Open AccessArticle
Microstructure and Dielectric Properties of PTFE-Based Composites Filled by Micron/Submicron-Blended CCTO
Crystals 2017, 7(5), 126; doi:10.3390/cryst7050126 (registering DOI) -
Abstract
This paper investigated a polymer-based composite by homogeneously embedding calcium copper titanate (CaCu3Ti4O12; CCTO) fillers into a polytetrafluoroethylene matrix. We observed the composite filled by CCTO powder at different sizes. The particle size effects of the CCTO
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This paper investigated a polymer-based composite by homogeneously embedding calcium copper titanate (CaCu3Ti4O12; CCTO) fillers into a polytetrafluoroethylene matrix. We observed the composite filled by CCTO powder at different sizes. The particle size effects of the CCTO filling, including single-size particle filling and co-blending filling, on the microstructure and dielectric properties of the composite were discussed. The dielectric performance of the composite was investigated within the frequency range of 100 Hz to 1 MHz. Results showed that the composite filled by micron/submicron-blended CCTO particles had the highest dielectric constant (εr = 25.6 at 100 Hz) and almost the same dielectric loss (tanδ = 0.1 at 100 Hz) as the composite filled by submicron CCTO particles at the same volume percentage content. We researched the theoretical reason of the high permittivity and low dielectric loss. We proved that it was effective in improving the dielectric property of the polymer-based composite by co-blending filling in this experiment. Full article
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Open AccessArticle
Synthesis and Characterization of New Sr3(BO3)2 Crystal for Stimulated Raman Scattering Applications
Crystals 2017, 7(5), 125; doi:10.3390/cryst7050125 -
Abstract
A new kind of borate crystalline material Sr3(BO3)2 with a similar calcite type structure was designed and synthesized by solid state reaction method, moreover, the single crystal growth was attempted with traditional Czochralski pulling method. Crystal phase of
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A new kind of borate crystalline material Sr3(BO3)2 with a similar calcite type structure was designed and synthesized by solid state reaction method, moreover, the single crystal growth was attempted with traditional Czochralski pulling method. Crystal phase of Sr3(BO3)2 was investigated by using X-Ray powder diffraction (XRPD) at room temperature and found similar to Ca3(BO3)2 crystal with space group of R-3c. The phase stability was studied by means of thermogravimetric differential thermal analysis (TG/DTA) and high temperature XRPD up to 1350 °C, where an obvious endothermic peak was observed in DTA curve around 1250 °C, and weak splits of diffraction peaks were found at temperatures above 1250 °C, indicating the existence of structure transformation for Sr3(BO3)2 crystal. Raman properties were studied experimentally and theoretically by using density functional perturbation theory, though the strongest frequency shift of Sr3(BO3)2 crystal (900 cm−1) was comparable to that of Ca3(BO3)2 (927 cm−1), the line width of the strongest Raman peak obtained for Sr3(BO3)2 (5.72 cm−1) was much lower than Ca3(BO3)2 (7.01 cm−1), indicating a larger Raman gain for Sr3(BO3)2 crystal, which would be favorable for stimulated Raman scattering application. Full article
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Open AccessArticle
Nanoscale Sensing Using Point Defects in Single-Crystal Diamond: Recent Progress on Nitrogen Vacancy Center-Based Sensors
Crystals 2017, 7(5), 124; doi:10.3390/cryst7050124 -
Abstract
Individual, luminescent point defects in solids, so-called color centers, are atomic-sized quantum systems enabling sensing and imaging with nanoscale spatial resolution. In this overview, we introduce nanoscale sensing based on individual nitrogen vacancy (NV) centers in diamond. We discuss two central challenges of
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Individual, luminescent point defects in solids, so-called color centers, are atomic-sized quantum systems enabling sensing and imaging with nanoscale spatial resolution. In this overview, we introduce nanoscale sensing based on individual nitrogen vacancy (NV) centers in diamond. We discuss two central challenges of the field: first, the creation of highly-coherent, shallow NV centers less than 10 nm below the surface of a single-crystal diamond; second, the fabrication of tip-like photonic nanostructures that enable efficient fluorescence collection and can be used for scanning probe imaging based on color centers with nanoscale resolution. Full article
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Open AccessArticle
Topology and Holonomy in Discrete-time Quantum Walks
Crystals 2017, 7(5), 122; doi:10.3390/cryst7050122 -
Abstract
We present a research article which formulates the milestones for the understanding and characterization of holonomy and topology of a discrete-time quantum walk architecture, consisting of a unitary step given by a sequence of two non-commuting rotations in parameter space. Unlike other similar
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We present a research article which formulates the milestones for the understanding and characterization of holonomy and topology of a discrete-time quantum walk architecture, consisting of a unitary step given by a sequence of two non-commuting rotations in parameter space. Unlike other similar systems recently studied in detail in the literature, this system does not present continous 1D topological boundaries, it only presents a discrete number of Dirac points where the quasi-energy gap closes. At these discrete points, the topological winding number is not defined. Therefore, such discrete points represent topological boundaries of dimension zero, and they endow the system with a non-trivial topology. We illustrate the non-trivial character of the system by calculating the Zak phase. We discuss the prospects of this system, we propose a suitable experimental scheme to implement these ideas, and we present preliminary experimental data. Full article
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Open AccessArticle
Effects of Al and N2 Flow Sequences on the Interface Formation of AlN on Sapphire by EVPE
Crystals 2017, 7(5), 123; doi:10.3390/cryst7050123 -
Abstract
The interface formation mechanisms of AlN films on sapphire substrates grown by the elementary source vapor phase epitaxy (EVPE) method, which is a new AlN bulk fabrication method using Al and N2 as precursors, are investigated. Supplying N2 after the substrate
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The interface formation mechanisms of AlN films on sapphire substrates grown by the elementary source vapor phase epitaxy (EVPE) method, which is a new AlN bulk fabrication method using Al and N2 as precursors, are investigated. Supplying N2 after the substrate temperature reaches the growth temperature [Process N2(GT)] causes the interface to become rough due to the thermal decomposition of sapphire. Self-separation occasionally occurs with the Process N2(GT), suggesting that the rough interface generates self-separating films with little strain. On the other hand, supplying N2 beginning at room temperature forms a relatively smooth interface with voids, which can be realized by the reaction between a nitrided sapphire surface and an Al source. Full article
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Open AccessFeature PaperArticle
Crystal and Magnetic Structures in Layered, Transition Metal Dihalides and Trihalides
Crystals 2017, 7(5), 121; doi:10.3390/cryst7050121 -
Abstract
Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study
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Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study of low dimensional physics and the design of functional heterostructures. Binary compounds with the compositions MX2 and MX3 where M is a metal cation and X is a halogen anion often form such structures. Magnetism can be incorporated by choosing a transition metal with a partially filled d-shell for M, enabling ferroic responses for enhanced functionality. Here a brief overview of binary transition metal dihalides and trihalides is given, summarizing their crystallographic properties and long-range-ordered magnetic structures, focusing on those materials with layered crystal structures and partially filled d-shells required for combining low dimensionality and cleavability with magnetism. Full article
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Open AccessFeature PaperArticle
Validation of the Concurrent Atomistic-Continuum Method on Screw Dislocation/Stacking Fault Interactions
Crystals 2017, 7(5), 120; doi:10.3390/cryst7050120 -
Abstract
Dislocation/stacking fault interactions play an important role in the plastic deformation of metallic nanocrystals and polycrystals. These interactions have been explored in atomistic models, which are limited in scale length by high computational cost. In contrast, multiscale material modeling approaches have the potential
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Dislocation/stacking fault interactions play an important role in the plastic deformation of metallic nanocrystals and polycrystals. These interactions have been explored in atomistic models, which are limited in scale length by high computational cost. In contrast, multiscale material modeling approaches have the potential to simulate the same systems at a fraction of the computational cost. In this paper, we validate the concurrent atomistic-continuum (CAC) method on the interactions between a lattice screw dislocation and a stacking fault (SF) in three face-centered cubic metallic materials—Ni, Al, and Ag. Two types of SFs are considered: intrinsic SF (ISF) and extrinsic SF (ESF). For the three materials at different strain levels, two screw dislocation/ISF interaction modes (annihilation of the ISF and transmission of the dislocation across the ISF) and three screw dislocation/ESF interaction modes (transformation of the ESF into a three-layer twin, transformation of the ESF into an ISF, and transmission of the dislocation across the ESF) are identified. Our results show that CAC is capable of accurately predicting the dislocation/SF interaction modes with greatly reduced DOFs compared to fully-resolved atomistic simulations. Full article
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Open AccessArticle
HPHT Diamond Crystallization in the Mg-Si-C System: Effect of Mg/Si Composition
Crystals 2017, 7(5), 119; doi:10.3390/cryst7050119 -
Abstract
Crystallization of diamond in the Mg-Si-C system has been studied at 7.5 GPa and 1800 °C with the Mg-Si compositions spanning the range from Mg-C to Si-C end-systems. It is found that as Si content of the system increases from 0 to 2
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Crystallization of diamond in the Mg-Si-C system has been studied at 7.5 GPa and 1800 °C with the Mg-Si compositions spanning the range from Mg-C to Si-C end-systems. It is found that as Si content of the system increases from 0 to 2 wt %, the degree of the graphite-to-diamond conversion increases from about 50 to 100% and remains at about this level up to 20 wt % Si. A further increase in Si content of the system leads to a decrease in the graphite-to-diamond conversion degree down to complete termination of diamond synthesis at Si content >50 wt %. Depending on the Si content crystallization of diamond, joint crystallization of diamond and silicon carbide and crystallization of silicon carbide only are found to take place. The cubic growth of diamond, typical of the Mg-C system, transforms to the cube-octahedron upon adding 1 wt % Si and then to the octahedron at a Si content of 2 wt % and higher. The crystallized diamonds are studied by a suite of optical spectroscopy techniques and the major characteristics of their defect-and-impurity structure are revealed. The correlations between the Si content of the Mg-Si-C system and the properties of the produced diamond crystals are established. Full article
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Open AccessArticle
Study on Micro Morphology of Potassium Dihydrogen Phosphate Crystals Grown at Elevated Temperatures
Crystals 2017, 7(4), 118; doi:10.3390/cryst7040118 -
Abstract
Potassium dihydrogen phosphate crystals were grown around 65 °C. The growth rate of KDP (KH2PO4) crystal prismatic faces was measured by laser polarization interference system. The surface micro morphology of KDP crystals grown at various supersaturation were comparatively observed
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Potassium dihydrogen phosphate crystals were grown around 65 °C. The growth rate of KDP (KH2PO4) crystal prismatic faces was measured by laser polarization interference system. The surface micro morphology of KDP crystals grown at various supersaturation were comparatively observed by atomic force microscope systematically. The results show that the growth rate increased with the rise of supersaturation, the aspect ratio reduced. The steps bunching height occurred at σ = 0.03 and 0.04. At this point, the steps height reached maximum value about 11.3~24.4 nm. The step bunching was almost constant at higher supersaturation. A few pits and holes appeared on the crystal surface at high supersaturation. In our opinion, the forming of these pits and holes might be related with the step movement and 2D nuclei mechanism. Full article
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Open AccessReview
The Synergetic Effects of Combining Structural Biology and EPR Spectroscopy on Membrane Proteins
Crystals 2017, 7(4), 117; doi:10.3390/cryst7040117 -
Abstract
Protein structures as provided by structural biology such as X-ray crystallography, cryo-electron microscopy and NMR spectroscopy are key elements to understand the function of a protein on the molecular level. Nonetheless, they might be error-prone due to crystallization artifacts or, in particular in
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Protein structures as provided by structural biology such as X-ray crystallography, cryo-electron microscopy and NMR spectroscopy are key elements to understand the function of a protein on the molecular level. Nonetheless, they might be error-prone due to crystallization artifacts or, in particular in case of membrane-imbedded proteins, a mostly artificial environment. In this review, we will introduce different EPR spectroscopy methods as powerful tools to complement and validate structural data gaining insights in the dynamics of proteins and protein complexes such that functional cycles can be derived. We will highlight the use of EPR spectroscopy on membrane-embedded proteins and protein complexes ranging from receptors to secondary active transporters as structural information is still limited in this field and the lipid environment is a particular challenge. Full article
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Open AccessArticle
Thermochemical Wear of Single Crystal Diamond Catalyzed by Ferrous Materials at Elevated Temperature
Crystals 2017, 7(4), 116; doi:10.3390/cryst7040116 -
Abstract
Single crystal diamond has been recognized as the optimal tool material in ultra-precision machining. However, the excessive tool wear prevents it from cutting ferrous materials. This paper conducts a series of thermal analysis tests under the conditions of different gas atmospheres, heating temperatures,
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Single crystal diamond has been recognized as the optimal tool material in ultra-precision machining. However, the excessive tool wear prevents it from cutting ferrous materials. This paper conducts a series of thermal analysis tests under the conditions of different gas atmospheres, heating temperatures, crystallographic planes and workpiece materials, in order to clarify the details of thermochemical wear of diamond catalyzed by iron at elevated temperature. Raman scattering analysis was performed to identify the transformation of diamond crystal structure. Energy dispersive X-ray analysis was used to detect the change in chemical composition of the work material. X-ray photoelectron spectroscopy was adopted to confirm the resultants of interfacial thermochemical reactions. The experimental results revealed that the diamond wear included the graphitization, diffusion and oxidation. Temperature was considered as the key factor affecting these wear mechanisms. The initial graphitization temperatures of diamond catalyzed by iron under different conditions were obtained, and the graphitized degree relied heavily on the crystallographic plane while being insensitive to the workpiece material. The diffusion wear rule was preliminarily achieved by the established prediction model of the carbon atoms diffusing into the iron lattice, and the types and resultants of interfacial chemical reactions were deduced. Full article
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Open AccessReview
Morphological Control of Polymer Spherulites via Manipulating Radial Lamellar Organization upon Evaporative Crystallization: A Mini Review
Crystals 2017, 7(4), 115; doi:10.3390/cryst7040115 -
Abstract
Various spherulites or spherulitic crystals are widely encountered in polymeric materials when crystallized from viscous melts or concentrated solutions. However, the microstructures and growth processes are quite complicated and remain unclear and, thus, the formation mechanisms are rather elusive. Here, diverse kinds of
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Various spherulites or spherulitic crystals are widely encountered in polymeric materials when crystallized from viscous melts or concentrated solutions. However, the microstructures and growth processes are quite complicated and remain unclear and, thus, the formation mechanisms are rather elusive. Here, diverse kinds of spherulitic growths and patterns of typical polyesters via evaporative crystallization of solution-cast thin films are delineated after detailed investigating the microstructures and in situ following the developing processes. The spherulitic crystals produced under different evaporation conditions reflect variously optical features, such as the usual Maltese Cross, non-birefringent or half-birefringent concentric-rings, extinction spiral banding, and even a nested ring-banded pattern. Polymer spherulites are composed of stacks of radial fibrillar lamellae, and the diversity of bewitchingly spherulitic morphologies is dominated by the arrangement and organization of radial lamellae, which is predicted to be tunable by modulating the evaporative crystallization processes. The emergence of various types of spherulitic morphologies of the same polymer is attributed to a precise manipulation of the radial lamellar organization by a coupling of structural features and specific crystal evolving courses under confined evaporation environments. The present findings improve dramatically the understanding of the structural development and crystallization mechanism for emergence of diverse polymer spherulitic morphologies. Full article
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Open AccessArticle
Fabrication of Low Dislocation Density, Single-Crystalline Diamond via Two-Step Epitaxial Lateral Overgrowth
Crystals 2017, 7(4), 114; doi:10.3390/cryst7040114 -
Abstract
Continuous diamond films with low dislocation density were obtained by two-step epitaxial lateral overgrowth (ELO). Grooves were fabricated by inductively coupled plasma etching. Mo/Pd stripes sputtered in the grooves were used to inhibit the propagation of dislocations originating from the diamond substrate. Coalescent
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Continuous diamond films with low dislocation density were obtained by two-step epitaxial lateral overgrowth (ELO). Grooves were fabricated by inductively coupled plasma etching. Mo/Pd stripes sputtered in the grooves were used to inhibit the propagation of dislocations originating from the diamond substrate. Coalescent diamond films were achieved by ELO via microwave plasma-enhanced chemical vapor deposition. Etch-pits were formed intentionally to characterize the quality of the epitaxial films and distinguish different growth areas, as dislocations served as preferential sites for etching. In the window regions, a high density of dislocations, displayed as dense etch-pits, was generated. By contrast, the etch-pit density was clearly lower in the overgrowth regions. After the second ELO step, the dislocation density was further decreased. Raman spectroscopy analysis suggested that the lateral overgrowth of diamond is a promising method for achieving low dislocation density films. Full article
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Open AccessArticle
Synthesis, Crystal Structure, Photoluminescence Properties and Antibacterial Activity of a Zn(II) Coordination Polymer Based on a Paddle-Wheel Cluster
Crystals 2017, 7(4), 112; doi:10.3390/cryst7040112 -
Abstract
A binuclear Zn(II) complex of formula {[Zn(BCPPO)H2O]•3C2H5OH}n (1) [H2BCPPO = Bis 4-carboxyphenyl phenyl phosphine oxide] has been synthesized and structurally characterized by single crystal X-ray diffraction, Powder X-ray diffraction (PXRD), Thermogravimetric analysis (TG), Elemental analysis (EA) and Infrared spectroscopy (IR). As revealed by
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A binuclear Zn(II) complex of formula {[Zn(BCPPO)H2O]•3C2H5OH}n (1) [H2BCPPO = Bis 4-carboxyphenyl phenyl phosphine oxide] has been synthesized and structurally characterized by single crystal X-ray diffraction, Powder X-ray diffraction (PXRD), Thermogravimetric analysis (TG), Elemental analysis (EA) and Infrared spectroscopy (IR). As revealed by the single crystal X-ray diffraction, in the binuclear Zn(II) complex, two paddle-wheel-type Zn2 units were connected by four BCPPO ligands to form one-dimensional chains. Their antibacterial activity was evaluated by using a minimal bactericidal concentration (MBC) benchmark. The binuclear Zn(II) complex shows excellent and long-term antibacterial activity against Escherichia coli and Staphylococcus aureus. In addition, the Photoluminescence properties of the binuclear Zn(II) complex was also investigated. Full article
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Open AccessArticle
Chiral Optical Tamm States: Temporal Coupled-Mode Theory
Crystals 2017, 7(4), 113; doi:10.3390/cryst7040113 -
Abstract
The chiral optical Tamm state (COTS) is a special localized state at the interface of a handedness-preserving mirror and a structurally chiral medium such as a cholesteric liquid crystal or a chiral sculptured thin film. The spectral behavior of COTS, observed as reflection
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The chiral optical Tamm state (COTS) is a special localized state at the interface of a handedness-preserving mirror and a structurally chiral medium such as a cholesteric liquid crystal or a chiral sculptured thin film. The spectral behavior of COTS, observed as reflection resonances, is described by the temporal coupled-mode theory. Mode coupling is different for two circular light polarizations because COTS has a helical structure replicating that of the cholesteric. The mode coupling for co-handed circularly polarized light exponentially attenuates with the cholesteric layer thickness since the COTS frequency falls into the stop band. Cross-handed circularly polarized light freely goes through the cholesteric layer and can excite COTS when reflected from the handedness-preserving mirror. The coupling in this case is proportional to anisotropy of the cholesteric and theoretically only anisotropy in magnetic permittivity can ultimately cancel this coupling. These two couplings being equal result in a polarization crossover (the Kopp–Genack effect) for which a linear polarization is optimal to excite COTS. The corresponding cholesteric thickness and scattering matrix for COTS are generally described by simple expressions. Full article
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Open AccessFeature PaperReview
Functionalisation of Colloidal Transition Metal Sulphides Nanocrystals: A Fascinating and Challenging Playground for the Chemist
Crystals 2017, 7(4), 110; doi:10.3390/cryst7040110 -
Abstract
Metal sulphides, and in particular transition metal sulphide colloids, are a broad, versatile and exciting class of inorganic compounds which deserve growing interest and attention ascribable to the functional properties that many of them display. With respect to their oxide homologues, however, they
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Metal sulphides, and in particular transition metal sulphide colloids, are a broad, versatile and exciting class of inorganic compounds which deserve growing interest and attention ascribable to the functional properties that many of them display. With respect to their oxide homologues, however, they are characterised by noticeably different chemical, structural and hence functional features. Their potential applications span several fields, and in many of the foreseen applications (e.g., in bioimaging and related fields), the achievement of stable colloidal suspensions of metal sulphides is highly desirable or either an unavoidable requirement to be met. To this aim, robust functionalisation strategies should be devised, which however are, with respect to metal or metal oxides colloids, much more challenging. This has to be ascribed, inter alia, also to the still limited knowledge of the sulphides surface chemistry, particularly when comparing it to the better established, though multifaceted, oxide surface chemistry. A ground-breaking endeavour in this field is hence the detailed understanding of the nature of the complex surface chemistry of transition metal sulphides, which ideally requires an integrated experimental and modelling approach. In this review, an overview of the state-of-the-art on the existing examples of functionalisation of transition metal sulphides is provided, also by focusing on selected case studies, exemplifying the manifold nature of this class of binary inorganic compounds. Full article
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Open AccessArticle
First-Principles Investigations on Structural and Elastic Properties of Orthorhombic TiAl under Pressure
Crystals 2017, 7(4), 111; doi:10.3390/cryst7040111 -
Abstract
The effects of pressure on the structural and elastic properties of orthorhombic TiAl are investigated using first-principles calculations based on density functional theory within the projector augmented wave method. The calculated lattice parameters at 0 GPa are in good agreement with the available
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The effects of pressure on the structural and elastic properties of orthorhombic TiAl are investigated using first-principles calculations based on density functional theory within the projector augmented wave method. The calculated lattice parameters at 0 GPa are in good agreement with the available experimental data. The pressure dependence of the normalized lattice parameters and the single crystal elastic constants are investigated. By the elastic stability criteria under pressure, it is found that orthorhombic TiAl is mechanically stable under pressure up to 100 GPa. The elastic moduli and Poisson’s ratio under pressure up to 100 GPa are calculated using the Hill average method. The ductility/brittleness under pressure are evaluated, and a critical pressure for brittle-to-ductile transition is found to be 40 GPa. The elastic anisotropy and Debye temperature under different pressure are estimated from the calculations. Full article
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Open AccessArticle
Li2HgMS4 (M = Si, Ge, Sn): New Quaternary Diamond-Like Semiconductors for Infrared Laser Frequency Conversion
Crystals 2017, 7(4), 107; doi:10.3390/cryst7040107 -
Abstract
A new family of quaternary diamond-like semiconductors (DLSs), Li2HgMS4 (M = Si, Ge, Sn), were successfully discovered for the first time. All of them are isostructural and crystallize in the polar space group (Pmn21). Seen from
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A new family of quaternary diamond-like semiconductors (DLSs), Li2HgMS4 (M = Si, Ge, Sn), were successfully discovered for the first time. All of them are isostructural and crystallize in the polar space group (Pmn21). Seen from their structures, they exhibit a three-dimensional (3D) framework structure that is composed of countless 2D honeycomb layers stacked along the c axis. An interesting feature, specifically, that the LiS4 tetrahedra connect with each other to build a 2D layer in the ac plane, is also observed. Experimental investigations show that their nonlinear optical responses are about 0.8 for Li2HgSiS4, 3.0 for Li2HgGeS4, and 4.0 for Li2HgSnS4 times that of benchmark AgGaS2 at the 55–88 μm particle size, respectively. In addition, Li2HgSiS4 and Li2HgGeS4 also have great laser-damage thresholds that are about 3.0 and 2.3 times that of powdered AgGaS2, respectively. The above results indicate that title compounds can be expected as promising IR NLO candidates. Full article
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Open AccessArticle
Comparative Interrelationship of the Structural, Nonlinear-Optical and Other Acentric Properties for Oxide, Borate and Carbonate Crystals
Crystals 2017, 7(4), 109; doi:10.3390/cryst7040109 -
Abstract
The structure and the maximal nonlinear optical (NLO) susceptibility χ(2) are tabulated for more 700 acentric binary oxides, 220 crystals of simple, binary and complex borates and for the same set of 110 carbonates, tartrates, formates, oxalates, acetates and fluoride-carbonates used in
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The structure and the maximal nonlinear optical (NLO) susceptibility χ(2) are tabulated for more 700 acentric binary oxides, 220 crystals of simple, binary and complex borates and for the same set of 110 carbonates, tartrates, formates, oxalates, acetates and fluoride-carbonates used in ultraviolet and deep ultraviolet optoelectronics. According to the chemical formula, the structural symbols of these crystals have been plotted on the plane of two minimal oxide bond lengths (OBL). It is shown that acentric crystals are positioned on such plane inside the vertical, horizontal and slope intersected ellipses of “acentricity”. The oxide and borate crystals with moderate NLO susceptibility are found in the central parts of these ellipses intersection and, with low susceptibility, on top, at the bottom and border of the ellipses rosette. The nonpolar fluoride-carbonate crystals with high NLO susceptibility are found in the curve-side rhombic parts of the slope ellipse of “acentricity”. The unmonotonous fuzzy dependence “χ(2)” on the OBL of these crystals is observed, and their clear-cut taxonomy on compounds with π– or σ–oxide bonds is also established. It is shown that the correlations of χ(2) with other acentric properties are nonlinear for the whole set of the oxide crystals having their clear maximum at a certain value of the piezoelectric or electro-optic coefficient. The correlation “hardness–thermoconductivity-fusibility” is plotted for oxide crystals, part of which is used at the creation of self-frequency-doubling solid state lasers. Full article
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Open AccessArticle
Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals
Crystals 2017, 7(4), 108; doi:10.3390/cryst7040108 -
Abstract
Organic charge-transfer (CT) crystals constitute an important class of functional materials, characterized by the directional charge-transfer interaction between π-electron Donor (D) and Acceptor (A) molecules, with the formation of one-dimensional ...DADAD... stacks. Among the many different and often unique phenomena displayed by
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Organic charge-transfer (CT) crystals constitute an important class of functional materials, characterized by the directional charge-transfer interaction between π-electron Donor (D) and Acceptor (A) molecules, with the formation of one-dimensional ...DADAD... stacks. Among the many different and often unique phenomena displayed by this class of crystals, Neutral-Ionic phase transition (NIT) occupies a special place, as it implies a collective electron transfer along the stack. The analysis of such a complex yet fascinating phenomenon has required many years of investigation, and still presents some open questions and challenges. We present an updated and extensive summary of the phenomenology of the temperature induced NIT, with emphasis on the spectroscopic signatures of the transition. A much shorter summary is given for the NIT induced by pressure. Finally, we report on the exploration, by chemical substitution, of the phase space of ...DADAD... CT crystals, aimed at finding materials with important semiconducting or ferroelectric properties, and at understanding the subtle factors determining the crystal packing. Full article
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