray diffraction (PXRD) patterns of clathrates. Here, we conducted Rietveld analysis with the direct space method for the structural determination of binary tetrahydrofuran (THF) + O2 and 3-hydroxytetrahydrofuran (3-OH THF) + O2 clathrate hydrates in order to identify the hydroxyl substituent effect on interactions between the host framework and the cyclic ether guest molecules. The refined PXRD results reveal that the hydroxyl groups are hydrogen-bonded to host hexagonal rings of water molecules in the 51264 cage, while any evidences of hydrogen bonding between THF guests and the host framework were not observed from PXRD at 100 K. This guest-host hydrogen bonding is thought to induce slightly larger 512 cages in the 3-OH THF hydrate than those in the THF hydrate. Consequently, the disorder dynamics of the secondary guest molecules also can be affected by the hydrogen bonding of larger guest molecules. The structural information of binary clathrate hydrates reported here can improve the understanding of the host-guest interactions occurring in clathrate hydrates and the specialized methodologies for crystal structure determination of clathrate hydrates. Full article
(This article belongs to the Special Issue Rietveld Refinement in the Characterization of Crystalline Materials)
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Open AccessArticle Multiple Fault Location in a Photovoltaic Array Using Bidirectional Hetero-Associative Memory Network in Micro-Distribution Systems
Crystals 2018, 8(8), 327; https://doi.org/10.3390/cryst8080327
Received: 24 May 2018 / Revised: 12 August 2018 / Accepted: 13 August 2018 / Published: 15 August 2018
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Abstract
In manual maintenance inspections of large-scaled photovoltaic (PV) or rooftop PV systems, several days are required to survey the entire PV field. To improve reliability and shorten the amount of time involved, this study proposes an electrical examination-based method for locating multiple faults
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In manual maintenance inspections of large-scaled photovoltaic (PV) or rooftop PV systems, several days are required to survey the entire PV field. To improve reliability and shorten the amount of time involved, this study proposes an electrical examination-based method for locating multiple faults in the PV array. The maximum power point tracking (MPPT) algorithm is used to estimate the maximum power of each PV panel; this is then compared with metering the output power of PV array. Power degradation indexes as input variables are parameterized to quantify the degradation between estimated maximum PV output power and metered PV output power, which can be categorized into normal condition, grounded faults, open-circuit faults, bridged faults, and mismatch faults. Bidirectional hetero-associative memory (BHAM) networks are then used to associate the inputs and locate multiple faults as output variables within the PV array. For a rooftop PV system with two strings, experimental results demonstrate that the proposed model has computational efficiency in learning and detection accuracies for real-time applications, and that its algorithm is easily implemented in a mobile intelligent vehicle. Full article
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Open AccessArticle Reducing the Induction Time Using Ultrasound and High-Shear Mixing in a Continuous Crystallization Process
Crystals 2018, 8(8), 326; https://doi.org/10.3390/cryst8080326
Received: 30 June 2018 / Revised: 30 July 2018 / Accepted: 31 July 2018 / Published: 14 August 2018
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Abstract
Continuous crystallization in tubular crystallizers is of particular interest to the pharmaceutical industry to accurately control average particle size, particle size distribution, and (polymorphic) shape. However, these types of crystallizers require fast nucleation, and thus, short induction times at the beginning of the
[...] Read more.
Continuous crystallization in tubular crystallizers is of particular interest to the pharmaceutical industry to accurately control average particle size, particle size distribution, and (polymorphic) shape. However, these types of crystallizers require fast nucleation, and thus, short induction times at the beginning of the flow process, which is challenging for larger and complex organic molecules. High shear and/or the presence of bubbles were identified to influence the nucleation behavior. This work investigates the effects of both high-shear mixing and ultrasound on the anti-solvent crystallization of paracetamol in acetone–water. Both devices generate intense amounts of shear and gas bubbles. Generally, the results show that increasing input power decreases the induction time significantly for both the rotor–stator mixer and ultrasound probe. However, the induction time is almost independent of the supersaturation for the ultrasound probe, while the induction time significantly increases with decreasing supersaturation for the rotor–stator mixer. In contrast, the particle size distribution for the rotor–stator mixer is independent of the supersaturation, while increasing supersaturation decreases the particle size for the ultrasound probe. Full article
(This article belongs to the Special Issue Advances in Ultrasound Stimulated Crystallization)
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Open AccessFeature PaperReview Metal–Organic Framework Hybrid Materials and Their Applications
Crystals 2018, 8(8), 325; https://doi.org/10.3390/cryst8080325
Received: 24 July 2018 / Revised: 8 August 2018 / Accepted: 10 August 2018 / Published: 14 August 2018
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Abstract
The inherent porous nature and facile tunability of metal–organic frameworks (MOFs) make them ideal candidates for use in multiple fields. MOF hybrid materials are derived from existing MOFs hybridized with other materials or small molecules using a variety of techniques. This led to
[...] Read more.
The inherent porous nature and facile tunability of metal–organic frameworks (MOFs) make them ideal candidates for use in multiple fields. MOF hybrid materials are derived from existing MOFs hybridized with other materials or small molecules using a variety of techniques. This led to superior performance of the new materials by combining the advantages of MOF components and others. In this review, we discuss several hybridization methods for the preparation of various MOF hybrids with representative examples from the literature. These methods include covalent modifications, noncovalent modifications, and using MOFs as templates or precursors. We also review the applications of the MOF hybrids in the fields of catalysis, drug delivery, gas storage and separation, energy storage, sensing, and others. Full article
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Open AccessArticle Stress Relaxation Properties and Microscopic Deformation Structure in Bending of the C7025 and C7035 Alloy
Crystals 2018, 8(8), 324; https://doi.org/10.3390/cryst8080324
Received: 27 June 2018 / Revised: 8 August 2018 / Accepted: 9 August 2018 / Published: 14 August 2018
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Abstract
Stress relaxation tests in cantilever bending were performed on the C7025 and C7035 alloys at 298 K and 393 K, respectively. The effect of stress-relief treatments on stress relaxation properties was investigated. The structural changes associated with the stress relaxation process were examined
[...] Read more.
Stress relaxation tests in cantilever bending were performed on the C7025 and C7035 alloys at 298 K and 393 K, respectively. The effect of stress-relief treatments on stress relaxation properties was investigated. The structural changes associated with the stress relaxation process were examined using transmission electron microscopy. The stress relaxation curve fits well to empirical formula σ* = [K’ln(t + α0) + C]−n for stress relaxation. The curves can be split into two stages. The stress drops fast at first and then it gets slower in the second stage, and tends towards a certain limited value after a long time. The curve and microstructure reveal that the C7035 alloy has a lower rate of stress relaxation and a higher anti-stress relaxation capacity than the C7025. The first reason is that the movement of vacancies required by spinodal decomposition is inhibited, and the quantity of cobalt-containing vacancies decreases dramatically in the C7035 alloy. The other reason is that the precipitated phases became uniformly diffused in the C7035 alloy. The precipitate phase is uniformly distributed in the grain boundaries and the matrix, during the relaxed condition, and thus the dislocation movement is blocked by the precipitate. Full article
(This article belongs to the Special Issue Recrystallization of Metallic Materials)
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Open AccessArticle Simultaneous Generation of Two Pairs of Stokes and Terahertz Waves from Coupled Optical Parametric Oscillations with Quasi-Phase-Matching
Crystals 2018, 8(8), 323; https://doi.org/10.3390/cryst8080323
Received: 23 July 2018 / Revised: 11 August 2018 / Accepted: 11 August 2018 / Published: 14 August 2018
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Abstract
We present a theoretical investigation on simultaneous generation of two pairs of Stokes and terahertz (THz) waves from coupled optical parametric oscillations (OPOs) with a quasi-phase-matching (QPM) scheme. The two pairs of Stokes and THz waves are generated by stimulated polariton scattering (SPS)
[...] Read more.
We present a theoretical investigation on simultaneous generation of two pairs of Stokes and terahertz (THz) waves from coupled optical parametric oscillations (OPOs) with a quasi-phase-matching (QPM) scheme. The two pairs of Stokes and THz waves are generated by stimulated polariton scattering (SPS) from periodically-inverted GaP. By analyzing the QPM conditions of coupled OPOs we find that the two THz waves with any frequency below the transverse optical (TO) mode frequency of GaP can be simultaneously generated with a suitable pump wavelength. We calculate the photon flux densities of the two THz waves by solving the coupled wave equations. The calculation results indicate that the two THz waves can be efficiently generated with high pump intensities, particularly in lower THz frequency band. Full article
(This article belongs to the Section Crystal Engineering)
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Open AccessArticle Photorefractive Properties of Molybdenum and Hafnium Co-Doped LiNbO3 Crystals
Crystals 2018, 8(8), 322; https://doi.org/10.3390/cryst8080322
Received: 10 July 2018 / Revised: 8 August 2018 / Accepted: 10 August 2018 / Published: 13 August 2018
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Abstract
A series of LiNbO3: Mo, Hf crystals with 0.5 mol % fixed MoO3 and various HfO2 concentrations (0.0, 2.0, and 3.5 mol %) were grown by the Czochralski technique. The photorefractive properties of the LiNbO3: Mo, Hf
[...] Read more.
A series of LiNbO3: Mo, Hf crystals with 0.5 mol % fixed MoO3 and various HfO2 concentrations (0.0, 2.0, and 3.5 mol %) were grown by the Czochralski technique. The photorefractive properties of the LiNbO3: Mo, Hf crystals were investigated by two-wave coupling measurements and the beam distortion method was employed to obtain the optical damage resistance ability. The UV-visible and OH absorption spectra were also studied. The experimental results imply that the photorefractive properties of LiNbO3: Mo crystals at laser wavelengths of 532, 488, and 442 nm can be greatly enhanced by doping HfO2 over the threshold concentration. At 442 nm especially, the response time of LN: Mo, Hf3.5 can be shortened to 0.9 s with a diffraction efficiency of 46.07% and a photorefractive sensitivity reaching 6.28 cm/J. Besides this, the optical damage resistance at 532 nm is 3 orders of magnitude higher than that of the mono-doped LiNbO3: Mo crystal, which is beneficial for applying it in the field of high-intensity lasers. Full article
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Open AccessArticle Study of Nanoscratching Process of GaAs Using Molecular Dynamics
Crystals 2018, 8(8), 321; https://doi.org/10.3390/cryst8080321
Received: 14 July 2018 / Revised: 3 August 2018 / Accepted: 9 August 2018 / Published: 11 August 2018
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Abstract
In this paper, molecular dynamics method was employed to investigate the nanoscratching process of gallium arsenide (GaAs) in order to gain insights into the material deformation and removal mechanisms in chemical mechanical polishing of GaAs. By analyzing the distribution of hydrostatic pressure and
[...] Read more.
In this paper, molecular dynamics method was employed to investigate the nanoscratching process of gallium arsenide (GaAs) in order to gain insights into the material deformation and removal mechanisms in chemical mechanical polishing of GaAs. By analyzing the distribution of hydrostatic pressure and coordination number of GaAs atoms, it was found that phase transformation and amorphization were the dominant deformation mechanisms of GaAs in the scratching process. Furthermore, anisotropic effect in nanoscratching of GaAs was observed. The diverse deformation behaviors of GaAs with different crystal orientations were due to differences in the atomic structure of GaAs. The scratching resistance of GaAs(001) surface was the biggest, while the friction coefficient of GaAs(111) surface was the smallest. These findings shed light on the mechanical wear mechanism in chemical mechanical polishing of GaAs. Full article
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Open AccessArticle Antisolvent Sonocrystallisation of Sodium Chloride and the Evaluation of the Ultrasound Energy Using Modified Classical Nucleation Theory
Crystals 2018, 8(8), 320; https://doi.org/10.3390/cryst8080320
Received: 30 July 2018 / Revised: 6 August 2018 / Accepted: 8 August 2018 / Published: 10 August 2018
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Abstract
The crystal nucleation rate of sodium chloride in ethanol was investigated by measuring the induction time at various supersaturation ratios under silent and ultrasound irradiation at frequencies between 22 and 500 kHz. Under silent conditions, the data follows the classical nucleation theory showing
[...] Read more.
The crystal nucleation rate of sodium chloride in ethanol was investigated by measuring the induction time at various supersaturation ratios under silent and ultrasound irradiation at frequencies between 22 and 500 kHz. Under silent conditions, the data follows the classical nucleation theory showing both the homogeneous and heterogeneous regions and giving an interfacial surface tension of 31.0 mN m−2. Sonication led to a non-linearity in the data and was fitted by a modified classical nucleation theory to account for the additional free energy being supplemented by sonication. For 98 kHz, this free energy increased from 1.33 × 108 to 1.90 × 108 J m−3 for sonication powers of 2 to 15 W, respectively. It is speculated that the energy was supplemented by the localised bubble collapses and collisions. Increasing the frequency from 22 to 500 kHz revealed that a minimum induction time was obtained at frequencies between 44 and 98 kHz, which has been attributed to the overall collapse intensity being the strongest at these frequencies. Full article
(This article belongs to the Special Issue Advances in Ultrasound Stimulated Crystallization)
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Open AccessArticle Structural Chemistry of Halide including Thallides A8Tl11X1−n (A = K, Rb, Cs; X = Cl, Br; n = 0.1–0.9)
Crystals 2018, 8(8), 319; https://doi.org/10.3390/cryst8080319
Received: 23 July 2018 / Revised: 7 August 2018 / Accepted: 7 August 2018 / Published: 10 August 2018
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Abstract
A8Tl11 (A = alkali metal) compounds have been known since the investigations of Corbett et al. in 1995 and are still a matter of current discussions as the compound includes one extra electron referred to the charge of the Tl
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A8Tl11 (A = alkali metal) compounds have been known since the investigations of Corbett et al. in 1995 and are still a matter of current discussions as the compound includes one extra electron referred to the charge of the Tl117− cluster. Attempts to substitute this additional electron by incorporation of a halide atom succeeded in the preparation of single crystals for the lightest triel homologue of the group, Cs8Ga11Cl, and powder diffraction experiments for the heavier homologues also suggested the formation of analogous compounds. However, X-Ray single crystal studies on A8Tl11X to prove this substitution and to provide a deeper insight into the influence on the thallide substructure have not yet been performed, probably due to severe absorption combined with air and moisture sensitivity for this class of compounds. Here, we present single crystal X-Ray structure analyses of the new compounds Cs8Tl11Cl0.8, Cs8Tl11Br0.9, Cs5Rb3Tl11Cl0.5, Cs5.7K2.3Tl11Cl0.6 and K4Rb4Tl11Cl0.1. It is shown that a (partial) incorporation of halide can also be indirectly determined by examination of the Tl-Tl distances, thereby the newly introduced cdd/cdav ratio allows to evaluate the degree of distortion of Tl117− clusters. Full article
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Open AccessArticle Neutron RTOF Stress Diffractometer FSD at the IBR-2 Pulsed Reactor
Crystals 2018, 8(8), 318; https://doi.org/10.3390/cryst8080318
Received: 18 July 2018 / Revised: 4 August 2018 / Accepted: 6 August 2018 / Published: 9 August 2018
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Abstract
The diffraction of thermal neutrons is a powerful tool for investigations of residual stresses in various structural materials and bulk industrial products due to the non-destructive character of the method and high penetration depth of neutrons. Therefore, for conducting experiments in this research
[...] Read more.
The diffraction of thermal neutrons is a powerful tool for investigations of residual stresses in various structural materials and bulk industrial products due to the non-destructive character of the method and high penetration depth of neutrons. Therefore, for conducting experiments in this research field, the neutron Fourier stress diffractometer FSD has been constructed at the IBR-2 pulsed reactor in FLNP JINR (Dubna, Russia). Using a special correlation technique at the long-pulse neutron source, a high resolution level of the instrument has been achieved (Δd/d ≈ 2 ÷ 4 × 10−3) over a wide range of interplanar spacing dhkl at a relatively short flight distance between the chopper and sample position (L = 5.55 m). The FSD design satisfies the requirements of a high luminosity, high resolution, and specific sample environment. In this paper, the current status of the FSD diffractometer is reported and examples of performed experiments are given. Full article
(This article belongs to the Special Issue Neutron Diffractometers for Single Crystals and Powders)
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Open AccessArticle Investigation of Si Dendrites by Electron-Beam-Induced Current
Crystals 2018, 8(8), 317; https://doi.org/10.3390/cryst8080317
Received: 14 July 2018 / Revised: 3 August 2018 / Accepted: 5 August 2018 / Published: 7 August 2018
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This paper reports on electron-beam-induced current (EBIC) characterization of special multicrystalline Si ingot by dendritic growth under high undercooling. Grain boundaries (GBs), dislocations, and their interaction with carbon related precipitates were investigated. The difference between grains from dendrite and non-dendrite growth was compared.
[...] Read more.
This paper reports on electron-beam-induced current (EBIC) characterization of special multicrystalline Si ingot by dendritic growth under high undercooling. Grain boundaries (GBs), dislocations, and their interaction with carbon related precipitates were investigated. The difference between grains from dendrite and non-dendrite growth was compared. In dendrite grains, parallel twins were frequently found. In non-dendrite grains, irregular GBs of various characters co-existed. Both parallel twins and irregular GBs exhibited dark EBIC contrast at room temperature, indicating the presence of minority carrier recombination centers due to impurity contamination. However, sometimes in non-dendrite grains GBs were visualized with bright EBIC contrast with enhanced collection of charge carriers. The origin of the abnormal bright EBIC contrast was explored and it turned out to be SiC related precipitates, which made GBs conduction channels for electron transport. Full article
(This article belongs to the Special Issue Growth and Evaluation of Multicrystalline Silicon)
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Open AccessFeature PaperReview Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor
Crystals 2018, 8(8), 316; https://doi.org/10.3390/cryst8080316
Received: 30 January 2018 / Revised: 19 May 2018 / Accepted: 19 May 2018 / Published: 6 August 2018
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Abstract
Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing
[...] Read more.
Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight. Full article
(This article belongs to the Special Issue Integration of 2D Materials for Electronics Applications)
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Open AccessArticle Analysis of Switching Current Data during Polarization Reversal in KTP Single Crystals with Surface Dielectric Layer
Crystals 2018, 8(8), 315; https://doi.org/10.3390/cryst8080315
Received: 27 June 2018 / Revised: 26 July 2018 / Accepted: 2 August 2018 / Published: 4 August 2018
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Abstract
Studies of polarization reversal processes in potassium titanyl phosphate (KTiOPO4, KTP) single crystals with surface dielectric layer are important due to their potential applications as the basis of bottom-up technology for creation of periodically poled nonlinear-optical crystals. We present the results
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Studies of polarization reversal processes in potassium titanyl phosphate (KTiOPO4, KTP) single crystals with surface dielectric layer are important due to their potential applications as the basis of bottom-up technology for creation of periodically poled nonlinear-optical crystals. We present the results of switching currents analysis accompanied by in situ visualization of domain kinetics during polarization reversal in KTP with 3 m-thick photoresist dielectric layer. Qualitative change of the switching current shape has been revealed as compared to the polarization reversal without surface dielectric layer. Two stages of domain structure evolution have been distinguished by in situ visualization of domain kinetics. The formation of submicron domain streamers in front of the moving domain walls has been revealed. The broadening of the domain streamers (1D domain growth) was observed at the second stage. The switching currents were approximated by the modified Kolmogorov-Avrami formula taking into account the change of the growth dimensionality (“geometrical catastrophe”). The sufficient input of the 1D growth to the switching process decreased with increase of the switching field. The obtained results were attributed to the domain wall shape instability induced by retardation of the depolarization field screening in ferroelectric with surface dielectric layer. Full article
(This article belongs to the Special Issue KTP Crystal for Nonlinear Optical and Electrooptic Applications)
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Open AccessEditorial Computer Simulations: Essential Tools for Crystal Growth Studies
Crystals 2018, 8(8), 314; https://doi.org/10.3390/cryst8080314
Received: 31 July 2018 / Revised: 2 August 2018 / Accepted: 2 August 2018 / Published: 4 August 2018
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This special issue discusses recent advances in computer simulation studies of crystal growth. Crystal growth is a key to innovation in science and technology. Owing to recent progress in computer performance, computer simulation studies of crystal growth have become increasingly important. This special
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This special issue discusses recent advances in computer simulation studies of crystal growth. Crystal growth is a key to innovation in science and technology. Owing to recent progress in computer performance, computer simulation studies of crystal growth have become increasingly important. This special issue covers a variety of simulation methods, including the Monte Carlo, molecular dynamics, first-principles, multiscale, and continuum simulation methods, which are used for studies on the fundamentals and applications of crystal growth and related phenomena for different materials, such as hard-sphere systems, ice, organic crystals, semiconductors, and graphene. Full article
Open AccessArticle Growth and Thermal Properties of Mg-Doped Lithium Isotope Niobate (Mg:7LiNbO3) Crystal
Crystals 2018, 8(8), 313; https://doi.org/10.3390/cryst8080313
Received: 10 July 2018 / Revised: 1 August 2018 / Accepted: 1 August 2018 / Published: 3 August 2018
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Abstract
An Mg-doped isotope lithium niobate (Mg:7LiNbO3) crystal was successfully grown from 7LiOH, Nb2O5, and MgO using the Crozchralski method. The weight of the as-grown crystal with good quality was about 40 g. The crystal
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An Mg-doped isotope lithium niobate (Mg:7LiNbO3) crystal was successfully grown from 7LiOH, Nb2O5, and MgO using the Crozchralski method. The weight of the as-grown crystal with good quality was about 40 g. The crystal structure was determined as an R3c space group using the X-ray powder diffraction (XRPD) method, and the crystal composition (Li%) determined using the Raman mode linewidth method was 49.29%. The average transmittance of the crystal in the range of 500–2500 nm was approximately 72%. Various thermal properties, including the specific heat (Cp), the thermal expansion coefficient (α), the thermal diffusion coefficient (λ), and the thermal conductivity (κ), were carefully determined and calculated, and the value divergences among Mg:7LiNbO3, the undoped isotope lithium niobate (7LiNbO3), and natural lithium niobate (LiNbO3) crystals were mainly related to the differences in microstructure caused by the crystal composition. Full article
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Open AccessArticle Effect of Charge on the Structures of Zn(II) Coordination Polymers with Triazole-carboxylate Ligands: Syntheses, Structures, and Luminescent Properties
Crystals 2018, 8(8), 312; https://doi.org/10.3390/cryst8080312
Received: 10 July 2018 / Revised: 22 July 2018 / Accepted: 27 July 2018 / Published: 1 August 2018
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Abstract
Three new Zn(II) coordination polymers, {[Zn(L1)2(H2O)2]·4H2O}n (1), {[Zn(L2)(H2O)2]·0.5H2O}n (2), and {[Zn1.5(L1)(o-bdc)(H2O)]·0.5H2O}n (3) (HL1 =
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Three new Zn(II) coordination polymers, {[Zn(L1)2(H2O)2]·4H2O}n (1), {[Zn(L2)(H2O)2]·0.5H2O}n (2), and {[Zn1.5(L1)(o-bdc)(H2O)]·0.5H2O}n (3) (HL1 = 3,5-di(1,2,4-triazol-1-yl)benzoic acid, H2L2 = 5-(triazol-1-yl)isophthalic acid, o-H2bdc = 1,2-benzenedicarboxylic acid), were successfully synthesized and characterized. Compound 1 exhibits 1D chain structure containing [Zn(L1)2] unit. Compound 2 shows a 2D plane with the ratio of Zn:L2 was 1:1 since L2 can match the positive charge of Zn(II). When L1 and o-bdc2– were presented together, Zn(II) centers are linked by L1 to form 2D double layers, then o-bdc2– further connected the adjacent layers to generate final 3D framework of 3. The ligand charges may play key roles in tuning the frameworks of compounds 13. The solid state luminescence of compounds 13 and all ligands have been investigated here. Full article
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Open AccessArticle MOCVD Growth of InGaAs/GaAs/AlGaAs Laser Structures with Quantum Wells on Ge/Si Substrates
Crystals 2018, 8(8), 311; https://doi.org/10.3390/cryst8080311
Received: 4 July 2018 / Revised: 20 July 2018 / Accepted: 26 July 2018 / Published: 31 July 2018
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Abstract
The paper presents the results of the application of MOCVD growth technique for formation of the GaAs/AlAs laser structures with InGaAs quantum wells on Si substrates with a relaxed Ge buffer. The fabricated laser diodes were of micro-striped type designed for the operation
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The paper presents the results of the application of MOCVD growth technique for formation of the GaAs/AlAs laser structures with InGaAs quantum wells on Si substrates with a relaxed Ge buffer. The fabricated laser diodes were of micro-striped type designed for the operation under the electrical pumping. Influence of the Si substrate offcut from the [001] direction, thickness of a Ge buffer and insertion of the AlAs/GaAs superlattice between Ge and GaAs on the structural and optical properties of fabricated samples was studied. The measured threshold current densities at room temperatures were 5.5 kA/cm2 and 20 kA/cm2 for lasers operating at 0.99 μm and 1.11 μm respectively. In order to obtain the stimulated emission at wavelengths longer than 1.1 μm, the InGaAs quantum well laser structures with high In content and GaAsP strain-compensating layers were grown both on Ge/Si and GaAs substrates. Structures grown on GaAs exhibited stimulated emission under optical pumping at the wavelengths of up to 1.24 μm at 300 K while those grown on Ge/Si substrates emitted at shorter wavelengths of up to 1.1 μm and only at 77 K. The main reasons for such performance worsening and also some approaches to overcome them are discussed. The obtained results have shown that monolithic integration of direct-gap A3B5 compounds on Si using MOCVD technology is rather promising approach for obtaining the Si-compatible on-chip effective light source. Full article
(This article belongs to the Special Issue MOVPE Growth of Crystalline Film)
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Open AccessFeature PaperArticle Optical Properties of KTP Crystals and Their Potential for Terahertz Generation
Crystals 2018, 8(8), 310; https://doi.org/10.3390/cryst8080310
Received: 30 June 2018 / Revised: 23 July 2018 / Accepted: 28 July 2018 / Published: 31 July 2018
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Abstract
High nonlinearity, wide transparency range and optical quality allowed potassium titanyl phosphate (KTiOPO4, KTP) crystals to be used in a wide range of nonlinear applications. The success of KTP usage in the visible and infrared (IR) ranges drives interest in applying
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High nonlinearity, wide transparency range and optical quality allowed potassium titanyl phosphate (KTiOPO4, KTP) crystals to be used in a wide range of nonlinear applications. The success of KTP usage in the visible and infrared (IR) ranges drives interest in applying it at longer wavelengths, that is, in the terahertz (THz) range. We use THz optical properties of KTP crystals measured by terahertz time-domain spectrometer (THz-TDS) and refractive index approximated in the form of Sellmeier equations to investigate KTP application possibilities for IR-to-THz and THz-to-THz wave conversion. As a result, phase matching for sf → f and sf → s types of difference frequency generation (DFG) of Ti:Sapphire laser (at the wavelengths of 0.65, 0.8 and 1.1 µm) is found possible, as well as second harmonic generation (SHG) of THz waves by f + sf type of interaction in the XZ principle plane of the crystal. Terahertz wave generation by phase-matched parametric processes in KTP demonstrates evident advantages in comparison with that of widely used MgO-doped LiNbO3 crystals. Full article
(This article belongs to the Special Issue KTP Crystal for Nonlinear Optical and Electrooptic Applications)
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Open AccessArticle Mechanochemical Synthesis of a Cocrystal of Two Supramolecular Hydrogen-Bonded Aggregates of 1,3,6,8-Tetraazatricyclo[4.3.1.13,8]undecane (TATU) with 4-tert-Butylphenol Bearing Different Hydrogen Bonding Interactions
Crystals 2018, 8(8), 309; https://doi.org/10.3390/cryst8080309
Received: 2 July 2018 / Revised: 13 July 2018 / Accepted: 20 July 2018 / Published: 30 July 2018
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Abstract
The synthesis and single crystal structure of a new cocrystal, which is composed of OHphenolic∙∙∙OHphenolic∙∙∙Naminalic supramolecular heterosynthons assembled from 4-tert-butylphenol and the macrocyclic aminal TATU, is presented. This cocrystal was prepared by solvent-free assisted grinding, which is a commonly used mechanochemical
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The synthesis and single crystal structure of a new cocrystal, which is composed of OHphenolic∙∙∙OHphenolic∙∙∙Naminalic supramolecular heterosynthons assembled from 4-tert-butylphenol and the macrocyclic aminal TATU, is presented. This cocrystal was prepared by solvent-free assisted grinding, which is a commonly used mechanochemical method. Crystal structure, supramolecular assembly through hydrogen bonding interactions as well as the physical and spectroscopic properties of the title cocrystal are presented in this paper. Full article
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Open AccessArticle Habit Change of Monoclinic Hydroxyapatite Crystals Growing from Aqueous Solution in the Presence of Citrate Ions: The Role of 2D Epitaxy
Crystals 2018, 8(8), 308; https://doi.org/10.3390/cryst8080308
Received: 9 July 2018 / Revised: 25 July 2018 / Accepted: 25 July 2018 / Published: 27 July 2018
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Abstract
Calcium hydroxyapatite (HAp) has been crystallized from aqueous solutions in the presence of citrate ions, in two temperature intervals. At lower temperature, where citrate could form the stable 3D-ordered phase Ca-citrate-tetrahydrate (Ca-Cit-TH), only the monoclinic (P21/c) HAp polymorph occurs
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Calcium hydroxyapatite (HAp) has been crystallized from aqueous solutions in the presence of citrate ions, in two temperature intervals. At lower temperature, where citrate could form the stable 3D-ordered phase Ca-citrate-tetrahydrate (Ca-Cit-TH), only the monoclinic (P21/c) HAp polymorph occurs and assumes the shape of fence-like aggregates, built by sharply [010] elongated lamellae dominated by the pinacoid {001}. This pronounced anisotropic growth habit is compared with the usually considered rod-like pseudo-hexagonal occurring in pure aqueous solution growth. The habit change is interpreted by assuming that 2D islands of Ca-citrate-tetrahydrate can be adsorbed as epi- monolayers of thickness d001 onto the different growth forms: {001}, {100}, {102¯}, {010}, and {101¯} of HAp. A comparison is made among the corresponding coincidence lattices, in order to explain on reticular basis the selective adsorption of citrate on the {001} HAp form. The role exerted by the 2D-epitaxially adsorbed Ca-Cit-TH as a “mortar” in the monoclinic HAp “brick” assembly is outlined as well. Full article
(This article belongs to the Special Issue Biomimetic Growth of Calcium Phosphate Crystals)
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Open AccessArticle The Mechanical Properties and Elastic Anisotropies of Cubic Ni3Al from First Principles Calculations
Crystals 2018, 8(8), 307; https://doi.org/10.3390/cryst8080307
Received: 21 June 2018 / Revised: 20 July 2018 / Accepted: 22 July 2018 / Published: 25 July 2018
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Abstract
Ni3Al-based superalloys have excellent mechanical properties which have been widely used in civilian and military fields. In this study, the mechanical properties of the face-centred cubic structure Ni3Al were investigated by a first principles study based on density functional
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Ni3Al-based superalloys have excellent mechanical properties which have been widely used in civilian and military fields. In this study, the mechanical properties of the face-centred cubic structure Ni3Al were investigated by a first principles study based on density functional theory (DFT), and the generalized gradient approximation (GGA) was used as the exchange-correlation function. The bulk modulus, Young’s modulus, shear modulus and Poisson’s ratio of Ni3Al polycrystal were calculated by Voigt-Reuss approximation method, which are in good agreement with the existing experimental values. Moreover, directional dependences of bulk modulus, Young’s modulus, shear modulus and Poisson’s ratio of Ni3Al single crystal were explored. In addition, the thermodynamic properties (e.g., Debye temperature) of Ni3Al were investigated based on the calculated elastic constants, indicating an improved accuracy in this study, verified with a small deviation from the previous experimental value. Full article
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Open AccessArticle Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels?
Crystals 2018, 8(8), 306; https://doi.org/10.3390/cryst8080306
Received: 6 July 2018 / Revised: 20 July 2018 / Accepted: 23 July 2018 / Published: 25 July 2018
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Abstract
Secondary cracks are known to absorb energy, retard primary crack propagation and initiate at lower loads than primary cracks. They are observed more often in hot-rolled than in hot-extruded ODS steels. In this work, the microstructural factors responsible for this observation are investigated.
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Secondary cracks are known to absorb energy, retard primary crack propagation and initiate at lower loads than primary cracks. They are observed more often in hot-rolled than in hot-extruded ODS steels. In this work, the microstructural factors responsible for this observation are investigated. Better understanding of these factors can lead to tailoring of improved ODS steels. Fracture toughness testing of two batches of 13Cr ODS steel, one hot-rolled and the other hot-extruded, was carried out. The fracture behaviour of secondary cracks was investigated using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Crystallographic texture and grain morphology play a predominant role in preventing secondary cracks in hot-extruded ODS steels. At lower temperatures, secondary cracks occur predominantly via transgranular cleavage. The fracture mode changes to ductile and intergranular at higher temperatures. Full article
(This article belongs to the Special Issue Nanostructured Ferritic Alloys)
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Open AccessCommunication Solvothermal Synthesis, Crystal Structure, and Characterization of a Heterometallic Iodoplumbate
Crystals 2018, 8(8), 305; https://doi.org/10.3390/cryst8080305
Received: 28 June 2018 / Revised: 17 July 2018 / Accepted: 23 July 2018 / Published: 25 July 2018
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Abstract
The design and synthesis of heterometallic iodoplumbates have attracted much interest due to the diverse structures and significant physical properties. A Pb/CuI/I heterometallic iodoplumbate, [PbCu2I4(bipy)]n (1) (bipy = 2,2′-bipyridine), has been synthesized by solvothermal
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The design and synthesis of heterometallic iodoplumbates have attracted much interest due to the diverse structures and significant physical properties. A Pb/CuI/I heterometallic iodoplumbate, [PbCu2I4(bipy)]n (1) (bipy = 2,2′-bipyridine), has been synthesized by solvothermal reaction of PbI2, Cu(NO3)2, bipy, and HI, and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analysis. This compound exhibits one-dimensional neutral structure, which is built upon the linkages of Pb/I chains, Cu/I chains, and Cu(bipy) units. The yellow compound has an optical bandgap of 2.32 eV and shows fluorescent emission at 610 nm which is assigned to iodide-to-copper charge transfer. Full article
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Open AccessArticle Simultaneous Generation of Two Orthogonally Polarized Terahertz Waves by Stimulated Polariton Scattering with a Periodically Poled LiNbO3 Crystal
Crystals 2018, 8(8), 304; https://doi.org/10.3390/cryst8080304
Received: 7 April 2018 / Revised: 20 July 2018 / Accepted: 20 July 2018 / Published: 24 July 2018
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Abstract
We present a theoretical investigation of the simultaneous generation of two orthogonally polarized terahertz (THz) waves by stimulated polariton scattering (SPS) with a periodically poled LiNbO3 (PPLN) crystal. The two orthogonally polarized THz waves are generated from SPS with A1 and
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We present a theoretical investigation of the simultaneous generation of two orthogonally polarized terahertz (THz) waves by stimulated polariton scattering (SPS) with a periodically poled LiNbO3 (PPLN) crystal. The two orthogonally polarized THz waves are generated from SPS with A1 and E symmetric transverse optical (TO) modes in a LiNbO3 crystal, respectively. The parallel polarized THz wave is generated from A1 symmetric TO modes with type-0 phase-matching of e = e + e, and the perpendicular polarized THz wave is generated from E symmetric TO modes with type-I phase-matching of e = o + o. The two types of phase-matching of e = e + e and e = o + o can be almost satisfied simultaneously by accurately selecting the poling period of the PPLN crystal. We calculate the photon flux density of the two orthogonally polarized THz waves by solving the coupled wave equations. The calculation results indicate that the two orthogonally polarized THz waves can be efficiently generated, and the relative intensities between the two orthogonally polarized THz waves can be modulated. Full article
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