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Crystals, Volume 7, Issue 5 (May 2017)

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Cover Story We analysed topologic and holonomic aspects of discrete-time quantum walk architectures. In [...] Read more.
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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
Received: 31 March 2017 / Revised: 21 April 2017 / Accepted: 23 April 2017 / Published: 25 April 2017
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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
(This article belongs to the Special Issue Diamond Crystals)
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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
Received: 7 March 2017 / Revised: 10 April 2017 / Accepted: 19 April 2017 / Published: 26 April 2017
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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
[...] Read more.
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
(This article belongs to the Special Issue Plasticity of Crystals and Interfaces)
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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
Received: 20 March 2017 / Revised: 18 April 2017 / Accepted: 23 April 2017 / Published: 27 April 2017
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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 M X 2 and M X 3 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
(This article belongs to the Special Issue Crystallography of Functional Materials)
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Open AccessArticle Topology and Holonomy in Discrete-time Quantum Walks
Crystals 2017, 7(5), 122; doi:10.3390/cryst7050122
Received: 24 January 2017 / Revised: 28 March 2017 / Accepted: 18 April 2017 / Published: 28 April 2017
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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
[...] Read more.
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
(This article belongs to the Special Issue Topological Crystalline Insulators: Current Progress and Prospects)
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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
Received: 15 March 2017 / Revised: 18 April 2017 / Accepted: 21 April 2017 / Published: 27 April 2017
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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
[...] Read more.
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
(This article belongs to the Special Issue Advances in GaN Crystals and Their Applications)
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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
Received: 8 April 2017 / Revised: 21 April 2017 / Accepted: 24 April 2017 / Published: 28 April 2017
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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
[...] Read more.
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
(This article belongs to the Special Issue Diamond Crystals)
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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
Received: 10 March 2017 / Revised: 24 April 2017 / Accepted: 24 April 2017 / Published: 28 April 2017
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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
[...] Read more.
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
(This article belongs to the Section Crystal Engineering)
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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
Received: 12 March 2017 / Revised: 20 April 2017 / Accepted: 25 April 2017 / Published: 30 April 2017
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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
[...] Read more.
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
(This article belongs to the Special Issue Crystal Structure of Electroceramics) Printed Edition available
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Open AccessArticle Crystallography and Growth of Epitaxial Oxide Films for Fundamental Studies of Cathode Materials Used in Advanced Li-Ion Batteries
Crystals 2017, 7(5), 127; doi:10.3390/cryst7050127
Received: 13 March 2017 / Revised: 25 April 2017 / Accepted: 28 April 2017 / Published: 8 May 2017
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Abstract
Li-ion battery systems, synthesized as epitaxial thin films, can provide powerful insights into their electrochemical processes. Crystallographic analysis shows that many important cathode oxides have an underlying similarity: their structures can be considered as different ordering schemes of Li and transition metal ions
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Li-ion battery systems, synthesized as epitaxial thin films, can provide powerful insights into their electrochemical processes. Crystallographic analysis shows that many important cathode oxides have an underlying similarity: their structures can be considered as different ordering schemes of Li and transition metal ions within a pseudo-cubic sublattice of oxygen anions arranged in a face-center cubic (FCC) fashion. This oxygen sublattice is compatible with SrTiO3 and similar perovskite oxides, thus perovskites can be used as supporting substrates for growing epitaxial cathode films. The predicted epitaxial growth and crystallographic relations were experimentally verified for different oxide films deposited by pulsed laser deposition (PLD) on SrTiO3 or SrRuO3/SrTiO3 of different orientations. The results based on cross-sectional high-resolution TEM of the following films are presented in the paper: (a) trigonal LiCoO2; (b) orthorhombic LiMnO2; (c) monoclinic Li2MnO3; (d) compositionally-complex monoclinic Li1.2Mn0.55Ni0.15Co0.1O2. All results demonstrated the feasibility of epitaxial growth for these materials, with the growth following the predicted cube-on-cube orientation relationship between the cubic and pseudo-cubic oxygen sublattices of a substrate and a film, respectively. Full article
(This article belongs to the Special Issue Crystallography of Functional Materials)
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Open AccessArticle Determination of Supercooling Degree, Nucleation and Growth Rates, and Particle Size for Ice Slurry Crystallization in Vacuum
Crystals 2017, 7(5), 128; doi:10.3390/cryst7050128
Received: 7 April 2017 / Revised: 28 April 2017 / Accepted: 29 April 2017 / Published: 5 May 2017
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Abstract
Understanding the crystallization behavior of ice slurry under vacuum condition is important to the wide application of the vacuum method. In this study, we first measured the supercooling degree of the initiation of ice slurry formation under different stirring rates, cooling rates and
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Understanding the crystallization behavior of ice slurry under vacuum condition is important to the wide application of the vacuum method. In this study, we first measured the supercooling degree of the initiation of ice slurry formation under different stirring rates, cooling rates and ethylene glycol concentrations. Results indicate that the supercooling crystallization pressure difference increases with increasing cooling rate, while it decreases with increasing ethylene glycol concentration. The stirring rate has little influence on supercooling crystallization pressure difference. Second, the crystallization kinetics of ice crystals was conducted through batch cooling crystallization experiments based on the population balance equation. The equations of nucleation rate and growth rate were established in terms of power law kinetic expressions. Meanwhile, the influences of suspension density, stirring rate and supercooling degree on the process of nucleation and growth were studied. Third, the morphology of ice crystals in ice slurry was obtained using a microscopic observation system. It is found that the effect of stirring rate on ice crystal size is very small and the addition of ethylene glycoleffectively inhibits the growth of ice crystals. The results in this paper can provide theoretical guidance and technical support for the development of vacuum icemakers. Full article
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Open AccessArticle Structure Determination and Luminescent Property Studies of the Single Crystal Na3Sm(BO3)2
Crystals 2017, 7(5), 129; doi:10.3390/cryst7050129
Received: 30 January 2017 / Revised: 25 April 2017 / Accepted: 28 April 2017 / Published: 5 May 2017
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Abstract
Sodium samarium borate Na3Sm(BO3)2, was prepared by a flux method and structurally characterized by single-crystal structure analysis for the first time. The results show that it crystallizes in the monoclinic system P21/n,
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Sodium samarium borate Na3Sm(BO3)2, was prepared by a flux method and structurally characterized by single-crystal structure analysis for the first time. The results show that it crystallizes in the monoclinic system P21/n, with a = 6.5667(3) Å, b = 8.7675(4) Å, c = 10.1850(5), β = 90.86°, V = 586.32(5) Å3 and Z = 4. The structure contains NaO7, NaO6, NaO5, SmO8, and BO3 units, which are interconnected via corner- or edge-sharing O atoms into a three-dimensional structure. The excitation spectra, emission spectra, decay time, and Commission International de l’Éclairage (CIE) chromaticity index of Na3Sm(BO3)2 were studied. Under near light excitation (406 nm), the powdered Na3Sm(BO3)2 shows the orange-red emission, which originates from the 4G5/26H9/2 and 4G5/26H7/2 transformation of Sm3+ ion. Full article
(This article belongs to the Special Issue Crystallography of Functional Materials)
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Open AccessArticle A Diagram of the Structure Evolution of Pb(Zn1/3Nb2/3) O3-9%PbTiO3 Relaxor Ferroelectric Crystals with Excellent Piezoelectric Properties
Crystals 2017, 7(5), 130; doi:10.3390/cryst7050130
Received: 27 March 2017 / Revised: 30 April 2017 / Accepted: 1 May 2017 / Published: 8 May 2017
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Abstract
Piezoelectric properties are of significant importance to medical ultrasound, actuators, sensors, and countless other device applications. The mechanism of piezoelectric properties can be deeply understood in light of structure evolutions. In this paper, we report a diagram of the structure evolutions of Pb(Zn
[...] Read more.
Piezoelectric properties are of significant importance to medical ultrasound, actuators, sensors, and countless other device applications. The mechanism of piezoelectric properties can be deeply understood in light of structure evolutions. In this paper, we report a diagram of the structure evolutions of Pb(Zn1/3Nb2/3)0.91Ti0.09O3 (PZN-9PT) crystals with excellent piezoelectric properties among orthorhombic, tetragonal, and cubic phases, with a temperature increasing from room temperature to 220 °C. Through fitting the temperature-dependent XRD curves with Gauss and Lorenz functions, we obtained the evolutions of the content ratio of three kinds of phases (orthorhombic, tetragonal and cubic) and the lattice parameters of the PZN-9PT system with the changes of temperature. The XRD fitting results together with Raman and dielectric spectra show that the phase transitions of PZN-9PT are a typical continuous evolution process. Additionally, resonance and anti-resonance spectra show the excellent piezoelectric properties of these crystals, which probably originate from the nano twin domains, as demonstrated by TEM images. Of particular attention is that the thickness electromechanical coupling factor kt is up to 72%. Full article
(This article belongs to the Special Issue Crystal Structure of Electroceramics) Printed Edition available
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Open AccessArticle Two-Stage Crystallizer Design for High Loading of Poorly Water-Soluble Pharmaceuticals in Porous Silica Matrices
Crystals 2017, 7(5), 131; doi:10.3390/cryst7050131
Received: 30 March 2017 / Revised: 2 May 2017 / Accepted: 5 May 2017 / Published: 9 May 2017
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Abstract
While porous silica supports have been previously studied as carriers for nanocrystalline forms of poorly water-soluble active pharmaceutical ingredients (APIs), increasing the loading of API in these matrices is of great importance if these carriers are to be used in drug formulations. A
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While porous silica supports have been previously studied as carriers for nanocrystalline forms of poorly water-soluble active pharmaceutical ingredients (APIs), increasing the loading of API in these matrices is of great importance if these carriers are to be used in drug formulations. A dual-stage mixed-suspension, mixed-product removal (MSMPR) crystallizer was designed in which the poorly soluble API fenofibrate was loaded into the porous matrices of pore sizes 35 nm–300 nm in the first stage, and then fed to a second stage in which the crystals were further grown in the pores. This resulted in high loadings of over 50 wt % while still producing nanocrystals confined to the pores without the formation of bulk-sized crystals on the surface of the porous silica. The principle was extended to another highly insoluble API, griseofulvin, to improve its loading in porous silica in a benchtop procedure. This work demonstrates a multi-step crystallization principle API in porous silica matrices with loadings high enough to produce final dosage forms of these poorly water-soluble APIs. Full article
(This article belongs to the Special Issue Effects of Confinement and Topography on Crystallization)
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Open AccessArticle Specific Internal Structure of Diamonds from Zarnitsa Kimberlite Pipe
Crystals 2017, 7(5), 133; doi:10.3390/cryst7050133
Received: 30 March 2017 / Revised: 4 May 2017 / Accepted: 7 May 2017 / Published: 11 May 2017
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Abstract
The Zarnitsa kimberlite pipe is one of the largest pipes of the Yakutian diamondiferous province. Currently, some limited published data exists on the diamonds from this deposit. Among the diamond population of this pipe there is a specific series of dark gray to
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The Zarnitsa kimberlite pipe is one of the largest pipes of the Yakutian diamondiferous province. Currently, some limited published data exists on the diamonds from this deposit. Among the diamond population of this pipe there is a specific series of dark gray to black diamonds with transition morphologies between octahedron and rounded rhombic dodecahedron. These diamonds have specific zonal and sectorial mosaic-block internal structures. The inner parts of these crystals have polycrystalline structure with significant misorientations between sub-individuals. The high consistency of the mechanical admixtures (inclusions) in the diamonds cores can cause a high grid stress of the crystal structure and promote the block (polycrystalline) structure of the core components. These diamond crystals have subsequently been formed due to crystallization of bigger sub-individuals on the polycrystalline cores according to the geometric selection law. Full article
(This article belongs to the Special Issue Diamond Crystals)
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Open AccessArticle On the Role of AlN Insertion Layer in Stress Control of GaN on 150-mm Si (111) Substrate
Crystals 2017, 7(5), 134; doi:10.3390/cryst7050134
Received: 12 April 2017 / Revised: 5 May 2017 / Accepted: 9 May 2017 / Published: 12 May 2017
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Abstract
In this study, low-temperature (LT) and high-temperature (HT) AlN insertion layers (ILs) grown at 680 and 970 °C were integrated with 3.7-μm GaN-based heterostructure grown on 150-mm Si (111) substrates by metalorganic chemical vapor deposition. Under a V/III flow ratio of 1960, the
[...] Read more.
In this study, low-temperature (LT) and high-temperature (HT) AlN insertion layers (ILs) grown at 680 and 970 °C were integrated with 3.7-μm GaN-based heterostructure grown on 150-mm Si (111) substrates by metalorganic chemical vapor deposition. Under a V/III flow ratio of 1960, the GaN epilayer with a continuous interface resulting from the LT AlN IL was subject to a compressive stress of −0.109 GPa. However, the GaN epilayer with discontinuous interfaces resulting from the HT AlN IL growth under the same flow ratio was subject to a tensile stress of 0.174 GPa. To realize continuous interfaces between the GaN epilayer and HT AlN IL, a higher V/III ratio of 5960 was utilized to suppress the decomposition of GaN. It results in changing the stress state of the GaN-based heterostructure from tensile to compressive. This strategic finding indicates that a stress-controllable GaN on Si can be achieved via the incorporation of HT AlN ILs. A minimum curvature at 5 km−1 is demonstrated for the 3.7-μm GaN-based heterostructure on a 150-mm Si (111) substrate, which has high potential for power switching device applications. Full article
(This article belongs to the Special Issue Advances in GaN Crystals and Their Applications)
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Open AccessArticle Phase Transition Behavior of the Layered Perovskite CsBi0.6La0.4Nb2O7: A Hybrid Improper Ferroelectric
Crystals 2017, 7(5), 135; doi:10.3390/cryst7050135
Received: 20 April 2017 / Revised: 9 May 2017 / Accepted: 10 May 2017 / Published: 13 May 2017
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Abstract
The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between the temperatures of 25 < T < 850 °C. At ambient temperature, this material
[...] Read more.
The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between the temperatures of 25 < T < 850 °C. At ambient temperature, this material adopts the polar space group P21am; this represents an example of hybrid improper ferroelectricity caused by the interaction of two distinct octahedral tilt modes. Within the limits of our data resolution, the thermal evolution of the crystal structure is consistent with a first-order transition between 700 and 750 °C, with both tilt modes vanishing simultaneously, leading to the aristotype space group P4/mmm. This apparent “avalanche transition” behavior resembles that seen in the related Aurivillius phase SrBi2Nb2O9. Full article
(This article belongs to the Special Issue Crystal Structure of Electroceramics) Printed Edition available
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Open AccessArticle Domain Patterning in Ion-Sliced LiNbO3 Films by Atomic Force Microscopy
Crystals 2017, 7(5), 137; doi:10.3390/cryst7050137
Received: 7 April 2017 / Revised: 2 May 2017 / Accepted: 11 May 2017 / Published: 14 May 2017
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Abstract
Photonic structures denoted as LNOI (LiNbO3-on-insulator) are of considerable interest for integrated optics due to a high refractive-index contrast provided by the interface LiNbO3/insulator. A topical problem for LNOI-based optical waveguides is optical-frequency conversion, in particular realized on ferroelectric
[...] Read more.
Photonic structures denoted as LNOI (LiNbO3-on-insulator) are of considerable interest for integrated optics due to a high refractive-index contrast provided by the interface LiNbO3/insulator. A topical problem for LNOI-based optical waveguides is optical-frequency conversion, in particular realized on ferroelectric domains on the basis of quasi phase-matching principle. This paper presents extended studies on the fabrication of domain patterns by atomic force microscopy (AFM) methods (raster lithography, piezo-force microscopy, conductive AFM) in single-crystal ion-sliced LiNbO3 films forming LNOI sandwiches. A body of data obtained on writing characteristics of domains and specified 1D and 2D domain patterns permitted us to manipulate the domain sizes and shapes. Of special importance is the stability of created patterns, which persist with no degradation during observation times of months. The domain coalescence leading to the transformation of a discrete domain pattern to a continuous one was investigated. This specific effect—found in thin LiNbO3 layers for the first time—was attributed to the grounding of space-charges accumulated on domain walls. Observations of an enhanced static conduction at domain walls exceeding that in surrounding areas by not less than by five orders of magnitude supports this assumption. AFM domain writing in ion-sliced films serves as a basis for studies in nonlinear photonic crystals in integrated optical schemes. Full article
(This article belongs to the Special Issue Lithium Niobate Crystals)
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Open AccessArticle Using Neutron Diffraction to Investigate Texture Evolution During Consolidation of Deuterated Triaminotrinitrobenzene (d-TATB) Explosive Powder
Crystals 2017, 7(5), 138; doi:10.3390/cryst7050138
Received: 10 April 2017 / Revised: 9 May 2017 / Accepted: 10 May 2017 / Published: 14 May 2017
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Abstract
Triaminotrinitrobenzene (TATB) is a highly anisotropic molecular crystal used in several plastic-bonded explosive (PBX) formulations. A complete understanding of the orientation distribution of TATB particles throughout a PBX charge is required to understand spatially variable, anisotropic macroscale properties of the charge. Although texture
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Triaminotrinitrobenzene (TATB) is a highly anisotropic molecular crystal used in several plastic-bonded explosive (PBX) formulations. A complete understanding of the orientation distribution of TATB particles throughout a PBX charge is required to understand spatially variable, anisotropic macroscale properties of the charge. Although texture of these materials can be measured after they have been subjected to mechanical or thermal loads, measuring texture evolution in situ is important in order to identify mechanisms of crystal deformation and reorientation used to better inform thermomechanical models. Neutron diffraction measurements were used to estimate crystallographic reorientation while deuterated TATB (d-TATB) powder was consolidated into a cylindrical pellet via a uniaxial die-pressing operation at room temperature. Both the final texture of the pressed pellet and the in situ evolution of texture during pressing were measured, showing that the d-TATB grains reorient such that (001) poles become preferentially aligned with the pressing direction. A compaction model is used to predict the evolution of texture in the pellet during the pressing process, finding that the original model overpredicted the texture strength compared to these measurements. The theory was extended to account for initial particle shape and pore space, bringing the results into good agreement with the data. Full article
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Open AccessArticle Preparation, Crystal structure and Luminescence Properties of Lanthanide Complexes with 2,4,6-tri(pyridin-2-yl)-1,3,5-triazine and Organic Carboxylic Acid
Crystals 2017, 7(5), 139; doi:10.3390/cryst7050139
Received: 1 April 2017 / Revised: 9 May 2017 / Accepted: 10 May 2017 / Published: 14 May 2017
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Abstract
Five crystal complexes {[Eu2(TPTZ)2(mNBA)6(H2O)2]·2CH3OH}n (1), [Eu(TPTZ)(CF3COO)(H2O)5]·Cl2·CH3CH2OH (2), {[Yb2(TPTZ)2(BDC)3]·2H2O}n (3), [Yb(TPTZ)Cl(H2O)4]·Cl2 (4) and [Er(TPTZ)(TTA)Cl2] (5) (mNBA = m-nitro benzoate, BDC = terephthalate, TTA = thenoyltrifluoroacetone, TPTZ = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) have been synthesized. The single X-ray diffraction reveals that TPTZ is mainly in the trident coordination
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Five crystal complexes {[Eu2(TPTZ)2(mNBA)6(H2O)2]·2CH3OH}n (1), [Eu(TPTZ)(CF3COO)(H2O)5]·Cl2·CH3CH2OH (2), {[Yb2(TPTZ)2(BDC)3]·2H2O}n (3), [Yb(TPTZ)Cl(H2O)4]·Cl2 (4) and [Er(TPTZ)(TTA)Cl2] (5) (mNBA = m-nitro benzoate, BDC = terephthalate, TTA = thenoyltrifluoroacetone, TPTZ = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) have been synthesized. The single X-ray diffraction reveals that TPTZ is mainly in the trident coordination mode and organic aromatic carboxylic acids are in the multiple coordination modes in the crystals. The composition of solvents, reaction temperature and reactant ratios all affect the composition and structure of the formed crystals. Crystals 1 and 3 belong to triclinic system, while the other three belong to monoclinic system. Among them, Crystal complexes 1 and 3 are coordination polymers. The other three crystals are mononuclear complexes with LnШ ions in the asymmetric environment. Both of the Crystal complexes 1 and 2 show strong luminescence emissions of Eu3+. The luminescence lifetimes of the two complexes are 0.761 ms and 0.447 ms, respectively. In addition, their luminescence quantum efficiencies are 25.0% and 16.7%, respectively. Full article
(This article belongs to the Special Issue Luminescent Properties of Lanthanoid Doped Crystals)
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Open AccessArticle A New Hemihydrate of Valacyclovir Hydrochloride
Crystals 2017, 7(5), 140; doi:10.3390/cryst7050140
Received: 28 March 2017 / Revised: 9 May 2017 / Accepted: 11 May 2017 / Published: 16 May 2017
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Abstract
Several crystal forms of valacyclovir hydrochloride, including two anhydrous and three hydrates, were investigated in this study. At the same time, a new hemihydrate of valacyclovir hydrochloride was first discovered and its properties were characterized by PXRD, TGA, DSC, and Raman in this
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Several crystal forms of valacyclovir hydrochloride, including two anhydrous and three hydrates, were investigated in this study. At the same time, a new hemihydrate of valacyclovir hydrochloride was first discovered and its properties were characterized by PXRD, TGA, DSC, and Raman in this study. The hemihydrate shows a distinctive PXRD pattern and a melting point of 209 °C with a water weight loss of 2.42% from the thermal analysis. The Raman spectra show a few distinctive peaks in the region of 1250–1400 cm−1 due to different crystal forms. The thermostability testing suggests it is a stable crystal form and remain the same for several months under high temperature and humidity. All these crystal forms show good dissolubility in the water at room temperature with excess 100 mg/mL. Full article
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Open AccessArticle Synthesis and Thermoelectric Properties of Copper Sulfides via Solution Phase Methods and Spark Plasma Sintering
Crystals 2017, 7(5), 141; doi:10.3390/cryst7050141
Received: 27 April 2017 / Revised: 12 May 2017 / Accepted: 13 May 2017 / Published: 16 May 2017
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Abstract
Large-scale Cu2S tetradecahedrons microcrystals and sheet-like Cu2S nanocrystals were synthesized by employing a hydrothermal synthesis (HS) method and wet chemistry method (WCM), respectively. The morphology of α-Cu2S powders prepared by the HS method is a tetradecahedron with
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Large-scale Cu2S tetradecahedrons microcrystals and sheet-like Cu2S nanocrystals were synthesized by employing a hydrothermal synthesis (HS) method and wet chemistry method (WCM), respectively. The morphology of α-Cu2S powders prepared by the HS method is a tetradecahedron with the size of 1–7 μm. The morphology of β-Cu2S is a hexagonal sheet-like structure with a thickness of 5–20 nm. The results indicate that the morphologies and phase structures of Cu2S are highly dependent on the reaction temperature and time, even though the precursors are the exact same. The polycrystalline copper sulfides bulk materials were obtained by densifying the as-prepared powders using the spark plasma sintering (SPS) technique. The electrical and thermal transport properties of all bulk samples were measured from 323 K to 773 K. The pure Cu2S bulk samples sintered by using the powders prepared via HS reached the highest thermoelectric figure of merit (ZT) value of 0.38 at 573 K. The main phase of the bulk sample sintered by using the powder prepared via WCM changed from β-Cu2S to Cu1.8S after sintering due to the instability of β-Cu2S during the sintering process. The Cu1.8S bulk sample with a Cu1.96S impurity achieved the highest ZT value of 0.62 at 773 K. Full article
(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)
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Open AccessArticle Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules
Crystals 2017, 7(5), 144; doi:10.3390/cryst7050144
Received: 3 April 2017 / Revised: 2 May 2017 / Accepted: 7 May 2017 / Published: 16 May 2017
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Abstract
Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π-electron donors (D) and acceptors (A) that form mixed stacks ... D+ρAρD+ρAρD+ρAρ ... with
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Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π -electron donors (D) and acceptors (A) that form mixed stacks ... D+ρAρD+ρAρD+ρAρ ... with variable ionicity 0 < ρ < 1 and electron transfer t along the stack. The microscopic NIT model presented here combines a modified Hubbard model for strongly correlated electrons delocalized along the stack with Coulomb intermolecular interactions treated in mean field. It also accounts for linear coupling of electrons to a harmonic molecular vibration and to the Peierls phonon. This simple framework captures the observed complexity of NITs with continuous and discontinuous ρ on cooling or under pressure, together with the stack’s instability to dimerization. The interplay of charge, molecular and lattice degrees of freedom at NIT amplifies the nonlinearity of responses, accounts for the dielectric anomaly, and generates strongly anharmonic potential energy surfaces (PES). Dynamics on the ground state PES address vibrational spectra using time correlation functions. When extended to the excited state PES, the NIT model describes the early (<1 ps) dynamics of transient NIT induced by optical CT excitation with a fs pulse. Although phenomenological, the model parameters are broadly consistent with density functional calculations. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Open AccessArticle AlGaN/GaN MOS-HEMTs with Corona-Discharge Plasma Treatment
Crystals 2017, 7(5), 146; doi:10.3390/cryst7050146
Received: 29 April 2017 / Revised: 13 May 2017 / Accepted: 16 May 2017 / Published: 18 May 2017
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Abstract
The effects of a corona-discharge plasma treatment on the performance of an AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor fabricated onto Si substrates were studied. The threshold voltage shifted from −8.15 to −4.21 V when the device was treated with an Al2O3
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The effects of a corona-discharge plasma treatment on the performance of an AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor fabricated onto Si substrates were studied. The threshold voltage shifted from −8.15 to −4.21 V when the device was treated with an Al2O3 layer. The leakage current was reduced from 2.9 × 10−5 to 4.2 × 10−7 mA/mm, and the ION/IOFF ratio increased from 8.3 × 106 to 7.3 × 108 using the corona-discharge plasma treatment, which exhibited an increase of about two orders of magnitude. The device exhibited excellent performance with a subthreshold swing of 78 mV/dec and a peak gain of 47.92 mS/mm at VGS = 10 V. Full article
(This article belongs to the Special Issue Advances in GaN Crystals and Their Applications)
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Open AccessArticle Synthesis, Crystal Structure and Nonlinear Optical Property of RbHgI3
Crystals 2017, 7(5), 148; doi:10.3390/cryst7050148
Received: 15 January 2017 / Revised: 15 May 2017 / Accepted: 16 May 2017 / Published: 22 May 2017
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Abstract
Searching for new nonlinear optical (NLO) crystals to be used in the infrared (IR) region is still a challenge. This paper presents the synthesis, crystal structure and properties of a new halide, RbHgI3. Its non-centrosymmetric single crystal can be grown in
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Searching for new nonlinear optical (NLO) crystals to be used in the infrared (IR) region is still a challenge. This paper presents the synthesis, crystal structure and properties of a new halide, RbHgI3. Its non-centrosymmetric single crystal can be grown in solution. In its crystal structure, all the polar [HgI4]2− groups align in such a way that brings a favorable net polarization. The measurement by Kurtz–Perry powder technique indicates that RbHgI3 shows a phase-matchable second harmonic generation (SHG) property seven times stronger than that of KH2PO4 (KDP). RbHgI3 displays excellent transparency in the range of 0.48–25 μm with relatively good thermal stability. The UV absorption implies that this yellow compound’s band gap is about 2.56 eV, close to that of AgGaS2. A preliminary measurement indicates that the laser-induced damage threshold of the crystal is about 28.3 MW/cm2. These preliminary experimental data reveal that RbHgI3 is a new candidate as nonlinear optical material in the infrared region. Full article
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Open AccessReview Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral–Ionic Transitions
Crystals 2017, 7(5), 132; doi:10.3390/cryst7050132
Received: 1 April 2017 / Revised: 30 April 2017 / Accepted: 8 May 2017 / Published: 11 May 2017
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Abstract
Mixed-stacked organic molecular compounds near the neutral–ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature or applied
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Mixed-stacked organic molecular compounds near the neutral–ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature or applied pressure, which is called an NI transition. This NI transition can also be induced by photoirradiation, in which case it is known as a prototypical ‘photoinduced phase transition’. In this paper, we focus on the ultrafast electron and molecular dynamics in the transition between the N and I states induced by irradiation by a femtosecond laser pulse and a terahertz electric-field pulse in TTF-CA. In the first half of the paper, we review the photoinduced N-to-I transition in TTF-CA studied by femtosecond-pump-probe reflection spectroscopy. We show that in the early stage of the transition, collective charge transfers occur within 20 fs after the photoirradiation, and microscopic one-dimensional (1D) I domains are produced. These ultrafast I-domain formations are followed by molecular deformations and displacements, which play important roles in the stabilization of photogenerated I domains. In the photoinduced I-to-N transition, microscopic 1D N domains are also produced and stabilized by molecular deformations and displacements. However, the time characteristics of the photoinduced N-to-I and I-to-N transitions in the picosecond time domain are considerably different from each other. In the second half of this paper, we review two phenomena induced by a strong terahertz electric-field pulse in TTF-CA: the modulation of a ferroelectric polarization in the I phase and the generation of a large macroscopic polarization in the N phase. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Open AccessReview Crystal Structures from Powder Diffraction: Principles, Difficulties and Progress
Crystals 2017, 7(5), 142; doi:10.3390/cryst7050142
Received: 27 April 2017 / Revised: 10 May 2017 / Accepted: 11 May 2017 / Published: 16 May 2017
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Abstract
The structure solution from powder diffraction has undergone an intense evolution during the last 20 years, but is far from being routine. Current challenges of powder crystallography include ab initio crystal structure determination on real samples of new materials with specific microstructures, characterization
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The structure solution from powder diffraction has undergone an intense evolution during the last 20 years, but is far from being routine. Current challenges of powder crystallography include ab initio crystal structure determination on real samples of new materials with specific microstructures, characterization of intermediate reaction products from in situ, in operando studies and novel phases from in situ studies of phase diagrams. The intense evolution of electron diffraction in recent years, providing an experimental (precession) and theoretical (still under intense development) solution to strong dynamic scattering of electrons, smears the traditional frontier between poly- and single-crystal diffraction. Novel techniques like serial snapshot X-ray crystallography point in the same direction. Finally, for the computational chemistry, theoreticians hand-in-hand with crystallographers develop tools where the theory meets experiment for crystal structure refinement, which becomes an unavoidable step in the validation of crystal structures obtained from powder diffraction. Full article
(This article belongs to the Special Issue Crystal Structure of Electroceramics) Printed Edition available
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Open AccessReview Structure Property Relationships and Cationic Doping in [Ca24Al28O64]4+ Framework: A Review
Crystals 2017, 7(5), 143; doi:10.3390/cryst7050143
Received: 20 March 2017 / Revised: 8 May 2017 / Accepted: 10 May 2017 / Published: 16 May 2017
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Abstract
Ca12Al14O33 (C12A7, 12CaO·7Al2O3, or [Ca12Al14O32]2+:O2) is a material with a clathrate cage framework, positively charged and stabilized by anions occluded
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Ca12Al14O33 (C12A7, 12CaO·7Al2O3, or [ Ca 12 Al 14 O 32 ] 2 + : O 2 ) is a material with a clathrate cage framework, positively charged and stabilized by anions occluded within 17% of the cages. The occluded anion is modular and can be elemental, polyatomic, and electronic in nature. This review focuses on the electride C12A7 ( [ Ca 24 Al 28 O 64 ] 4 + : ( 4 * ) e ( 2 ) O 2 ), where O2− anions are replaced with electrons, and compliments previous structural and electronic property reviews to illuminate the structure–property relationships. Electride formation is updated with new findings in carbonaceous reduction methods. Most importantly, an extensive compilation of cationic doped C12A7 isostructural compounds is presented as motivation to study doped C12A7 electrides. Cationic dopants have profound impacts on the electronic properties due to changes in the density of states, localized electron behavior, and structural distortions. Full article
(This article belongs to the Special Issue Crystallography of Functional Materials)
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Open AccessReview Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U3O8 Types of Layers
Crystals 2017, 7(5), 145; doi:10.3390/cryst7050145
Received: 12 April 2017 / Revised: 12 May 2017 / Accepted: 14 May 2017 / Published: 18 May 2017
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Abstract
A series of mixed-metal oxide structures based on the stacking of α-U3O8 type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition Am+((n+1)/m)
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A series of mixed-metal oxide structures based on the stacking of α-U3O8 type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition Am+((n+1)/m)B(3n+1)O(8n+3) (e.g., A = Ag, Bi, Ca, Cu, Ce, Dy, Eu, Gd K, La, Nd, Pb, Pr, Sr, Y; B = Nb, Ta; m = 1–3; n = 1, 1.5, 2), and the edge-shared BO7 pentagonal pyramid single, double, and/or triple layers are differentiated by the average thickness, (i.e., 1 ≤ n ≤ 2), of the BO7 layers and the local coordination environment of the “A” site cations. Temperature dependent polymorphism has been investigated for structures containing single layered (n = 1) monovalent (m = 1) “A” site cations (e.g., Ag2Nb4O11, Na2Nb4O11, and Cu2Ta4O11). Furthermore, symmetry lowering distortions were observed for the Pb ion-exchange synthesis of Ag2Ta4O11 to yield PbTa4O11. Several members within the subset of the family have been constructed with optical and electronic properties that are suitable for the conversion of solar energy to chemical fuels via water splitting. Full article
(This article belongs to the Special Issue Crystallography of Functional Materials)
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Open AccessReview Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems
Crystals 2017, 7(5), 147; doi:10.3390/cryst7050147
Received: 10 April 2017 / Revised: 15 May 2017 / Accepted: 15 May 2017 / Published: 19 May 2017
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Abstract
Due to the effects of microphase separation and physical dimensions, confinement widely exists in the multi-component polymer systems (e.g., polymer blends, copolymers) and the polymers having nanoscale dimensions, such as thin films and nanofibers. Semicrystalline polymers usually show different crystallization kinetics, crystalline structure
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Due to the effects of microphase separation and physical dimensions, confinement widely exists in the multi-component polymer systems (e.g., polymer blends, copolymers) and the polymers having nanoscale dimensions, such as thin films and nanofibers. Semicrystalline polymers usually show different crystallization kinetics, crystalline structure and morphology from the bulk when they are confined in the nanoscale environments; this may dramatically influence the physical performances of the resulting materials. Therefore, investigations on the crystalline and spherulitic morphology of semicrystalline polymers in confined systems are essential from both scientific and technological viewpoints; significant progresses have been achieved in this field in recent years. In this article, we will review the recent research progresses on the crystalline and spherulitic morphology of polymers crystallized in the nanoscale confined environments. According to the types of confined systems, crystalline, spherulitic morphology and morphological evolution of semicrystalline polymers in the ultrathin films, miscible polymer blends and block copolymers will be summarized and reviewed. Full article
(This article belongs to the Special Issue Crystal Morphology and Assembly in Spherulites)
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Open AccessReview Material and Device Architecture Engineering Toward High Performance Two-Dimensional (2D) Photodetectors
Crystals 2017, 7(5), 149; doi:10.3390/cryst7050149
Received: 1 April 2017 / Revised: 4 May 2017 / Accepted: 10 May 2017 / Published: 22 May 2017
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Abstract
Photodetectors based on two-dimensional (2D) nanostructures have led to a high optical response, and a long photocarrier lifetime because of spatial confinement effects. Since the discovery of graphene, many different 2D semiconductors have been developed and utilized in the ultrafast and ultrasensitive detection
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Photodetectors based on two-dimensional (2D) nanostructures have led to a high optical response, and a long photocarrier lifetime because of spatial confinement effects. Since the discovery of graphene, many different 2D semiconductors have been developed and utilized in the ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges. This review presents a comprehensive summary of recent breakthroughs in constructing high-performance photodetectors based on 2D materials. First, we give a general overview of 2D photodetectors based on various single-component materials and their operating wavelength (ultraviolet to terahertz regime). Then, we summarize the design and controllable synthesis of heterostructure material systems to promote device photoresponse. Subsequently, special emphasis is put on the accepted methods in rational engineering of device architectures toward the photoresponse improvements. Finally, we conclude with our personal viewpoints on the challenges and promising future directions in this research field. Full article
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