Crystals

12 pages, 3147 KiB

Open AccessArticle

Transitions and Geometric Evolution of Cu₃₀₉ Nanocluster during Slow Cooling Process

by Pengfei Ji, Yi Zhao, Mingli Wan, Jinna He, Mingli Tian, Yueli Song and Yong Li

Crystals 2018, 8(5), 231; https://doi.org/10.3390/cryst8050231 - 21 May 2018

Cited by 2 | Viewed by 2855

Since the nucleation and growth of clusters is usually a non-equilibrium condensation process, a distribution of structural isomers for a given cluster size may be encountered even under the same conditions. In this work, molecular dynamics simulations are performed on sets of molten [...] Read more.

Since the nucleation and growth of clusters is usually a non-equilibrium condensation process, a distribution of structural isomers for a given cluster size may be encountered even under the same conditions. In this work, molecular dynamics simulations are performed on sets of molten clusters of Cu₃₀₉ to study their structures at low temperatures while controlling the cooling rate. Several different final structures including perfect icosahedra (ICO), imperfect Mark’ decahedra (MDEC) and imperfect FCC truncated octahedra (TOCT) are obtained even at the same cooling rate. It is calculated that the most favorable structure is icosahedra, which becomes more and more favorable as the cooling rate is slowed. To better understand the process of crystallization, several techniques, including potential-temperature curves, common neighbor analysis (CNA) and radial distribution function (RDF), are used to analyze and study the structural transition. Results show that different structures are obtained under identical conditions due to the stochastic nature of nucleation and relatively small energy difference between isomers. The process of geometrical evolution for icosahedra is given by comparing and analyzing the time evolution of the root-mean-square deviation (RMSD) of atoms located in every shell. Full article

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17 pages, 8008 KiB

Open AccessArticle

Self-Assembly of 3d-4f Zn^II-Ln^III (Ln = Ho and Er) Bis(salamo)-Based Complexes: Controlled Syntheses, Structures and Fluorescence Properties

by Xiu-Yan Dong, Qing Zhao, Quan-Peng Kang, Xiao-Yan Li and Wen-Kui Dong

Crystals 2018, 8(5), 230; https://doi.org/10.3390/cryst8050230 - 20 May 2018

Cited by 36 | Viewed by 3416

Abstract

Two new hetero-trinuclear 3d-4f complexes [Zn₂(L)Ho(μ₂-OAc)₂(OAc)(MeOH)]·CH₂Cl₂ (1) and [Zn₂(L)Er(μ₂-OAc)₂]OAc (2), derived from a bis(salamo)-based ligand H₄L, were synthesized and characterized via elemental [...] Read more.

Two new hetero-trinuclear 3d-4f complexes [Zn₂(L)Ho(μ₂-OAc)₂(OAc)(MeOH)]·CH₂Cl₂ (1) and [Zn₂(L)Er(μ₂-OAc)₂]OAc (2), derived from a bis(salamo)-based ligand H₄L, were synthesized and characterized via elemental analyses, IR, UV–Vis, fluorescence spectra and X-ray crystallography. The X-ray crystal structure analyses demonstrated that two μ₂-acetateanions bridge the Zn^II and Ln^III (Ln = Ho (1) and Er (2)) atoms in a μ₂-fashion forming similar hetero-trinuclear structures, respectively. In complex 1, one methanol molecule as coordinating solvent participates in the coordination, the two Zn^II atoms are six- and five-coordinated and have geometries of slightly distorted tetragonal pyramid and octahedron, and the Ho^III atom is nine-coordinated and has the geometry of a mono-capped square antiprism. In complex 2, the two Zn^II atoms both possess five-coordinated tetragonal pyramid geometries, and the Er^III atom is eight-coordinated with a square antiprism geometry. Furthermore, the fluorescence properties of complexes 1 and 2 were determined. Full article

(This article belongs to the Section Crystal Engineering)

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11 pages, 1868 KiB

Open AccessArticle

A New Family of Heterometallic Ln^III[12-MC_Fe^III_N(shi)-4] Complexes: Syntheses, Structures and Magnetic Properties

by Tingting Lou, Hua Yang, Suyuan Zeng, Dacheng Li and Jianmin Dou

Crystals 2018, 8(5), 229; https://doi.org/10.3390/cryst8050229 - 19 May 2018

Cited by 8 | Viewed by 2973

Abstract

A new family of Ln^III [12-Metallacrown-4] compounds of formulas (C₅H₆N) [LnFe₄(shi)₄(C₆H₅COO)₄(Py)₄]·3.5Py [Ln = Eu^III (1); Gd^III (2); Tb^III ( [...] Read more.

A new family of Ln^III [12-Metallacrown-4] compounds of formulas (C₅H₆N) [LnFe₄(shi)₄(C₆H₅COO)₄(Py)₄]·3.5Py [Ln = Eu^III (1); Gd^III (2); Tb^III (3); Dy^III (4); and, H₃shi = salicylhydroxamic acid] were obtained through one-pot reactions with H₃shi, Fe(NO₃)₃·9H₂O, and, Ln(NO₃)₃·6H₂O as reagents. Single-crystal X-ray analyses show that they are isostructural and have the similar [12-MC_Fe^III _N(shi)-4] core, with four benzoate molecules bridging the central Ln^III ion to the ring Fe^III ions. The negative charge of the 12-MC-4 metallacrown is balanced by one pyridinium cation, which forms the hydrogen bond with an adjacent solvent pyridine molecule. Magnetic measurements demonstrate antiferromagnetic coupling interactions and field-induced slow magnetic relaxation in complex 4. Full article

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7 pages, 2310 KiB

Open AccessArticle

Effect of Lithium Doping on Microstructural and Optical Properties of ZnO Nanocrystalline Films Prepared by the Sol-Gel Method

by Hung-Pin Hsu, Der-Yuh Lin, Cheng-Ying Lu, Tsung-Shine Ko and Hone-Zern Chen

Crystals 2018, 8(5), 228; https://doi.org/10.3390/cryst8050228 - 19 May 2018

Cited by 14 | Viewed by 3557

Abstract

The Zn_1−xLi_xO (x = 0, 0.01, 0.03, and 0.05) nanocrystalline films were synthesized on silicon (Si) substrates by using the sol-gel method. The crystal structure and surface morphology of these films were investigated by X-ray diffraction (XRD) and field [...] Read more.

The Zn_1−xLi_xO (x = 0, 0.01, 0.03, and 0.05) nanocrystalline films were synthesized on silicon (Si) substrates by using the sol-gel method. The crystal structure and surface morphology of these films were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). We observed that the average grain size was gradually reduced with increasing doping Li content. Photoluminescence (PL) spectra show that increasing the Li content will deteriorate the crystalline quality and result in the decrease of ultraviolet emission from the excitonic recombination and the enhancement of visible emission from the recombination between the intrinsic defects. The current-voltage properties of Zn_1−xLi_xO nanocrystalline films were also studied under dark and photo-illumination for photo-detection applications. The normalized photo-to-dark-current ratio (I_photo − I_dark)/I_dark has been enhanced from 315 to 4161 by increasing the Li content of the Zn_1−xLi_xO nanocrystalline films from zero to 0.05. Full article

(This article belongs to the Special Issue Functional Oxide Based Thin-Film Materials)

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13 pages, 5339 KiB

Open AccessCommunication

Structural and Hirshfeld Surface Analyses of a Novel Hetero-Tetranuclear Cu^II-Na^I Bis(Salamo)-Based Coordination Compound

by Ya-Xuan Sun, Ling-Zhi Liu, Fei Wang, Xiao-Ya Shang, Le Chen and Wen-Kui Dong

Crystals 2018, 8(5), 227; https://doi.org/10.3390/cryst8050227 - 18 May 2018

Cited by 10 | Viewed by 2746

Abstract

The newly designed butterfly-shaped hetero-tetranuclear Cu^II-Na^I coordination compound, [Cu₃(HL)₂Na]∙Pic (Pic⁻ is abbreviation of picrate) (1) which is derived from a naphthalenediol-based bis(Salamo)-type chelating ligand H₄L have been synthesized and characterized by [...] Read more.

The newly designed butterfly-shaped hetero-tetranuclear Cu^II-Na^I coordination compound, [Cu₃(HL)₂Na]∙Pic (Pic⁻ is abbreviation of picrate) (1) which is derived from a naphthalenediol-based bis(Salamo)-type chelating ligand H₄L have been synthesized and characterized by elemental analyses, UV-vis spectra, IR spectra analysis, and Hirshfeld surface analysis. X-ray crystallographic analyses revealed that the coordination compound 1 is a novel hetero-tetranuclear Cu^II-Na^I bis(Salamo)-type coordination compound and it differs from heterotrinuclear Cu^II-Na^I bis(Salamo)-type coordination compound reported earlier. The Cu1 and Cu3 atoms are tetra-coordinated with geometries of distorted square pyramid, while Cu2 atom are hexa-coordinated with the geometry of a distorted octahedron. The Na^I atom is octa-coordinated with the geometry of a distorted hexagonal bipyramid. Furthermore, the supramolecular structure and Hirshfeld surface analyses have been discussed in detail. Full article

(This article belongs to the Section Crystal Engineering)

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14 pages, 8211 KiB

Open AccessReview

Brief Review of Surface Passivation on III-V Semiconductor

by Lu Zhou, Baoxue Bo, Xingzhen Yan, Chao Wang, Yaodan Chi and Xiaotian Yang

Crystals 2018, 8(5), 226; https://doi.org/10.3390/cryst8050226 - 18 May 2018

Cited by 70 | Viewed by 9717

Abstract

The III-V compound semiconductor, which has the advantage of wide bandgap and high electron mobility, has attracted increasing interest in the optoelectronics and microelectronics field. The poor electronic properties of III-V semiconductor surfaces resulting from a high density of surface/interface states limit III-V [...] Read more.

The III-V compound semiconductor, which has the advantage of wide bandgap and high electron mobility, has attracted increasing interest in the optoelectronics and microelectronics field. The poor electronic properties of III-V semiconductor surfaces resulting from a high density of surface/interface states limit III-V device technology development. Various techniques have been applied to improve the surface and interface quality, which cover sulfur-passivation, plasmas-passivation, ultrathin film deposition, and so on. In this paper, recent research of the surface passivation on III-V semiconductors was reviewed and compared. It was shown that several passivation methods can lead to a perfectly clean surface, but only a few methods can be considered for actual device integration due to their effectiveness and simplicity. Full article

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26 pages, 4591 KiB

Open AccessEditor’s ChoiceReview

The Crystal Orbital Hamilton Population (COHP) Method as a Tool to Visualize and Analyze Chemical Bonding in Intermetallic Compounds

by Simon Steinberg and Richard Dronskowski

Crystals 2018, 8(5), 225; https://doi.org/10.3390/cryst8050225 - 18 May 2018

Cited by 198 | Viewed by 15503

Abstract

Recognizing the bonding situations in chemical compounds is of fundamental interest for materials design because this very knowledge allows us to understand the sheer existence of a material and the structural arrangement of its constituting atoms. Since its definition 25 years ago, the [...] Read more.

Recognizing the bonding situations in chemical compounds is of fundamental interest for materials design because this very knowledge allows us to understand the sheer existence of a material and the structural arrangement of its constituting atoms. Since its definition 25 years ago, the Crystal Orbital Hamilton Population (COHP) method has been established as an efficient and reliable tool to extract the chemical-bonding information based on electronic-structure calculations of various quantum-chemical types. In this review, we present a brief introduction into the theoretical background of the COHP method and illustrate the latter by diverse applications, in particular by looking at representatives of the class of (polar) intermetallic compounds, usually considered as “black sheep” in the light of valence-electron counting schemes. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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8 pages, 2212 KiB

Open AccessArticle

An Analytical Solution to Lumped Parameter Equivalent Circuit Model of Organic Solar Cells

by Gongyi Huang, Fei Yu and Chuanzhong Xu

Crystals 2018, 8(5), 224; https://doi.org/10.3390/cryst8050224 - 18 May 2018

Cited by 10 | Viewed by 3596

Abstract

In this paper, an analytical and closed-form solution to the lumped parameter equivalent circuit model of organic solar cells is proposed to complete the simulations of the S-shaped I-V characteristics. Based on the model previously proposed by Mazhari, the set of [...] Read more.

In this paper, an analytical and closed-form solution to the lumped parameter equivalent circuit model of organic solar cells is proposed to complete the simulations of the S-shaped I-V characteristics. Based on the model previously proposed by Mazhari, the set of terminal current and voltage equations describing the three diodes is solved and the effects from the model parameters are illustrated. Our solutions are verified by being compared with the least square method results and experimental data, respectively. Good agreements show that our solution calculation scheme is not only both accurate and efficient, but also valid in the whole operation regime of solar cells, especially for the S-shaped kink on the condition where the terminal voltage is larger than the open circuit voltage. Such an analytical solution can play an important role in the simulations for I-V characteristics of solar cells, fast extractions of the model parameters, and implements into practical photovoltaic device simulators. Full article

(This article belongs to the Special Issue Selected Papers from Taiwan Association for Academic Innovation, TAAI 2018)

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29 pages, 88206 KiB

Open AccessReview

One-Dimensional Zinc Oxide Nanomaterials for Application in High-Performance Advanced Optoelectronic Devices

by Meng Ding, Zhen Guo, Lianqun Zhou, Xuan Fang, Lili Zhang, Leyong Zeng, Lina Xie and Hongbin Zhao

Crystals 2018, 8(5), 223; https://doi.org/10.3390/cryst8050223 - 18 May 2018

Cited by 54 | Viewed by 9939

Abstract

Unlike conventional bulk or film materials, one-dimensional (1D) semiconducting zinc oxide (ZnO) nanostructures exhibit excellent photoelectric properties including ultrahigh intrinsic photoelectric gain, multiple light confinement, and subwavelength size effects. Compared with polycrystalline thin films, nanowires usually have high phase purity, no grain boundaries, [...] Read more.

Unlike conventional bulk or film materials, one-dimensional (1D) semiconducting zinc oxide (ZnO) nanostructures exhibit excellent photoelectric properties including ultrahigh intrinsic photoelectric gain, multiple light confinement, and subwavelength size effects. Compared with polycrystalline thin films, nanowires usually have high phase purity, no grain boundaries, and long-distance order, making them attractive for carrier transport in advanced optoelectronic devices. The properties of one-dimensional nanowires—such as strong optical absorption, light emission, and photoconductive gain—could improve the performance of light-emitting diodes (LEDs), photodetectors, solar cells, nanogenerators, field-effect transistors, and sensors. For example, ZnO nanowires behave as carrier transport channels in photoelectric devices, decreasing the loss of the light-generated carrier. The performance of LEDs and photoelectric detectors based on nanowires can be improved compared with that of devices based on polycrystalline thin films. This article reviews the fabrication methods of 1D ZnO nanostructures—including chemical vapor deposition, hydrothermal reaction, and electrochemical deposition—and the influence of the growth parameters on the growth rate and morphology. Important applications of 1D ZnO nanostructures in optoelectronic devices are described. Several approaches to improve the performance of 1D ZnO-based devices, including surface passivation, localized surface plasmons, and the piezo-phototronic effect, are summarized. Full article

(This article belongs to the Special Issue Functional Oxide Based Thin-Film Materials)

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8 pages, 6879 KiB

Open AccessArticle

Effect of Calcium and Phosphate on Compositional Conversion from Dicalcium Hydrogen Phosphate Dihydrate Blocks to Octacalcium Phosphate Blocks

by Yuki Sugiura and Kunio Ishikawa

Crystals 2018, 8(5), 222; https://doi.org/10.3390/cryst8050222 - 17 May 2018

Cited by 15 | Viewed by 4848

Abstract

Octacalcium phosphate (OCP) has attracted much attention as an artificial bone substitute because of its excellent osteoconductive and bone replacement properties. Although numerous studies have investigated OCP powder fabrication, there are only a few studies on OCP block fabrication. Therefore, in this study, [...] Read more.

Octacalcium phosphate (OCP) has attracted much attention as an artificial bone substitute because of its excellent osteoconductive and bone replacement properties. Although numerous studies have investigated OCP powder fabrication, there are only a few studies on OCP block fabrication. Therefore, in this study, the feasibility of optimizing dicalcium hydrogen phosphate dihydrate (DCPD) blocks, as a precursor for OCP block fabrication, under a pH 6 adjusted acetate buffer solution at 70 °C for 2 days was investigated. When a DCPD block was immersed in acetate buffer, the block was partially converted to OCP, with a large amount of dicalcium hydrogen phosphate anhydrate (DCPA), and its macroscopic structure was maintained. When the DCPD block was immersed in a Ca-containing solution, it was converted to mainly hydroxyapatite (HAp) with DCPA. On the other hand, when the DCPD block was immersed in a PO₄-containing solution, the block was converted to OCP, and its macroscopic structure was maintained. In other words, the PO₄-induced calcium phosphate with a Ca/P molar ratio lower than 1.0 may represent an intermediate phase during the compositional transformation from a DCPD block to an OCP block through the dissolution–precipitation reaction. Full article

(This article belongs to the Special Issue Biomimetic Growth of Calcium Phosphate Crystals)

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10 pages, 891 KiB

Open AccessArticle

Synthesis and Crystal Structures of Cadmium(II) Cyanide with Branched-Butoxyethanol

by Takeshi Kawasaki and Takafumi Kitazawa

Crystals 2018, 8(5), 221; https://doi.org/10.3390/cryst8050221 - 17 May 2018

Cited by 2 | Viewed by 3443

Abstract

Two novel 3D cadmium(II) cyanide coordination polymers with branched-butoxyethanol compounds (iBucel = iso-butoxyethanol, tBucel = tert-butoxyethanol), [{Cd(CN)₂(iBucel)₂}{Cd(CN)₂(H₂O)(iBucel)}₂{Cd(CN)₂}₆∙2(iBucel)]_n I and [{Cd(CN)₂(H₂O)_1.06(tBucel)_0.94 [...] Read more.

Two novel 3D cadmium(II) cyanide coordination polymers with branched-butoxyethanol compounds (iBucel = iso-butoxyethanol, tBucel = tert-butoxyethanol), [{Cd(CN)₂(iBucel)₂}{Cd(CN)₂(H₂O)(iBucel)}₂{Cd(CN)₂}₆∙2(iBucel)]_n I and [{Cd(CN)₂(H₂O)_1.06(tBucel)_0.94}{Cd(CN)₂(tBucel)}₂{Cd(CN)₂}₂∙1.06(tBucel)]_n II, were synthesized and characterized by structural determination. Complex I contains two distinct Cd(II) coordination geometries: octahedral and tetrahedral. In contrast, complex II contains three distinct Cd(II) coordination geometries: octahedral, square-pyramidal, and tetrahedral. In the two complexes, branched-butoxyethanol molecules behave as both a ligand and a guest in the Cd(CN)₂ cavities. The framework in I contains octahedral and tetrahedral Cd(II) in a 3:6 ratio. In I, the coordination environments of octahedral Cd(II) are cis-O-Cd-O. The framework in II contains octahedral, square-pyramidal, and tetrahedral Cd(II) in a 1:2:2 ratio. In II, the coordination environment of octahedral Cd(II) is disordered trans-O-Cd-O and the axial oxygen ligand is either a water or tBucel molecule. In II, the square-pyramidal Cd(II) geometry is formed by one tBucel ligand and four cyanide ligands. The Cd(CN)₂ frameworks of the two complexes exhibit different structures. Full article

(This article belongs to the Special Issue Crystal Structure Analysis of Supramolecular and Porous Solids)

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22 pages, 2790 KiB

Open AccessArticle

Metal Halide Perovskite Single Crystals: From Growth Process to Application

by Shuigen Li, Chen Zhang, Jiao-Jiao Song, Xiaohu Xie, Jian-Qiao Meng and Shunjian Xu

Crystals 2018, 8(5), 220; https://doi.org/10.3390/cryst8050220 - 17 May 2018

Cited by 31 | Viewed by 13014

Abstract

As a strong competitor in the field of optoelectronic applications, organic-inorganic metal hybrid perovskites have been paid much attention because of their superior characteristics, which include broad absorption from visible to near-infrared region, tunable optical and electronic properties, high charge mobility, long exciton [...] Read more.

As a strong competitor in the field of optoelectronic applications, organic-inorganic metal hybrid perovskites have been paid much attention because of their superior characteristics, which include broad absorption from visible to near-infrared region, tunable optical and electronic properties, high charge mobility, long exciton diffusion length and carrier recombination lifetime, etc. It is noted that perovskite single crystals show remarkably low trap-state densities and long carrier diffusion lengths, which are even comparable with the best photovoltaic-quality silicon, and thus are expected to provide better optoelectronic performance. This paper reviews the recent development of crystal growth in single-, mixed-organic-cation and fully inorganic halide perovskite single crystals, in particular the solution approach. Furthermore, the application of metal hybrid perovskite single crystals and future perspectives are also highlighted. Full article

(This article belongs to the Special Issue Metal Halide Perovskite Crystals: Growth Techniques, Properties and Emerging Applications)

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12 pages, 2002 KiB

Open AccessArticle

Recent Insights into Protein Crystal Nucleation

by Christo N. Nanev

Crystals 2018, 8(5), 219; https://doi.org/10.3390/cryst8050219 - 17 May 2018

Cited by 14 | Viewed by 3517

Abstract

Homogeneous nucleation of protein crystals in solution is tackled from both thermodynamic and energetic perspectives. The entropic contribution to the destructive action of water molecules which tend to tear up the crystals and to their bond energy is considered. It is argued that, [...] Read more.

Homogeneous nucleation of protein crystals in solution is tackled from both thermodynamic and energetic perspectives. The entropic contribution to the destructive action of water molecules which tend to tear up the crystals and to their bond energy is considered. It is argued that, in contrast to the crystals’ bond energy, the magnitude of destructive energy depends on the imposed supersaturation. The rationale behind the consideration presented is that the critical nucleus size is determined by the balance between destructive and bond energies. By summing up all intra-crystal bonds, the breaking of which is needed to disintegrate a crystal into its constituting molecules, and using a crystallographic computer program, the bond energy of the closest-packed crystals is calculated (hexagonal closest-packed crystals are given as an example). This approach is compared to the classical mean work of separation (MWS) method of Stranski and Kaischew. While the latter is applied merely for the so-called Kossel-crystal and vapor grown crystals, the approach presented can be used to establish the supersaturation dependence of the protein crystal nucleus size of arbitrary lattice structures. Full article

(This article belongs to the Special Issue Biological Crystallization)

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12 pages, 14802 KiB

Open AccessArticle

One-Step Synthesis of Cu–ZnO@C from a 1D Complex [Cu_0.02Zn_0.98(C₈H₃NO₆)(C₁₂H₈N₂)]_n for Catalytic Hydroxylation of Benzene to Phenol

by Guanghui Wang, Zheng Yan, Li Song, Lei Li, Jie Zhu and Haidong Wang

Crystals 2018, 8(5), 218; https://doi.org/10.3390/cryst8050218 - 16 May 2018

Cited by 1 | Viewed by 3530

Abstract

A novel one-dimensional bimetallic complex [Cu_0.02Zn_0.98(C₈H₃NO₆)(C₁₂H₈N₂)]_n (“Complex”) has been synthesized by a hydrothermal method. A Cu–ZnO@C composite was obtained by a one-step pyrolysis of Complex. Correlated [...] Read more.

A novel one-dimensional bimetallic complex [Cu_0.02Zn_0.98(C₈H₃NO₆)(C₁₂H₈N₂)]_n (“Complex”) has been synthesized by a hydrothermal method. A Cu–ZnO@C composite was obtained by a one-step pyrolysis of Complex. Correlated with the characterization results, it is confirmed that both metallic Cu⁰ and ZnO nanoparticles were highly dispersed on/in the carbon substrate. This simple one-step pyrolysis method avoids the high-temperature pretreatment under H₂ commonly required for preparation of such Cu–ZnO catalysts. The Cu–ZnO@C composite was tested with respect to its catalytic activities for the hydroxylation of benzene to phenol with H₂O₂. The results indicate that the benzene conversion, phenol yield, and phenol selectivity reached the maximum values (55.7%, 32%, and 57.5%, respectively) at Complex carbonized at 600 °C, and were higher than those of the commercial mixed sample. Compared with the other candidate catalysts, the turnover frequency (TOF) of our Cu–ZnO@C catalyst (117.9 mmol mol⁻¹ s⁻¹) can be ranked at the top. The higher catalytic activities should be due to the highly dispersed metallic Cu⁰ and ZnO particles as well as their synergistic interaction. Full article

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18 pages, 3242 KiB

Open AccessReview

High-Pressure, High-Temperature Behavior of Silicon Carbide: A Review

by Kierstin Daviau and Kanani K. M. Lee

Crystals 2018, 8(5), 217; https://doi.org/10.3390/cryst8050217 - 16 May 2018

Cited by 69 | Viewed by 12096

Abstract

The high-pressure behavior of silicon carbide (SiC), a hard, semi-conducting material commonly known for its many polytypic structures and refractory nature, has increasingly become the subject of current research. Through work done both experimentally and computationally, many interesting aspects of high-pressure SiC have [...] Read more.

The high-pressure behavior of silicon carbide (SiC), a hard, semi-conducting material commonly known for its many polytypic structures and refractory nature, has increasingly become the subject of current research. Through work done both experimentally and computationally, many interesting aspects of high-pressure SiC have been measured and explored. Considerable work has been done to measure the effect of pressure on the vibrational and material properties of SiC. Additionally, the transition from the low-pressure zinc-blende B3 structure to the high-pressure rocksalt B1 structure has been measured by several groups in both the diamond-anvil cell and shock communities and predicted in numerous computational studies. Finally, high-temperature studies have explored the thermal equation of state and thermal expansion of SiC, as well as the high-pressure and high-temperature melting behavior. From high-pressure phase transitions, phonon behavior, and melting characteristics, our increased knowledge of SiC is improving our understanding of its industrial uses, as well as opening up its application to other fields such as the Earth sciences. Full article

(This article belongs to the Special Issue High-Pressure Studies of Crystalline Materials)

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18 pages, 7064 KiB

Open AccessReview

Twin Domains in Organometallic Halide Perovskite Thin-Films

by Wei Liu, Yang Liu, Ju Wang, Cuncun Wu, Congyue Liu, Lixin Xiao, Zhijian Chen, Shufeng Wang and Qihuang Gong

Crystals 2018, 8(5), 216; https://doi.org/10.3390/cryst8050216 - 16 May 2018

Cited by 16 | Viewed by 7479

Abstract

The perovskite is a class of material with crystalline structure similar to CaTiO₃. In recent years, the organic-inorganic hybrid metallic halide perovskite has been widely investigated as a promising material for a new generation photovoltaic device, whose power conversion efficiency (PCE) [...] Read more.

The perovskite is a class of material with crystalline structure similar to CaTiO₃. In recent years, the organic-inorganic hybrid metallic halide perovskite has been widely investigated as a promising material for a new generation photovoltaic device, whose power conversion efficiency (PCE) record reaches 22.7%. One of its underlying morphological characteristics is the twin domain within those sub-micron sized crystal grains in perovskite thin films. This is important for discussion since it could be the key for understanding the fundamental mechanism of the device’s high performance, such as long diffusion distance and low recombination rate. This review aims to summarize studies on twin domains in perovskite thin films, in order to figure out its importance, guide the current studies on mechanism, and design new devices. Firstly, we introduce the research history and characteristics of widely known twin domains in inorganic perovskite BaTiO₃. We then focus on the impact of the domain structure emerging in hybrid metallic halide perovskite thin films, including the observation and discussion on ferroelectricity/ferroelasity. The theoretical analysis is also presented in this review. Finally, we present a spectroscopic method, which can reveal the generality of twin domains within perovskite thin films. We anticipate that this summary on the structural and physical properties of organometallic halide perovskite will help to understand and improve the high-performance of photovoltaic devices. Full article

(This article belongs to the Special Issue Metal Halide Perovskite Crystals: Growth Techniques, Properties and Emerging Applications)

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9 pages, 3299 KiB

Open AccessArticle

Mild Synthesis and Structural Characterization of a Novel Vanadyl Selenite-Hydrogen Selenite Phase, Na[VO(SeO₃)(HSeO₃)]·1,5H₂O

by Stefano Canossa, Giovanni Predieri and Claudia Graiff

Crystals 2018, 8(5), 215; https://doi.org/10.3390/cryst8050215 - 15 May 2018

Cited by 1 | Viewed by 3229

Abstract

Single crystals of Na[VO(SeO₃)(HSeO₃)]·1,5H₂O have been prepared by solvent slow evaporation from an equimolar aqueous mixture of sodium selenite and vanadyl sulfate. The overall arrangement consists of VO₆ octahedra, [SeO₃]²⁻ and [HSeO₃ [...] Read more.

Single crystals of Na[VO(SeO₃)(HSeO₃)]·1,5H₂O have been prepared by solvent slow evaporation from an equimolar aqueous mixture of sodium selenite and vanadyl sulfate. The overall arrangement consists of VO₆ octahedra, [SeO₃]²⁻ and [HSeO₃]⁻ anions forming a strong backbone with channel-like voids, while sodium cations participate in the framework, ensuring the overall charge balance. The crystal packing of the material features channels with a star-shaped section showing a mean aperture of 6.10 Å with the oxygen atoms of the V=O moieties pointing towards the interior of the cavities. Their dimensions are the narrowest reported until now in vanadium selenite compounds. Disordered sodium cations with occupancy factor of 0.5 are found in the star-shaped section cavities together with disordered water molecules, interacting with each other via hydrogen bonds. Full article

(This article belongs to the Special Issue Crystal Structures of Compounds Containing Ions Selenite)

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48 pages, 3005 KiB

Open AccessArticle

Polaron-Mediated Luminescence in Lithium Niobate and Lithium Tantalate and Its Domain Contrast

by Philipp Reichenbach, Thomas Kämpfe, Alexander Haußmann, Andreas Thiessen, Theo Woike, Robin Steudtner, Laura Kocsor, Zsuzsanna Szaller, László Kovács and Lukas M. Eng

Crystals 2018, 8(5), 214; https://doi.org/10.3390/cryst8050214 - 15 May 2018

Cited by 15 | Viewed by 5510

Abstract

In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as [...] Read more.

In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as on bipolarons. We discuss the absorption measurements on reduced as well as on doped LNO that made the characterization of the formed polaron states possible by their absorption bands. Next, we proceed by reporting on the two polaron-mediated photoluminescence bands that have been observed in LNO: (1) A near-infrared luminescence band in the range of 1.5 eV shows a mono-exponential decay and a strong dependence on iron doping. This luminescence is emitted by bound polarons returning from an excited state to the ground state. (2) A luminescence band at visible wavelengths with a maximum at 2.6 eV shows a stretched-exponential decay and is strongly enhanced by optical damage resistant doping around the doping threshold. This luminescence stems from the recombination of free electron and hole polarons. The next major topic of this review are domain contrasts of the visible photoluminescence that have been observed after electrical poling of the substrate, as singly inverted domains show a slightly reduced and faster decaying luminescence. Subsequent annealing results in an exponential decrease of that domain contrast. We show that this contrast decay is strongly related to the mobility of lithium ions, thus confirming the role of polar defect complexes, including lithium vacancies, for these domain contrasts. Finally we discuss the extension of our investigations to lithium tantalate (LTO) samples. While the results on the domain contrast and its decay are similar to LNO, there are remarkable differences in their luminescence spectra. Full article

(This article belongs to the Special Issue Lithium Niobate: Bulk Crystals, Composites, Thin Films and Nanocrystals)

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11 pages, 5250 KiB

Open AccessArticle

New Polymorphs of the Phase-Change Material Sodium Acetate

by Birger Dittrich, Justin Bergmann, Peter Roloff and Guido J. Reiss

Crystals 2018, 8(5), 213; https://doi.org/10.3390/cryst8050213 - 15 May 2018

Cited by 4 | Viewed by 4696

Abstract

Two new polymorphs of the phase-change material sodium acetate were characterized by single-crystal X-ray diffraction. A tetragonal form was found first. It converted to a orthorhombic form after measurement of a single crystal of the tetragonal form at 100 K and subsequent warming [...] Read more.

Two new polymorphs of the phase-change material sodium acetate were characterized by single-crystal X-ray diffraction. A tetragonal form was found first. It converted to a orthorhombic form after measurement of a single crystal of the tetragonal form at 100 K and subsequent warming to ambient temperature. Hirshfeld surface fingerprint plots show the different packing environments of the two new compared to the two known orthorhombic polymorphs Forms I and II. The accuracy and precision of the structures were improved compared to conventional independent atom model refinement through the use of aspherical scattering factors of the invariom database. We think that the layered nature of all sodium acetate forms, and the thereby limited (“quantized”) availability of vibrational modes, is related to the phenomenon of supersaturation, which is connected to its phase-change properties. Full article

(This article belongs to the Special Issue Experimental and Theoretical Electron Density Analysis of Crystals)

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31 pages, 13543 KiB

Open AccessReview

Peak Broadening Anisotropy and the Contrast Factor in Metal Alloys

by Thomas Hadfield Simm

Crystals 2018, 8(5), 212; https://doi.org/10.3390/cryst8050212 - 13 May 2018

Cited by 37 | Viewed by 6900

Abstract

Diffraction peak profile analysis (DPPA) is a valuable method to understand the microstructure and defects present in a crystalline material. Peak broadening anisotropy, where broadening of a diffraction peak doesn’t change smoothly with 2θ or d-spacing, is an important aspect of these [...] Read more.

Diffraction peak profile analysis (DPPA) is a valuable method to understand the microstructure and defects present in a crystalline material. Peak broadening anisotropy, where broadening of a diffraction peak doesn’t change smoothly with 2θ or d-spacing, is an important aspect of these methods. There are numerous approaches to take to deal with this anisotropy in metal alloys, which can be used to gain information about the dislocation types present in a sample and the amount of planar faults. However, there are problems in determining which method to use and the potential errors that can result. This is particularly the case for hexagonal close packed (HCP) alloys. There is though a distinct advantage of broadening anisotropy in that it provides a unique and potentially valuable way to develop crystal plasticity and work-hardening models. In this work we use several practical examples of the use of DPPA to highlight the issues of broadening anisotropy. Full article

(This article belongs to the Special Issue Crystal Dislocations: Their Impact on Physical Properties of Crystals)

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10 pages, 2427 KiB

Open AccessArticle

Optimization of Ca₁₄MgSb₁₁ through Chemical Substitutions on Sb Sites: Optimizing Seebeck Coefficient and Resistivity Simultaneously

by Yufei Hu, Kathleen Lee and Susan M. Kauzlarich

Crystals 2018, 8(5), 211; https://doi.org/10.3390/cryst8050211 - 13 May 2018

Cited by 9 | Viewed by 3671

Abstract

In thermoelectric materials, chemical substitutions are widely used to optimize thermoelectric properties. The Zintl phase compound, Yb₁₄MgSb₁₁, has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca₁₄AlSb₁₁ with space [...] Read more.

In thermoelectric materials, chemical substitutions are widely used to optimize thermoelectric properties. The Zintl phase compound, Yb₁₄MgSb₁₁, has been demonstrated as a promising thermoelectric material at high temperatures. It is iso-structural with Ca₁₄AlSb₁₁ with space group I4₁/acd. Its iso-structural analog, Ca₁₄MgSb₁₁, was discovered to be a semiconductor and have vacancies on the Sb(3) sites, although in its nominal composition it can be described as consisting of fourteen Ca²⁺ cations with one [MgSb₄]⁹⁻ tetrahedron, one Sb₃⁷⁻ linear anion and four isolated Sb³⁻ anions (Sb(3) site) in one formula unit. When Sn substitutes Sb in Ca₁₄MgSb₁₁, optimized Seebeck coefficient and resistivity were achieved simultaneously although the Sn amount is small (<2%). This is difficult to achieve in thermoelectric materials as the Seebeck coefficient and resistivity are inversely related with respect to carrier concentration. Thermal conductivity of Ca₁₄MgSb_11-xSn_x remains almost the same as Ca₁₄MgSb₁₁. The calculated zT value of Ca₁₄MgSb_10.80Sn_0.20 reaches 0.49 at 1075 K, which is 53% higher than that of Ca₁₄MgSb₁₁ at the same temperature. The band structure of Ca₁₄MgSb₇Sn₄ is calculated to simulate the effect of Sn substitutions. Compared to the band structure of Ca₁₄MgSb₁₁, the band gap of Ca₁₄MgSb₇Sn₄ is smaller (0.2 eV) and the Fermi-level shifts into the valence band. The absolute values for density of states (DOS) of Ca₁₄MgSb₇Sn₄ are smaller near the Fermi-level at the top of valence band and 5p-orbitals of Sn contribute most to the valence bands near the Fermi-level. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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7 pages, 567 KiB

Open AccessArticle

Lattice Parameters of Optical Damage Resistant In-Doped LiNbO₃ Crystals

by Serguei Sulyanov and Tatyana Volk

Crystals 2018, 8(5), 210; https://doi.org/10.3390/cryst8050210 - 13 May 2018

Cited by 9 | Viewed by 3444

Abstract

The lattice parameters in optical damage resistant crystal LiNbO₃-In were measured for the first time using the X-ray powder method with an internal standard, which provides a high accuracy of the results. The lattice parameters vs. In concentration were obtained in [...] Read more.

The lattice parameters in optical damage resistant crystal LiNbO₃-In were measured for the first time using the X-ray powder method with an internal standard, which provides a high accuracy of the results. The lattice parameters vs. In concentration were obtained in the concentration range from 0.24 to 3.2 at % In in the crystal. The results are discussed in the framework of currently accepted model of the LiNbO₃ intrinsic defect structure. Full article

(This article belongs to the Special Issue Lithium Niobate: Bulk Crystals, Composites, Thin Films and Nanocrystals)

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21 pages, 4265 KiB

Open AccessArticle

High-Pressure Elastic, Vibrational and Structural Study of Monazite-Type GdPO₄ from Ab Initio Simulations

by Alfonso Muñoz and Placida Rodríguez-Hernández

Crystals 2018, 8(5), 209; https://doi.org/10.3390/cryst8050209 - 10 May 2018

Cited by 10 | Viewed by 3550

Abstract

The GdPO₄ monazite-type has been studied under high pressure by first principles calculations in the framework of density functional theory. This study focuses on the structural, dynamical, and elastic properties of this material. Information about the structure and its evolution under pressure, [...] Read more.

The GdPO₄ monazite-type has been studied under high pressure by first principles calculations in the framework of density functional theory. This study focuses on the structural, dynamical, and elastic properties of this material. Information about the structure and its evolution under pressure, the equation of state, and its compressibility are reported. The evolution of the Raman and Infrared frequencies, as well as their pressure coefficients are also presented. Finally, the study of the elastic constants provides information related with the elastic and mechanical properties of this compound. From our results, we conclude that monazite-type GdPO₄ becomes mechanically unstable at 54 GPa; no evidence of soft phonons has been found up to this pressure at the zone center. Full article

(This article belongs to the Special Issue High-Pressure Studies of Crystalline Materials)

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7 pages, 1017 KiB

Open AccessArticle

Optical Absorption and Reflectivity of a Molecular Cluster of Lithium Niobate Adsorbed on a Graphene Layer

by Osiris Salas, Eric Garces and Luis Fernando Magana

Crystals 2018, 8(5), 208; https://doi.org/10.3390/cryst8050208 - 10 May 2018

Cited by 3 | Viewed by 3474

Abstract

We used density functional theory to study the adsorption of a molecular cluster of lithium niobate (LiNbO₃) on a graphene layer. The cluster size is about 1.2 nm, and it has 11 molecules. We optimized the cluster, and then we calculated [...] Read more.

We used density functional theory to study the adsorption of a molecular cluster of lithium niobate (LiNbO₃) on a graphene layer. The cluster size is about 1.2 nm, and it has 11 molecules. We optimized the cluster, and then we calculated its interaction with the optimized graphene layer. We found that the cluster is adsorbed on the graphene layer with an adsorption energy of −2.2213 eV (−0.068 eV/atom). Afterwards, we calculated the reflectivity, and the optical absorption coefficients of the system cluster-graphene and of a graphene layer alone, to make a comparison. We found large differences in the values of these properties of the new system, with respect to the corresponding ones of graphene. We performed our calculations using the general gradient approximation (GGA), and the GGA modified for van der Waals interactions in order to take into account the long-range correlations. Full article

(This article belongs to the Special Issue Lithium Niobate: Bulk Crystals, Composites, Thin Films and Nanocrystals)

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7 pages, 1642 KiB

Open AccessArticle

Synthesis, Crystal Structures, and Anti-Liver Cancer Activity Studies on Three Similar Coordination Polymers

by Bao-Hua Song, Xing Ding, Chen Li and Gui-Feng An

Crystals 2018, 8(5), 207; https://doi.org/10.3390/cryst8050207 - 10 May 2018

Cited by 3 | Viewed by 3261

Abstract

By employment of a pre-designed molecular building block [(pzTp)Fe(CN)₃], one cyano-bridged trinuclear complex [MnMe₄TACD][(pzTp)Fe(CN)₃]₂(H₂O)₁₆ 1, [pzTp = tetrakis(pyrazoly)borate, Me₄TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane] and two 1D chain-like coordination polymers {[(pzTp)Fe(CN)₃ [...] Read more.

By employment of a pre-designed molecular building block [(pzTp)Fe(CN)₃], one cyano-bridged trinuclear complex [MnMe₄TACD][(pzTp)Fe(CN)₃]₂(H₂O)₁₆ 1, [pzTp = tetrakis(pyrazoly)borate, Me₄TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane] and two 1D chain-like coordination polymers {[(pzTp)Fe(CN)₃]₂[M(Bim)₂](H₂O)₂}_n [M = Cu(II) for 2 and Mn(II) for 3, Bim = bis(1-imidazolyl)methane] were synthesized by making use of different nitrogen-containing organic linkers. Furthermore, in vitro cytotoxicity of compounds 1–3 was studied against four human liver cancer cells (HepG2, Bel-7402, SMMC7721, and MHCC97) via the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium (MTT) assay experiments. Full article

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24 pages, 6514 KiB

Open AccessReview

Layered Indium Selenide under High Pressure: A Review

by Alfredo Segura

Crystals 2018, 8(5), 206; https://doi.org/10.3390/cryst8050206 - 09 May 2018

Cited by 31 | Viewed by 8011

Abstract

This paper intends a short review of the research work done on the structural and electronic properties of layered Indium Selenide (InSe) and related III–VI semiconductors under high pressure conditions. The paper will mainly focus on the crucial role played by high pressure [...] Read more.

This paper intends a short review of the research work done on the structural and electronic properties of layered Indium Selenide (InSe) and related III–VI semiconductors under high pressure conditions. The paper will mainly focus on the crucial role played by high pressure experimental and theoretical tools to investigate the electronic structure of InSe. This objective involves a previous revision of results on the pressure dependence of the InSe crystal structure and related topics such as the equation of state and the pressure-temperature crystal phase diagram. The main part of the paper will be devoted to reviewing the literature on the optical properties of InSe under high pressure, especially the absorption experiments that led to the identification of the main optical transitions, and their assignment to specific features of the electronic structure, with the help of modern first-principles band structure calculations. In connection with these achievements we will also review relevant results on the lattice dynamical, dielectric, and transport properties of InSe, as they provided very useful supplementary information on the electronic structure of the material. Full article

(This article belongs to the Special Issue High-Pressure Studies of Crystalline Materials)

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18 pages, 4065 KiB

Open AccessArticle

Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄: Two More Germanides among Polar Intermetallics

by Riccardo Freccero, Pavlo Solokha, Davide Maria Proserpio, Adriana Saccone and Serena De Negri

Crystals 2018, 8(5), 205; https://doi.org/10.3390/cryst8050205 - 05 May 2018

Cited by 12 | Viewed by 3535

Abstract

In this study, two novel Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄ polar intermetallics were prepared by direct synthesis of pure constituents. Their crystal structures were determined by single crystal X-ray diffraction analysis: Lu₅Pd₄ [...] Read more.

In this study, two novel Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄ polar intermetallics were prepared by direct synthesis of pure constituents. Their crystal structures were determined by single crystal X-ray diffraction analysis: Lu₅Pd₄Ge₈ is monoclinic, P2₁/m, mP34, a = 5.7406(3), b = 13.7087(7), c = 8.3423(4) Å, β = 107.8(1), Z = 2; Lu₃Pd₄Ge₄ is orthorhombic, Immm, oI22, a = 4.1368(3), b = 6.9192(5), c = 13.8229(9) Å, Z = 2. The Lu₅Pd₄Ge₈ analysed crystal is one more example of non-merohedral twinning among the rare earth containing germanides. Chemical bonding DFT studies were conducted for these polar intermetallics and showing a metallic-like behavior. Gathered results for Lu₅Pd₄Ge₈ and Lu₃Pd₄Ge₄ permit to described both of them as composed by [Pd–Ge]^δ– three dimensional networks bonded to positively charged lutetium species. From the structural chemical point of view, the studied compounds manifest some similarities to the Zintl phases, containing well-known covalent fragment i.e., Ge dumbbells as well as unique cis-Ge₄ units. A comparative analysis of molecular orbital diagrams for Ge₂^6– and cis-Ge^10– anions with COHP results supports the idea of the existence of complex Pd–Ge polyanions hosting covalently bonded partially polarised Ge units. The palladium atoms have an anion like behaviour and being the most electronegative cause the noticeable variation of Ge species charges from site to site. Lutetium charges oscillate around +1.5 for all crystallographic positions. Obtained results explained why the classical Zintl-Klemm concept can’t be applied for the studied polar intermetallics. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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21 pages, 3383 KiB

Open AccessArticle

Mixed Sr and Ba Tri-Stannides/Plumbides A^II(Sn_1−xPb_x)₃

by Michael Langenmaier, Michael Jehle and Caroline Röhr

Crystals 2018, 8(5), 204; https://doi.org/10.3390/cryst8050204 - 04 May 2018

Cited by 2 | Viewed by 3606

Abstract

The continuous substitution of tin by lead (M

^{IV}

) allows for the exploration geometric criteria for the stability of the different stacking variants of alkaline-earth tri-tetrelides

A^{II} M_{3}^{IV}

. A series of ternary Sr and Ba mixed tri-stannides/plumbides [...] Read more.

The continuous substitution of tin by lead (M

^{IV}

) allows for the exploration geometric criteria for the stability of the different stacking variants of alkaline-earth tri-tetrelides

A^{II} M_{3}^{IV}

. A series of ternary Sr and Ba mixed tri-stannides/plumbides A

^{II}

(Sn

_{1 - x}

Pb

_{x}

)

_{3}

(A

^{II}

= Sr, Ba) was synthesized from stoichiometric mixtures of the elements. Their structures were determined by means of single crystal X-ray data. All structures exhibit close packed ordered

A M_{3}

layers containing M kagomé nets. Depending on the stacking sequence, the resulting M polyanion resembles the oxygen substructure of the hexagonal (face-sharing octahedra, h stacking, Ni

_{3}

Sn-type, border compound BaSn

_{3}

) or the cubic (corner-sharing octahedra, c stacking, Cu

_{3}

Au-type, border compound SrPb

_{3}

) perovskite. In the binary compound BaSn

_{3}

(Ni

_{3}

Sn-type) up to 28% of Sn can be substituted against Pb (hP8, P $6_{3} /$ mmc, x = 0.28(4): a = 726.12(6), c = 556.51(6) pm, R1 = 0.0264). A further increased lead content of 47 to 66% causes the formation of the BaSn

_{2.57}

Bi

_{0.43}

-type structure with a

{(hhhc)}_{2}

stacking [hP32, P $6_{3} /$ mmc, x = 0.47(3): a = 726.80(3), c = 2235.78(14) pm, R1 = 0.0437]. The stability range of the BaPb

_{3}

-type sequence

{(hhc)}_{3}

starts at a lead proportion of 78% (hR36, R $\bar{3}$ m, a = 728.77(3), c = 2540.59(15) pm, R1= 0.0660) and reaches up to the pure plumbide BaPb

_{3}

. A second new polymorph of BaPb

_{3}

forms the Mg

_{3}

In-type structure with a further increased amount of cubic sequences [

{(hhcc)}_{3}

; hR48, a = 728.7(2), c = 3420.3(10) pm, R1 = 0.0669] and is thus isotypic with the border phase SrSn

_{3}

of the respective strontium series. For the latter, a Pb content of 32% causes a small existence region of the PuAl

_{3}

-type structure [hP24, P $6_{3} /$ mmc, a = 696.97(6), c = 1675.5(2) pm, R1 = 0.1182] with a

{(hcc)}_{2}

stacking. The series is terminated by the pure c stacking of SrPb

_{3}

, the stability range of this structure type starts at 75% Pb (cP4, Pm $\bar{3}$ m; a = 495.46(9) pm, R1 = 0.0498). The stacking of the close packed layers is evidently determined by the ratio of the atomic radii of the contributing elements. The Sn/Pb distribution inside the polyanion (’coloring’) is likewise determined by size criteria. The electronic stability ranges, which are discussed on the basis of the results of FP-LAPW band structure calculations are compared with the Zintl concept and Wade’s/mno electron counting rules. Still, due to the presence of only partially occupied steep M-p bands the compounds are metals exhibiting pseudo band gaps close to the Fermi level. Thus, this structure family represents an instructive case for the transition from polar ionic/covalent towards (inter)metallic chemistry. Full article

(This article belongs to the Special Issue Compounds with Polar Metallic Bonding)

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18 pages, 1271 KiB

Open AccessArticle

The Rietveld Refinement in the EXPO Software: A Powerful Tool at the End of the Elaborate Crystal Structure Solution Pathway

by Angela Altomare, Francesco Capitelli, Nicola Corriero, Corrado Cuocci, Aurelia Falcicchio, Anna Moliterni and Rosanna Rizzi

Crystals 2018, 8(5), 203; https://doi.org/10.3390/cryst8050203 - 04 May 2018

Cited by 12 | Viewed by 6755

Abstract

The Rietveld method is the most reliable and powerful tool for refining crystal structure when powder diffraction data are available. It requires that the structure model to be adjusted is as close as possible to the true structure. The Rietveld method usually represents [...] Read more.

The Rietveld method is the most reliable and powerful tool for refining crystal structure when powder diffraction data are available. It requires that the structure model to be adjusted is as close as possible to the true structure. The Rietveld method usually represents the final step of the powder solution process, in particular when a new structure is going to be determined and published. EXPO is a software able to execute all the steps of the solution process in a mostly automatic way, by starting from the chemical formula and the experimental diffraction pattern, passing through computational methods for locating the structure model and optimizing it, and ending to the Rietveld refinement. In this contribution, we present the most recent solution strategies in EXPO, both in reciprocal and direct space, aiming at obtaining models suitable to be refined by the Rietveld method. Examples of Rietveld refinements are described, whose results are related to different solution procedures and types of compounds (organic and inorganic). Full article

(This article belongs to the Special Issue Rietveld Refinement in the Characterization of Crystalline Materials)

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17 pages, 1903 KiB

Open AccessArticle

On the Phase Diagrams of ⁴He Adsorbed on Graphene and Graphite from Quantum Simulation Methods

by Thomas L. Badman and Jeffrey M. McMahon

Crystals 2018, 8(5), 202; https://doi.org/10.3390/cryst8050202 - 04 May 2018

Cited by 7 | Viewed by 3732

Abstract

The ground-state phase diagrams of

^{4}

He adsorbed on graphene and graphite are calculated using quantum simulation methods. In this work, a systematic investigation of the approximations used in such simulations is carried out. Particular focus is placed on the helium–helium (He–He) and [...] Read more.

The ground-state phase diagrams of

^{4}

He adsorbed on graphene and graphite are calculated using quantum simulation methods. In this work, a systematic investigation of the approximations used in such simulations is carried out. Particular focus is placed on the helium–helium (He–He) and helium–carbon (He–C) interactions, as well as their modern approximations. On careful consideration of other approximations and convergence, the simulations are otherwise (numerically) exact. The He–He interaction as approximated by a sum of pairwise potentials is quantitatively assessed. A similar analysis is made for the He–C interaction, but more thoroughly and with a focus on surface corrugation. The importance of many-body effects is discussed. Altogether, the results provide “reference data” for the considered systems. Using comparisons with experiments and first-principle calculations, conclusions are drawn regarding the quantitative accuracy of these modern approximations to these interactions. Full article

(This article belongs to the Special Issue Quantum Crystals)

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