Crystal Structure Search Research Articles

Ground state and phase stability of GaP-ZnS and GaP-ZnSe quaternary systems from first-principles

Quaternary alloys systems with large direct bandgap are promising for electronic devices and energy applications. Here we performed a crystal structure search for GaP-ZnS and GaP-ZnSe quaternary alloys using an ab initio evolutionary algorithm. The analysis of structures reveals the existence of trigonal (R3m) phases for GaPZnS and GaPZnSe alloys, which are thermodynamically slightly unstable, but dynamically and mechanically stable. State-of-the-art local density approximation half-occupation (LDA-1/2) approach divulges that both materials exhibit wide band-gap character (EG(x)∼2.78eV, and 2.48eV for GaPZnS and GaPZnSe respectively). Moreover, GaPZnS is found to be indirect; whereas GaPZnSe evinces a quasi-direct band-gap character. Besides, we predict a relatively weak band-gap bowing of 0.84eV for GaPZnS and 0.96eV for GaPZnSe quaternary alloys.

Tutorial: Systematic development of polynomial machine learning potentials for elemental and alloy systems

Machine learning potentials (MLPs) developed from extensive datasets constructed from density functional theory calculations have become increasingly appealing to many researchers. This paper presents a framework of polynomial-based MLPs, called polynomial MLPs. The systematic development of accurate and computationally efficient polynomial MLPs for many elemental and binary alloy systems and their predictive powers for various properties are also demonstrated. Consequently, many polynomial MLPs are available in a repository website [A. Seko, Polynomial Machine Learning Potential Repository at Kyoto University, https://sekocha.github.io]. The repository will help many scientists perform accurate and efficient large-scale atomistic simulations and crystal structure searches.

A noise-robust data assimilation method for crystal structure determination using powder diffraction intensity.

Crystal structure prediction for a given chemical composition has long been a challenge in condensed-matter science. We have recently shown that experimental powder x-ray diffraction (XRD) data are helpful in a crystal structure search using simulated annealing, even when they are insufficient for structure determination by themselves [Tsujimoto et al., Phys. Rev. Mater. 2, 053801 (2018)]. In the method, the XRD data are assimilated into the simulation by adding a penalty function to the physical potential energy, where a crystallinity-type penalty function, defined by the difference between experimental and simulated diffraction angles was used. To improve the success rate and noise robustness, we introduce a correlation-coefficient-type penalty function adaptable to XRD data with significant experimental noise. We apply the new penalty function to SiO2 coesite and ɛ-Zn(OH)2 to determine its effectiveness in the data assimilation method.

Predicted superconductivity and superionic state in the electride Li5N under high pressure

Recently, electrides have received increasing attention due to their multifunctional properties as superconducting, catalytic, insulating, and electrode materials, with potential to offer other performance and possess novel physical states. This work uncovers that Li5N as an electride possess four novel physical states simultaneously: electride state, super-coordinated state, superconducting state, and superionic state. By obtaining high-pressure phase diagrams of the Li–N system at 150–350 GPa using a crystal structure search algorithm, we find that Li5N can remain stable as P6/mmm structure and has a 14-fold super-coordination number, as verified by Bader charge and electron localization function analysis. Its superconducting transition temperature reaches the highest at 150 GPa (T c = 48.97 K). Besides, Li5N exhibits the superionic state at 3000 K, in which N atoms act like solid, while some Li atoms flow like liquid. The above results are further verified at a macroscopic level by using deep learning potential molecular dynamics simulations.

Theoretical Study on the Vapochromic Ni(II)-Quinonoid Complex: One-Dimensional Stacking Structure-Based Color Switching.

Vapochromic crystals of Ni(II)-quinonoid complexes were theoretically investigated using density functional theory (DFT) calculations. Kato et al. previously reported that the purple crystals of a four-coordinate Ni(II)-quinonoid complex (1P) exhibited vapochromic characteristics upon exposure to methanol gas, resulting in orange crystals of the six-coordinate methanol-bound complex (1O) [Angew. Chem., Int. Ed.2017, 56, 2345-2349]. However, the authors did not characterize the crystal structure of 1P. In the present study, we computationally predicted the crystal structure of 1P by performing a crystal structure search with classical force-field computations followed by optimization using DFT calculations. The simulated powder X-ray diffraction pattern of the DFT-optimized structure agreed with experimental observations, indicating that our predicted crystal structure is reliable. Investigation of the optimized crystal structure of 1P revealed that its color change arose from changes in its 1D-band structure, which consists of Ni 3d orbitals and quinonoid π-orbitals. Intermolecular interactions were weakened upon the binding of methanol to the Ni(II) center in 1O. Consequently, the intermolecular 3d-π interaction in 1P lowered the band gap and induced the red-shifting of the monomeric four-coordinate Ni(II)-quinonoid complex. Meanwhile, the obtained absorption spectrum of 1O closely corresponded to that of the monomeric six-coordinate Ni(II)-quinonoid complex. Our study provides a new strategy for accurately predicting molecular crystal structures and reveals a new insight into vapochromism based on band structure color switching.

A Novel Two-Dimensional ZnSiP2 Monolayer as an Anode Material for K-Ion Batteries and NO2 Gas Sensing.

Using the crystal-structure search technique and first-principles calculation, we report a new two-dimensional semiconductor, ZnSiP2, which was found to be stable by phonon, molecular-dynamic, and elastic-moduli simulations. ZnSiP2 has an indirect band gap of 1.79 eV and exhibits an anisotropic character mechanically. Here, we investigated the ZnSiP2 monolayer as an anode material for K-ion batteries and gas sensing for the adsorption of CO, CO2, SO2, NO, NO2, and NH3 gas molecules. Our calculations show that the ZnSiP2 monolayer possesses a theoretical capacity of 517 mAh/g for K ions and an ultralow diffusion barrier of 0.12 eV. Importantly, the ZnSiP2 monolayer exhibits metallic behavior after the adsorption of the K-atom layer, which provides better conductivity in a period of the battery cycle. In addition, the results show that the ZnSiP2 monolayer is highly sensitive and selective to NO2 gas molecules.

As-Li electrides under high pressure: Superconductivity, plastic, and superionic states

Inorganic electrides are a new class of compounds catering to the interest of scientists due to the multiple usages exhibited by interstitial electrons in the lattice. However, the influence of the shape and distribution of interstitial electrons on physical properties and new forms of physical states is still unknown. In this work, crystal structure search algorithms are employed to explore the possibility of forming unique electrides in the As-Li system, where interstitial electrons behave as one-dimensional (1D) electron chains (1D electride) in the $Pmmm$ phase of $\mathrm{As}{\mathrm{Li}}_{7}$ and transform into zero-dimensional (0D) electron clusters (0D electride) in the $P6/mmm$ phase at 80 GPa. The $P6/mmm$ phase has relatively high superconductivity at 150 GPa $({T}_{c}=38.4\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ compared to classical electrides, even at moderate pressure with ${T}_{c}=16.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition, a Dirac cone in the band has been observed, expanding the sources of Dirac materials. The survival of $\mathrm{As}{\mathrm{Li}}_{7}$ at room temperature is confirmed by molecular dynamics simulation at 300 K. At 1000 K, the As atoms in the system act like a solid, while a portion of the Li atoms cycle around the As atoms, and another portion of the Li atoms flow freely like a liquid, showing the unusual physical phenomenon of the coexistence of the plastic and superionic states. This suggests that the superionic and plastic states cannot only be found in hydrides but also in the electride. Our results indicate that superconducting electride $\mathrm{As}{\mathrm{Li}}_{7}$ with superionic and plastic states can exist in Earth's interior.

Stability and electronic properties of five new ternary tantalum carbonitrides

The crystal structures, mechanical properties, and electronic structures of tantalum carbonitride TaxCyNz have been predicted via crystal structure search combined with the first-principles calculations. Ground-state structures of five stoichiometric TaxCyNz (Ta2CN, Ta3C2N, Ta3CN2, Ta4C3N, and Ta4CN3) were first uncovered, and their stabilities were confirmed by the elastic constants and phonon dispersions calculations. The mechanical properties of these five new phases were then systematically studied. The hardness calculations suggested that Ta4C3N possesses the largest hardness (which reaches a value up to 25.8 GPa) among these five compositions. The metallic nature and the atomic bonding behavior were evaluated based on the electronic structures. Furthermore, thermodynamic assessment established that these designed phases can be synthesized at readily attainable pressures.

Pressure-induced phase transitions and mechanical properties of ternary nanolaminated carbide Mo2Ga2C from first-principles calculations

The newly synthesized ternary carbide Mo2Ga2C (P63/mmc, Z = 2) with novel double-Ga layers, the firstly reported member of the well-known MAX-like phases, is stimulating extensive research interests. Here, motivated by the recent experimental and theoretical works, the high-pressure structure evolution behavior of Mo2Ga2C up to 150 GPa has been extensively investigated using an efficient crystal structure search approach combined with first-principles calculations. Besides the experimental high-pressure trigonal P-3m1 phase, a new orthorhombic Amm2 phase was firstly identified and was confirmed to be dynamically stable above 90 GPa. The occurrence of this Amm2 phase follows the strong twist of CMo6 octahedrons in P-3m1 phase and the coordination number of Mo increases from six to seven under compression. Pressure-induced phase transition from P63/mmc to P-3m1 at 22 GPa and P-3m1 to Amm2 at 90 GPa was characterized as first order with a volume reduction of 2.3% and 2.5%, respectively. Inspections of electronic and crystal structures suggested an enhanced Ga–Ga covalent hybridization in the predicted Amm2 phase, which becomes much less compressible under compression. The mechanical parameters and elastic anisotropy behaviors of both P63/mmc and P-3m1 phases were then systematically studied, and the (101¯0)[1¯21¯0] directions were found to be their possible dislocations slip system.

Prediction of Above-Room-Temperature Superconductivity in Lanthanide/Actinide Extreme Superhydrides.

Achieving room-temperature superconductivity has been an enduring scientific pursuit driven by broad fundamental interest and enticing potential applications. The recent discovery of high-pressure clathrate superhydride LaH10 with superconducting critical temperatures (Tc) of 250-260 K made it tantalizingly close to realizing this long-sought goal. Here, we report a remarkable finding based on an advanced crystal structure search method of a new class of extremely hydrogen-rich clathrate superhydride MH18 (M: rare-earth/actinide atom) stoichiometric compounds stabilized at an experimentally accessible pressure of 350 GPa. These compounds are predicted to host Tc up to 330 K, which is well above room temperature. The bonding and electronic properties of these MH18 clathrate superhydrides closely resemble those of atomic metallic hydrogen, giving rise to the highest Tc hitherto found in a thermodynamically stable hydride compound. An in-depth study of these extreme superhydrides offers insights for elucidating phonon-mediated superconductivity above room temperature in hydrogen-rich and other low-Z materials.

High-energy-density metal nitrides with armchair chains

Polymeric nitrogen has attracted much attention owing to its possible application as an environmentally safe high-energy-density material. Based on a crystal structure search method accelerated by the use of machine learning and graph theory and on first-principles calculations, we predict a series of metal nitrides with chain-like polynitrogen (P21-AlN6, P21-GaN6, P-1-YN6, and P4/mnc-TiN8), all of which are estimated to be energetically stable below 40.8 GPa. Phonon calculations and ab initio molecular dynamics simulations at finite temperature suggest that these nitrides are dynamically stable. We find that the nitrogen in these metal nitrides can polymerize into two types of poly-N42− chains, in which the π electrons are either extended or localized. Owing to the presence of the polymerized N4 chains, these metal nitrides can store a large amount of chemical energy, which is estimated to range from 4.50 to 2.71 kJ/g. Moreover, these compounds have high detonation pressures and detonation velocities, exceeding those of conventional explosives such as TNT and HMX.

Superionic Silica-Water and Silica-Hydrogen Compounds in the Deep Interiors of Uranus and Neptune.

Silica, water, and hydrogen are known to be the major components of celestial bodies, and have significant influence on the formation and evolution of giant planets, such as Uranus and Neptune. Thus, it is of fundamental importance to investigate their states and possible reactions under the planetary conditions. Here, using advanced crystal structure searches and first-principles calculations in the Si-O-H system, we find that a silica-water compound (SiO_{2})_{2}(H_{2}O) and a silica-hydrogen compound SiO_{2}H_{2} can exist under high pressures above 450 and 650GPa, respectively. Further simulations reveal that, at high pressure and high temperature conditions corresponding to the interiors of Uranus and Neptune, these compounds exhibit superionic behavior, in which protons diffuse freely like liquid while the silicon and oxygen framework is fixed as solid. Therefore, these superionic silica-water and silica-hydrogen compounds could be regarded as important components of the deep mantle or core of giants, which also provides an alternative origin for their anomalous magnetic fields. These unexpected physical and chemical properties of the most common natural materials at high pressure offer key clues to understand some abstruse issues including demixing and erosion of the core in giant planets, and shed light on building reliable models for solar giants and exoplanets.

The new stable phases of Fe2Pd crystal alloy and their properties

This work presents a combination of particle swarm optimization crystal structure search method and first-principles calculations to investigate new stable crystal structures of Fe2Pd. Three structurally stable tetragonal phases (with space groups I4/mmm, P4/mmm, P4/nmm) and a structurally stable cubic phase (C15 Laves phase (Fd3¯m)) are predicted at 0 GPa. The enthalpy of formation reveals that these four phases are thermodynamically stable in the ferromagnetic state, and the tetragonal P4/nmm phase has the highest stability. Calculated phonon spectra indicates that the remaining three predicted structures are dynamically stable except for I4/mmm structure. From the elastic constant, all of the predicted structures are mechanically stable phases. Among these studied structures, the three tetragonal phases with similar values of bulk modulus have high uniaxial magneto-crystalline anisotropy, high thermal expansion coefficient, large bulk modulus, and good ductility. However, the cubic Fd3¯m phase is characterized by low magneto-crystalline anisotropy, small thermal expansion coefficient, high shear and Young’s modulus, and brittleness. Finally, the high-pressure stability of these four structures is investigated. The theoretical prediction indicates a structural transition from tetragonal P4/nmm phase to ductile Fd3¯m structure at 26.8 GPa, while the I4/mmm is the most stable above 80 GPa.

A B2N monolayer: a direct band gap semiconductor with high and highly anisotropic carrier mobility.

Two-dimensional materials with a planar lattice, suitable direct band gap, and high and highly anisotropic carrier mobility are desirable for the development of advanced field-effect transistors. Here we predict three thermodynamically stable B-rich 2D B-N compounds with the stoichiometries of B2N, B3N, and B4N using a combination of crystal structure searches and first-principles calculations. Among them, B2N has an ultraflat surface and consists of eight-membered B6N2 and pentagonal B3N2 rings. The eight-membered B6N2 rings are linked to each other through both edge-sharing (in the y direction) and bridging B3N2 pentagons (in the x direction). B2N is a semiconductor with a direct band gap of 1.96 eV, and the nature of the direct band gap is well preserved in bilayer B2N. The hole mobility of B2N is as high as 0.6 × 103 cm2 V-1 s-1 along the y direction, 7.5 times that in the x direction. These combined novel properties render the B2N monolayer as a natural example in the field of two-dimensional functional materials with broad application potential for use in field-effect transistors.

New high-pressure monoclinic phase of Sn

High-pressure structure phase transitions of Sn were studied by using the unbiased CALYPSO structure prediction method combined with first-principles calculations. The experimental structures of Fd-3m, I41/amd, I4/mmm, Immm, Im-3m and P63/mmc were successfully reproduced in our crystal structure searches. Furthermore, we firstly uncovered a novel high-pressure structure of Sn with space group P21/m symmetry stabilized at the pressure range of 6.1–21.5 GPa. The results of elastic constants and phonon dispersion curves show that the P21/m structure is dynamically and mechanically stable at the considered pressures. Atomic structure of the P21/m phase was very similar to that of I4/mmm (body-centered tetragonal) under the same pressure. Our findings enrich the high pressure phases of Sn and lay the foundation for further theoretical and experimental research.

Stability and mechanical properties of W1−xMoxB4.2 ( x=0.0–1.0 ) from first principles

Heavy transition-metal tetraborides (e.g., tungsten tetraboride, molybdenum tetraboride, and molybdenum-doped tungsten tetraboride) exhibit superior mechanical properties, but solving their complex crystal structures has been a long-standing challenge. Recent experimental x-ray and neutron diffraction measurements combined with first-principles structural searches have identified a complex structure model for tungsten tetraboride that contains a boron trimer as an unusual structural unit with a stoichiometry of 1:4.2. In this paper, we expand the study to binary ${\mathrm{MoB}}_{4.2}$ and ternary ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ ($x=0.0--1.0$) compounds to assess their thermodynamic stability and mechanical properties using a tailor-designed crystal structure search method in conjunction with first-principles energetic calculations. Our results reveal that an orthorhombic ${\mathrm{MoB}}_{4.2}$ structure in $Cmcm$ symmetry matches well the experimental x-ray diffraction patterns. For the synthesized ternary Mo-doped tungsten tetraborides, a series of ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ structures are theoretically designed using a random substitution approach by replacing the W to Mo atoms in the $Cmcm$ binary crystal structure. This approach leads to the discovery of several ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ structures that are energetically superior and stable against decomposition into binary ${\mathrm{WB}}_{4.2}$ and ${\mathrm{MoB}}_{4.2}$. The structural and mechanical properties of these low-energy ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ structures largely follow the Vegard's law. Under changing composition parameter $x=0.0--1.0$, the superior mechanical properties of ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ stay in a narrow range. This unusual phenomenon stems from the strong covalent network with directional bonding configurations formed by boron atoms to resist elastic deformation. The findings offer insights into the fundamental structural and physical properties of ternary ${\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{B}}_{4.2}$ in relation to the binary ${\mathrm{WB}}_{4.2}/{\mathrm{MoB}}_{4.2}$ compounds, which open a promising avenue for further rational optimization of the functional performance of transition-metal borides that can be synthesized under favorable experimental conditions for wide applications.

Icosahedral silicon boride: A potential hybrid photovoltaic-thermoelectric for energy harvesting

Boron-rich compounds have attracted significant attention due to their promising and diverse physical properties, which include ultrahardness, resistance to oxidation and corrosion, and even superconductivity. Here, using a crystal structure search method based on first-principles calculations, we find a boron-rich silicon compound $\mathrm{Si}{\mathrm{B}}_{12}$ that is stable under moderate pressure of around 20 GPa, and which we predict is recoverable to ambient pressure. This silicon boride, with space group Pnnm, is structurally related to the $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ boron phase. Specifically, the $\mathrm{Si}{\mathrm{B}}_{12}$ structure is formed by replacing the ${\mathrm{B}}_{2}$ pairs in $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ with silicon atoms. Our calculations show that this Pnnm $\mathrm{Si}{\mathrm{B}}_{12}$ phase exhibits good thermal stability at moderate pressures above 20 GPa and temperatures to 900 K. We suggest this structure has dynamic stability at ambient pressure and remains stable to temperatures as high as 2000 K. Impressively, this $\mathrm{Si}{\mathrm{B}}_{12}$ phase possesses good light absorption and thermoelectrical properties, which are enhanced by its small and indirect band gap, doubly degenerate bands, and low lattice thermal conductivity. Our predictions should stimulate further investigations of this class of boron-rich semiconductors, especially in view of their superior photovoltaic and thermoelectric properties which may be beneficial in energy applications.

Prediction of an unusual trigonal phase of superconducting LaH10 stable at high pressures

Based on evolutionary crystal structure searches in combination with ab initio calculations, we predict an unusual structural phase of the superconducting LaH$_{10}$ that is stable from about 250 GPa to 425 GPa pressure. This new phase belongs to a trigonal $R\bar{3}m$ crystal lattice with an atypical cell angle, $\alpha_{rhom}$ $\sim$ 24.56$^{\circ}$. We find that the new structure contains three units of LaH$_{10}$ in its primitive cell, unlike the previously known trigonal phase, where primitive cell contains only one LaH$_{10}$ unit. In this phase, a 32-H atoms cage encapsulates La atoms, analogous to the lower pressure face centred cubic phase. However, the hydrogen cages of the trigonal phase consist of quadrilaterals and hexagons, in contrast to the cubic phase, that exhibits squares and regular hexagons. Surprisingly, the shortest H-H distance in the new phase is shorter than that of the lower pressure cubic phase and of atomic hydrogen metal. We find a structural phase transition from trigonal to hexagonal at 425 GPa, where the hexagonal crystal lattice coincides with earlier predictions. Solving the anisotropic Migdal-Eliashberg equations we obtain that the predicted trigonal phase (for standard values of the Coulomb pseudopotential) is expected to become superconducting at a critical temperature of about 175 K, which is less than $T_c \sim$250 K measured for cubic LaH$_{10}$.

Direct H-He chemical association in superionic FeO2H2He at deep-Earth conditions.

abstractHydrogen and helium are known to play crucial roles in geological and astrophysical environments; however, they are inert toward each other across wide pressure-temperature (P-T) conditions. Given their prominent presence and influence on the formation and evolution of celestial bodies, it is of fundamental interest to explore the nature of interactions between hydrogen and helium. Using an advanced crystal structure search method, we have identified a quaternary compound FeO2H2He stabilized in a wide range of P-T conditions. Ab initio molecular dynamics simulations further reveal a novel superionic state of FeO2H2He hosting liquid-like diffusive hydrogen in the FeO2He sublattice, creating a conducive environment for H-He chemical association, at P-T conditions corresponding to the Earth's lowest mantle regions. To our surprise, this chemically facilitated coalescence of otherwise immiscible molecular species highlights a promising avenue for exploring this long-sought but hitherto unattainable state of matter. This finding raises strong prospects for exotic H-He mixtures inside Earth and possibly also in other astronomical bodies.

Accurate and flexible neural-network interatomic potential for mixed materials: TixZr1−xO2 from bulk to clusters and nanoparticles

Many interesting systems, such as interfaces, surfaces, grain boundaries, and nanoparticles, contain so many atoms that quantum-mechanical atomistic simulations become inconvenient or outright impossible. It is therefore desirable to develop accurate and flexible general-purpose interatomic potentials to make it possible to explore the potential energy surface of such structures. In this work we generate a neural-network potential through charge equilibration technique (CENT) for ${\text{Ti}}_{x}{\text{Zr}}_{1\ensuremath{-}x}{\text{O}}_{2}$ with $0\ensuremath{\le}x\ensuremath{\le}1$. Optimized symmetry functions for multicomponent systems make it possible to train the potential on less than 10 000 diverse structures containing different cation ratios $x$, from pure ${\text{TiO}}_{2}$ to ${\text{ZrO}}_{2}$, in free and periodic boundary conditions in the framework of density functional theory. The combination of the CENT potential with the symmetry functions generates a flexible and reliable method to reproduce the complexity of the energy landscape of these mixed materials with different boundary conditions at zero pressure. The reliability and transferability of the potential are verified by calculating some properties of bulk and slab configurations. Moreover, in order to investigate the performance of potential for different crystal phases and cluster configurations which are not included in our training data set, we performed a crystal structure search by minima hopping method. Beside reproducing known results in agreement with DFT calculations, we discovered novel crystal structures for bulk ${\text{TiZr}}_{3}{\text{O}}_{8}$ and ${\text{TiZrO}}_{4}$, as well as for small clusters and ${\text{ZrO}}_{2}$ nanoparticles.