Simulated X-ray Diffraction Patterns Research Articles

Investigation of new superhard carbon allotropes with promising electronic properties

During the systematic search for a new superhard carbon allotrope, we predicted three structures with promising physical properties. Our electronic structure calculations show that these materials have a semiconducting band gap and a high carrier mobility comparable with diamond. The simulated x-ray diffraction patterns of the proposed materials are in a good agreement with the experimental X-ray spectra. Evaluated phase transition pressures from graphite to the new proposed carbon phases are smaller than 25 GPa and close to the experimental values.

AlCoCrCuFeNi high entropy alloy cluster growth and annealing on silicon: A classical molecular dynamics simulation study

Molecular dynamics simulations are carried out for describing deposition and annealing processes of AlCoCrCuFeNi high entropy alloy (HEA) thin films. Deposition results in the growth of HEA clusters. Further annealing between 300K and 1500K leads to a coalescence phenomenon, as described by successive jump in the root mean square displacement of atoms. The simulated X-ray diffraction patterns during annealing reproduces the main feature of the experiments: a phase transition of the cluster structure from bcc to fcc.

Prediction of a novel monoclinic carbon allotrope

A novel allotrope of carbon with P2/m symmetry was identified during an ab initio minima-hopping structural search which we call M10-carbon. This structure is predicted to be more stable than graphite at pressures above 14.4 GPa and consists purely of s p 3 bonds. It has a high bulk modulus and is almost as hard as diamond. A comparison of the simulated X-ray diffraction pattern shows a good agreement with experimental results from cold compressed graphite.

Dynamic composition determination in heterogeneous ensembles using angular autocorrelation functions as signatures

Signatures of heterogeneous ensembles composed of multiple types of random oriented particles are represented by the angular autocorrelation functions of the systems. These signatures are retrieved by averaging oversampled angular correlation functions calculated from simulated high throughput X-ray diffraction patterns. Using a component signature matrix, the composition of the heterogeneous system will be directly calculated from the signature of the ensemble. Dynamic composition of a heterogeneous ensemble is determined by fluctuation X-ray scattering using the angular autocorrelation functions as signatures.

Pressure induced phase transitions in TiH2

Recent room temperature experiments on TiH2 [Kalita et al., J. Appl. Phys. 108, 043511 (2010)], an important compound in hydrogen storage research, revealed a cubic (fcc, Fm-3m) to tetragonal (bct, I4/mmm) phase transition at around 0.6 GPa, which was suggested to remain stable up to at least 90 GPa. However, the simulated X-ray diffraction (XRD) pattern of the I4/mmm structure cannot explain all the diffraction peaks observed at 90 GPa. In this article, we apply the recently developed particle swarm optimization algorithm for crystal structure prediction to propose that at 63 GPa TiH2 presents a further phase transition from I4/mmm to P4/nmm, which we have also confirmed is dynamically and enthalpically stable up to 294 GPa. Moreover, the XRD patterns and calculated lattice parameters of the P4/nmm structure are in good agreement with the experimental available data. Above 294 GPa, we predict a monoclinic P21/m structure to be stable.

Structures and stabilities of alkaline earth metal peroxides XO2 (X = Ca, Be, Mg) studied by a genetic algorithm

The structures and stabilities of alkaline earth metal peroxides XO2 (X = Ca, Be, Mg) were studied using an adaptive genetic algorithm (GA) for global structure optimization in combination with first-principles calculations. From the adaptive GA search, we obtained an orthorhombic structure for CaO2 with 12 atoms in the unit cell, which is energetically more favorable than the previously proposed structures. Reaction energy of the decomposition CaO2 → CaO + 1/2O2 determined by density functional theory (DFT) calculation shows that this orthorhombic calcium peroxide structure is thermodynamically stable. The simulated X-ray diffraction (XRD) pattern using our predicted structure is in excellent agreement with experimental data. We also show that crystal phase BeO2 is unlikely to exist under normal conditions. MgO2 has a cubic pyrite structure, but it is not stable against decomposition: MgO2 → MgO + 1/2O2.

Analysis of Rietveld Method Application for Gyrolite Crystal Structure Refinement

The several simulated X-ray diffraction patterns for calcium silicate hydrate – gyrolite were calculated with GSAS program using the structural model of natural mineral gyrolite and profile parameters values determined from refining crystal structure of real synthetic gyrolite. To determine the limits of the Rietveld method applicability for synthetic gyrolite crystal structure refinement, each simulated pattern was refined by using a biased starting structural model of gyrolite. The complete and precise refinement of all parameters of gyrolite crystal structure was achieved only using structural restraints on bond lengths in tetrahedral, octahedral and interlayer sheets of silicate. DOI: http://dx.doi.org/10.5755/j01.ms.18.4.3101

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.

Hybrid functional study rationalizes the simple cubic phase of calcium at high pressures

Simple cubic (SC) phase has been long experimentally determined as the high-pressure phase III of elemental calcium (Ca) since 1984. However, recent density functional calculations within semi-local approximation showed that this SC phase is structurally unstable by exhibiting severely imaginary phonons, and is energetically unstable with respect to a theoretical body-centered tetragonal I4(1)/amd structure over the pressure range of phase III. These calculations generated extensive debates on the validity of SC phase. Here we have re-examined the SC structure by performing more precise density functional calculations within hybrid functionals of Heyd-Scuseria-Erhzerhof and PBE0. Our calculations were able to rationalize fundamentally the phase stability of SC structure over all other known phases by evidence of its actual energetic stability above 33 GPa and its intrinsically dynamical stability without showing any imaginary phonons in the entire pressure range studied. We further established that the long-thought theoretical I4(1)/amd structure remains stable in a narrow pressure range before entering SC phase and is actually the structure of experimental Ca-III(') synthesized recently at low temperature 14 K as supported by the excellent agreement between our simulated x-ray diffraction patterns and the experimental data. Our results shed strong light on the crucial role played by the precise electron exchange energy in a proper description of the potential energy of Ca.

Lithium Dihydroborate: First-Principles Structure Prediction of LiBH2

We report a first-principles structure prediction of the LiBH(2), which structures are modeled by using four formula units per unit cell without symmetry restrictions. The computational methodology combines a simulated annealing approach and density functional total energy calculations for crystalline solid structures. The predicted lowest energy structure shows the formation of linear anionic chains, (∞)(1)[BH(2)], enthalpy of formation at 0 K equal to -90.07 kJ/mol. Ring structures, in particular with butterfly and planar square topologies, are found to be stable but well above the ground state by 20.26 and 12.92 kJ/mol, respectively. All convergent structures fall in the symmetry families monoclinic, tetragonal, and orthorhombic. For the representative structures of each family group, simulated X-ray diffraction patterns and infrared spectra are reported.

Structure and properties of the low-density phase ι-Al2O3from first principles

Of the various phases of transition alumina, iota-alumina (\ensuremath{\iota}-Al${}_{2}$O${}_{3}$) is the least well known. It is considered to be the end member of the mullite series in the limit of zero Si content. The structural details of \ensuremath{\iota}-Al${}_{2}$O${}_{3}$ are not available and its physical properties are totally unknown. Based on an appropriately modified structure of a high alumina content mullite phase close to the 9-1 mullite, we have successfully constructed a structural model for \ensuremath{\iota}-Al${}_{2}$O${}_{3}$. The simulated x-ray diffraction (XRD) pattern of this model agrees well with a measured XRD pattern obtained from samples that claim to belong to \ensuremath{\iota}-Al${}_{2}$O${}_{3}$. \ensuremath{\iota}-Al${}_{2}$O${}_{3}$ is a highly disordered ultralow-density phase of alumina with a theoretical density of 2854 kg/m${}^{3}$. The calculated total energy per Al${}_{2}$O${}_{3}$ is much higher than \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$ and the other well-known disordered phase, \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$. Using this theoretically constructed model, we have calculated the elastic, thermodynamic, electronic, and spectroscopic properties of \ensuremath{\iota}-Al${}_{2}$O${}_{3}$ and compared them with those of \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$ and \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$. It is shown that the mechanical strength of \ensuremath{\iota}-Al${}_{2}$O${}_{3}$ is much weaker with bulk and shear moduli that are only 57 and 42% of \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$. Phonon dispersion results and the subsequently calculated thermodynamic properties point to the fact that \ensuremath{\iota}-Al${}_{2}$O${}_{3}$ is an alumina phase preceding \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$ in the processing of alumina before reaching \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$. Electronic structure calculations show it to be an insulator with a direct band gap of only 3.0 eV at the \ensuremath{\Gamma} point and a higher ionic bonding character than \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$ and \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$. It has a calculated static dielectric constant of 2.73 and a corresponding refractive index of 1.65 that are in agreement with reported data. Also calculated are the Al-$K$, Al-${L}_{3}$, and O-$K$ edges of the x-ray absorption near edge structure in \ensuremath{\iota}-Al${}_{2}$O${}_{3}$, showing sensitive dependence on its local bonding environment. However, only the weighted average of these spectra can be directly compared with the measured ones which are currently unavailable.

Novel Superhard Carbon: C-Centered OrthorhombicC8

A novel carbon allotrope of C-centered orthorhombic C(8) (Cco-C(8)) is predicted by using a recently developed particle-swarm optimization method on structural search. Cco-C(8) adopts a sp(3) three-dimensional bonding network that can be viewed as interconnected (2,2) carbon nanotubes through 4- and 6-member rings and is energetically more favorable than earlier proposed carbon polymorphs (e.g., M carbon, bct-C(4), W carbon, and chiral C(6)) over a wide range of pressures studied (0-100 GPa). The simulated x-ray diffraction pattern, density, and bulk modulus of Cco-C(8) are in good accordance with the experimental data on structurally undetermined superhard carbon recovered from cold compression of carbon nanotube bundles. The simulated hardness of Cco-C(8) can reach a remarkably high value of 95.1 GPa, such that it is capable of cracking diamond.

Superconducting high-pressure phase of platinum hydride from first principles

Recently, a superconducting transition was found in silane in high-pressure experiments. However, the experimental x-ray diffraction (XRD) patterns do not match any one of the proposed silane structures, and it has been suggested to be originated from a platinum hydride. Here the underlying physics behind the anomalous superconducting transition in experiment is revealed by first-principles calculations. We predict a stable hexagonal $P{6}_{3}/mmc$ phase of platinum hydride at high pressure. Its lattice constants and simulated XRD pattern are in excellent agreement with the experimental results. The superconducting transition temperatures in this phase are found to agree with the measured values, too. This discovery provides insights into the unusual superconducting behavior reported for silane.

Stacking fault model of ∊-martensite and itsDIFFaXimplementation

Plastic deformation of highly alloyed austenitic transformation-induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of ∊-martensite within the original austenite. The ∊-martensite is often described as a phase having a hexagonal close-packed crystal structure. In this contribution, an alternative structure model is presented that describes ∊-martensite embedded in the austenitic matrixviaclustering of stacking faults in austenite. The applicability of the model was tested on experimental X-ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in theDIFFaXroutine; the faulted austenite and ∊-martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X-ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction.

Low-Temperature Phase Transformation from Graphite tosp3Orthorhombic Carbon

We identify by ab initio calculations an orthorhombic carbon polymorph in Pnma symmetry that has the lowest enthalpy among proposed cold-compressed graphite phases. This new phase contains alternating zigzag and armchair buckled carbon sheets transformed via a one-layer by three-layer slip mechanism. It has a wide indirect band gap and a large bulk modulus that are comparable to those of diamond. Its simulated x-ray diffraction pattern best matches the experimental data. Pressure plays a key role in lowering the kinetic barrier during the phase conversion process. These results provide a comprehensive understanding and an excellent account for experimental findings.

Multiscale Model Predictions of X-Ray Diffraction Patterns in Contracting Skeletal Muscle

In order to explain time-resolved x-ray diffraction data, enabled by recent advances in synchrotron small-angle diffraction instruments, we explored the feasibility of using dynamic 3D models of muscle contraction to predict x-ray diffraction patterns. This approach differs radically from previous attempts, which merely aimed to provide a “best fit” structure for defined quasi-static states, by providing a tool to generate families of structures that evolve in time that explains both the structural (x-ray) and the mechanical data simultaneously. Specifically, we exploit the computational platform MUSICO which was developed originally to model muscle mechanics data, by extending this framework to simulate x-ray diffraction patterns using 3D multiscale models. These models take into account (i) biochemical states of myosin interacting with actin; (ii) rate constants in the actomyosin ATP hydrolysis cycle; (iii) function of myosin molecular motors in a 3D sarcomere lattice; (iv) Ca2+ regulation of myosin binding to actin; (v) extensibility of actin and myosin filaments; and (vi) multiple sarcomeres in series and in parallel. The platform is conceived primarily as a hypothesis-testing tool in which model predictions are tested against the best available mechanical and x-ray diffraction data on the same system. Our preliminary simulations provided dynamic x-ray diffraction patterns during force development and relaxation in skeletal muscle. The simulated patterns generally predicted well changes in repetitive molecular spacings and displayed similarity with experimental data. Once fully developed, this tool will enable extraction of maximum information from the x-ray patterns, in combination with the physiological data, and therefore provide a template to test hypotheses concerning crossbridge and regulatory protein action in working muscle. Our approach can be extended to any muscle system, and it could ultimately provide an interpretive framework for studying the mechanisms of inherited or acquired diseases.

Amorphization and recrystallization study of lithium insertion into manganese dioxide

Various polymorphs of MnO(2) are widely used as electrode materials in Li/MnO(2) batteries. Electrolytic manganese dioxide (EMD) is the most electrochemically active form of MnO(2) and is very difficult to characterize. Their structural details are still largely unknown owing to the poor quality of X-ray diffraction (XRD) patterns obtained from most MnO(2) samples. Simulated amorphisation and crystallization technique was used to derive microstructural models for Li-MnO(2) which included most microstructural details that one would expect to find in the real material. Specifically, pyrolusite-MnO(2), comprising about 25,000 atoms, was amorphised (strain-induced) under molecular dynamics (MD) and different concentrations of lithium ions were inserted. Each system was then crystallized under MD simulation. The resulting models conformed to the pyrolusite polymorph, with microstructural features including: extensive micro-twinning and more general grain-boundaries, stacking faults, dislocations and isolated point defects and defect clusters. Molecular graphical images, showing the atom positions for the microstructural features together with simulated XRD patterns they give rise to, are presented and compared with measured XRD. The calculated XRD are in accord with experiment thus validating the structural models.

Automated Fitting of X-Ray Powder Diffraction Patterns from Interstratified Phyllosilicates

AbstractNEWMOD®, developed by R.C. Reynolds, Jr., has been an important tool for evaluating quantitatively X-ray diffraction (XRD) patterns from interstratified clay minerals for more than 20 years. However, a significant drawback to the NEWMOD® approach is that analyses are done by forward simulation, making results sensitive to user input and starting-model assumptions. In the present study, a reverse-fitting procedure was implemented in a new program, FITMOD, which automatically minimizes the differences between experimental and simulated XRD patterns. The differences are minimized by varying model parameters (such as Reichweite, crystal-size distribution, cation content, type of disorder, etc.) using the downhill simplex method. The downhill simplex method is a non-linear optimization technique for determining minima of functions. This method does not require calculation of the derivatives of the functions being minimized, an important consideration with many of the parameters in NEWMOD- type simulations. Instead, the downhill simplex method calculates pseudo-derivatives by evaluating sufficient points to define a derivative for each independent variable. The performance of FITMOD was evaluated by fitting a series of synthetic XRD patterns generated by NEWMOD+, yielding agreement factors, Rwp, of <0.3%. As long as the correct interstratified system was specified (e.g. illite-smectite), excellent fits were obtained irrespective of the starting parameters for the simulations. FITMOD was also tested using experimental XRD patterns, which gave very good fits, in agreement with previously published results. The optimization routine yields good fits for both synthetic and experimental XRD profiles in a reasonable time, with the possibility of varying all important structural parameters. FITMOD automatically provides optimum fits to experimental XRD data without operator bias, and fitting efficiency and accuracy were, therefore, significantly improved.

The structure of schwertmannite, a nanocrystalline iron oxyhydroxysulfate

Schwertmannite is a poorly crystalline mineral that forms ochre rusts and precipitates in acid mine environments. Despite its ubiquity and its role as scavenger of important contaminants such as arsenic or selenium, its structure has not been yet determined. Here, a structure for schwertmannite is presented based on pair distribution function (PDF) data, X-ray diffraction (XRD) analyses, and density functional theory (DFT) calculations. We propose a structure formed by a deformed frame of iron octahedra similar to that of akaganeite. Simulations of X-ray diffraction patterns unveil the presence of long-range order associated with the position of the sulfate molecules, providing a useful way to discern two types of sulfate complexes in the structure. The simulations suggest that two sulfate molecules per unit cell are present in the structure forming one outer sphere and one inner sphere complex inside the channels formed by iron octahedra. Knowledge of the positions of the sulfates in the structure will help to better understand exchange processes with oxyanions of trace contaminants, such as arsenate, chromate, or selenate, that strongly influence their biogeochemical cycling in mining ecosystems.

Superhard and superconducting structures of BC5

The crystal structures of the synthesized superhard diamondlike BC5 have been extensively explored through ab initio evolutionary algorithm. We uncovered seven intriguing low-energy structures all possessing sp3 hybridizations. After examining the dynamical stability, it is found that two Pmma structures (Pmma-1 and Pmma-2) are energetically more preferable. The simulated x-ray diffraction pattern, Raman modes, and Vickers hardness for Pmma-1 and Pmma-2 structures show remarkable agreement with the experimental data. Electronic and electron–phonon coupling calculations reveal that the two Pmma structures are hole conducting and superconducting with critical temperature ∼11–23 K.