Simulated X-ray Diffraction Spectra Research Articles

Combining experiments and molecular dynamics simulations to investigate the effects of water on the structure and mechanical properties of a coal gangue-based geopolymer

The effects of water on the structure and mechanical properties of a coal gangue-based geopolymer were investigated experimentally and compared with the results of molecular dynamics simulations. The chemical composition and crystalline phases of the raw materials were analyzed via X-ray fluorescence and X-ray diffraction (XRD) after 28 days of curing, and the mechanical properties were tested. Molecular structure models for H2O/Al ratios of 2.5, 3, and 3.5 were established using molecular dynamics simulation software, and structural optimization and molecular dynamics simulations were performed under the universal force field. Based on the obtained results, the energy and temperature change curves, simulated XRD spectrum, radial distribution function, hydrogen bonding, mean squared displacement, and mechanical properties of the amorphous geopolymer were analyzed. The rationality of the molecular model was verified through comparison with the results of the molecular dynamics simulations and experiments, and as the H2O/Al ratio increased within a given range, the energy and temperature stability time of the system are decreased, the area of the dispersion peak is decreased, the amorphous phase is increased, the diffusion degree of Si and Al in the system is decreased, the length of Si-O bond, Al-O bond, H-O bond and O-O bond is shortened, and the structure is more stable. The mechanical properties obtained from the experiment were also improved.

Topological semimetal in an sp2−sp3 hybridized carbon network with nodal rings

There have been several proposals that three-dimensional allotropes of carbon formed by graphene networks can support two types of topological nodal line semimetals: type A has closed nodal rings inside the first Brillouin zone (BZ), and type B has nodal lines traversing the whole BZ to be periodically connected. Among these proposals, it has been found that nearly all-$s{p}^{2}$ hybridized allotropes tend to hold A-type closed nodal rings, while the $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ hybridized allotropes tend to hold B-type periodic nodal lines. Here we identify by ab initio calculations an $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ hybridized carbon allotrope in $Pcca$ (${D}_{2h}^{8}$) symmetry as a topological nodal line semimetal with two A-type closed nodal rings on the ${k}_{y}=0$ and ${k}_{y}=\ensuremath{\pi}$ mirror planes, respectively. The projections of these two nodal rings onto (010) surfaces form concentric ellipses with different size. The drumhead surface states appear in the region enclosed by the bigger ring but disappear inside of the smaller ring. This is consistent with the distribution of the Berry phase calculated along the [010] periodic reciprocal lattice. An effective $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ model has been proposed, and the parameters have been obtained through fitting the ab initio results. The model can reproduce the nodal rings in the shape of an ellipse, satisfying the twofold rotational symmetry. Meanwhile, the simulated x-ray diffraction spectrum matches well with the recently reported distinct diffraction peaks found in the diamond-rich coatings on stainless steel substrate. These results establish a carbon phase with intriguing structural and electronic properties and expand our understandings about topological nodal lines in carbon networks.

A superhard orthorhombic carbon with all six-membered-ring in sp3 bonding networks

We identify by first-principles calculations a new diamond-like carbon phase with a 16-atom orthorhombic primitive cell in Pbcn (D2h14) symmetry. This new carbon allotrope consists of all sp3 six-membered rings like as the reported BC8, BC12 and R16 carbon, while energetically more stable than these carbon phases. Its dynamical stability has been confirmed by phonon mode analysis and molecular dynamics simulations. The calculated bulk modulus (435 GPa) and Vickers hardness (93 GPa) are comparable to that of diamond, showing as a superhard carbon material. Electronic band calculations reveal that it is an insulator with an indirect band gap of 4.35 eV. Simulated X-ray diffraction spectrum presents a complicated pattern, showing an amorphous form of diamond, reflecting its structural complexity stemming from its multiple bond lengths and bond angles, but matches well with the diffractions peaks found in the diamond-rich coatings on stainless steel substrate. These findings lay a foundation for further study of this new diamond-like carbon allotrope and its outstanding properties.

Wide band gap carbon allotropes: Inspired by zeolite-nets

Based on the topologies proposed for zeolites, six metastable semiconductor carbon allotropes with band gaps of 2.72–3.89 eV are predicted using ab initio density functional calculations. The hardnesses of these allotropes are about 90%–94% that of diamond, indicating that they may be superhard materials. We also present simulated X-ray diffraction spectra of these new carbon allotropes to provide a basis for possible experimental observations and synthesis. These new carbon structures with a range of band gaps and with hardnesses comparable to diamond could be potential targets for the synthesis of hard and transparent materials.

Ba2phenanthrene is the main component in the Ba-doped phenanthrene superconductor.

We have systematically investigated the crystal structure of Ba-doped phenanthrene with various Ba doping levels by the first-principles calculations combined with the X-ray diffraction (XRD) spectra simulations. Although the experimental stoichiometry ratio of Ba atom and phenanthrene molecule is 1.5:1, the simulated XRD spectra, space group symmetry and optimized lattice parameters of Ba1.5phenanthrene are not consistent with the experimental ones, while the results for Ba2phenanthrene are in good agreement with the measurements. The strength difference of a few XRD peaks can be explained by the existence of pristine phenanthrene. Our findings suggest that instead of uniform Ba1.5phenanthrene, there coexist Ba2phenanthrene and undoped phenanthrene in the superconducting sample. The electronic calculations indicate that Ba2phenanthrene is a semiconductor with a small energy gap less than 0.05 eV.

Diamond polytypes under high pressure: A first-principles study

The properties of diamond polytypes under high pressure have been investigated by means of the first-principles calculations. They are unveiled to be thermodynamically and kinetically more stable than hexagonal diamond (h-diamond) even under pressure. The electronic and elastic properties of the diamond polytypes are also explored, revealing very close cohesive energy, band gap and equilibrium density to those of cubic diamond (c-diamond). The Vickers hardness is uncovered to be comparable or even larger than those of the c- and h-diamond. Furthermore, they experience lower energy barriers than the h-diamond during the transforming process from graphite under pressure. Especially, we explored the characterized Raman modes under pressure from 0 to 40GPa for each allotrope, which can be used to identify the diamond polytypes from the c-diamond T2g background mode, and the result may serve as a useful guide for future experimental studies. In addition, the simulated X-ray diffraction spectra under pressure is also presented.

Structure determination of ultra dense magnesium borohydride: A first-principles study

Magnesium borohydride (Mg(BH4)2) is one of the potential hydrogen storage materials. Recently, two experiments [Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov, and H. Hagemann, Angew. Chem., Int. Ed. 50, 11162 (2011); L. George, V. Drozd, and S. K. Saxena, J. Phys. Chem. C 113, 486 (2009)] found that α-Mg(BH4)2 can irreversibly be transformed to an ultra dense δ-Mg(BH4)2 under high pressure. Its volumetric hydrogen content at ambient pressure (147 g/cm(3)) exceeds twice of DOE's (U.S. Department of Energy) target (70 g/cm(3)) and that of α-Mg(BH4)2 (117 g/cm(3)) by 20%. In this study, the experimentally proposed P4(2)nm structure of δ-phase has been found to be dynamically unstable. A new Fddd structure has been reported as a good candidate of δ-phase instead. Its enthalpy from 0 to 12 GPa is much lower than P4(2)nm structure and the simulated X-ray diffraction spectrum is in satisfied agreement with previous experiments. In addition, the previously proposed P-3m1 structure, which is denser than Fddd, is found to be a candidate of ε-phase due to the agreement of Raman shifts.

Sp3-Bonded silicon allotropes based on the Kelvin problem

The Kelvin problem, how to partition three-dimensional space into cells of equal volume with minimal area, is a fascinating one. Aggregations of bubbles are naturally physical illustrations of the Kelvin problem. And the superconductor Na8Si46 as an inspiration leads to an amazing discovery of the Weaire-Phelan (WP) structure of foam - the optimal solution to the Kelvin problem to date. Here based on the structural similarity between sp(3)-bonded silicon allotropes and the solutions to the Kelvin problem, a series of new sp(3)-hybridization silicon allotropes, named "Kelvin Silicons", are presented. Furthermore, the structural stability and electronic properties of these new silicon allotropes are investigated using density-functional theory (DFT) calculations. The results show that Kelvin Silicons are structurally stable semiconductors with indirect bandgaps in the range of 0.17-1.40 eV, and their bulk moduli are about 75.9-88.5% that of the diamond phase. The simulated X-ray diffraction spectra of the new silicon crystalline structures would provide more information for possible experimental observations and synthesis.

First-Principles Determination of the Structure of Magnesium Borohydride

The energy landscape of Mg(BH(4))(2) under pressure is explored by ab initio evolutionary calculations. Two new tetragonal structures, with space groups P4 and I4(1)/acd, are predicted to be lower in enthalpy by 15.4 and 21.2 kJ/mol, respectively, than the earlier proposed P4(2)nm phase. We have simulated x-ray diffraction spectra, lattice dynamics, and equations of state of these phases. The density, volume contraction, bulk modulus, and simulated x-ray diffraction patterns of I4(1)/acd and P4 structures are in excellent agreement with the experimental results.

InN/InGaN multiple quantum wells emitting at 1.5 μm grown by molecular beam epitaxy

This work reports on the growth by molecular beam epitaxy and characterization of InN/InGaN multiple quantum wells (MQWs) emitting at 1.5 μm. X-ray diffraction (XRD) spectra show satellite peaks up to the second order. Estimated values of well (3 nm) and barrier (9 nm) thicknesses were derived from transmission electron microscopy and the fit between experimental data and simulated XRD spectra. Transmission electron microscopy and XRD simulations also confirmed that the InGaN barriers are relaxed with respect to the GaN template, while the InN MQWs grew under biaxial compression on the InGaN barriers. Low temperature (14 K) photoluminescence measurements reveal an emission from the InN MQWs at 1.5 μm. Measurements as a function of temperature indicate the existence of localized states, probably due to InN quantum wells’ thickness fluctuations as observed by transmission electron microscopy.

Superhard diamondlikeBC5: A first-principles investigation

We perform first-principles density-functional calculations to identify the possible crystal structure of a superhard diamondlike ${\text{BC}}_{5}$ phase, which was recently synthesized under high-pressure and high-temperature conditions. Interestingly, we find only a small total-energy difference between the energetically most favorable ordered configuration and the fully disordered state of ${\text{BC}}_{5}$ modeled using a 54-atom special quasirandom structure, indicating a weak ordering tendency. It is thus likely that the ${\text{BC}}_{5}$ phase synthesized under experimental conditions is disordered in nature. Such a conclusion is further corroborated by the fact that the disordered ${\text{BC}}_{5}$ structure displays volume-per-atom, bulk modulus and its pressure derivative, and simulated x-ray diffraction spectrum in good agreements with experiments.

Imogolite Nanotubes: Stability, Electronic, and Mechanical Properties

The aluminosilicate mineral imogolite is composed of single-walled nanotubes with stoichiometry of (HO)(3)Al(2)O(3)SiOH and occurs naturally in soils of volcanic origin. In the present work we study the stability and the electronic and mechanical properties of zigzag and armchair imogolite nanotubes using the density-functional tight-binding method. The (12,0) imogolite tube has the highest stability of all tubes studied here. Uniquely for nanotubes, imogolite has a minimum in the strain energy for the optimum structure. This is in agreement with experimental data, as shown by comparison with the simulated X-ray diffraction spectrum. An analysis of the electronic densities of states shows that all imogolite tubes, independent on their chirality and size, are insulators.

Structure properties of Pd/V superlattices formed by the dual electron-beam system

Pd/V multilayered structures are produced by the sequential deposition of palladium and vanadium with two electron-beam guns in a UHV system. The crystallographic structure of the layered samples is analysed by the θ–2θ X-ray diffraction technique. The measured X-ray diffraction spectra show the peaks characteristic of superiattices. The structure of the samples is studied using a statistical model of the superlattice structure. Typical structural parameters of the Pd/V superlattices (the unit cell composition, the standard deviation of the numbers of monolayers in the unit cell, the grain thickness, the standard deviation of the grain thickness, the interface thickness) are estimated by a comparison of numerically simulated X-ray diffraction spectra with experimental spectra.

Ion beam mixing of Pd/V superlattices studied by X-ray diffractometry

The ion beam mixing technique was applied to the fabrication of Pd-V metallic alloys. Layered Pd(fcc)/V(bcc) thin films were produced by sequential deposition of palladium and vanadium on heated glass substrates. Measured X-ray diffraction spectra showed the peaks characteristic for superlattices. Numerically simulated X-ray diffraction spectra were compared with experimental data. The crystallographic structure of Pd/V superlattices after implantation has been studied as a function of the ion dose using the θ-2θ X-ray diffraction technique.

Studying the interface of Au-Cu superlattices

The interface region character of Au-Cu superlattices has been studied. Superlattice samples were vacuum deposited at room temperature. In order to increase the influence of the interface slope on X-ray diffraction spectra the superlattice unit cells of some samples consisted of two double layers of gold and copper. The structure of the samples was studied using a statistical model of the superlattice structure. The thickness of the interdiffused interface region has been estimated by comparison of experimental with simulated X-ray diffraction spectra. Information about the terrace growth of superlattices is presented. Results show that it is possible to perform a detailed analysis of the interface on the basis of X-ray diffraction measurements in spite of the obtention of an “averaged” image of a sample structure.