Crystal Structure Prediction Methods Research Articles

Structural, mechanical and electronic properties and hardness of ionic vanadium dihydrides under pressure from first-principles computations

Based on a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we explore the crystal structures of VH2 under the pressure range of 0−300 GPa. The cubic Fm-3m phase with regular VH8 cubes is predicted to transform into orthorhombic Pnma structure with fascinating distorted VH9 tetrakaidecahedrons at 47.36 GPa. Both the Fm-3m phase at 0 GPa and the Pnma phase at 100 GPa are mechanically and dynamically stable, as verified with the calculations of elastic constants and phonon dispersions, respectively. Moreover, the calculated electronic band structure and density of states indicate both stable phases are metallic. Remarkably, the analyses of the Poisson’s ratio, electron localization function (ELF) and Bader charge substantiate that both stable phases are ionic crystals on account of effective charges transferring from V atom to H. On the basis of the microscopic hardness model, the Fm-3m and Pnma crystals of VH2 are potentially incompressible and hard materials with the hardness values of 17.83 and 17.68 GPa, respectively.

Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature

Both helium and ammonia are main components of icy giant planets. While ammonia is very reactive, helium is the most inert element in the universe. It is of great interest whether ammonia and helium can react with each other under planetary conditions, and if so, what kinds of structures and states of matter can form. Here, using crystal structure prediction methods and first-principles calculations, we report three new stable stoichiometries and eight new stable phases of He−NH3 compounds under pressures up to 500 GPa. These structures may exhibit perovskitelike structures for HeNH3 and He2NH3, and a host-guest crystal structure for He(NH3)2. Superionic states are found in all these He−NH3 compounds under high pressures and temperatures in which the hydrogen atoms are diffusive while the nitrogen and helium atoms remain fixed. Such dynamical behavior in helium ammonia compounds is quite different from that in helium water compounds, where weakly interacting helium is more diffusive than stronger bound hydrogen. The low-density host-guest phase of space group I4cm is found to be stable at very low pressures (about 3 GPa) and it enters into a plastic state, characterized by freely rotating ammonia molecules. The present results suggest that plastic or superionic helium ammonia compounds may exist under planetary conditions, and helium contributes crucially to the exotic physics and chemistry observed under extreme conditions.Received 25 November 2019Revised 25 February 2020Accepted 10 March 2020DOI:https://doi.org/10.1103/PhysRevX.10.021007Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCrystal meltingCrystal structureLiquid-solid phase transitionPressure effectsPhysical SystemsCore of giant planetsMolecular solidsTechniquesFirst-principles calculationsMolecular dynamicsCondensed Matter, Materials & Applied Physics

Route to high- Tc superconductivity via CH4 -intercalated H3S hydride perovskites

While exploring potential superconductors in the C-S-H ternary system using first-principles crystal structure prediction methods, we uncovered a class of hydride perovskites based on the intercalation of methane into an ${\mathrm{H}}_{3}\mathrm{S}$ framework. These intriguing ${\mathrm{H}}_{3}\mathrm{S}\text{\ensuremath{-}}{\mathrm{CH}}_{4}$ structures emerge as metastable at $\ensuremath{\sim}100$ GPa. Electron-phonon coupling calculations indicate that phases with ${\mathrm{CSH}}_{7}$ stoichiometry are potential superconductors with ${T}_{c}$ values ranging from 100 K to 190 K at megabar pressures. The results are expected to guide the experimental search for new high-${T}_{c}$ superconductors, including those stable at lower pressures than previously documented superconducting hydrides such as ${\mathrm{H}}_{3}\mathrm{S}$ and ${\mathrm{LaH}}_{10}$.

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-Tc superconductivity. RE trihydrides (REH3) generally undergo a phase transition from ambient P63/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH3 (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH4, YH6, YH9, and CeH9 with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH3 have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH3 up to 220 GPa. Experiments observed the Fm3̅m phase of DyH3 to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH3 recovered under ambient conditions. Our calculations predicted a series of phases for DyH3 at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P63/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH3, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH3 is systematically compared with other REH3 compounds at high pressures. The phase transition pressure in REH3 shows an inverse relation with the ionic radius of RE atoms.

Identifying aspirin polymorphs from combined DFT-based crystal structure prediction and solid-state NMR.

A combined experimental and computational approach was used to distinguish between different polymorphs of the pharmaceutical drug aspirin. This method involves the use of ab initio random structure searching (AIRSS), a density functional theory (DFT)-based crystal structure prediction method for the high-accuracy prediction of polymorphic structures, with DFT calculations of nuclear magnetic resonance (NMR) parameters and solid-state NMR experiments at natural abundance. AIRSS was used to predict the crystal structures of form-I and form-II of aspirin. The root-mean-square deviation between experimental and calculated 1 H chemical shifts was used to identify form-I as the polymorph present in the experimental sample, the selection being successful despite the large similarities between the molecular environments in the crystals of the two polymorphs.

Database of novel magnetic materials for high-performance permanent magnet development

This paper describes the open Novamag database that has been developed for the design of novel Rare-Earth free/lean permanent magnets. Its main features as software technologies, friendly graphical user interface, advanced search mode, plotting tool and available data are explained in detail. Following the philosophy and standards of Materials Genome Initiative, it contains significant results of novel magnetic phases with high magnetocrystalline anisotropy obtained by three computational high-throughput screening approaches based on a crystal structure prediction method using an Adaptive Genetic Algorithm, tetragonally distortion of cubic phases and tuning known phases by doping. Additionally, it also includes theoretical and experimental data about fundamental magnetic material properties such as magnetic moments, magnetocrystalline anisotropy energy, exchange parameters, Curie temperature, domain wall width, exchange stiffness, coercivity and maximum energy product, that can be used in the study and design of new promising high-performance Rare-Earth free/lean permanent magnets. The results therein contained might provide some insights into the ongoing debate about the theoretical performance limits beyond Rare-Earth based magnets. Finally, some general strategies are discussed to design possible experimental routes for exploring most promising theoretical novel materials found in the database.

Rapid Structure Determination of Molecular Solids Using Chemical Shifts Directed by Unambiguous Prior Constraints.

NMR-based crystallography approaches involving the combination of crystal structure prediction methods, ab initio calculated chemical shifts and solid-state NMR experiments are powerful methods for crystal structure determination of microcrystalline powders. However, currently structural information obtained from solid-state NMR is usually included only after a set of candidate crystal structures has already been independently generated, starting from a set of single-molecule conformations. Here, we show with the case of ampicillin that this can lead to failure of structure determination. We propose a crystal structure determination method that includes experimental constraints during conformer selection. In order to overcome the problem that experimental measurements on the crystalline samples are not obviously translatable to restrict the single-molecule conformational space, we propose constraints based on the analysis of absent cross-peaks in solid-state NMR correlation experiments. We show that these absences provide unambiguous structural constraints on both the crystal structure and the gas-phase conformations, and therefore can be used for unambiguous selection. The approach is parametrized on the crystal structure determination of flutamide, flufenamic acid, and cocaine, where we reduce the computational cost by around 50%. Most importantly, the method is then shown to correctly determine the crystal structure of ampicillin, which would have failed using current methods because it adopts a high-energy conformer in its crystal structure. The average positional RMSE on the NMR powder structure is ⟨rav⟩ = 0.176 Å, which corresponds to an average equivalent displacement parameter Ueq = 0.0103 Å2.

Ternary mixed-anion semiconductors with tunable band gaps from machine-learning and crystal structure prediction

We report the computational investigation of a series of ternary X$_4$Y$_2$Z and X$_5$Y$_2$Z$_2$ compounds with X={Mg, Ca, Sr, Ba}, Y={P, As, Sb, Bi}, and Z={S, Se, Te}. The compositions for these materials were predicted through a search guided by machine learning, while the structures were resolved using the minima hopping crystal structure prediction method. Based on $\textit{ab initio}$ calculations, we predict that many of these compounds are thermodynamically stable. In particular, 21 of the X$_4$Y$_2$Z compounds crystallize in a tetragonal structure with $\textit{I-42d}$ symmetry, and exhibit band gaps in the range of 0.3 and 1.8 eV, well suited for various energy applications. We show that several candidate compounds (in particular X$_4$Y$_2$Te and X$_4$Sb$_2$Se) exhibit good photo absorption in the visible range, while others (e.g., Ba$_4$Sb$_2$Se) show excellent thermoelectric performance due to a high power factor and extremely low lattice thermal conductivities.

A New Family of Two-Dimensional Crystals: Open-Framework T3 X ( T = C, Si, Ge, Sn; X = O, S, Se, Te) Compounds with Tetrahedral Bonding.

To accelerate development of innovative materials, their modelings and predictions with useful functionalities are of vital importance. Here, based on a recently developed crystal structure prediction method, we find a new family of stable two-dimensional crystals with an open-channel tetrahedral bonding network, rendering a potential prototype for electronic and energy applications. The proposed structural prototype with a space group of Cmme hosts at least 13 different freestanding T3 X compounds with group IV ( T = C, Si, Ge, Sn) and VI ( X = O, S, Se, Te) elements. Moreover, the proposed materials display diverse electronic properties ranging from direct band gap semiconductor to topological insulator at their pristine forms, which are further tunable by mechanical strain.

Half-metallicity in uranium intermetallics: crystal structure prediction of a high-pressure phase of UCo

Evolutionary crystal structure prediction methods combined with density functional theory (DFT) calculations reveal a high-pressure (hp) phase of the intermetallic compound UCo crystalizing in the NaTl structure type (). We predict this compound to be formed at pressures below 9 GPa. Hp-UCo shows the same structural trends as the two experimentally known pseudo-binary compounds UCo0.2Rh0.8 and UNi0.8Pt0.2. We classify them as ordered solid solutions of a bcc lattice following Vegard’s law. We predict hp-UCo and its adjacent phases UFe and UNi to be itinerant magnets. In the limit of vanishing spin–orbit interactions UFe and hp-UCo are half-metallic ferrimagnets. Spin–orbit coupling generally reduces the spin-polarization at the Fermi level. In case of hp-UCo the decrease depends on the applied DFT functional making the prediction of actual occurrence of half-metallicity problematic. In case of UFe the results are less dependent on the DFT functional. We demonstrate with these calulations that against ‘common sense’ also materials with heavy elements can be interesting candidates for half-metallicity. We highlight that the NaTl structure type should be an interesting candidate for further investigations of half-metallicity.

Topology-based crystal structure generator

Crystal structure prediction is a new dynamically developing field. Two main types of crystal structure prediction methods exist: (1) based on global optimization and (2) based on data mining. It seems promising to hybridize data mining and global optimization techniques. The former generally involve no empirical information and are truly predictive, the latter rely on the databases of existing crystal structures, and are relatively fast, but prone to error, because the databases are far from complete. Furthermore, the theorist’s dream is to be as little dependent on empirical data as possible and be always capable of predicting new structures, not contained in the databases. Here we present an approach to generate an infinite number of crystal structures from a finite set of idealized periodic nets. The resulting structures are highly ordered, possess nontrivial symmetries, and often low energy. Topologically generated structures can be used for initializing evolutionary crystal structure prediction calculations, or on their own — as an extended data mining approach. The efficiency of the proposed approach in both scenarios is confirmed by a series of tests, which we also present here. As an additional enhancement to evolutionary algorithm we introduce a technique for adjusting fractions of variation operators on the fly. Tests show significant performance improvements due to both of these developments.

Control and prediction of the organic solid state: a challenge to theory and experiment†.

The ability of theoretical chemists to quantitatively model the weak forces between organic molecules is being exploited to predict their crystal structures and estimate their physical properties. Evolving crystal structure prediction methods are increasingly being used to aid the design of organic functional materials and provide information about thermodynamically plausible polymorphs of speciality organic materials to aid, for example, pharmaceutical development. However, the increasingly sophisticated experimental studies for detecting the range of organic solid-state behaviours provide many challenges for improving quantitative theories that form the basis for the computer modelling. It is challenging to calculate the relative thermodynamic stability of different organic crystal structures, let alone understand the kinetic effects that determine which polymorphs can be observed and are practically important. However, collaborations between experiment and theory are reaching the stage of devising experiments to target the first crystallization of new polymorphs or create novel organic molecular materials.

New potentials for accurate and efficient ab initio crystal structure prediction methods

New potentials for accurate and efficient <i>ab initio</i> crystal structure prediction methods

Structures and magnetic properties of iron silicide from adaptive genetic algorithm and first-principles calculations

We performed a systematic search for low-energy structures of binary iron silicide over a wide range of compositions using the crystal structure prediction method based on adaptive genetic algorithm. 36 structures with formation energies within 50 meV/atom (11 of them are within 20 meV) above the convex hull formed by experimentally known stable structures are predicted. Magnetic properties of these low-energy structures are investigated. Some of these structures can be promising candidates for rare-earth-free permanent magnet.

Successful Computationally Directed Templating of Metastable Pharmaceutical Polymorphs

A strategy of using crystal structure prediction (CSP) methods to determine which, if any, isostructural template could facilitate the first crystallization of a predicted polymorph by vapor deposition is extended to the fenamate family. Mefenamic acid (MFA) and tolfenamic acid (TFA) are used as molecules with minimal chemical differences, whereas flufenamic acid (FFA) shows greater differences in the substituents. The three crystal energy landscapes were calculated, and periodic electronic structure calculations were used to confirm the thermodynamic plausibility of possible isostructural polymorphs to experimentally obtainable crystals of the other molecules. As predicted, a new polymorph, TFA form VI, was found by sublimation onto isomorphous MFA form I, using a recently developed technique. MFA and TFA form a continuous solid solution with the structure of MFA I and TFA VI at the limits, but the isomorphous MFA/FFA solid solution does not extended to a new polymorph of FFA. The novel solid solution st...

Novel high-pressure calcium carbonates

Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using the evolutionary crystal structure prediction method uspex, we systematically explore the $\mathrm{MgO}\text{\ensuremath{-}}{\mathrm{CO}}_{2}$ and $\mathrm{CaO}\text{\ensuremath{-}}{\mathrm{CO}}_{2}$ systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While ${\mathrm{MgCO}}_{3}$ is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known ${\mathrm{CaCO}}_{3}$, and previously unknown ${\mathrm{Ca}}_{3}{\mathrm{CO}}_{5}$ and ${\mathrm{CaC}}_{2}{\mathrm{O}}_{5}$. ${\mathrm{Ca}}_{3}{\mathrm{CO}}_{5}$ polymorphs are found to contain isolated orthocarbonate ${{\mathrm{CO}}_{4}}^{4\ensuremath{-}}$ tetrahedra, and are stable at relatively low pressures $(&gt;11\phantom{\rule{0.16em}{0ex}}\mathrm{GPa})$, whereas ${\mathrm{CaC}}_{2}{\mathrm{O}}_{5}$ is stable above 33 GPa and its polymorphs feature polymeric motifs made of ${\mathrm{CO}}_{4}$ tetrahedra. Detailed analysis of the chemical stability of ${\mathrm{CaCO}}_{3},{\mathrm{Ca}}_{3}{\mathrm{CO}}_{5}$, and ${\mathrm{CaC}}_{2}{\mathrm{O}}_{5}$ in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. Instead, ${\mathrm{MgCO}}_{3}$ is the main host of oxidized carbon throughout the lower mantle.

Fine-grained optimization method for crystal structure prediction

Crystal structure prediction based on first-principles calculations is often achieved by applying relaxation to randomly generated initial structures. Relaxing a structure requires multiple optimization steps. It is time consuming to fully relax all the initial structures, but it is difficult to figure out which initial structure leads to the optimal solution in advance. In this paper, we propose a optimization method for crystal structure prediction, called Look Ahead based on Quadratic Approximation, that optimally assigns optimization steps to each candidate structure. It allows us to identify the most stable structure with a minimum number of total local optimization steps. Our simulations using known systems Si, NaCl, Y2Co17, Al2O3, and GaAs showed that the computational cost can be reduced significantly compared to random search. This method can be applied for controlling all kinds of local optimizations based on first-principles calculations to obtain best results under restricted computational resources.

Near-Ideal Xylene Selectivity in Adaptive Molecular Pillar[ n]arene Crystals.

The energy-efficient separation of alkylaromatic compounds is a major industrial sustainability challenge. The use of selectively porous extended frameworks, such as zeolites or metal–organic frameworks, is one solution to this problem. Here, we studied a flexible molecular material, perethylated pillar[n]arene crystals (n = 5, 6), which can be used to separate C8 alkylaromatic compounds. Pillar[6]arene is shown to separate para-xylene from its structural isomers, meta-xylene and ortho-xylene, with 90% specificity in the solid state. Selectivity is an intrinsic property of the pillar[6]arene host, with the flexible pillar[6]arene cavities adapting during adsorption thus enabling preferential adsorption of para-xylene in the solid state. The flexibility of pillar[6]arene as a solid sorbent is rationalized using molecular conformer searches and crystal structure prediction (CSP) combined with comprehensive characterization by X-ray diffraction and 13C solid-state NMR spectroscopy. The CSP study, which takes into account the structural variability of pillar[6]arene, breaks new ground in its own right and showcases the feasibility of applying CSP methods to understand and ultimately to predict the behavior of soft, adaptive molecular crystals.

Theoretical exploring the mechanical and electrical properties of tI12-B6C4O2

Utilizing the crystal structure prediction method (CALYPSO), a tetragonal BCO compound (tI12-B6C4O2 with I4¯m2 symmetric structure) was predicted. Computed formation enthalpies, elastic constants and phonon dispersion spectra certify that tI12-B6C4O2 is thermodynamically, dynamically and mechanically stable. Our results indicate that tI12-B6C4O2 has large mechanical moduli and high hardness (21.9 GPa). The directional dependences of the Young’s modulus, shear modulus and Poisson’s ratio have been visualized to analysis the mechanical anisotropy. The calculated band structure and partial density of state revealed that tI12-B6C4O2 is a typical for conductor with sp3 hybrid BC and BO covalent bonds.

First principles study of LiAlO2: new dense monoclinic phase under high pressure

In this work, we have systematically explored the crystal structures of LiAlO2 at high pressures using crystal structure prediction method in combination with the density functional theory calculations. Besides the reported α, β, γ, δ and ε-phases, here we propose a new monoclinic ζ-LiAlO2 (C2/m) structure, which becomes thermodynamically and dynamically stable above 27 GPa. It is found that the cation coordination number increases from 4 to 6 under compression. Consisting of the compact {LiO6} and {AlO6} octahedrons, the newly-discovered ζ-phase possesses a very high density. Further electronic calculations show that LiAlO2 is still an insulator up to 60 GPa, and its bandgap increases upon compression. The present study advances our understanding on the crystal structures and high-pressure phase transitions of LiAlO2 that may trigger applications in multiple areas of industry and provoke more related basic science research.