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Crystals — Open Access Journal of Crystallography

Crystals (ISSN 2073-4352; CODEN: CRYSBC) is an international peer-reviewed open access journal of crystallography. Crystals is published monthly online by MDPI.

Open Access free for readers, with article processing charges (APC) paid by authors or their institutions.
High visibility: Indexed in the Science Citation Index Expanded (Web of Science), Inspec (IET) and Scopus.
Rapid publication: manuscripts are peer-reviewed and a first decision provided to authors approximately 16 days after submission; acceptance to publication is undertaken in 4.8 days (median values for papers published in the first six months of 2018).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.

Impact Factor: 2.144 (2017) ; 5-Year Impact Factor: 1.928 (2017)

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Latest Articles

Open AccessArticle

Solvent Effects on the Spin-Transition in a Series of Fe(II) Dinuclear Triple Helicate Compounds

by Alexander R. Craze, Mohan M. Bhadbhade, Cameron J. Kepert, Leonard F. Lindoy, Christopher E. Marjo and Feng Li

Crystals 2018, 8(10), 376; https://doi.org/10.3390/cryst8100376 (registering DOI) - 23 September 2018

Abstract

[...] Read more.

This work explores the effect of lattice solvent on the observed solid-state spin-transition of a previously reported dinuclear Fe(II) triple helicate series 1–3 of the general form [Fe^II₂L₃](BF₄)₄(CH₃CN)_n, where L is the Schiff base condensation product of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L¹), 4,4′-diaminodiphenyl sulfide (L²) and 4,4′-diaminodiphenyl ether (L³) respectively, and 1 is the complex when L = L¹, 2 when L = L² and 3 when L = L³ (Craze, A.R.; Sciortino, N.F.; Bhadbhade, M.M.; Kepert, C.J.; Marjo, C.E.; Li, F. Investigation of the Spin Crossover Properties of Three Dinuclear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type. Inorganics.2017, 5 (4), 62). Desolvation of 1 and 2 during measurement resulted not only in a decrease in T_1/2 and completeness of spin-crossover (SCO) but also a change in the number of steps in the spin-profile. Compounds 1 and 2 were observed to change from a two-step 70% complete transition when fully solvated, to a single-step half complete transition upon desolvation. The average T_1/2 value of the two-steps in the solvated materials was equivalent to the single T_1/2 of the desolvated sample. Upon solvent loss, the magnetic profile of 3 experienced a transformation from a gradual SCO or weak antiferromagnetic interaction to a single half-complete spin-transition. Variable temperature single-crystal structures are presented and the effects of solvent molecules are also explored crystallographically and via a Hirshfeld surface analysis. The spin-transition profiles of 1–3 may provide further insight into previous discrepancies in dinuclear triple helicate SCO research reported by the laboratories of Hannon and Gütlich on analogous systems (Tuna, F.; Lees, M. R.; Clarkson, G. J.; Hannon, M. J. Readily Prepared Metallo-Supramolecular Triple Helicates Designed to Exhibit Spin-Crossover Behaviour. Chem. Eur. J.2004, 10, 5737–5750 and Garcia, Y.; Grunert, C. M.; Reiman, S.; van Campenhoudt, O.; Gütlich, P. The Two-Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy. Eur. J. Inorg. Chem.2006, 3333–3339). Full article

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Open AccessReview

Basic Concepts and Recent Advances of Crystallographic Orientation Determination of Graphene by Raman Spectroscopy

by Yucheng Lan, Mobolaji Zondode, Hua Deng, Jia-An Yan, Marieme Ndaw, Abdellah Lisfi, Chundong Wang and Yong-Le Pan

Crystals 2018, 8(10), 375; https://doi.org/10.3390/cryst8100375 (registering DOI) - 21 September 2018

Abstract

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Graphene is a kind of typical two-dimensional material consisting of pure carbon element. The unique material shows many interesting properties which are dependent on crystallographic orientations. Therefore, it is critical to determine their crystallographic orientations when their orientation-dependent properties are investigated. Raman spectroscopy has been developed recently to determine crystallographic orientations of two-dimensional materials and has become one of the most powerful tools to characterize graphene nondestructively. This paper summarizes basic aspects of Raman spectroscopy in crystallographic orientation of graphene nanosheets, determination principles, the determination methods, and the latest achievements in the related studies. Full article

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Open AccessCommunication

Crystal Structure of the Protonated Germanide Cluster [HGe₉]³⁻

by Corinna Lorenz and Nikolaus Korber

Crystals 2018, 8(10), 374; https://doi.org/10.3390/cryst8100374 - 21 September 2018

Abstract

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A single crystal X-ray diffraction study of the new compound [Rb([2.2.2]crypt)]₂[Rb([18]crown−6)][HGe₉]·4NH₃ revealed the presence of the first protonated nine-atom germanide cluster [HGe₉]³⁻. It forms from Rb₄Ge₉ in liquid ammonia, so that [Ge₉]⁴⁻ can be considered as the base and [HGe₉]³⁻ its formally conjugated acid. The H atom is attached to a germanium vertex atom of the basal square plane, as it is known for [RGe₉]³⁻ (R = C₅H₉, Mes, etc.) or [HE₉]³⁻ (E = Si, Sn). In addition, the proton could be located unambiguously in the Fourier difference map. [HGe₉]³⁻ also represents a nido cluster species with 22 cluster-bonding electrons, which can be considered the most stable structure for nine-atom cluster species for all group 14 elements. Full article

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Open AccessArticle

Propylene-Selective Thin Zeolitic Imidazolate Framework Membranes on Ceramic Tubes by Microwave Seeding and Solvothermal Secondary Growth

by Jingze Sun, Chen Yu and Hae-Kwon Jeong

Crystals 2018, 8(10), 373; https://doi.org/10.3390/cryst8100373 - 21 September 2018

Abstract

[...] Read more.

Zeolitic imidazolate framework (ZIF-8) membranes have attracted tremendous interest for their high-resolution kinetic separation of propylene/propane mixtures. Current polycrystalline ZIF-8 membranes are supported mostly on planar ceramic substrates (e.g., alumina disks) because of their high thermal, chemical, and mechanical stabilities and facile manufacturing in the labs. Planar supports are, however, not scalable for practical separation applications owing to their low packing density (typically 30–500 m²/m³). On the other hand, ceramic tubes provide order-of-magnitude higher packing densities than planar supports (i.e., much higher membrane areas per module). Here, we report polycrystalline ZIF-8 membranes with thicknesses of ~1.2 μm grown on the bore side of commercially-available ceramic tubes using the microwave seeding and secondary growth technique. The tubular ZIF-8 membranes showed excellent propylene/propane separation factors of ~80, exceeding all currently-reported ZIF-8 membranes on ceramic tubes. It was found that the secondary growth time was critical to enhance the propylene/propane separation factor of the membranes. Membranes were also grown on the shell side of tubular supports, showing the versatility of our technique. Full article

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Open AccessArticle

Synthesis, Crystal Structure, and Properties of a New Coordination Polymer Built from N/O-Donor Mixed Ligands

by Mei-An Zhu, Shuai-Shuai Han, Feng Deng, Jia-Le Li and Shui-Sheng Chen

Crystals 2018, 8(10), 372; https://doi.org/10.3390/cryst8100372 - 21 September 2018

Abstract

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The coordination polymer, namely, [Cd(H₂L)(nobda)]_n (1) was prepared by the reaction of Cd(NO₃)₂·4H₂O with 4-amino-1,2-benzenedicarboxylic acid (H₂nobda) and 1,4-di(1H-imidazol-4-yl)benzene (H₂L), and characterized by single-crystal X-ray diffraction, elemental analysis, infrared (IR) spectroscopy, thermogravimetric analysis, and powder X-ray diffraction (PXRD). The carboxylic acid of H₂nobda ligands was completely deprotonated to be nobda²⁻ anions, which act as tridentate ligand to connect the Cd²⁺ to form two-dimensional (2D) network, while the neutral H₂L ligands serve as a linear didentate bridge to connect two adjacent Cd²⁺ ions upper and down the 2D layer. The adjacent 2D layers were further linked into the three-dimensional (3D) supramolecular polymer by the weak interactions such as hydrogen bonds and π−π stacking interactions. The ultraviolet-visible (UV-vis) absorption spectra and luminescent properties in the solid state at room temperature have been investigated. Full article

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Open AccessArticle

Comparison of the Mesomorphic Behaviour of 1:1 and 1:2 Mixtures of Charged Gay-Berne GB(4.4,20.0,1,1) and Lennard-Jones Particles

by Tommaso Margola, Katsuhiko Satoh and Giacomo Saielli

Crystals 2018, 8(10), 371; https://doi.org/10.3390/cryst8100371 - 20 September 2018

Abstract

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We present a Molecular Dynamics study of mixtures of charged Gay-Berne (GB) ellipsoids and spherical Lennard-Jones (LJ) particles as models of ionic liquids and ionic liquid crystals. The GB system is highly anisotropic (GB(4.4,20.0,1,1)) and we observe a rich mesomorphism, with ionic nematic and smectic phases in addition to the isotropic mixed phase and crystalline phases with honeycomb structure. The systems have been investigated by analyzing the orientational and translational order parameters, as well as radial distribution functions. We have directly compared 1:1 mixtures, where the GB and LJ particles have a charge equal in magnitude and opposite in sign, and 1:2 mixtures where the number of LJ particles is twice as large compared to the GB and their charge half in magnitude. The results highlight the role of the long-range isotropic electrostatic interaction compared to the short-range van der Waals anisotropic contribution, and the effect of the stoichiometry on the stability of ionic mesophases. Full article

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Open AccessArticle

Quasicrystal Tilings in Three Dimensions and Their Empires

by Dugan Hammock, Fang Fang and Klee Irwin

Crystals 2018, 8(10), 370; https://doi.org/10.3390/cryst8100370 - 20 September 2018

Abstract

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The projection method for constructing quasiperiodic tilings from a higher dimensional lattice provides a useful context for computing a quasicrystal’s vertex configurations, frequencies, and empires (forced tiles). We review the projection method within the framework of the dual relationship between the Delaunay and Voronoi cell complexes of the lattice being projected. We describe a new method for calculating empires (forced tiles) which also borrows from the dualisation formalism and which generalizes to tilings generated projections of non-cubic lattices. These techniques were used to compute the vertex configurations, frequencies and empires of icosahedral quasicrystals obtained as a projections of the

D_{6}

and

Z^{6}

lattices to

R^{3}

and we present our analyses. We discuss the implications of this new generalization. Full article

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Open AccessReview

Non-Platinum Metal Complexes as Potential Anti-Triple Negative Breast Cancer Agents

by Eva María Domínguez-Martís, Diego Gabriel Mosteiro-Miguéns, Lucía Vigo-Gendre, David López-Ares, Manuel Freire-Garabal, María Jesús Núñez-Iglesias and Silvia Novío

Crystals 2018, 8(10), 369; https://doi.org/10.3390/cryst8100369 - 20 September 2018

Abstract

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Breast cancer (BC) is the most common cancer in women worldwide, with a mortality rate that has been forecasted to rise in the next decade. This is especially worrying for people with triple-negative BC (TNBC), because of its unresponsiveness to current therapies. Different drugs to treat TNBC have been assessed, and, although platinum chemotherapy drugs seem to offer some hope, their drawbacks have motivated extensive investigations into alternative metal-based BC therapies. This paper aims to: (i) describe the preliminary in vitro and in vivo anticancer properties of non-platinum metal-based complexes (NPMBC) against TNBC; and (ii) analyze the likely molecular targets involved in their anticancer activity. Full article

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Open AccessArticle

The Crystal Structure of Defect KBB’O₆ Pyrochlores (B,B’: Nb,W,Sb,Te) Revisited from Neutron Diffraction Data

by Sergio F. Mayer, Horacio Falcón, María Teresa Fernández-Díaz and José Antonio Alonso

Crystals 2018, 8(10), 368; https://doi.org/10.3390/cryst8100368 - 20 September 2018

Abstract

Three defect pyrochlores KNbWO₆·xH₂O, KNbTeO₆ and KSbWO₆ were synthesized by solid state reaction at 750 °C, from stoichiometric mixtures of K₂C₂O₄, Sb₂O₃, Nb₂O

[...] Read more.

Three defect pyrochlores KNbWO₆·xH₂O, KNbTeO₆ and KSbWO₆ were synthesized by solid state reaction at 750 °C, from stoichiometric mixtures of K₂C₂O₄, Sb₂O₃, Nb₂O₅, WO₃ and 20% excess TeO₂. A neutron powder diffraction (NPD) data analysis allowed unveiling some structural features. They are all defined in the cubic

F d \bar{3} m

space group symmetry, with α = 10.5068(1) Å, 10.2466(1) Å and 10.2377(1) Å, respectively. Difference Fourier synthesis for KNbWO₆·xH₂O clearly showed the presence of crystallization water, with extra O’ oxygen and H⁺ atoms that were located from NPD data. These O’ oxygen atoms are placed at 32e Wyckoff sites, conforming a K₂O’ sublattice interpenetrated with the covalent framework constituted by (Nb,W)O₆ octahedra. The H⁺ ions coordinate the O’ atoms at partially occupied 96g Wyckoff sites while K⁺ ions shift also along 32e sites, but closer to the 16c special site (0,0,0). By contrast, extra H₂O molecules are absent in the other two pyrochlores: in KNbTeO₆ and KSbWO₆ K⁺ ions are shifted along 32e (x,x,x) sites further away from the origin than for the previous material, and the higher covalency of the octahedral network determines more compact structures, with shorter B–O distances and narrower B–O–B angles in the proposed AB₂O₆ defect pyrochlore structure. Full article

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Open AccessArticle

Size-Controllable Synthesis of Zeolitic Imidazolate Framework/Carbon Nanotube Composites

by Fang Fu, Bin Zheng, Lin-Hua Xie, Huiling Du, Shuangming Du and Zhenhua Dong

Crystals 2018, 8(10), 367; https://doi.org/10.3390/cryst8100367 - 20 September 2018

Abstract

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Composite materials that combine the unique properties of zeolitic imidazolate frameworks (ZIFs) and carbon nanotubes (CNTs) can give rise to novel applications. Here, ZIF-8/CNT composites were successfully prepared with and without the addition of an agent template. The size of the ZIF-8 crystals in the composite materials was controlled by varying the template, feeding order, and concentration of reactants. Thus, ZIF-8 crystals with a wide variety of sizes (from nano- to micrometer size, which is range that differs by a factor of 10) were obtained, depending on the conditions. This size-controllable synthesis of ZIF-8 was achieved by modifying the number of nucleation sites on the CNTs, as revealed by density functional theory (DFT) calculations. This work provides an efficient method for preparing ZIF-8/CNT composites with controllable size and can pave the way for the synthesis of other metal-organic framework (MOF)/CNT composite materials. Full article

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Open AccessArticle

Selective Area Growth and Structural Characterization of GaN Nanostructures on Si(111) Substrates

by Alexana Roshko, Matt Brubaker, Paul Blanchard, Todd Harvey and Kris A. Bertness

Crystals 2018, 8(9), 366; https://doi.org/10.3390/cryst8090366 - 16 September 2018

Abstract

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Selective area growth (SAG) of GaN nanowires and nanowalls on Si(111) substrates with AlN and GaN buffer layers grown by plasma-assisted molecular beam epitaxy was studied. For N-polar samples filling of SAG features increased with decreasing lattice mismatch between the SAG and buffer. Defects related to Al–Si eutectic formation were observed in all samples, irrespective of lattice mismatch and buffer layer polarity. Eutectic related defects in the Si surface caused voids in N-polar samples, but not in metal-polar samples. Likewise, inversion domains were present in N-polar, but not metal-polar samples. The morphology of Ga-polar GaN SAG on nitride buffered Si(111) was similar to that of homoepitaxial GaN SAG. Full article

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Open AccessReview

Thermal and Chemical Expansion in Proton Ceramic Electrolytes and Compatible Electrodes

by Andreas Løken, Sandrine Ricote and Sebastian Wachowski

Crystals 2018, 8(9), 365; https://doi.org/10.3390/cryst8090365 - 14 September 2018

Abstract

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This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC). The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while the chemical expansion is explained based on crystallography and defect chemistry. Modelling methods are used to predict the thermochemical expansion of PCCs and MIECs with two examples: hydration of barium zirconate (BaZr_1−xY_xO_3−δ) and oxidation/reduction of La_1−xSr_xCo_0.2Fe_0.8O_3−δ. While it is unusual for a review paper, we conducted experiments to evaluate the influence of the heating rate in determining expansion coefficients experimentally. This was motivated by the discrepancy of some values in literature. The conclusions are that the heating rate has little to no effect on the obtained values. Models for the expansion coefficients of a composite material are presented and include the effect of porosity. A set of data comprising thermal and chemical expansion coefficients has been gathered from the literature and presented here divided into two groups: protonic electrolytes and mixed ionic-electronic conductors. Finally, the methods of mitigation of the thermal mismatch problem are discussed. Full article

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Open AccessArticle

Temperature Effects on the Elastic Constants, Stacking Fault Energy and Twinnability of Ni₃Si and Ni₃Ge: A First-Principles Study

by Lili Liu, Liwan Chen, Youchang Jiang, Chenglin He, Gang Xu and Yufeng Wen

Crystals 2018, 8(9), 364; https://doi.org/10.3390/cryst8090364 - 14 September 2018

Abstract

The volume versus temperature relations for Ni

_{3}

Si and Ni

_{3}

Ge are obtained by using the first principles calculations combined with the quasiharmonic approach. Based on the equilibrium volumes at temperature T, the temperature dependence of the elastic constants, generalized

[...] Read more.

The volume versus temperature relations for Ni

_{3}

Si and Ni

_{3}

_{3}

Si and Ni

_{3}

Ge are investigated by first principles calculations. The elastic constants, antiphase boundary energies, complex stacking fault energies, superlattice intrinsic stacking fault energies and twinning energy decrease with increasing temperature. The twinnability of Ni

_{3}

Si and Ni

_{3}

Ge are examined using the twinnability criteria. It is found that their twinnability decrease with increasing temperature. Furthermore, Ni

_{3}

Si has better twinnability than Ni

_{3}

Ge at different temperatures. Full article

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Open AccessArticle

Mosaicity of Spin-Crossover Crystals

by Sabine Lakhloufi, Elodie Tailleur, Wenbin Guo, Frédéric Le Gac, Mathieu Marchivie, Marie-Hélène Lemée-Cailleau, Guillaume Chastanet and Philippe Guionneau

Crystals 2018, 8(9), 363; https://doi.org/10.3390/cryst8090363 - 13 September 2018

Abstract

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Real crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state and others in the low spin (LS) state. Since the volume of HS and LS crystal packings are known to be very different, the assembly of domains within the crystal, i.e., its mosaicity, may be modified at the spin crossover. With little data available in the literature we propose an investigation into the temperature dependence of mosaicity in certain spin-crossover crystals. The study was preceded by the examination of instrumental factors, in order to establish a protocol for the measurement of mosaicity. The results show that crystal mosaicity appears to be strongly modified by thermal spin-crossover; however, the nature of the changes are probably sample dependent and driven, or masked, in most cases by the characteristics of the crystal (disorder, morphology …). No general relationship could be established between mosaicity and crystal properties. If, however, mosaicity studies in spin-crossover crystals are conducted and interpreted with great care, they could help to elucidate crucial crystal characteristics such as mechanical fatigability, and more generally to investigate systems where phase transition is associated with large volume changes. Full article

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Open AccessArticle

Inducing Crystallinity of Metal Thin Films with Weak Magnetic Fields without Thermal Annealing

by Stefan S. Ručman, Winita Punyodom, Jaroon Jakmunee and Pisith Singjai

Crystals 2018, 8(9), 362; https://doi.org/10.3390/cryst8090362 - 13 September 2018

Abstract

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Since the discovery of thin films, it has been known that higher crystallinity demands higher temperatures, making the process inadequate for energy-efficient and environmentally friendly methods of thin film fabrication. We resolved this problem by sparking metal wires in a 0.4 Tesla magnetic field at ambient conditions under ultra-pure nitrogen flow to replace the annealing of thin films, and thus designed an environmentally friendly and energy-efficient thin film fabrication method. We employed grazing incidence X-Ray Diffraction spectroscopy to characterize crystallinity of Iron, Nickel, Copper and Tungsten thin films prepared by a sparking discharge process in the presence of 0.4 T magnetic field at an ambient temperature of 25 °C. Control experiment was conducted by sparking without a magnetic field present and using ultra-pure nitrogen flow and ambient air containing oxygen. The Iron thin film prepared in ultra-pure nitrogen flow preserved crystallinity even after one year of ageing. Nickel exhibited higher crystallinity when sparked in nitrogen gas flow than when sparked in atmospheric air and was the only element to crystalize under atmospheric air. Tungsten successfully crystalized after just 40 min of sparking and aluminium failed to crystalize at all, even after 12 h of sparking under nitrogen flow. Full article

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Open AccessArticle

Molecular Dynamics Investigation of Graphene Nanoplate Diffusion Behavior in Poly-α-Olefin Lubricating Oil

by Lupeng Wu, Baoyu Song, Leon M. Keer and Le Gu

Crystals 2018, 8(9), 361; https://doi.org/10.3390/cryst8090361 - 13 September 2018

Abstract

[...] Read more.

Graphene as a type of novel additive significantly enhanced the tribological performance of blended lubricating oil. However, the dispersibility of graphene with long-term stability in lubricating oil is still a challenge. Chemical modification for graphene, rather than using surfactants, provided a better method to improve the dispersibility of graphene in lubricants. In this study, the equilibrium molecular dynamics (EMD) simulations were carried out to investigate the diffusion behavior of graphene nanoplates in poly-α-olefin (PAO) lubricating oil. The effects of graphene-size, edge-functionalization, temperature, and pressure on the diffusion coefficient were studied. In order to understand the influence of edge-functionalization, three different functional groups were grafted to the edge of graphene nanoplates: COOH, COON(CH₃)₂, CONH(CH₂)₈CH₃ (termed GO, MG, and AG, respectively). The EMD simulations results demonstrated that the relationships between diffusion coefficient and graphene-size and number of functional groups were linear while the temperature and pressure had a nonlinear influence on the diffusion coefficient. It was found that the larger dimension and more functional groups provided the lower diffusion coefficient. AG with eight CONH(CH₂)₈CH₃ groups exhibited the lowest diffusion coefficient. Furthermore, the experimental results and radial distribution function for graphene-PAO illustrated that the diffusion coefficient reflected the dispersibility of nanoparticles in nanofluids to some degree. To our best knowledge, this study is the first time the diffusion behavior of graphene in PAO lubricating oil was investigated using EMD simulations. Full article

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Open AccessArticle

Tracing Phase Transformation and Lattice Evolution in a TRIP Sheet Steel under High-Temperature Annealing by Real-Time In Situ Neutron Diffraction

by Dunji Yu, Yan Chen, Lu Huang and Ke An

Crystals 2018, 8(9), 360; https://doi.org/10.3390/cryst8090360 - 11 September 2018

Abstract

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Real-time in situ neutron diffraction was used to characterize the crystal structure evolution in a transformation-induced plasticity (TRIP) sheet steel during annealing up to 1000 °C and then cooling to 60 °C. Based on the results of full-pattern Rietveld refinement, critical temperature regions were determined in which the transformations of retained austenite to ferrite and ferrite to high-temperature austenite during heating and the transformation of austenite to ferrite during cooling occurred, respectively. The phase-specific lattice variation with temperature was further analyzed to comprehensively understand the role of carbon diffusion in accordance with phase transformation, which also shed light on the determination of internal stress in retained austenite. These results prove the technique of real-time in situ neutron diffraction as a powerful tool for heat treatment design of novel metallic materials. Full article

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Open AccessArticle

DFT Calculations and Mesophase Study of Coumarin Esters and Its Azoesters

by M. Hagar, H.A. Ahmed and O.A. Alhaddadd

Crystals 2018, 8(9), 359; https://doi.org/10.3390/cryst8090359 - 8 September 2018

Abstract

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Two groups of coumarin derivatives, 4-methyl-2-oxo-2H-chromen-7-yl 4-alkoxybenzoates (coumarin esters), I_n, and 4-methyl-2-oxo-2H-chromen-7-yl 4-(2-(4-alkoxyphenyl)diazenyl) benzoates (coumarin azoesters), II_n, were synthesized and investigated for their mesophase behavior and stability. Each group constitutes five series that differ from each other by length of the mesogenic part. Within each homologous series, the length of the terminal alkoxy group varies between 6, 8, 10, 12 and 16 carbons. Mesophase behavior was investigated by differential scanning calorimetry (DSC) and identified by polarized light microscopy (PLM). Density functional theory (DFT) calculations for coumarin derivatives were discussed. The results revealed that the incorporation of azo group incorporated in the mesogenic core decreases the energy differences, increases the dipole moments and stabilities of coumarin azoesters series more than coumarin esters. Full article

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Open AccessArticle

Investigation of Inverted Perovskite Solar Cells for Viscosity of PEDOT:PSS Solution

by Pao-Hsun Huang, Yeong-Her Wang, Chien-Wu Huang, Wen-Ray Chen and Chien-Jung Huang

Crystals 2018, 8(9), 358; https://doi.org/10.3390/cryst8090358 - 6 September 2018

Abstract

[...] Read more.

In this paper, we demonstrate that the inverted CH₃NH₃PbI₃ (perovskite) solar cells (PSCs) based on fullerene (C₆₀) as an acceptor is fabricated by applying an improved poly(3,4-ethlyenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution as a hole transport layer (HTL). The power conversion efficiency (PCE) of inverted PSCs is increased by 37.5% with stable values of open-circuit voltage (V_OC) and fill factor (FF) because we enhance the viscosity of the PEDOT:PSS solution, indicating the perfect effect on both external quantum efficiency (EQE) and surface grain size. The characteristics of the PEDOT:PSS solution, which is being improved through facile methods of obtaining excellent growth of PEDOT:PSS thin film, have a considerable impact on carrier transport. A series of further processing fabrications, including reliable and feasible heating and stirring techniques before the formation of the PEDOT:PSS thin film via spin-coating, not only evaporate the excess moisture but also obviously increase the conductivity. The raised collection of holes become the reason for the enhanced PCE of 3.0%—therefore, the stable performance of FF and V_OC are attributed to lower series resistance of devices and the high-quality film crystallization of perovskite and organic acceptors, respectively. Full article

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Open AccessReview

A Review of Current Development of Graphene Mechanics

by Qiang Cao, Xiao Geng, Huaipeng Wang, Pengjie Wang, Aaron Liu, Yucheng Lan and Qing Peng

Crystals 2018, 8(9), 357; https://doi.org/10.3390/cryst8090357 - 6 September 2018

Abstract

[...] Read more.

Graphene, a two-dimensional carbon in honeycomb crystal with single-atom thickness, possesses extraordinary properties and fascinating applications. Graphene mechanics is very important, as it relates to the integrity and various nanomechanical behaviors including flexing, moving, rotating, vibrating, and even twisting of graphene. The relationship between the strain and stress plays an essential role in graphene mechanics. Strain can dramatically influence the electronic and optical properties, and could be utilized to engineering those properties. Furthermore, graphene with specific kinds of defects exhibit mechanical enhancements and thus the electronic enhancements. In this short review, we focus on the current development of graphene mechanics, including tension and compression, fracture, shearing, bending, friction, and dynamics properties of graphene from both experiments and numerical simulations. We also touch graphene derivatives, including graphane, graphone, graphyne, fluorographene, and graphene oxide, which carve some fancy mechanical properties out from graphene. Our review summarizes the current achievements of graphene mechanics, and then shows the future prospects. Full article

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