Simulated X-ray Diffraction Patterns Research Articles

VISUALISASI STRUKTUR KRISTAL KERAMIK PEROVSKITE MENGGUNAKAN VESTA

Visualization of crystal structures and simulation of X-ray diffraction patterns of perovskite ceramic was successfully performed by VESTA software programs. The purpose of this research is to obtain the relation of lattice parameter, and composition to the diffraction pattern. The software program produces crystal structure information and a representative X-ray diffraction pattern for the ceramic materials. The program needs several input parameters such as the coordinates of each constituent atom, lattice parameters, and space symmetry. The obtained output of the software program are in the form of diffraction pattern graph and crystal structure data which gives the description of the profile and type (phase) of ceramic material. The results showed that the peak position and intensity of the diffraction pattern are influenced by the arrangement of the atoms within the unit cell. The addition of impurity atoms such as Sr on the Ba side in BaTiO3 causes the BaTiO3 structure changes from Orthorombic (a≠b≠c) to Tetragonal (a=b≠c) structure. Based on the simulation, it can be predicted that the critical concentration of the change of structure occur at Sr concentration about 0.4.

“XANSONS for COD”: a new small BOINC project in crystallography

Abstract “XANSONS for COD” (http://xansons4cod.com) is a new BOINC project aimed at creating the open-access database of simulated x-ray and neutron powder diffraction patterns for nanocrystalline phase of materials from the collection of the Crystallography Open Database (COD). The project uses original open-source software XaNSoNS to simulate diffraction patterns on CPU and GPU. This paper describes the scientific problem this project solves, the project’s internal structure, its operation principles and organization of the final database.

Irradiation damage in nuclear graphite at the atomic scale

Microstructures of nuclear graphite have been investigated based on the analysis of both X-ray diffraction patterns and High Resolution Transmission Electron Microscopy images (HRTEM) from experiments and from molecular dynamics simulations. One filler particle – composed of almost aligned crystallites separated by Mrozowski cracks – and the whole sequence of single crystal graphite irradiated from its virgin state up to highly damaged (64.5% of defects) have been simulated. Simulations are assessed against experiments based on the analysis of its microstructures extracted with Bragg's law and Scherrer's equation applied to simulated X-ray diffraction patterns. In particular, the evolution of cell parameters with defect concentration generated by irradiation agrees well with experimental observations. Simulated HRTEM images show many features observed in experimental images in both virgin and irradiated French nuclear graphite. Some of these features can be linked unequivocally to defined atomistic configurations. Basal grain boundaries (GBs), Mrozowski cracks, graphene sheets and their folding belong to this category. Conversely, some patterns in simulated HRTEM cannot be related to a unique atomistic configuration and might eventually give rise to misleading interpretation. This is evidenced for edge dislocations in virgin nuclear graphite as well as for residues of graphene layers in highly damaged graphite.

Electronic and mechanic properties of trigonal boron nitride by first-principles calculations

A new boron nitride allotrope with 6 atoms in a unit cell termed as trigonal BN (TBN), which belongs to P3121 space group, is theoretically investigated. Electronic structures, mechanic properties, phonon spectra and other properties were calculated by using first-principles based on density functional theory (DFT). The elastic constants reveal that TBN is mechanically stable. Furthermore, phonon dispersion indicates that TBN is dynamically stable. The calculated bulk modulus and shear modulus of TBN are 323 and 342 GPa, respectively. The calculated Young's modulus are Ex = Ey = 760 GPa, Ez = 959 GPa, indicating that TBN is a super-hard and brittle material. The universal anisotropy index, which is only 0.296, shows its weak anisotropy. Band structure states clearly that TBN is an indirect semiconductor with a band gap of 3.87 eV. The valence bands are mainly composed of N 2p states, and the conduction bands are mainly contributed by B 2p states. Simulated X-ray diffraction patterns (XRD) and Raman spectra were also provided for future experimental characterizations. Due to its band gap and super-hard properties, TBN may possess potential in super-hard, optical and electronic applications.

Single-bonded allotrope of nitrogen predicted at high pressure

An allotrope of nitrogen formed solely by N-N single bonds is predicted to exist between 100 and 150 GPa. The crystal structure has the Pccn symmetry and is characterized by a distorted tetrahedral network consisting of fused ${\mathrm{N}}_{8}, {\mathrm{N}}_{10}$, and ${\mathrm{N}}_{12}$ rings. Stability of this structure is established by phonon and vibrational free energy calculations at 0 K and finite temperatures. The simulated x-ray diffraction pattern of the Pccn phase is compared to the pattern of a recently synthesized nitrogen phase at the same $P\ensuremath{-}T$ conditions, which suggests that the Pccn phase is likely a minor component of the latter. The Pccn phase is expected to form above the stability field of the cubic gauche (cg) phase. The outstanding metastability of this phase is attributed to the intrinsic stability of the $s{p}^{3}$ bonding as well as the energetically favorable dihedral angles between N-N single bonds, in either gauche or trans conformation. The prediction of another single-bonded phase of nitrogen after the lab-synthesized cg phase will stimulate research on metastable phases of nitrogen and their applications as high-energy-density materials.

Pseudopolymorphs of LB30870, a Direct Thrombin Inhibitor: One-Dimensional Solvent Channel Structures Explain Reversible Hydration/Dehydration

Three different hydrates of LB30870, a new direct thrombin inhibitor, were identified during the screening of solid form, and their interconversion relationship and relative thermodynamic stabilities were investigated. Form I (hexahydrate) changes to Form II (dihydrate) or Form III (tetrahydrate) by dehydration, while Form II becomes Form I by hydration, and both Form II and Form III change to Form I in the aqueous slurry. Single crystals obtained from two different crystallization conditions, wet or air-dried, were found to be isostructural with a difference in the solvent channels, and based on the simulated and experimental powder X-ray diffraction patterns, the air-dried crystals are assigned as Form I and Form III, respectively. In all crystal structures, LB30870 is in a folded conformation forced by the presence of strong hydrogen bonds by two structural water molecules. The solvent channel formed can hold up to six additional hydration water sites per each LB30870, and the one-dimensional solvent c...

Adsorptive separation of carbon dioxide from flue gas using mesoporous MCM-41: A molecular simulation study

Realistic molecular models of MCM-41 have been developed and used for studying the separation of carbon dioxide from flue gas mixtures using grand canonical Monte Carlo simulations. The simulated X-ray diffraction pattern and surface area of the models are in good agreement with experimental results reported in literature. Adsorption of pure carbon dioxide was studied on the three different models at two different temperatures, 273.2 K and 303.2 K. Isosteric heats of adsorption of CO2 calculated from the simulations were in the range 20–25 kJ/mol, which matches well with reported experimental values. The simulated CO2 adsorption isotherms showed good agreement with experimental isotherms at both the temperatures for two of the models, which were selected for further mixture adsorption studies. Binary CO2/N2 adsorption simulations were performed at different bulk gas compositions, and the selectivities of CO2 over N2 were observed to be in the range 4-10. Further studies on adsorption of ternary and quaternary bulk gas mixtures containing water vapor and O2 in addition to CO2 and N2 did not reveal any significant effect on CO2 adsorption and CO2-N2 selectivity.

Molecular dynamics simulation and experimental validation by X-ray data of hydroxyapatite crystalline structures

Computer simulations have provide information about structural and dynamic properties of hydroxyapatite (HAp). However, the accuracy of simulated HAp in the description of experimental crystalline structures is not evaluated in many of theoretical studies. Here, molecular dynamics (MD) simulations of HAp in the isobaric-isothermal (NPT) ensemble are carried out using LAMMPS at temperature of 298 K and pressure of 1 atm. We evaluate the quality of two sets of force field parameters by comparison between simulated and experimental unit-cell geometry and powder X-ray diffraction (XRD) patterns. Presence of calcium vacancies is taken into account in the simulations in order to reproduce a non-stoichiometric HAp synthesized in our lab. By using the number of vacancies determined from X-ray fluorescence (XRF) results, we observe great similarity between experimental and simulated diffraction patterns and unit-cell parameters as well as in the atom positions inside the unit cell. Significant structural modifications are identified when an excessive number of calcium atoms was removed, which emphasizes the importance of considering reliable experimental data. We encourage the use of this experimental validation methodology to demonstrate the quality of any simulated crystalline structure.

Hydrothermal synthesis and Er3+ ion exchange of sodium GTS-type titanosilicate

Powder samples of Na-GTS have been prepared using a hydrothermal method. Er3+- exchanged Na-GTS [Na4(1-x)Er(4/3)xTi4Si3O16·nH2O] with the compositions in the range of 0.28 ≤ x ≤ 0.64 was obtained by shaking the single-phase sample of Na-GTS in the ErC13 aqueous solutions (25 mL, 0.01~0.1 M) at 25 °C for 6 hours. The chemical analyses by atomic absorption spectrometry (AAS) show that the maximum allowance of Er3+ incorporated into Na-GTS is x = 0.64. TG-DTA measurements show that the incorporation of Er3+ slightly decreases the water contents and the dehydration temperatures. The powder X-ray diffraction (XRD) patterns indicate that the diffraction-peak intensities vary systematically with increasing the Er3+ composition (x). The simulation of XRD patterns suggests the cation-distribution model that Er3+ and Na+ ions preferentially occupy the 4e and 6g sites, respectively.

Pressure-Stabilized Semiconducting Electrides in Alkaline-Earth-Metal Subnitrides.

High pressure is able to modify profoundly the chemical bonding and generate new phase structures of materials with chemical and physical properties not accessible at ambient conditions. We here report an unprecedented phenomenon on the pressure-induced formation of semiconducting electrides via compression of layered alkaline-earth subnitrides Ca2N, Sr2N, and Ba2N that are conducting electrides with loosely confined electrons in the interlayer voids at ambient pressure. Our extensive first-principles swarm structure searches identified the high-pressure semiconducting electride phases of a tetragonal I4̅2d structure for Ca2N and a monoclinic Cc structure shared by Sr2N and Ba2N, both of which contain atomic-size cavities with paring electrons distributed within. These electride structures are validated by the excellent agreement between the simulated X-ray diffraction patterns and the experimental data available. We attribute the emergence of the semiconducting electride phases to the p-d hybridization on alkaline-earth-metal atoms under compression as well as the filling of the p-d hybridized band due to the interaction between Ca and N. Our work provides a unique example of pressure-induced metal-to-semiconductor transition in compound materials and reveals unambiguously the electron-confinement topology change between different types of electrides.

Computational prediction of a simple cubic carbon allotrope consisting of C12 clusters.

We identify by ab initio calculations a new simple cubic carbon phase in Pa3¯ symmetry, which has a 48-atom unit cell in all-sp3 bonding networks, thus termed SC48 carbon. It can be viewed as a crystalline form of C12 clusters or a combined structure of SC24 and BC12 carbon, but it is energetically more stable than the recently reported cubic carbon phases such as BC8, SC24, BC12, and fcc-C12. The structural stability is verified by phonon mode analysis. Electronic band and density of state calculations reveal that SC48 carbon is an insulator with a large direct band gap of 4.40 eV. Moreover, simulated x-ray diffraction patterns provide an excellent match to the distinct diffraction peaks found in milled fullerene soot. These results provide a solid foundation for further exploration of this new carbon allotrope.

A metastable phase of shocked bulk single crystal copper: an atomistic simulation study

Structural phase transformation in bulk single crystal Cu in different orientation under shock loading of different intensities has been investigated in this article. Atomistic simulations, such as, classical molecular dynamics using embedded atom method (EAM) interatomic potential and ab-initio based molecular dynamics simulations, have been carried out to demonstrate FCC-to-BCT phase transformation under shock loading of 〈100〉 oriented bulk single crystal copper. Simulated x-ray diffraction patterns have been utilized to confirm the structural phase transformation before shock-induced melting in Cu(100).

Superhard three-dimensional carbon with metallic conductivity

There has been a long-standing search for 3D metallic, superhard carbon, stable under ambient conditions. Here, we report the discovery of a new 3D orthorhombic phase (denoted C14-diamond below) from first-principles calculations. This allotrope is metallic and superhard and is built from nano-layered sp3 carbon with the cubic diamond structure, connected by proper sp2-bonded ethene-type CC links. This unique configuration of sp2 carbons makes C14-diamond conductive and allows one-dimensional electronic conduction along the c-axis. The high hardness of C14-diamond is further confirmed by its higher density compared with other metallic carbon allotropes. It is dynamically stable and more favorable than most other theoretically predicted carbons in energy. Interestingly, the simulated x-ray diffraction pattern of C14-diamond is similar to that of the experimentally reported while yet unresolved carbon phase obtained by shock-compression of tetracyanoethylene (TCE) powder, indicating that it could be a potential candidate for this carbon phase. We further propose a possible transition process from TCE to C14-diamond, suggesting that conductive superhard material might be synthesized by this way.

A potential descriptor for the CO2 hydrogenation to CH4 over Al2O3 supported Ni and Ni-based alloy catalysts

Supported Ni, Cu, Fe, and Ni-M (M=Cu or Fe) catalysts of the same total metal loading and different Ni to M ratios are examined for the CO2 hydrogenation to CH4. Substituting Ni with Cu or Fe in a Ni/Al2O3 catalyst leads to smaller average particle size, sharper particle size distribution and alloy formation, which were determined from transmission electron microscopy (TEM) and X-ray diffraction (XRD). Surface properties of the Ni/Al2O3 and Ni-M/Al2O3 catalysts are found to be different as revealed by hydrogen temperature programmed desorption (H2-TPD). Furthermore, the surface of the supported Ni-Cu catalyst was enriched with Cu. The catalytic activity increases for 75Ni25Fe/Al2O3 relative to Ni/Al2O3, but decreases for 75Ni25Cu/Al2O3. In both these catalysts, the Ni to M ratio was 3:1. The supported Ni and the two Ni3M alloy catalysts are modelled successfully as the shift in 2θ between experimental and simulated XRD pattern were in accordance. Moreover, the trend in shifts in experimental and simulated optimized lattice constants relative to Ni was similar. Density functional theory (DFT) calculations revealed changes in the electronic properties. The electronic properties calculated based on the d-density of states (d-DOS) of surface Ni atoms are the d-band center (εd) and number of d-DOS at Fermi level (NEF). An excellent correlation between TOFCH4and NEFexists, whereas εd did not satisfactorily capture the trends inTOFCH4. Thus, the NEFbest describes the catalytic activity trend of the supported Ni and two Ni-based alloy catalysts.

Novel Superhard sp^{3} Carbon Allotrope from Cold-Compressed C_{70} Peapods.

Design and synthesis of new carbon allotropes have always been important topics in condensed matter physics and materials science. Here we report a new carbon allotrope, formed from cold-compressed C_{70} peapods, which most likely can be identified with a fully sp^{3}-bonded monoclinic structure, here named V carbon, predicted from our simulation. The simulated x-ray diffraction pattern, near K-edge spectroscopy, and phonon spectrum agree well with our experimental data. Theoretical calculations reveal that V carbon has a Vickers hardness of 90GPa and a bulk modulus ∼400 GPa, which well explains the "ring crack" left on the diamond anvils by the transformed phase in our experiments. The V carbon is thermodynamically stable over a wide pressure range up to 100GPa, suggesting that once V carbon forms, it is stable and can be recovered to ambient conditions. A transition pathway from peapod to V carbon has also been suggested. These findings suggest a new strategy for creating new sp^{3}-hybridized carbon structures by using fullerene@nanotubes carbon precursor containing odd-numbered rings in the structures.

Domain structure of CoIr nanoalloys

X-ray diffraction (XRD) pattern of nanosized equimolar solid solution CoIr prepared by thermolysis of [Co(NH3)6][Ir(C2O4)3] contains peaks characteristic of both face-centered cubic (fcc) and hexagonal close-packed (hcp) structure. Moreover, 101 peak of hcp modification is substantially wider than 100 and 002 peaks, 102 and 103 are very broad and almost invisible. Peak 200 of fcc structure is wider than the other peaks of this modification and slightly shifted toward lower angles. It was shown by simulation of XRD patterns that particles of CoIr alloy are nanoheterogeneous and consist of lamellar domains having fcc and hcp structures. The best fit was obtained for the following model parameters: an average crystallites size is about 10 nm, average thicknesses of the fcc and hcp domains are 1.7 and 1.1 respectively. The presence of domain structure was confirmed by transmission electron microscopy data.

Semimetallic carbon allotrope with a topological nodal line in mixed sp2-sp3 bonding networks

Graphene is known as a two-dimensional Dirac semimetal, in which electron states are described by the Dirac equation of relativistic quantum mechanics. Three-dimensional analogs of graphene are characterized by Dirac points or lines in momentum space, which are protected by symmetry. Here, we report a novel 3D carbon allotrope belonging to a class of topological nodal line semimetals, discovered using an evolutionary structure search method. The new carbon phase in the monoclinic C2/m space group, termed m-C8, consists of five-membered rings with sp3 bonding interconnected by sp2-bonded carbon networks. Enthalpy calculations reveal that m-C8 is more favorable than recently reported topological semimetallic carbon allotropes, and the dynamic stability of m-C8 is verified by phonon spectra and molecular dynamics simulations. Simulated X-ray diffraction patterns indicate that m-C8 could be one of the unidentified carbon phases observed in detonation shoot. The analysis of electronic properties indicates that m-C8 exhibits a nodal line protected by both inversion and time-reversal symmetries in the absence of spin-orbit coupling and the surface band connecting the projected nodal points. Our results may help design new carbon allotropes with exotic electronic properties.

X-ray diffraction data-assisted structure searches

X-ray diffraction (XRD) is an important technique for structure determination. However, in traditional methods, estimated structural information (e.g., unit cell parameters and space group) is required to determine the precise structure from XRD data. We propose a versatile global search method for determining crystal structures from experimental powder XRD data without guessed structural information. Unlike in traditional structure prediction, which uses total energy as the fitness function, the fitness in our method is the degree of dissimilarity between the simulated and the experimental XRD patterns. To improve the efficiency of structure searching using powder XRD data, several techniques are implemented, including background subtraction, profile parameter optimization by simulated annealing, and structure evolution using CALYPSO. The efficiency and robustness of our method is demonstrated here using predictions of previous known structures (hexagonal ZnO and anatase TiO2). The method is also used to explore the high-pressure phases of binary compounds CaLi2 and Ca3C2; it uncovers candidate structures (C2221 for CaLi2 at 54 GPa and Cm2m for Ca3C2 at 12.5 GPa) whose simulated XRD patterns agree with their experimental patterns.

Thermal evolution of Mg-Al and Ni-Al layered double hydroxides: the structure of the dehydrated phase.

Simulation of X-ray diffraction patterns on the basis of the models of one-dimensional disordered crystals was used to investigate the structure of the dehydrated phase produced by dehydration of Mg-Al and Ni-Al layered double hydroxides at a temperature of ∼473-498 K. It was found that the removal of water molecules transforms the initial structure, which is a mixture of 3R1 and 2H1 polytypes, into a structure that comprises preferentially fragments of 3R2 and 1H polytypes and has some turbostratic disorder.

Package of double helical bromine chains inside single-walled carbon nanotubes

The helicity of stable double helical bromine chains inside single-walled carbon nanotubes (SWCNTs) was studied through the calculation of systematic interaction energy, using the van der Waals interaction potential. The results presented clear images of stable double helical structures inside SWCNTs. The optimal helical radius and helical angle of chain structure increase and decrease, respectively, with the increase of tube radius. The detailed analysis indicated that some metastable structures in SWCNTs may also co-exist with the optimal structures, but not within the same tubes. In addition, a detailed simulation of X-ray diffraction patterns was performed for the obtained optimal helical structures.