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Crystals, Volume 6, Issue 4 (April 2016)

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Research

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Open AccessArticle The Adhesive Properties of Coherent and Semicoherent NiAl/V Interfaces Within the Peierls-Nabarro Model
Crystals 2016, 6(4), 32; doi:10.3390/cryst6040032
Received: 22 January 2016 / Revised: 13 March 2016 / Accepted: 22 March 2016 / Published: 25 March 2016
Cited by 1 | PDF Full-text (633 KB) | HTML Full-text | XML Full-text
Abstract
The work of adhesion and the interface energy of NiAl/V coherent interface systems have been investigated using first-principles methods. The adhesion of the Ni-terminated interface is larger than the Al-terminated interface. The difference in charge density and the density of states show that
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The work of adhesion and the interface energy of NiAl/V coherent interface systems have been investigated using first-principles methods. The adhesion of the Ni-terminated interface is larger than the Al-terminated interface. The difference in charge density and the density of states show that the Ni-terminated interface is dominated by metallic bonds, and the Al-terminated interface is dominated by metallic and covalent bonds. To account for the effects of misfit dislocations on the semicoherent interfaces, the Peierls–Nabarro model combined with generalized stacking fault energy is employed to determine the interface energy. It is found that misfit dislocations can reduce the adhesion of the interface, and the reduction increases with the maximum of the restoring force. Full article
(This article belongs to the Special Issue Crystal Dislocations)
Open AccessArticle Degradation of Polypropylene Membranes Applied in Membrane Distillation Crystallizer
Crystals 2016, 6(4), 33; doi:10.3390/cryst6040033
Received: 20 January 2016 / Revised: 9 March 2016 / Accepted: 23 March 2016 / Published: 28 March 2016
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Abstract
The studies on the resistance to degradation of capillary polypropylene membranes assembled in a membrane crystallizer were performed. The supersaturation state of salt was achieved by evaporation of water from the NaCl saturated solutions using membrane distillation process. A high feed temperature (363
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The studies on the resistance to degradation of capillary polypropylene membranes assembled in a membrane crystallizer were performed. The supersaturation state of salt was achieved by evaporation of water from the NaCl saturated solutions using membrane distillation process. A high feed temperature (363 K) was used in order to enhance the degradation effects and to shorten the test times. Salt crystallization was carried out by the application of batch or fluidized bed crystallizer. A significant membrane scaling was observed regardless of the method of realized crystallization. The SEM-EDS, DSC, and FTIR methods were used for investigations of polypropylene degradation. The salt crystallization onto the membrane surface accelerated polypropylene degradation. Due to a polymer degradation, the presence of carbonyl groups on the membranes’ surface was identified. Besides the changes in the chemical structure a significant mechanical damage of the membranes, mainly caused by the internal scaling, was also found. As a result, the membranes were severely damaged after 150 h of process operation. A high level of salt rejection was maintained despite damage to the external membrane surface. Full article
(This article belongs to the Special Issue Membrane-Assisted Crystallization)
Open AccessArticle Very Strong Parallel Interactions Between Two Saturated Acyclic Groups Closed with Intramolecular Hydrogen Bonds Forming Hydrogen-Bridged Rings
Crystals 2016, 6(4), 34; doi:10.3390/cryst6040034
Received: 29 January 2016 / Revised: 12 March 2016 / Accepted: 17 March 2016 / Published: 31 March 2016
Cited by 1 | PDF Full-text (2622 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Saturated acyclic four-atom groups closed with a classic intramolecular hydrogen bond, generating planar five-membered rings (hydrogen-bridged quasi-rings), in which at least one of the ring atoms is bonded to other non-ring atoms that are not in the ring plane and, thus, capable to
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Saturated acyclic four-atom groups closed with a classic intramolecular hydrogen bond, generating planar five-membered rings (hydrogen-bridged quasi-rings), in which at least one of the ring atoms is bonded to other non-ring atoms that are not in the ring plane and, thus, capable to form intermolecular interactions, were studied in this work, in order to find the preferred mutual positions of these species in crystals and evaluate strength of intermolecular interactions. We studied parallel interactions of these rings by analysing crystal structures in the Cambridge Structural Database (CSD) and by quantum chemical calculations. The rings can have one hydrogen atom out of the ring plane that can form hydrogen bonds between two parallel rings. Hence, in these systems with parallel rings, two types of hydrogen bonds can be present, one in the ring, and the other one between two parallel rings. The CSD search showed that 27% of the rings in the crystal structures form parallel interactions. The calculations at very accurate CCSD(T)/CBS level revealed strong interactions, in model systems of thiosemicarbazide, semicarbazide and glycolamide dimers the energies are −9.68, −7.12 and −4.25 kcal/mol. The hydrogen bonds between rings, as well as dispersion interactions contribute to the strong interaction energies. Full article
(This article belongs to the Special Issue Analysis of Hydrogen Bonds in Crystals) Printed Edition available
Open AccessArticle PRINCEPS: A Computer-Based Approach to the Structural Description and Recognition of Trends within Structural Databases, and Its Application to the Ce-Ni-Si System
Crystals 2016, 6(4), 35; doi:10.3390/cryst6040035
Received: 6 December 2015 / Revised: 17 March 2016 / Accepted: 22 March 2016 / Published: 1 April 2016
Cited by 1 | PDF Full-text (10151 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Intermetallic crystal structures offer an enormous structural diversity, with an endless array of structural motifs whose connection to stability and physical properties are often mysterious. Making sense of the often complex crystal structures that arise here, developing a clear structural description, and identifying
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Intermetallic crystal structures offer an enormous structural diversity, with an endless array of structural motifs whose connection to stability and physical properties are often mysterious. Making sense of the often complex crystal structures that arise here, developing a clear structural description, and identifying connections to other phases can be laborious and require an encyclopedic knowledge of structure types. In this Article, we present PRINCEPS, an algorithm based on a new coordination environment projection scheme that facilitates the structural analysis and comparison of such crystal structures. We demonstrate the potential of this approach by applying it to the complex Ce-Ni-Si ternary system, whose 17 binary and 21 ternary phases would present a daunting challenge to one seeking to understand the system by manual inspection (but has nonetheless been well-described through the heroic efforts of previous researchers). With the help of PRINCEPS, most of the ternary phases in this system can be rationalized as intergrowths of simple structural fragments, and grouped into a handful of structural series (with some outliers). These results illustrate how the PRINCEPS approach can be used to organize a vast collection of crystal structures into structurally meaningful families, and guide the description of complex atomic arrangements. Full article
(This article belongs to the Special Issue Intermetallics)
Open AccessArticle Integrated Membrane Desalination Systems with Membrane Crystallization Units for Resource Recovery: A New Approach for Mining from the Sea
Crystals 2016, 6(4), 36; doi:10.3390/cryst6040036
Received: 28 January 2016 / Revised: 9 March 2016 / Accepted: 29 March 2016 / Published: 1 April 2016
Cited by 5 | PDF Full-text (2117 KB) | HTML Full-text | XML Full-text
Abstract
The mining industry is facing problems of clean production in terms of mineral processing, pollution, water consumption, and renewable energy. An interesting outlook can be to combine the mining industry with membrane-based desalination in the logic of mining from the sea. In fact,
[...] Read more.
The mining industry is facing problems of clean production in terms of mineral processing, pollution, water consumption, and renewable energy. An interesting outlook can be to combine the mining industry with membrane-based desalination in the logic of mining from the sea. In fact, several of the drawbacks found in both mining and desalination can be minimized or overcome, which includes hindering mineral depletion, water production instead of water consumption, smart usage of brine instead of disposal, and low energy consumption, etc. Recently, membrane crystallization (MCr) has been developed to recover minerals from highly concentrated solutions. This study suggests MCr for the treatment of nanofiltration (NF) retentate and reverse osmosis (RO) brine leaving membrane-based desalination system. Thermodynamic modeling has been carried out to predict at which water recovery factor and which amount of minerals can be recovered. Theoretical results deviate only 2.09% from experimental results. Multivalent components such as barium, strontium, and magnesium are easier to recover from NF retentate with respect to RO brine. KCl and NiCl2 might be recovered from both NF retentate and RO brine, whereas lithium can only be recovered from RO brine. Moreover, copper and manganese compounds might also be recovered from desalination brine in perspectives. Full article
(This article belongs to the Special Issue Membrane-Assisted Crystallization)
Open AccessArticle Isomorphous Crystals from Diynes and Bromodiynes Involved in Hydrogen and Halogen Bonds
Crystals 2016, 6(4), 37; doi:10.3390/cryst6040037
Received: 4 February 2016 / Revised: 24 March 2016 / Accepted: 24 March 2016 / Published: 2 April 2016
Cited by 1 | PDF Full-text (4214 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Isomorphous crystals of two diacetylene derivatives with carbamate functionality (BocNH-CH2-diyne-X, where X = H or Br) have been obtained. The main feature of these structures is the original 2D arrangement (as supramolecular sheets or walls) in which the H bond and
[...] Read more.
Isomorphous crystals of two diacetylene derivatives with carbamate functionality (BocNH-CH2-diyne-X, where X = H or Br) have been obtained. The main feature of these structures is the original 2D arrangement (as supramolecular sheets or walls) in which the H bond and halogen bond have a prominent effect on the whole architecture. The two diacetylene compounds harbor neighboring carbamate (Boc protected amine) and conjugated alkyne functionalities. They differ only by the nature of the atom located at the penultimate position of the diyne moiety, either a hydrogen atom or a bromine atom. Both of them adopt very similar 2D wall organizations with antiparallel carbamates (as in antiparallel beta pleated sheets). Additional weak interactions inside the same walls between molecular bricks are H bond interactions (diyne-H···O=C) or halogen bond interactions (diyne-Br···O=C), respectively. Based on crystallographic atom coordinates, DFT (B3LYP/6-31++G(d,p)) and DFT (M06-2X/6-31++G(d,p)) calculations were performed on these isostructural crystals to gain insight into the intermolecular interactions. Full article
(This article belongs to the Special Issue Analysis of Hydrogen Bonds in Crystals) Printed Edition available
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Open AccessArticle Synthesis, Structures and Properties of Cobalt Thiocyanate Coordination Compounds with 4-(hydroxymethyl)pyridine as Co-ligand
Crystals 2016, 6(4), 38; doi:10.3390/cryst6040038
Received: 29 February 2016 / Revised: 22 March 2016 / Accepted: 24 March 2016 / Published: 2 April 2016
Cited by 8 | PDF Full-text (3176 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Reaction of Co(NCS)2 with 4-(hydroxymethyl)pyridine (hmpy) leads to the formation of six new coordination compounds with the composition [Co(NCS)2(hmpy))4] (1), [Co(NCS)2(hmpy)4] × H2O (1-H2O), [Co(NCS)2
[...] Read more.
Reaction of Co(NCS)2 with 4-(hydroxymethyl)pyridine (hmpy) leads to the formation of six new coordination compounds with the composition [Co(NCS)2(hmpy))4] (1), [Co(NCS)2(hmpy)4] × H2O (1-H2O), [Co(NCS)2(hmpy)2(EtOH)2] (2), [Co(NCS)2(hmpy)2(H2O)2] (3), [Co(NCS)2(hmpy)2]n∙4 H2O (4) and [Co(NCS)2(hmpy)2]n (5). They were characterized by single crystal and powder X-ray diffraction experiments, thermal and elemental analysis, IR and magnetic measurements. Compound 1 and 1-H2O form discrete complexes, in which the Co(II) cations are octahedrally coordinated by two terminal thiocyanato anions and four 4-(hydroxymethyl)pyridine ligands. Discrete complexes were also observed for compounds 2 and 3 where two of the hmpy ligands were substituted by solvent, either water (3) or ethanol (2). In contrast, in compounds 4 and 5, the Co(II) cations are linked into chains by bridging 4-(hydroxymethyl)pyridine ligands. The phase purity was checked with X-ray powder diffraction. Thermogravimetric measurements showed that compound 3 transforms into 5 upon heating, whereas the back transformation occurs upon resolvation. Magnetic measurements did not show any magnetic exchange via the hmpy ligand for compound 5. Full article
(This article belongs to the Special Issue Crystal Structure of Complex Compounds)
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Open AccessArticle High-Spin Chains and Crowns from Double-Exchange Mechanism
Crystals 2016, 6(4), 39; doi:10.3390/cryst6040039
Received: 5 December 2015 / Revised: 23 March 2016 / Accepted: 28 March 2016 / Published: 6 April 2016
Cited by 1 | PDF Full-text (1845 KB) | HTML Full-text | XML Full-text
Abstract
This article addresses the question of the possibility of obtaining high-spin chains and crowns of magnetic units s = 1 from doped (by a hole) antiferromagnetic architectures. It aims at determining the range of values of the double-exchange model interactions for which these
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This article addresses the question of the possibility of obtaining high-spin chains and crowns of magnetic units s = 1 from doped (by a hole) antiferromagnetic architectures. It aims at determining the range of values of the double-exchange model interactions for which these molecules exhibit a high-spin ground state. Several chains and crowns of sizes varying between three to seven magnetic sites have been studied using a refined double-exchange model. It is shown that, for physical values of the parameters, linear chains of three, four and five sites are likely to adopt the highest spin state. For chains of six sites, small values of magnetic couplings are needed to get the highest spin, but it would be easy to get an S = 3/2 ground state. For systems of seven (or slightly more) sites, the highest spin state becomes non accessible but S = 5/2 states are likely to be obtained. Surprisingly, the physics of crowns is substantially different. The same trends are observed for even-number systems but with a larger double-exchange regime. At variance, odd-number systems do not exhibit a double-exchange mechanism for low values of the magnetic couplings. These observations are rationalized from an analysis of the computed spectra and wave functions. Full article
(This article belongs to the Special Issue High Spin Molecules)
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Open AccessArticle 2-Azidoimidazolium Ions Captured by N-Heterocyclic Carbenes: Azole-Substituted Triazatrimethine Cyanines
Crystals 2016, 6(4), 40; doi:10.3390/cryst6040040
Received: 29 January 2016 / Revised: 15 February 2016 / Accepted: 17 February 2016 / Published: 8 April 2016
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Abstract
1,3-Disubstituted 2-azidoimidazolium salts (substituents = methyl, methoxy; anion = PF6) reacted with N-heterocyclic carbenes to yield yellow 2-(1-(azolinylidene)triazen-3-yl)-1,3-R2-imidazolium salts (azole = 1,3-dimethylimidazole, 1,3-dimethoxyimidazole, 4-dimethylamino-1-methyl-1,2,4-triazole; R = methyl, methoxy; anion = PF6). Crystal structures of three cationic
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1,3-Disubstituted 2-azidoimidazolium salts (substituents = methyl, methoxy; anion = PF6) reacted with N-heterocyclic carbenes to yield yellow 2-(1-(azolinylidene)triazen-3-yl)-1,3-R2-imidazolium salts (azole = 1,3-dimethylimidazole, 1,3-dimethoxyimidazole, 4-dimethylamino-1-methyl-1,2,4-triazole; R = methyl, methoxy; anion = PF6). Crystal structures of three cationic triazenes were determined. Numerous interionic C–H···F contacts were observed. Solvatochromism of the triazenes in polar solvents was investigated by UV-Vis spectroscopy, involving the dipolarity π* and hydrogen-bond donor acidity α of the solvent. Cyclovoltammetry showed irreversible reduction of the cations to uncharged radicals. Thermoanalysis showed exothermal decomposition. Full article
(This article belongs to the Special Issue Nitrogen-Rich Salts)
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Open AccessArticle Raman Identification of Polymorphs in Pentacene Films
Crystals 2016, 6(4), 41; doi:10.3390/cryst6040041
Received: 21 January 2016 / Revised: 10 March 2016 / Accepted: 5 April 2016 / Published: 8 April 2016
Cited by 4 | PDF Full-text (973 KB) | HTML Full-text | XML Full-text
Abstract
We use Raman spectroscopy to characterize thin films of pentacene grown on Si/SiO x by Supersonic Molecular Beam Deposition (SuMBD). We find that films up to a thickness of about 781 Å (∼ 52 monolayers) all belong to the so-called thin-film (TF) phase. The appearance with strong intensity of some lattice phonons suggests that the films are characterized by good intra-layer order. A comparison of the Raman spectra in the lattice and CH bending spectral regions of the TF polymorph with the corresponding ones of the high-temperature (HT) and low-temperature (LT) bulk pentacene polymorphs provides a quick and nondestructive method to identify the different phases. Full article
(This article belongs to the Section Biomolecular Crystals)
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Open AccessArticle Rb2Na(NO3)3: A Congruently Melting UV-NLO Crystal with a Very Strong Second-Harmonic Generation Response
Crystals 2016, 6(4), 42; doi:10.3390/cryst6040042
Received: 26 January 2016 / Revised: 26 March 2016 / Accepted: 7 April 2016 / Published: 13 April 2016
Cited by 6 | PDF Full-text (2162 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Crystals of congruently melting noncentrosymmetric (NCS) mixed alkali metal nitrate, Rb2Na(NO3)3, have been grown through solid state reactions. The material possesses layers with NaO8 hexagonal bipyramids and NO3 triangular units. Rb+ cations are residing
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Crystals of congruently melting noncentrosymmetric (NCS) mixed alkali metal nitrate, Rb2Na(NO3)3, have been grown through solid state reactions. The material possesses layers with NaO8 hexagonal bipyramids and NO3 triangular units. Rb+ cations are residing in the interlayer space. Each NaO8 hexagonal bipyramid shares its corners and edges with two and three NO3 units, respectively, in order to fulfill a highly dense stacking in the unit cell. The NaO8 groups share their six oxygen atoms in equatorial positions with three different NO3 groups to generate a NaO6-NO3 layer with a parallel alignment. The optimized arrangement of the NO3 groups and their high density in the structure together produce a strong second-harmonic generation (SHG) response. Powder SHG measurements indicate that Rb2Na(NO3)3 has a strong SHG efficiency of five times that of KH2PO4 (KDP) and is type I phase-matchable. The calculated average nonlinear optical (NLO) susceptibility of Rb2Na(NO3)3 turns out to be the largest value among the NLO materials composed of only [NO3] anion. In addition, Rb2Na(NO3)3 exhibits a wide transparency region ranging from UV to near IR, which suggests that the compound is a promising NLO material. Full article
(This article belongs to the Special Issue Nonlinear Optical Crystals)
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Open AccessArticle The Role of Hydrogen Bond in Designing Molecular Optical Materials
Crystals 2016, 6(4), 43; doi:10.3390/cryst6040043
Received: 2 February 2016 / Revised: 6 April 2016 / Accepted: 7 April 2016 / Published: 13 April 2016
Cited by 1 | PDF Full-text (1935 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
In this perspective article, we revise some of the empirical and semi-empirical strategies for predicting how hydrogen bonding affects molecular and atomic polarizabilities in aggregates. We use p-nitroaniline and hydrated oxalic acid as working examples to illustrate the enhancement of donor and
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In this perspective article, we revise some of the empirical and semi-empirical strategies for predicting how hydrogen bonding affects molecular and atomic polarizabilities in aggregates. We use p-nitroaniline and hydrated oxalic acid as working examples to illustrate the enhancement of donor and acceptor functional-group polarizabilities and their anisotropy. This is significant for the evaluation of electrical susceptibilities in crystals; and the properties derived from them like the refractive indices. Full article
(This article belongs to the Special Issue Analysis of Hydrogen Bonds in Crystals) Printed Edition available
Open AccessFeature PaperArticle One-Dimensional Mass-Spring Chains Supporting Elastic Waves with Non-Conventional Topology
Crystals 2016, 6(4), 44; doi:10.3390/cryst6040044
Received: 26 February 2016 / Revised: 4 April 2016 / Accepted: 13 April 2016 / Published: 16 April 2016
Cited by 2 | PDF Full-text (2240 KB) | HTML Full-text | XML Full-text
Abstract
There are two classes of phononic structures that can support elastic waves with non-conventional topology, namely intrinsic and extrinsic systems. The non-conventional topology of elastic wave results from breaking time reversal symmetry (T-symmetry) of wave propagation. In extrinsic systems, energy is injected into
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There are two classes of phononic structures that can support elastic waves with non-conventional topology, namely intrinsic and extrinsic systems. The non-conventional topology of elastic wave results from breaking time reversal symmetry (T-symmetry) of wave propagation. In extrinsic systems, energy is injected into the phononic structure to break T-symmetry. In intrinsic systems symmetry is broken through the medium microstructure that may lead to internal resonances. Mass-spring composite structures are introduced as metaphors for more complex phononic crystals with non-conventional topology. The elastic wave equation of motion of an intrinsic phononic structure composed of two coupled one-dimensional (1D) harmonic chains can be factored into a Dirac-like equation, leading to antisymmetric modes that have spinor character and therefore non-conventional topology in wave number space. The topology of the elastic waves can be further modified by subjecting phononic structures to externally-induced spatio-temporal modulation of their elastic properties. Such modulations can be actuated through photo-elastic effects, magneto-elastic effects, piezo-electric effects or external mechanical effects. We also uncover an analogy between a combined intrinsic-extrinsic systems composed of a simple one-dimensional harmonic chain coupled to a rigid substrate subjected to a spatio-temporal modulation of the side spring stiffness and the Dirac equation in the presence of an electromagnetic field. The modulation is shown to be able to tune the spinor part of the elastic wave function and therefore its topology. This analogy between classical mechanics and quantum phenomena offers new modalities for developing more complex functions of phononic crystals and acoustic metamaterials. Full article
(This article belongs to the Special Issue Phononic Crystals)
Open AccessArticle Analysis of Longitudinal Waves in Rod-Type Piezoelectric Phononic Crystals
Crystals 2016, 6(4), 45; doi:10.3390/cryst6040045
Received: 14 February 2016 / Revised: 28 March 2016 / Accepted: 12 April 2016 / Published: 18 April 2016
Cited by 1 | PDF Full-text (7649 KB) | HTML Full-text | XML Full-text
Abstract
Phononic crystals can be used to control elastic waves due to their frequency bands. This paper analyzes the passive and active control as well as the dispersion properties of longitudinal waves in rod-type piezoelectric phononic crystals over large frequency ranges. Based on the
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Phononic crystals can be used to control elastic waves due to their frequency bands. This paper analyzes the passive and active control as well as the dispersion properties of longitudinal waves in rod-type piezoelectric phononic crystals over large frequency ranges. Based on the Love rod theory for modeling the longitudinal wave motions in the constituent rods and the method of reverberation-ray matrix (MRRM) for deriving the member transfer matrices of the constituent rods, a modified transfer matrix method (MTMM) is proposed for the analysis of dispersion curves by combining with the Floquet–Bloch principle and for the calculation of transmission spectra. Numerical examples are provided to validate the proposed MTMM for analyzing the band structures in both low and high frequency ranges. The passive control of longitudinal-wave band structures is studied by discussing the influences of the electrode’s thickness, the Poisson’s effect and the elastic rod inserts in the unit cell. The influences of electrical boundaries (including electric-open, applied electric capacity, electric-short and applied feedback control conditions) on the band structures are investigated to illustrate the active control scheme. From the calculated comprehensive frequency spectra over a large frequency range, the dispersion properties of the characteristic longitudinal waves in rod-type piezoelectric phononic crystals are summarized. Full article
(This article belongs to the Special Issue Phononic Crystals)
Open AccessArticle The Hydrogen Bonded Structures of Two 5-Bromobarbituric Acids and Analysis of Unequal C5–X and C5–X′ Bond Lengths (X = X′ = F, Cl, Br or Me) in 5,5-Disubstituted Barbituric Acids
Crystals 2016, 6(4), 47; doi:10.3390/cryst6040047
Received: 15 December 2015 / Revised: 14 April 2016 / Accepted: 18 April 2016 / Published: 22 April 2016
PDF Full-text (4383 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The crystal structure of the methanol hemisolvate of 5,5-dibromobarbituric acid (1MH) displays an H-bonded layer structure which is based on N–H∙∙∙O=C, N–H∙∙∙O(MeOH) and (MeOH)O–H∙∙∙O interactions. The barbiturate molecules form an H-bonded substructure which has the fes topology. 5,5′-Methanediylbis(5-bromobarbituric acid) 2, obtained from a
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The crystal structure of the methanol hemisolvate of 5,5-dibromobarbituric acid (1MH) displays an H-bonded layer structure which is based on N–H∙∙∙O=C, N–H∙∙∙O(MeOH) and (MeOH)O–H∙∙∙O interactions. The barbiturate molecules form an H-bonded substructure which has the fes topology. 5,5′-Methanediylbis(5-bromobarbituric acid) 2, obtained from a solution of 5,5-dibromobarbituric acid in nitromethane, displays a N–H···O=C bonded framework of the sxd type. The conformation of the pyridmidine ring and the lengths of the ring substituent bonds C5–X and C5–X′ in crystal forms of 5,5-dibromobarbituric acid and three closely related analogues (X = X′ = Br, Cl, F, Me) have been investigated. In each case, a conformation close to a C5-endo envelope is correlated with a significant lengthening of the axial C5–X′ in comparison to the equatorial C5–X bond. Isolated molecule geometry optimizations at different levels of theory confirm that the C5-endo envelope is the global conformational energy minimum of 5,5-dihalogenbarbituric acids. The relative lengthening of the axial bond is therefore interpreted as an inherent feature of the preferred envelope conformation of the pyrimidine ring, which minimizes repulsive interactions between the axial substituent and pyrimidine ring atoms. Full article
(This article belongs to the Special Issue Analysis of Hydrogen Bonds in Crystals) Printed Edition available
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Review

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Open AccessReview Colloidal Crystallization in 2D for Short-Ranged Attractions: A Descriptive Overview
Crystals 2016, 6(4), 46; doi:10.3390/cryst6040046
Received: 5 November 2015 / Revised: 13 April 2016 / Accepted: 14 April 2016 / Published: 20 April 2016
Cited by 1 | PDF Full-text (24517 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
With the aid of 2D computer simulations, the whole colloidal crystallization process for particles interacting with a short-ranged attractive potential is described, emphazising the visualization of the different subprocesses at the particle level. Starting with a supercooled homogeneous fluid, the system undergoes a
[...] Read more.
With the aid of 2D computer simulations, the whole colloidal crystallization process for particles interacting with a short-ranged attractive potential is described, emphazising the visualization of the different subprocesses at the particle level. Starting with a supercooled homogeneous fluid, the system undergoes a metastable fluid-fluid phase separation. Afterwards, crystallite nucleation is observed and we describe the obtainment of the critical crystallite size and other relevant quantities for nucleation. After the crystal formation, we notice the shrinking and eventual disappearance of the smaller crystals, which are close to larger ones; a manifestation of Ostwald ripening. When two growing crystal grains impinge on each other, the formation of grain boundaries is found; it is appreciated how a grain boundary moves, back and forth, not only on a perpendicular direction to the boundary, but with a rotation and a deformation. Subsequently, after the healing of the two extremes of the boundary, the two grains end up as a single imperfect grain that contains a number of complex dislocations. If these dislocations are close to the boundary with the fluid, they leave the crystal to make it more perfect. Otherwise, they migrate randomly inside the grain until they get close enough to the boundary to leave the grain. This last process of healing, trapping and getting rid of complex dislocations occurs preferentially for low-angle grain boundaries. If the angle between the symmetry axes of the two grains is not low, we end up with a polycrystal made of several touching crystal grains. Full article
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