Interface structure prediction via CALYPSO method

Wei Y., Zhou Z., Fang W., Long R.

TiO2 is an excellent photocatalytic and photovoltaic material but suffers low efficiency because of deep trap states giving rise to fast charge and energy losses. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we demonstrate that grain boundaries (GBs), which are common in polycrystalline TiO2, accelerate nonradiative electron-hole recombination by a factor of 3. Despite GBs increase the band gap without creating deep trap states, and accelerate coherence loss, they enhance nonadiabatic electron-phonon coupling, and facilitate the relaxation. Importantly, electrons accumulated at the boundaries together with the relatively long-lived excite state favor photocatalytic reaction. Our study rationalizes the experimental observations and provides valuable perspectives for improving the device performance by defect engineering.

Lv J., Xu M., Lin S., Shao X., Zhang X., Liu Y., Wang Y., Chen Z., Ma Y.

Crystalline silicon is dominating the current solar cell market due to the significant efficiency improvement and cost reduction in last decades. However, its indirect band gap nature leads to inefficient visible-light absorption, which seriously impedes further performance enhancement in silicon-based photovoltaic devices. Thus, it is highly desirable to develop direct band gap silicon materials. Herein, by means of ab initio swarm-intelligence structure-searching method, we predicted a quasi-direct gap semiconducting tri-layer silicene structure consisting of alternating arrays of six-membered Si rings, which can be converted into a direct gap semiconductor of 0.86 eV by applying a low tensile strain (~ 2.5%). Our calculations revealed that the photovoltaic efficiency of the tri-layer silicene reaches 29% at 1.0 µm, which is comparable to that of bulk GaAs with the highest conversion efficiency among thin-film solar cell absorbers.

Zhu Q., Samanta A., Li B., Rudd R.E., Frolov T.

The study of grain boundary phase transitions is an emerging field until recently dominated by experiments. The major bottleneck in the exploration of this phenomenon with atomistic modeling has been the lack of a robust computational tool that can predict interface structure. Here we develop a computational tool based on evolutionary algorithms that performs efficient grand-canonical grain boundary structure search and we design a clustering analysis that automatically identifies different grain boundary phases. Its application to a model system of symmetric tilt boundaries in Cu uncovers an unexpected rich polymorphism in the grain boundary structures. We find new ground and metastable states by exploring structures with different atomic densities. Our results demonstrate that the grain boundaries within the entire misorientation range have multiple phases and exhibit structural transitions, suggesting that phase behavior of interfaces is likely a general phenomenon. The atomic structure of grain boundary phases remains unknown and is difficult to investigate experimentally. Here, the authors use an evolutionary algorithm to computationally explore interface structures in higher dimensions and predict low-energy configurations, showing interface phases may be ubiquitous.

Tang H., Deng Z., Lin Z., Wang Z., Chu I., Chen C., Zhu Z., Zheng C., Ong S.P.

We present an exposition of first-principles approaches to elucidating interfacial reactions in all-solid-state sodium-ion batteries. We will demonstrate how thermodynamic approximations based on assumptions of fast alkali diffusion and multispecies equilibrium can be used to effectively screen combinations of Na-ion electrodes, solid electrolytes, and buffer oxides for electrochemical and chemical compatibility. We find that exchange reactions, especially between simple oxides and thiophosphate groups to form PO43–, are the main cause of large driving forces for cathode/solid electrolyte interfacial reactions. A high reactivity with large volume changes is also predicted at the Na anode/solid electrolyte interface, while the Na2Ti3O7 anode is predicted to be much more stable against a broad range of solid electrolytes. We identify several promising binary oxides, Sc2O3, SiO2, TiO2, ZrO2, and HfO2, that are similarly or more chemically compatible with most electrodes and solid electrolytes than the common...

Jelver L., Larsen P.M., Stradi D., Stokbro K., Jacobsen K.W.

Xu M., Shao S., Gao B., Lv J., Li Q., Wang Y., Wang H., Zhang L., Ma Y.

Titanium dioxide has been widely used as an efficient transition metal oxide photocatalyst. However, its photocatalytic activity is limited to the ultraviolet spectrum range due to the large bandgap beyond 3 eV. Efforts to reduce the bandgap to achieve a broader spectrum range of light absorption have been successfully attempted via the experimental synthesis of dopant-free metastable surface structures of rutile-type TiO2 (011) 2 × 1. This new surface phase possesses a reduced bandgap of ∼2.1 eV, showing great potential for an excellent photocatalyst covering a wide range of visible light. There is a need to establish the atomistic structure of this metastable surface to understand the physical cause for the bandgap reduction and to improve the future design of photocatalysts. Here, we report computational investigations in an effort to unravel this surface structure via swarm structure-searching simulations. The established structure adopts the anatase (101)-like structure model, where the topmost 2-fold O atoms form a quasi-hexagonal surface pattern and bond with the unsaturated 5-fold and 4-fold Ti atoms in the next layer. The predicted anatase (101)-like surface model can naturally explain the experimental observation of the STM images, the electronic bandgap, and the oxidation state of Ti4+. Dangling bonds on the anatase (101)-like surface are abundant making it a superior photocatalyst. First-principles molecular dynamics simulations have supported the high photocatalytic activity by showing that water and formic acid molecules dissociate spontaneously on the anatase (101)-like surface.

Kiyohara S., Oda H., Miyata T., Mizoguchi T.

A virtual screening method achieved a maximum boost in speed of several tens of thousands–fold while determining the interface structure.

Mathew K., Singh A.K., Gabriel J.J., Choudhary K., Sinnott S.B., Davydov A.V., Tavazza F., Hennig R.G.

A Materials Project based open-source Python tool, MPInterfaces, has been developed to automate the high-throughput computational screening and study of interfacial systems. The framework encompasses creation and manipulation of interface structures for solid/solid hetero-structures, solid/implicit solvents systems, nanoparticle/ligands systems; and the creation of simple system-agnostic workflows for in depth computational analysis using density-functional theory or empirical energy models. The package leverages existing open-source high-throughput tools and extends their capabilities towards the understanding of interfacial systems. We describe the various algorithms and methods implemented in the package. Using several test cases, we demonstrate how the package enables high-throughput computational screening of advanced materials, directly contributing to the Materials Genome Initiative (MGI), which aims to accelerate the discovery, development, and deployment of new materials.

Wang H., Wang Y., Lv J., Li Q., Zhang L., Ma Y.

Atomistic structure prediction from “scratch” is one of the central issues in physical, chemical, materials and planetary science, and it will inevitably play a critical role in accelerating materials discovery. Along this thrust, CALYPSO structure prediction method by taking advantage of structure smart learning in a swarm was recently developed in Prof. Yanming Ma’s group, and it has been demonstrated through a wide range of applications to be highly efficient on searching ground state or metastable structures of materials with only the given knowledge of chemical composition. The purpose of this paper is to provide an overview of the basic theory and main features of the CALYPSO method, as well as its versatile applications (limited only to a few works done in Ma’s group) on design of a broad range of materials including those of isolated clusters/nanoparticles, two-dimensional reconstructed surfaces, and three-dimensional bulks (at ambient or high pressure conditions) with a variety of functional properties. It is to say that CALYPSO has become a major structure prediction technique in the field, with which the door for a functionality-driven design of materials is now opened up.

Gao B., Shao X., Lv J., Wang Y., Ma Y.

The adsorption of atoms is one of the efficient approaches for functionalizing two-dimensional (2D) layer materials with desirable properties. The structural knowledge of atoms adsorbed on 2D layer materials is crucial for understanding their functional performance. Here we propose a versatile method for predicting the structures of atoms adsorbed on 2D materials via the swarm-intelligence-based CALYPSO structure-prediction method. Several techniques are implemented to improve the efficiency of structure searching, including fixed adsorption sites, constraints of symmetry and distance during structure generation, and the constrained particle swarm-optimization algorithm for structure evolution. The method is successfully applied to investigate the well-studied systems of hydrogenated and oxidized graphene. The energetically most stable structures of single-sided hydrogenated graphene are predicted for different contents of hydrogen; altering the hydrogen content appears to effectively tune the band gap. A...

Sun R., Wang Z., Saito M., Shibata N., Ikuhara Y.

Grain boundary (GB) phase transformations often occur in polycrystalline materials while exposed to external stimuli and are universally implicated in substantially affecting their properties, yet atomic-scale knowledge on the transformation process is far from developed. In particular, whether GBs loaded with defects due to treatments can still be conventionally considered as disordered areas with kinetically trapped structure or turn ordered is debated. Here we combine advanced electron microscopy, spectroscopy and first-principles calculations to probe individual TiO2 GB subject to different atmosphere, and to demonstrate that stimulated structural defects can self-assemble at GB, forming an ordered structure, which results in GB nonstoichiometry and structural transformations at the atomic scale. Such structural transformation is accompanied with electronic transition at GB. The three-dimensional transformations afford new perspectives on the structural defects at GBs and on the development of strategies to manipulate practically significant GB transformations. Grain boundaries in polycrystalline materials strongly influence their mechanical properties. Here, the authors investigate polycrystalline TiO2by high-resolution electron microscopy and observe that structural defects form ordered structures at grain boundaries influencing their properties.

Asahi R., Morikawa T., Irie H., Ohwaki T.

Raghunathan R., Johlin E., Grossman J.C.

In photovoltaic devices, the bulk disorder introduced by grain boundaries (GBs) in polycrystalline silicon is generally considered to be detrimental to the physical stability and electronic transport of the bulk material. However, at the extremum of disorder, amorphous silicon is known to have a beneficially increased band gap and enhanced optical absorption. This study is focused on understanding and utilizing the nature of the most commonly encountered Σ3 GBs, in an attempt to balance incorporation of the advantageous properties of amorphous silicon while avoiding the degraded electronic transport of a fully amorphous system. A combination of theoretical methods is employed to understand the impact of ordered Σ3 GBs on the material properties and full-device photovoltaic performance.

Lu J., Gomes L.C., Nunes R.W., Castro Neto A.H., Loh K.P.

Heteroepitaxy of two-dimensional (2D) crystals, such as hexagonal boron nitride (BN) on graphene (G), can occur at the edge of an existing heterointerface. Understanding strain relaxation at such 2D laterally fused interface is useful in fabricating heterointerfaces with a high degree of atomic coherency and structural stability. We use in situ scanning tunneling microscopy to study the 2D heteroepitaxy of BN on graphene edges on a Ru(0001) surface with the aim of understanding the propagation of interfacial strain. We found that defect-free, pseudomorphic growth of BN on a graphene edge “substrate” occurs only for a short distance (

Schusteritsch G., Pickard C.J.

We present here a fully first-principles method for predicting the atomic structure of interfaces. Our method is based on the {\it ab initio} random structure searching (AIRSS) approach, applied here to treat two dimensional defects. The method relies on repeatedly generating random structures in the vicinity of the interface and relaxing them within the framework of density functional theory (DFT). The method is simple, requiring only a small set of parameters that can be easily connected to the chemistry of the system of interest, and efficient, ideally adapted to high-throughput first-principles calculations on modern parallel architectures. Being first-principles, our method is transferable, an important requirement for a generic computational method for the determination of the structure of interfaces. Results for two structurally and chemically very different interfaces are presented here, grain boundaries in graphene and grain boundaries in strontium titanate (SrTiO$_3$). We successfully find a previously unknown low energy grain boundary structure for the graphene system, as well as recover the previously known higher energy structures. For the SrTiO$_3$ system we study both stoichiometric and non-stoichiometric compositions near the grain boundary and find previously unknown low energy structures for all stoichiometries. We predict that these low energy structures have long-range distortions to the ground state crystal structure emanating into the bulk from the interface.

Lv Y., Li J., Zhang Z., Liu Y., Yuan J., Lin J., Wang X.

Abstract As an independent thermodynamic parameter, pressure significantly influences interatomic distances, leading to an increase in material density. In this work, we employ the CALYPSO structure search and density functional theory calculations to explore the structural phase transitions and electronic properties of calcium–sulfur compounds (Ca x S1−x , where x = 1/4, 1/3, 1/2, 2/3, 3/4, 4/5) under 0–1200 GPa. The calculated formation enthalpies suggest that Ca x S1−x compounds undergo multiple phase transitions and eventually decompose into elemental Ca and S, challenging the traditional view that pressure stabilizes and densifies compounds. The analysis of formation enthalpy indicates that an increase in pressure leads to a rise in internal energy and the PV term, resulting in thermodynamic instability. Bader charge analysis reveals that this phenomenon is attributed to a decrease in charge transfer under high pressure. The activation of Ca-3d orbitals is significantly enhanced under pressure, leading to competition with Ca-4s orbitals and S-3p orbitals. This may cause the formation enthalpy minimum on the convex hull to shift sequentially from CaS to CaS3, then to Ca3S and Ca2S, and finally back to CaS. These findings provide critical insights into the behavior of alkaline-earth metal sulfides under high pressure, with implications for the synthesis and application of novel materials under extreme conditions and for understanding element distribution in planetary interiors.

Zhang P., Cui W., Hao J., Shi J., Li Y.

Calcium, one of the most abundant elements in the Earth’s mantle, does not react easily with noble gases (e.g., He and Xe) under ambient conditions. However, high pressure can alter electron configurations in atoms, leading to the formation of unconventional compounds. In this study, we systematically investigate Ca–Xe compounds across pressures of 0–150 GPa using calypso structure prediction methods combined with first-principles calculations. We identify four novel Ca–Xe compounds Pm3̄m-CaXe, P4/mmm-CaXe2, I4/m-Ca3Xe, and P4/mmm-Ca2Xe3 that demonstrate stability over a wide pressure range from 37.5 to 150 GPa. All these compounds exhibit metallic properties and are dynamically stable, as indicated by the absence of imaginary frequencies in their phonon dispersion spectra. Ionic bonding between Ca and Xe is observed due to electron transfer from Ca to Xe. Ab initio molecular dynamics simulations show that Pm3̄m-CaXe, P4/mmm-CaXe2, and P4/mmm-Ca2Xe3 remain solid up to pressures of 135 GPa and temperatures of 4000 K. In contrast, I4/m-Ca3Xe undergoes a transition from solid to liquid at temperatures above 3500 K due to weakened Ca–Xe bonds. The findings suggest that these Ca–Xe compounds could potentially be synthesized experimentally under high-pressure conditions. The results offer theoretical guidance for discovering new high-pressure Xe compounds and provide valuable insights into Xe chemistry.

Wang C., Liu P., Zhang D., Wei Y., Cui T., Liu Z.

Bandi H., Dahule R., Kakarla A.K., Maezono R., Narsimulu D., Akram S.W., Shanthappa R., Yu J.S.

Xu W., Wang Q., Zeng Q., Li X., Shi J., Hao J., Cui W., Li Y.

Yang Y., Song J., Zhang H., Li Z., Liu S., Wang Y., Su X.

Pressure-induced nitrogen-rich compounds hold significant application prospects in high-energy-density materials. Utilizing first-principles calculations and swarm-intelligence structure search methods, we have identified ten new types of Gd-N compounds with different configurations, such as one-dimensional N-chains composed of N6 rings or N8 rings, and two-dimensional N-layers constructed of N14 rings, N18 rings, or N18 + N6 rings. Moreover, the predicted Gd-N compounds exhibit different magnetic properties, and a magnetic phase diagram is constructed in the pressure range of 0 to 200 GPa. Remarkably, the volumetric energy density (11.58–17.79 kJ/cm3) of Gd polynitrides with high nitrogen content, including P-1(I)-GdN6, P-1(II)-GdN6, R-3-GdN8, C2mm-GdN9, and P1-GdN10, surpassed that of TNT (7.05 kJ/cm3), making them promising candidates for energetic materials. The discovery of diverse chain-like and layered structures in the GdNx compounds highlights the role of gadolinium in inducing the diversity and complexity of nitrogen arrangements.

Dahule R., Gao B., Hongo K., Panda E., Ma Y., Maezono R.

Toso S., Dardzinski D., Manna L., Marom N.

Sun C., Zhang Y., Xu M., Wang F., Cui W., Niu C., Li Y.

We predict that the hex-BeP2 monolayer exhibits a high electron mobility on the order of 105 cm2 V−1 s−1, along with a remarkable photovoltaic efficiency of 29.3%.

Zhang B., Xu M., Wang Z., Hao J., Li Y.

Nagappan N., Priyanga G.S., Thomas T.

Wang X., Wang Y., Wang Z., Zhang Y., Gu J.

Abstract The phase stability, elastic anisotropy, and minimum thermal conductivity of MnB2 in different crystal structures have been investigated by first-principles calculations based on density functional theory. The results found that P63/mmc (hP6-MnB2), P6/mmm (hP3-MnB2), Pmmn (oP6-MnB2), R 3 ¯ m (hR3-MnB2), Pnma (oP12-MnB2), and Immm (oI18-MnB2) all exhibit mechanical and dynamic stability under environmental conditions, and the sequence of phase stability was hP6 > hR3 > oP6 > oI18 > oP12 > hP3. In addition, Vickers hardness calculations indicated that hP6, hR3, oP6, and oI18 of MnB2 have potential as hard materials, while hP3 and oP12 are not suitable as hard materials. Moreover, the elastic anisotropy of different MnB2 phases were also comprehensively investigated. It is found that the anisotropic order of bulk modulus is oP12 > hP3 > hP6 > hR3 > oI18 > oP6, while that of Young’s modulus is oP12 > hR3 > hP6 > oP6 > hP3 > oI18. Furthermore, the minimum thermal conductivity of different MnB2 phases was evaluated by means of Clarke’s and Cahill’s models. The results suggested that these MnB2 diborides are all not suitable as thermal barrier coating materials.

Zhang C., Xu H., Xu X., Zheng W.

The structures and chemical bond evolution of ditantalum doped carbon clusters Ta2Cn−/0 (n = 1–7) were studied via size-selected anion photoelectron spectroscopy and theoretical calculations. It is found that Ta2C−/0 has a triangular structure and Ta2C2−/0 has a quasi-rhombus structure with C2v symmetry. Ta2C3− has a quasi-planar structure with a carbon atom and a C2 unit interacting with two tantalum atoms, and the lowest-energy isomer of neutral Ta2C3 has a triangular bipyramid structure with three carbon atoms around the Ta2 unit. Ta2C4−/0 has two C2 units connected with the Ta2 unit in parallel. Two isomers of Ta2C5− are observed, where both isomers have one carbon atom and two C2 units bound to the Ta2 unit in different ways. The most stable structure of neutral Ta2C5 has one carbon atom added on top of the Ta2C4 cluster. The most stable structures of Ta2C6-7−/0 can be viewed as a C2 unit and a C3 unit capping a butterfly like Ta2C4 structure, respectively. Molecular orbital analysis shows that neutral Ta2C3 has a large gap between its highest occupied molecular orbital and lowest unoccupied molecular orbital. Chemical bonding analysis reveals that the Ta–Ta interactions in Ta2Cn−/0 (n = 1–7) clusters are slightly weaker than the Ta–Ta interaction in bare Ta2 due to the participation in forming multicenter bonds.

Shao S., Lv J., Li X., Li L., Liu P., Liu Z., Chen C., Wang Y., Ma Y.

Mahatara S., Lany S.

Epitaxial lattice matching is an important condition for the formation of coherent interfaces with low defect densities. However, lattice-matched substrates with the same crystal structure as the active layer are often not available, suggesting opportunities for utilizing heterostructural interfaces. For example, at high Al contents that are interesting for ultrawide-gap applications in power electronics, AlxGa1−xN semiconductor alloys in the (0001) orientation of the wurtzite (wz) structure become lattice-matched to (111)-oriented rocksalt (rs) TaC substrates. To predict the expected interface atomic structures under different synthesis conditions, we perform high-throughput density-functional-theory calculations, using an algorithm for systematic sampling of the possible stacking sequences of the atomic layers on the in-plane hexagonal lattice. The approach considers octahedral, tetrahedral, and prismatic coordination motifs, and is generally applicable for the modeling of commensurate rs/wz heterostructural interfaces. Our results provide guidance for synthesis control of substrate-film bonding and the polarity of ultrawide-gap AlxGa1−xN alloys on TaC substrates. Published by the American Physical Society 2024