Search For Crystal Structures Research Articles

Crystal structure search with principal invariants

In this study, we propose a general way of finding a new crystal structure starting from a database. We also introduce a concise representation of the data based on principal invariants derived from the spatial arrangement of atoms in the crystal structure. Since the selection of the present representation does not depend on the size and selection method of the unit-cell defining the crystal structure, it is ideal for measuring similarity between two arbitrarily different crystal structures. For a more general search for crystal structures, the known crystal structure space is better defined using a variational autoencoder. Furthermore, we used either Pearson's correlation or the square root of the Jensen-Shannon divergence to directly search similar crystal structures for a wide variety of space groups. Our novel method uses extended database information, the similarity measure, and first-principles electronic structure calculation to predict several new metastable crystal structures of SnSe, P, and SnS.

Large‐Scale Synthesis and Applications of Hafnium–Tantalum Carbides

AbstractComprehensive theoretical and experimental studies are performed to discover a new way of synthesis of HfTaC coatings. Here, an evolutionary search for stable crystal structures in the ternary HfTaC system with subsequent selective large‐scale experimental synthesis of coatings using a unique plasma dynamic experimental setup is performed. Optimization of the experimental process allows us to perform selective synthesis of coatings made of hafnium–tantalum carbides with predefined stoichiometry, crystal structure, and properties. Along with more than 70 compounds, the HfTaC system belongs to ternary and quaternary carbides of group IV and V transition metals, and this study opens the door to synthesis of a large number of functional coatings composed of other carbides including high‐entropy carbides.

High-Pressure Mg–Sc–H Phase Diagram and Its Superconductivity from First-Principles Calculations

In this work, global search for crystal structures of ternary Mg-Sc-H hydrides (Mg$_x$Sc$_y$H$_z$) under high pressure ($100 \le P \le 200$ GPa) were performed using the evolutionary algorithm and first-principles calculations. Based on them, we computed the thermodynamic convex hull and pressure-dependent phase diagram of Mg$_x$Sc$_y$H$_z$ for $z/(x+y) < 4$. We have identified the stable crystal structures of four thermodynamically stable compounds with the higher hydrogen content, i.e., $R\bar{3}m$-MgScH$_{6}$, $C2/m$-Mg$_{2}$ScH$_{10}$, $Immm$-MgSc$_{2}$H$_{9}$ and $Pm\bar{3}m$-Mg(ScH$_{4}$)$_{3}$. Their superconducting transition temperatures were computationally predicted by the McMillan-Allen-Dynes formula combined with first-principles phonon calculations. They were found to exhibit superconductivity; among them, $R\bar{3}m$-MgScH$_{6}$ was predicted to have the highest $T_{c}$ (i.e. 23.34 K) at 200 GPa.

Generating Cocrystal Polymorphs with Information Entropy Driven by Molecular Dynamics-Based Enhanced Sampling.

Predicting structures of organic molecular cocrystals is a challenging task when considering the immense number of possible intermolecular orientations. Use of the Shannon information entropy, constructed from an intermolecular orientational spatial distribution function, to drive a search for crystal structures via enhanced molecular dynamics can be an efficient way to map out a landscape of putative polymorphs. Here, the Shannon entropy is used to generate a set of collective variables for differentiating polymorphs of a 1:1 cocrystal of resorcinol and urea. We show that driven adiabatic free energy dynamics, a particular enhanced-sampling approach, combined with these entropy variables, can transform the stable phase into alternate polymorphs. Density functional theory calculations confirm that a structure obtained from the enhanced molecular dynamics is stable at pressures above 1 GPa. We thus show that enhanced sampling should be considered an integral component of crystal structure searching protocols for systems with multiple independent molecules.

Materials informatics based on evolutionary algorithms: Application to search for superconducting hydrogen compounds

We present materials informatics approach to search for superconducting hydrogen compounds, which is based on a genetic algorithm and a genetic programming. This method consists of four stages: (i) search for stable crystal structures of materials by a genetic algorithm, (ii) collection of physical and chemical property data by first-principles calculations, (iii) development of superconductivity predictor based on the database by a genetic programming, and (iv) discovery of potential candidates by regression analysis. By repeatedly performing the process as (i) $\rightarrow$ (ii) $\rightarrow$ (iii) $\rightarrow$ (iv) $\rightarrow$ (i) $\rightarrow$ $\dots$, the superconductivity of the discovered candidates is validated by first-principles calculations, and the database and predictor are further improved, which leads to an efficient search for superconducting materials. We applied this method to hypothetical ternary hydrogen compounds and predicted KScH$_{12}$ with a modulated hydrogen cage showing the superconducting critical temperature of 122 K at 300 GPa and GaAsH$_{6}$ showing 98 K at 180 GPa.

The Symmetric and Topological Code of the Cluster Self-Assembly of the Icosahedral Structure of (Rb13)(Rb2O)3 (Fm-3c, cF184) Metal Oxide

A search for crystal structures of A n O m metal oxides containing icosahedral i–A@A12 cluster precursors is performed (TOPOS program package, ICSD and CRYSTMET databases). Among 1802 metal oxides, the local region represented by i–Cs@Cs12 is determined in the Cs7O (P-6m2) metal oxide. In the case of the (Rb13)(Rb2O)3 (Fm-3c, cF184, V = 12409.8 A3) metal oxide, the i–Rb@Rb12 cluster precursor with the symmetry m-3 and cluster spacers in the form of Rb–O–Rb chains, which occupy the pores in the threedimensional framework, are identified. Cluster i–Rb@Rb12 occupies position 8b with the highest possible crystallographic symmetry m-3 for icosahedron. The topological type of the basic 3D network, which characterizes the packing of cluster precursors Rb13, corresponds to a simple cubic 3D network P c (Pm-3m, cP1) with CN = 6. The symmetric and topological codes of the self-assembly processes of the crystal structure from the nanocluster precursors S 3 0 is fully reconstructed in the following form: primary chain S 3 1 → microlayer S 3 2 → microframework S 3 3 . Cluster precursors in the primary chain are flipped through 90° and are characterized by the maximum possible number of Rb–Rb bonds corresponding to 8 and this mechanism of local binding is realized at all stages of the self-assembly of the 3D framework structure. During the assembly of the primary chain and microlayer, there is additional binding of the Rb@Rb12 icosahedra via the Rb atoms of three-atomic cluster spacers Rb–O–Rb. In the 3D framework structure, in the local environment of Rb@Rb12, there are 12 Rb–O–Rb cluster spacers.

Stochastic generation of complex crystal structures combining group and graph theory with application to carbon

A method is introduced to stochastically generate crystal structures with defined structural characteristics. Reasonable quotient graphs for symmetric crystals are constructed using a random strategy combined with space group and graph theory. Our algorithm enables the search for large-size and complex crystal structures with a specified connectivity, such as threefold ${\mathrm{sp}}^{2}$ carbons, fourfold ${\mathrm{sp}}^{3}$ carbons, as well as mixed ${\mathrm{sp}}^{2}\text{\ensuremath{-}}{\mathrm{sp}}^{3}$ carbons. To demonstrate the method, we randomly construct initial structures adhering to space groups from 75 to 230 and a range of lattice constants, and we identify 281 new ${\mathrm{sp}}^{3}$ carbon crystals. First-principles optimization of these structures show that most of them are dynamically and mechanically stable and are energetically comparable to those previously proposed. Some of the new structures can be considered as candidates to explain the experimental cold compression of graphite.

Discovery of zirconium dioxides for the design of better oxygen-ion conductors using efficient algorithms beyond data mining.

It is important to find crystal structures with low formation (Ev) and migration-barrier (Em) energies for oxygen vacancies for the development of fast oxygen-ion conductors. To identify crystal structures with lower Ev and Em than those of ground-state ZrO2, we first reoptimize the crystal structures of various oxides reported in the database, and then directly construct them using an evolutionary algorithm. For efficient searching, we employ the linearized ridge regression model for Ev using descriptors obtained from density functional theory calculations of the unit cells and apply the predicted Ev as a fitness value in the evolutionary algorithm. We also find a correlation between the Ev and Em for the crystal structures of ZrO2. On the basis of this correlation, we confirm that the newly constructed crystal structures, as well as certain reoptimized structures from the database, that possess low Ev also have Em lower than that of ground-state ZrO2. Our successful strategy consisting of a combination of the evolutionary algorithm, first-principles calculations, and machine-learning techniques may be applicable to other oxide systems in finding crystal structures with low Ev and Em.

Global search for low-lying crystal structures using the artificial force induced reaction method: A case study on carbon

We propose an approach to perform the global search for low-lying crystal structures from first principles, by combining the artificial force induced reaction (AFIR) method and the periodic boundary conditions (PBCs). The AFIR method has been applied extensively to molecular systems to elucidate the mechanism of chemical reactions such as homogeneous catalysis. The present PBC/AFIR approach found 274 local minima for carbon crystals in the ${\mathrm{C}}_{8}$ unit cell described by the generalized gradient approximation--Perdew-Burke-Ernzerhof functional. Among many newly predicted structures, three low-lying structures, which exhibit somewhat higher energy compared with those previously predicted, such as $\mathrm{Cco}\ensuremath{-}{\mathrm{C}}_{8}$ ($Z$-carbon) and $M$-carbon, are further discussed with calculations of phonon and band dispersion curves. Furthermore, approaches to systematically explore two- or one-dimensional periodic structures are proposed and applied to the ${\mathrm{C}}_{8}$ unit cell with the slab model. These results suggest that the present approach is highly promising for predicting crystal structures.

Theoretical search for possible Au-Si crystal structures using a genetic algorithm

We performed a global search for possible Au-Si crystal structures using a genetic algorithm (GA) combined with density functional theory (DFT) calculations. Two Au-Si structures, $\mathrm{A}{\mathrm{u}}_{8}\mathrm{S}{\mathrm{i}}_{8}$ and $\mathrm{A}{\mathrm{u}}_{16}\mathrm{S}{\mathrm{i}}_{8}$, were found to be energetically stable and have no imaginary frequencies by phonon calculations. The formation energies of all the studied structures gave a convex hull of the Au-Si system, showing that the most stable composition was $\mathrm{Au}\ensuremath{-}\mathrm{Si}=1:1$ and that the Si-rich structures were much less stable than the Au-rich ones.

CRYSTAL CHEMISTRY OF THREE-COMPONENT WHITE DWARFS AND NEUTRON STAR CRUSTS: PHASE STABILITY, PHASE STRATIFICATION, AND PHYSICAL PROPERTIES

ABSTRACT A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are “bred” by a genetic algorithm and optimized at constant pressure under the assumption of linear response (Thomas–Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not previously predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low-density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase-layering diagram and mass–radius-composition dependence, both of which are reported for He–C–O and C–O–Ne white dwarfs. Finite thickness interfacial phases (“interphases”) show up at the boundaries between single-component body-centered cubic (bcc) crystalline regions, some of which have lower lattice symmetry than cubic. A second application—quasi-static settling of heavy nuclei in white dwarfs—builds on our equilibrium phase-layering method. Tests of this nonequilibrium method reveal extra phases that play the role of transient host phases for the settling species.

Exsolution Process as a Route toward Extremely Low Thermal Conductivity in Cu12Sb4–xTexS13 Tetrahedrites

Achieving extremely low lattice thermal conductivity is an essential requirement for improving the performance of thermoelectric materials. Engineered nanostructures in bulk materials and the search for complex crystal structures that inherently poorly conduct heat are the two main areas of research being currently pursued to achieve this objective. Tetrahedrites, a class of widely studied minerals, show intrinsically very low thermal conductivity values on the order of 0.5 W·m–1·K–1 at 300 K, leading to interesting thermoelectric properties around 700 K. Here, we report on the low-temperature transport properties of a series of synthetic tetrahedrites Cu12Sb4–xTexS13 and demonstrate that, at low Te concentrations, an exsolution process sets in near 250 K, resulting in the coexistence of two tetrahedrite phases, likely at the submicrometer length scale. Remarkably, this mechanism triggers a significant reduction of ∼40% in thermal conductivity, which drops to 0.25 W·m–1·K–1 below 200 K. This exceptionally...

Carbonic Acid: From Polyamorphism to Polymorphism

Layers of glassy methanolic (aqueous) solutions of KHCO3 and HCl were deposited sequentially at 78 K on a CsI window, and their reaction on heating in vacuo in steps from 78 to 230 K was followed by Fourier transform infrared (FTIR) spectroscopy. After removal of solvent and excess HCl, IR spectra revealed formation of two distinct states of amorphous carbonic acid (H2CO3), depending on whether KHCO3 and HCl had been dissolved in methanol or in water, and of their phase transition to either crystalline alpha- or beta-H2CO3. The main spectral features in the IR spectra of alpha- and beta-H2CO3 are observable already in those of the two amorphous H2CO3 forms. This indicates that H-bond connectivity or conformational state in the two crystalline phases is on the whole already developed in the two amorphous forms. The amorphous nature of the precursors to the two crystalline polymorphs is confirmed using powder X-ray diffraction. These diffractograms also show that alpha- and beta-amorphous H2CO3 are two distinct structural states. The variety of structural motifs found within a few kJ/mol in a computational search for possible crystal structures provides a plausible rationalization for (a) the observation of more than one amorphous form and (b) the retention of the motif observed in the amorphous form in the corresponding crystalline form. The polyamorphism inferred for carbonic acid from our FTIR spectroscopic and powder X-ray diffraction studies is special since two different crystalline states are linked to two distinct amorphous states. We surmise that the two amorphous states of H2CO3 are connected by a first-order-like phase transition.

The Discovery of New Crystal Forms of 5-Fluorocytosine Consistent with the Results of Computational Crystal Structure Prediction

A computational search for low-energy crystal structures of 5-fluorocytosine was performed in conjunction with a manual crystallization screen to determine whether solid forms other than the known monohydrate structure could be discovered. The predicted low-energy structures were based on a hydrogen-bonded ribbon, which was observed in two newly determined anhydrous polymorphic crystal structures, a hemipentahydrate, and two solvates, as well as the known monohydrate. An alternative ribbon structure present in some less-stable predicted structures was found in a disordered monohydrate structure. The anhydrous crystal structure prediction was a success, by the criteria of the international crystal structure prediction blind tests. Thus we can rationalize the crystal structure behavior of 5-fluorocytosine as having a strongly preferred two-dimensional ribbon structure, which exhibits versatile methods of packing, leading to polymorphism and a number of closely related solvate structures.

On the lack of hydrogen bonds in the crystal structure of alloxan

The crystal structure of alloxan ((CO) 3.NH.CO.NH) is unusual in that it contains no hydrogen bonds. We account for this by using a realistic model for the intermolecular forces which includes a distributed multipole electrostatic model, to find minimum energy dimer and crystal structures. Alloxan dimer structures with close (CO) … (CO) interactions are predicted to have similar energies to the hydrogen bonded dimers. An extensive systematic search for possible crystal structures for alloxan found the observed crystal packing to have the lowest lattice energy and hydrogen bonded polymorphs to be less stable by more than 5 kJ mol −1. This illustrates the limitations of the functional group approach to predicting molecular crystal structures.