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Crystals, Volume 8, Issue 1 (January 2018)

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Cover Story (view full-size image) Not all is at it should be! A survey of the structures formed between zinc and cadmium [...] Read more.
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Open AccessArticle Anthracene-Based Lanthanide Metal-Organic Frameworks: Synthesis, Structure, Photoluminescence, and Radioluminescence Properties
Crystals 2018, 8(1), 53; https://doi.org/10.3390/cryst8010053
Received: 11 December 2017 / Revised: 10 January 2018 / Accepted: 17 January 2018 / Published: 22 January 2018
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Abstract
Four anthracene-based lanthanide metal-organic framework structures (MOFs) were synthesized from the combination of the lanthanide ions, Eu3+, Tb3+, Er3+, and Tm3+, with 9,10-anthracenedicarboxylic acid (H2ADC) in dimethylformamide (DMF) under hydrothermal conditions. The 3-D
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Four anthracene-based lanthanide metal-organic framework structures (MOFs) were synthesized from the combination of the lanthanide ions, Eu3+, Tb3+, Er3+, and Tm3+, with 9,10-anthracenedicarboxylic acid (H2ADC) in dimethylformamide (DMF) under hydrothermal conditions. The 3-D networks crystalize in the triclinic system with P-1 space group with the following compositions: (i) {{[Ln2(ADC)3(DMF)4·DMF]}n, Ln = Eu (1) and Tb (2)} and (ii) {{[Ln2(ADC)3(DMF)2(OH2)2·2DMF·H2O]}n, Ln = Er (3) and Tm (4)}. The metal centers exist in various coordination environments; nine coordinate in (i), while seven and eight coordinate in (ii). The deprotonated ligand, ADC, assumes multiple coordination modes, with its carboxylate functional groups severely twisted away from the plane of the anthracene moiety. The structures show ligand-based photoluminescence, which appears to be significantly quenched when compared with that of the parent H2ADC solid powder. Structure 2 is the least quenched and showed an average photoluminescence lifetime from bi-exponential decay of 0.3 ns. On exposure to ionizing radiation, the structures show radioluminescence spectral features that are consistent with the isolation of the ligand units in its 3-D network. The spectral features vary among the 3-D networks and appear to suggest that the latter undergo significant changes in their molecular and/or electronic structure in the presence of the ionizing radiation. Full article
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Open AccessArticle Crystal Chemistry of Zinc Quinaldinate Complexes with Pyridine-Based Ligands
Crystals 2018, 8(1), 52; https://doi.org/10.3390/cryst8010052
Received: 1 December 2017 / Revised: 15 January 2018 / Accepted: 16 January 2018 / Published: 19 January 2018
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Abstract
Substitution of methanol in [Zn(quin)2(CH3OH)2] (quin denotes an anionic form of quinoline-2-carboxylic acid, also known as quinaldinic acid) with pyridine (Py) or its substituted derivatives, 3,5-lutidine (3,5-Lut), nicotinamide (Nia), 3-hydroxypyridine (3-Py-OH), 3-hydroxymethylpyridine (3-Hmpy), 4-hydroxypyridine (4-Py-OH) and
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Substitution of methanol in [Zn(quin)2(CH3OH)2] (quin denotes an anionic form of quinoline-2-carboxylic acid, also known as quinaldinic acid) with pyridine (Py) or its substituted derivatives, 3,5-lutidine (3,5-Lut), nicotinamide (Nia), 3-hydroxypyridine (3-Py-OH), 3-hydroxymethylpyridine (3-Hmpy), 4-hydroxypyridine (4-Py-OH) and 4-hydroxymethylpyridine (4-Hmpy), afforded a series of novel heteroleptic complexes with compositions [Zn(quin)2(Py)2] (1), [Zn(quin)2(3,5-Lut)2] (2), [Zn(quin)2(Nia)2]·2CH3CN (3), [Zn(quin)2(3-Py-OH)2] (4), [Zn(quin)2(3-Hmpy)2] (5), [Zn(quin)2(4-Pyridone)] (6) (4-Pyridone = a keto tautomer of 4-hydroxypyridine), and [Zn(quin)2(4-Hmpy)2] (7). In all reactions, the {Zn(quin)2} structural fragment with quinaldinate ions bound in a bidentate chelating manner retained its structural integrity. With the exception of [Zn(quin)2(4-Pyridone)] (6), all complexes feature a six-numbered coordination environment of metal ion that may be described as a distorted octahedron. The arrangement of ligands is trans. The coordination sphere of zinc(II) in the 4-pyridone complex consists of only three ligands, two quinaldinates, and one secondary ligand. The metal ion thereby attains a five-numbered coordination environment that is best described as a distorted square-pyramid (τ parameter equals 0.39). The influence of substituents on the pyridine-based ligand over intermolecular interactions in the solid state is investigated. Since pyridine and 3,5-lutidine are not able to form hydrogen-bonding interactions, the solid state structures of their complexes, [Zn(quin)2(Py)2] (1) and [Zn(quin)2(3,5-Lut)2] (2), are governed by π···π stacking, C–H∙∙∙π, and C–H∙∙∙O intermolecular interactions. With other pyridine ligands possessing amide or hydroxyl functional groups, the connectivity patterns in the crystal structures of their complexes are governed by hydrogen bonding interactions. Thermal decomposition studies of novel complexes have shown the formation of zinc oxide as the end product. Full article
(This article belongs to the Special Issue Crystal Chemistry of Zinc, Cadmium and Mercury)
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Open AccessArticle Determination of the Correct Composition of “Hydrous Lead(II) Oxotellurate(IV)” as PbTeO3, Crystallizing as a New Polymorph
Crystals 2018, 8(1), 51; https://doi.org/10.3390/cryst8010051
Received: 21 December 2017 / Revised: 16 January 2018 / Accepted: 17 January 2018 / Published: 19 January 2018
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Abstract
In previous studies, it has been reported that the crystalline product precipitated from the reaction of aqueous solutions of lead(II) salts with sodium oxotellurate(IV) is hydrous lead(II) oxotellurate(IV); however, there have been conflicting specifications of the water content, and the crystal structure of
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In previous studies, it has been reported that the crystalline product precipitated from the reaction of aqueous solutions of lead(II) salts with sodium oxotellurate(IV) is hydrous lead(II) oxotellurate(IV); however, there have been conflicting specifications of the water content, and the crystal structure of the product is yet undetermined. During the present study, it was shown that the precipitated material does not contain any structural water and in fact represents the third modification (denoted as γ-) of PbTeO3, as revealed by thermal analysis, vibrational spectroscopy, single crystal and powder X-ray diffraction. This modification crystallizes in the space group P 1 ¯ with five formula units in the asymmetric unit, comprising off-centred coordination polyhedra around the PbII cations (coordination numbers: 5–7 with Pb-O distances ranging from 2.3–3.0 Å), and trigonal-pyramidal TeO32− units. The thermal behaviour and structural phase transitions of PbTeO3 were investigated by means of temperature-dependent X-ray powder diffraction and complementary thermal analysis measurements. In addition, the crystal structure of β-PbTeO3 was redetermined, and a comparison was made between the three known polymorphs of PbTeO3. Full article
(This article belongs to the Special Issue Synthesis and Structural Investigations of Polymorphic Compounds)
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Open AccessReview Amorphous Phase Mediated Crystallization: Fundamentals of Biomineralization
Crystals 2018, 8(1), 48; https://doi.org/10.3390/cryst8010048
Received: 18 December 2017 / Revised: 10 January 2018 / Accepted: 11 January 2018 / Published: 19 January 2018
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Abstract
Many biomineralization systems start from transient amorphous precursor phases, but the exact crystallization pathways and mechanisms remain largely unknown. The study of a well-defined biomimetic crystallization system is key for elucidating the possible mechanisms of biomineralization and monitoring the detailed crystallization pathways. In
[...] Read more.
Many biomineralization systems start from transient amorphous precursor phases, but the exact crystallization pathways and mechanisms remain largely unknown. The study of a well-defined biomimetic crystallization system is key for elucidating the possible mechanisms of biomineralization and monitoring the detailed crystallization pathways. In this review, we focus on amorphous phase mediated crystallization (APMC) pathways and their crystallization mechanisms in bio- and biomimetic-mineralization systems. The fundamental questions of biomineralization as well as the advantages and limitations of biomimetic model systems are discussed. This review could provide a full landscape of APMC systems for biomineralization and inspire new experiments aimed at some unresolved issues for understanding biomineralization. Full article
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Open AccessArticle Effect of Sonication Output Power on the Crystal Structure and Magnetism of SrFe12O19 Nanoparticles
Crystals 2018, 8(1), 45; https://doi.org/10.3390/cryst8010045
Received: 24 November 2017 / Revised: 17 December 2017 / Accepted: 16 January 2018 / Published: 19 January 2018
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Abstract
We reported the effect of the sonication output power (SOP), from 120, 180, to 240 W, on the crystal structure, morphology, and magnetic properties of SrFe12O19 nanoparticles synthesized by sonochemical process assisted with heat treatment. X-ray Diffraction analysis of the
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We reported the effect of the sonication output power (SOP), from 120, 180, to 240 W, on the crystal structure, morphology, and magnetic properties of SrFe12O19 nanoparticles synthesized by sonochemical process assisted with heat treatment. X-ray Diffraction analysis of the obtained powder showed the formation of Fe3O4 with low crystallinity degree, which increased with the increase in SOP, together in a crystalline phase identified as SrCO3. The formation of SrFe12O19 started at 1073 K, and was completed at 1173 K. However, hexaferrite was obtained with the secondary phases α-Fe2O3 and SrFeO2.5. At 1323 K, the secondary phases vanished, and a single phase SrFe12O19 was detected. Vibrating Sample Magnetometry analysis showed that the SrFeO2.5 phase caused the formation of a hysteresis loop known as the Perminvar magnetic hysteresis loop. At 1323 K, the powder synthesized at 120 W showed a specific magnetization of 67.15 Am2/kg at 1.43 × 106 A/m, and coercivity of 4.69 × 104 A/m, with a spherical-like morphology and average particle size of 56.81 nm obtained by Scanning Electron Microscopy analysis. The increment of SOP promoted a high degree of crystallinity and decrease in crystal size. Additionally, it promoted the formation of secondary phases, induced agglomeration, and modified the morphology of the particles. Full article
(This article belongs to the Special Issue Crystal Structure of Magnetic Materials)
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Open AccessArticle A 12-Fold ThSi2 Interpenetrated Network Utilizing a Glycine-Based Pseudopeptidic Ligand
Crystals 2018, 8(1), 47; https://doi.org/10.3390/cryst8010047
Received: 21 November 2017 / Revised: 4 January 2018 / Accepted: 15 January 2018 / Published: 18 January 2018
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Abstract
We report the synthesis and characterization of a 3D Cu(II) coordination polymer, [Cu3(L1)2(H2O)8]·8H2O (1), with the use of a glycine-based tripodal pseudopeptidic ligand (H3L1 = N
[...] Read more.
We report the synthesis and characterization of a 3D Cu(II) coordination polymer, [Cu3(L1)2(H2O)8]·8H2O (1), with the use of a glycine-based tripodal pseudopeptidic ligand (H3L1 = N,N′,N″-tris(carboxymethyl)-1,3,5-benzenetricarboxamide or trimesoyl-tris-glycine). This compound presents the first example of a 12-fold interpenetrated ThSi2 (ths) net. We attempt to justify the unique topology of 1 through a systematic comparison of the synthetic parameters in all reported structures with H3L1 and similar tripodal pseudopeptidic ligands. We additionally explore the catalytic potential of 1 in the A3 coupling reaction for the synthesis of propargylamines. The compound acts as a very good heterogeneous catalyst with yields up to 99% and loadings as low as 3 mol %. Full article
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Open AccessArticle Monoclinic Paracetamol vs. Paracetamol-4,4′-Bipyridine Co-Crystal; What Is the Difference? A Charge Density Study
Crystals 2018, 8(1), 46; https://doi.org/10.3390/cryst8010046
Received: 22 December 2017 / Revised: 8 January 2018 / Accepted: 15 January 2018 / Published: 18 January 2018
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Abstract
Paracetamol (PCM) has two well-documented polymorphic forms at room temperature; monoclinic Form I is more stable than the other orthorhombic Form II. Form II exhibits improved tabletting properties compared to Form I due to low shearing forces; however, difficulties in its manufacture have
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Paracetamol (PCM) has two well-documented polymorphic forms at room temperature; monoclinic Form I is more stable than the other orthorhombic Form II. Form II exhibits improved tabletting properties compared to Form I due to low shearing forces; however, difficulties in its manufacture have limited its use in industrial manufacture. Previous studies have found that the introduction of a co-former to form co-crystals would allow the PCM molecule to exist in a conformation similar to that of the orthorhombic form while being more stable at room temperature. Experimental charge density analysis of the paracetamol-4,4′-bipyridine (PCM-44BP) co-crystal system, and its constituent molecules, has been carried out to examine the forces that drive the formation and stabilisation of the co-crystal, while allowing PCM to maintain a packing motif similar to that found in Form II. It is hoped studies on this well-known compound will help apply the knowledge gained to other drug molecules that are less successful. The PCM molecules in the co-crystal were found to exhibit similar packing motifs to that found in Form I, however, intercalation of the 44BP molecule between the PCM layers resulted in a shallower angle between molecular planes, which could result in the required lateral shear. Topological analysis identified more weak interactions in the co-crystal compared to the individual molecules, thus allowing for greater stability as evidenced by the lattice energies. Weak interactions in the PCM-44BP co-crystal were found to range in strength from 4.08–84.33 kJ mol−1, and this variety allowed the PCM-44BP planes to be held together, while a weak π–π interaction (15.14 kJ mol−1) allowed lateral shear to occur, thus mimicking the planes found in Form II PCM and offering the possibility of improved tabletting properties. A comparison of integrated atomic charges between partitions of the PCM molecules in the single and co-crystal found that the hydroxyl and amide groups were involved in greater hydrogen bonding in the co-crystal, resulting in a charge redistribution across the molecule evidenced by a larger molecular dipole moment (µ = 12.34D). These findings, in addition to the co-crystal having the largest lattice energy, form a potential basis with which to predict that the co-crystal exhibits improved solubility and stability profiles. It is anticipated that these findings will contribute to improvements in the formulation and other physical properties of PCM and other pharmaceutical compounds. Full article
(This article belongs to the Special Issue Experimental and Theoretical Electron Density Analysis of Crystals)
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Open AccessArticle A Feasible and Effective Post-Treatment Method for High-Quality CH3NH3PbI3 Films and High-Efficiency Perovskite Solar Cells
Crystals 2018, 8(1), 44; https://doi.org/10.3390/cryst8010044
Received: 14 December 2017 / Revised: 13 January 2018 / Accepted: 13 January 2018 / Published: 18 January 2018
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Abstract
The morphology control of CH3NH3PbI3 (MAPbI3) thin-film is crucial for the high-efficiency perovskite solar cells, especially for their planar structure devices. Here, a feasible and effective post-treatment method is presented to improve the quality of MAPbI
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The morphology control of CH3NH3PbI3 (MAPbI3) thin-film is crucial for the high-efficiency perovskite solar cells, especially for their planar structure devices. Here, a feasible and effective post-treatment method is presented to improve the quality of MAPbI3 films by using methylamine (CH3NH2) vapor. This post-treatment process is studied thoroughly, and the perovskite films with smooth surface, high preferential growth orientation and large crystals are obtained after 10 s treatment in MA atmosphere. It enhances the light absorption, and increases the recombination lifetime. Ultimately, the power conversion efficiency (PCE) of 15.3% for the FTO/TiO2/MAPbI3/spiro-OMeTAD/Ag planar architecture solar cells is achieved in combination with this post-treatment method. It represents a 40% improvement in PCE compared to the best control cell. Moreover, the whole post-treatment process is simple and cheap, which only requires some CH3NH2 solution in absolute ethanol. It is beneficial to control the reaction rate by changing the volume of the solution. Therefore, we are convinced that the post-treatment method is a valid and essential approach for the fabrication of high-efficiency perovskite solar cells. Full article
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Open AccessArticle Trinuclear Co(II) and Mononuclear Ni(II) Salamo-Type Bisoxime Coordination Compounds
Crystals 2018, 8(1), 43; https://doi.org/10.3390/cryst8010043
Received: 19 December 2017 / Revised: 13 January 2018 / Accepted: 15 January 2018 / Published: 17 January 2018
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Abstract
One trinuclear Co(II) coordination compound [{CoL1(OAc)(CH3COCH3)}2Co] (1) and one unprecedented mononuclear Ni(II) coordination compound [Ni(L2)2] (2), constructed from a Salamo-type ligand H2L1 were synthesized
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One trinuclear Co(II) coordination compound [{CoL1(OAc)(CH3COCH3)}2Co] (1) and one unprecedented mononuclear Ni(II) coordination compound [Ni(L2)2] (2), constructed from a Salamo-type ligand H2L1 were synthesized and characterized by elemental analyses, IR, UV-vis spectra, and single crystal X-ray diffraction analyses. The results show that the Co(II) atoms have no significant distortion in CoO6 or CoO4N2 octahedrons in coordination compound 1. Interestingly, in coordination compound 2, the desired tri- or mono-nuclear Salamo-type Ni(II) coordination compound was not obtained, but an unprecedented Ni(II) coordination compound [Ni(L2)2] was synthesized, the Ni1 atom having no significant distortion in the NiO2N2 planar quadrilateral geometry. Furthermore, the antimicrobial activities of coordination compound 1 and previously reported coordination compound [{CoL1(OAc)(MeOH)}2Co]·2MeOH (3) are discussed. Full article
(This article belongs to the Special Issue Crystal Structures of Boron Compounds)
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Open AccessEditorial Analysis of Halogen and Other σ-Hole Bonds in Crystals
Crystals 2018, 8(1), 42; https://doi.org/10.3390/cryst8010042
Received: 10 January 2018 / Revised: 10 January 2018 / Accepted: 10 January 2018 / Published: 17 January 2018
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Abstract
Schneider has observed that [1]: “ . . . the chemistry of the last century was largely the chemistry of covalent bonding, whereas that of the present century is more likely to be the chemistry of noncovalent binding.”[...] Full article
(This article belongs to the Special Issue Analysis of Halogen and Other σ-Hole Bonds in Crystals)
Open AccessArticle Salts and Co-Crystalline Assemblies of Tetra(4-Pyridyl)Ethylene with Di-Carboxylic Acids
Crystals 2018, 8(1), 41; https://doi.org/10.3390/cryst8010041
Received: 22 December 2017 / Revised: 9 January 2018 / Accepted: 15 January 2018 / Published: 17 January 2018
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Abstract
Tetraarylethylene derivatives are emerging as an increasingly important family of supramolecular building blocks in both solution phase and the solid state. The utility of tetraarylethylenes stems from appealing structural features (rigidity and symmetry) and their propensity to exhibit aggregation induced emission (AIE). In
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Tetraarylethylene derivatives are emerging as an increasingly important family of supramolecular building blocks in both solution phase and the solid state. The utility of tetraarylethylenes stems from appealing structural features (rigidity and symmetry) and their propensity to exhibit aggregation induced emission (AIE). In an effort to investigate the luminescent sensing ability of heteroaromatic tetraarylethylenes, we previously prepared tetra(4-pyridyl)ethylene and characterized its solution phase AIE properties. We here report the successful incorporation of tetra(4-pyridyl)ethylene into three distinct salts and co-crystalline assemblies with three organic di-carboxylic acids (oxalic acid, malonic acid, and fumaric acid). Interactions between the tetra(pyridyl)ethylene and di-acid components were found to vary from conventional to charge-assisted hydrogen bonding according to the extent of proton transfer between the acid and pyridine groups. Notably, the formation of pyridinium-carboxylate adducts in the salts does not appear to be strongly correlated with acid pKa. Three distinct network topologies were observed, and all featured the bridging of two or three tetra(pyridyl)ethylene groups through di-acid linkers. Crystalline assemblies also retained the AIE activity of tetra(pyridyl)ethylene and were luminescent under UV light. As tetra(4-pyridyl)ethylene features four Lewis basic and potentially metal ligating pyridine rings in a relatively well-defined geometry, this compound represents an attractive building block for the design of additional crystalline organic and metal–organic functional materials. Full article
(This article belongs to the Special Issue Crystal Structure Analysis of Supramolecular and Porous Solids)
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Open AccessArticle Effect of MnO2 Concentration on the Conductivity of Ce0.9Gd0.1MnxO2−δ
Crystals 2018, 8(1), 40; https://doi.org/10.3390/cryst8010040
Received: 15 November 2017 / Revised: 12 January 2018 / Accepted: 15 January 2018 / Published: 17 January 2018
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Abstract
Samples with the composition Ce0.9Gd0.1MnxO2−δ with x = 0.01, 0.02, and 0.05 Mn-addition were prepared by mixed oxide route from Ce0.9Gd0.1O2−δ and MnO2 and sintered at 1300 °C. The electronic
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Samples with the composition Ce0.9Gd0.1MnxO2−δ with x = 0.01, 0.02, and 0.05 Mn-addition were prepared by mixed oxide route from Ce0.9Gd0.1O2−δ and MnO2 and sintered at 1300 °C. The electronic conductivity was measured using a modified Hebb-Wagner technique, the electrical conductivity was investigated by impedance spectroscopy, and oxygen permeation was measured for the sample with x = 0.05. An increase of the electronic partial conductivity with increasing Mn addition was observed, which can be attributed to an additional Mn 3d-related state between the top of the valence band and the bottom of the Ce 4f band. The grain boundary conductivity was found to be suppressed for low Mn contents, but enhanced for the sample with x = 0.05. Full article
(This article belongs to the Special Issue Ceramic Conductors)
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Open AccessArticle Temperature Dependence of the Thermal, Electrical Resistivity, Dielectric and Piezoelectric Properties of CaYAl3O7 Crystal
Crystals 2018, 8(1), 34; https://doi.org/10.3390/cryst8010034
Received: 13 December 2017 / Revised: 3 January 2018 / Accepted: 10 January 2018 / Published: 17 January 2018
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Abstract
Calcium yttrium aluminate (CaYAl3O7) crystal was grown and characterized in detail for high temperature piezoelectric sensors for the first time. The thermal properties of the CaYAl3O7 (CYAM) crystal were investigated systematically. In particular, the CYAM crystal
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Calcium yttrium aluminate (CaYAl3O7) crystal was grown and characterized in detail for high temperature piezoelectric sensors for the first time. The thermal properties of the CaYAl3O7 (CYAM) crystal were investigated systematically. In particular, the CYAM crystal exhibits considerably high resistivity along X- and Z- direction in the order of 6.96 × 107 Ω·cm and 2.86 × 108 Ω·cm at 600 °C, respectively. The temperature dependence of the electromechanical properties of CYAM crystal were investigated over the temperature range of 25–500 °C. The high thermal stability of piezoelectric properties together with its high electrical resistivity, makes CaYAl3O7 crystal a promising candidate for high temperature piezoelectric applications. Full article
(This article belongs to the Special Issue Crystal Growth for Optoelectronic and Piezoelectric Applications)
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Open AccessArticle CO2 Capture and Crystallization of Ammonia Bicarbonate in a Lab-Scale Scrubber
Crystals 2018, 8(1), 39; https://doi.org/10.3390/cryst8010039
Received: 21 November 2017 / Revised: 9 January 2018 / Accepted: 12 January 2018 / Published: 16 January 2018
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Abstract
A lab-scale bubble-column scrubber is used to capture CO2 gas and produce ammonia bicarbonate (ABC) using aqueous ammonia as an absorbent under a constant pH and temperature. The CO2 concentration is adjusted by mixing N2 and CO2 in the
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A lab-scale bubble-column scrubber is used to capture CO2 gas and produce ammonia bicarbonate (ABC) using aqueous ammonia as an absorbent under a constant pH and temperature. The CO2 concentration is adjusted by mixing N2 and CO2 in the range of 15–60 vol % at 55 °C. The process variables are the pH of the solution, temperature, gas-flow rate and the concentration of gas. The effects of the process variables on the removal efficiency (E), absorption rate (RA) and overall mass-transfer coefficient (KGa) were explored. A multiple-tube mass balance model was used to determine RA and KGa, in which RA and KGa were in the range of 2.14 × 10−4–1.09 × 10−3 mol/(s·L) and 0.0136–0.5669 1/s, respectively. Results found that, RA showed an obvious increase with the increase in pH, inlet gas concentration and gas temperature, while KGa decreased with an increase in inlet gas concentration. Using linear regression, an empirical expression for KGa/E was obtained. On the other hand, ammonia bicarbonate crystals could be produced at a pH of 9.5 when the gas concentration was higher than 30% and γ (=Fg/FA, the gas-liquid molar flow rate ratio) ≥ 1.5. Full article
(This article belongs to the Special Issue Carbonates)
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Open AccessArticle Effects of Disorder on the Pressure-Induced Mott Transition in κ-(BEDT-TTF)2Cu[N(CN)2]Cl
Crystals 2018, 8(1), 38; https://doi.org/10.3390/cryst8010038
Received: 14 December 2017 / Revised: 9 January 2018 / Accepted: 11 January 2018 / Published: 16 January 2018
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Abstract
We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt κ-(BEDT-TTF)2Cu[N(CN)2]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals
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We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals to X-ray irradiation. The crystals were then fine-tuned across the Mott transition by the application of continuously controllable He-gas pressure at low temperatures. Measurements of the thermal expansion and resistance show that the first-order character of the Mott transition prevails for low irradiation doses achieved by irradiation times up to 100 h. For these crystals with a moderate degree of disorder, we find a first-order transition line which ends in a second-order critical endpoint, akin to the pristine crystals. Compared to the latter, however, we observe a significant reduction of both, the critical pressure p c and the critical temperature T c . This result is consistent with the theoretically-predicted formation of a soft Coulomb gap in the presence of strong correlations and small disorder. Furthermore, we demonstrate, similar to the observation for the pristine sample, that the Mott transition after 50 h of irradiation is accompanied by sizable lattice effects, the critical behavior of which can be well described by mean-field theory. Our results demonstrate that the character of the Mott transition remains essentially unchanged at a low disorder level. However, after an irradiation time of 150 h, no clear signatures of a discontinuous metal-insulator transition could be revealed anymore. These results suggest that, above a certain disorder level, the metal-insulator transition becomes a smeared first-order transition with some residual hysteresis. Full article
(This article belongs to the Special Issue Advances in Organic Conductors and Superconductors)
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