Motivated by the high superconducting transition temperature (TC) shown by monolayer FeSe on cubic perovskite SrTiO3(001) and SrTiO3(001)-2×1 reconstructed surfaces, in this study, we explore the atomic and electronic structures of monolayer FeSe on various SrTiO3(001)-2×1 surface reconstructions using the CALYPSO method and first-principles calculations. Our search reveals two new Ti2O2 and Ti2O reconstructed surface structures, besides the Ti2O3 and double TiO2 layer reconstructed surfaces, and the two new Ti2O2 and Ti2O reconstructed surface structures are more stable under Ti-rich conditions than under Ti-poor conditions. The Fermi-surface topology of an FeSe monolayer on Ti2O3- and Ti2O2-type reconstructed STO surfaces is different from that of an FeSe monolayer on a Ti2O-type STO reconstructed surface. The established structure of monolayer FeSe on a Ti2O-type STO(001) reconstructed surface can naturally explain the experimental observation of the electronic band structure on the monolayer FeSe superconductor and obtained electrons counting per Fe atom. Surface states in the mid-gap induced by various STO surface reconstructions will result in band bending. The surface-state-induced band bending is also responsible for the electron transfer from the STO substrate to the FeSe films.

An anatase TiO2(001) surface has shown great potential as an ideal and powerful photocatalyst due to its chemical stability, nontoxicity, and high reactivity. However, the fundamental atomic structure of the reconstructed anatase (001)-(1 × 4) surface is still under debate, which greatly impedes further exploration of its chemical activity. Herein, the anatase (001)-(1 × 4) surface reconstruction and the photocatalytic reactivity have been extensively studied using an effective surface structure searching method in combination with the first-principles calculations. Our study reveals that there exist two stable (1 × 4) reconstructed surfaces, i.e., the previously proposed “ad-molecule” (ADM) and oxidized ridge (OR) surface structures, and their simulated STM images are in good agreement with experimental observations. Moreover, we find that the ADM surface has superior photocatalytic reactivity to the OR surface and a small number of water can be dissociated at the terrace at one water monolayer coverage, which has never been found before. These findings can not only be applied to solve the experimental controversies about the atomic structure of the reconstructed anatase (001)-(1 × 4) surface but also provide a theoretical basis for exploring the intrinsic properties of the surface.

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.Keywords: anatase; CALYPSO; dopant-free; reduced bandgap; surface reconstruction; titanium dioxide;

High pressure can fundamentally alter the electronic structure of elemental metals, leading to the unexpected formation of intermetallics with unusual structural features. In the present study, the phase stabilities and structural changes of Na–Fe intermetallics under pressure were studied using unbiased structure searching methods, combined with density functional theory calculations. Two intermetallics with stoichiometries Na3Fe and Na4Fe are found to be thermodynamically stable at pressures above 120 and 155 GPa, respectively. An interesting structural feature is that both have form a host–guest-like structure with Na sublattices constructed from small and large polygons similar to the host framework of the self-hosting incommensurate phases observed in Group I and II elements. Apart from the one-dimensional (1D) Fe chains running through the large channels, more interestingly, electrides are found to localize in the small channels between the layers. Electron topological analysis shows secondary bonding interactions between the Fe atoms and the interstitial electrides help to stabilize these structures.

Carbon has the capability of forming various bonding states that affect the structures and properties of transition metal carbides. In this work, structural search was performed to explore the structural diversity of LaC2 at pressures of 0.0–30.0 GPa. Five stable structures of LaC2 reveal a variety of carbon structural units ranging from a dimer to bent C3, zigzag C4 and armchair polymer chains. A series of pressure-induced structural transformations are predicted, I4/mmm (i.e. experimental α phase) → C2/c → Pnma → Pmma, which involve the catenation of carbon from a dimer to zigzag C4 units and further to armchair polymer chains. The bent C3 unit appears in a novel Immm structure. This structure is the theoretical ground state of LaC2 under ambient conditions, but is kinetically inaccessible from the experimental α phase. LaC2 becomes thermodynamically metastable relative to La2C3 + diamond above 17.1 GPa, and eventually decomposes into constituent elements above 35.6 GPa. The presented results indicate that catenation of carbon can be realized even in simple inorganic compounds under nonambient conditions.

Pressure can induce significant changes in atomic and electronic structures and can in some cases even induce compound formation between elements that do not bond under ambient conditions. Here, we have extensively explored the Li–Fe system at high pressure using the effective CALYPSO algorithm in combination with first-principles calculations. Strikingly, our results show that the stoichiometries of LiFe, Li3Fe and Li3Fe2 have stability regimes on a phase diagram. It is found that both LiFe and Li3Fe2 adopted a fully developed three-dimensional framework. With an increasing Li content, a Li bonding pattern evolves from occupying diamond sublattice to armchair chains, and to the host frameworks constructed by 3- and 6-membered planar rings and 4- and 7-membered planar rings in which linear Fe chains are located in small channels. Our findings put forward a further understanding of the crystal structures and electronic properties of Li–Fe compounds at high pressures.

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.

•The synthesized ε-MoH was demonstrated to be energetically stable up to 100 GPa.•New Mo hydrides with stoichiometry MoH2 were predicted to be stable at high pressure.•The phase transition sequence of MoH2 at high pressure was proposed.•MoH and MoH2 are metallic at high pressure.We present results from first-principles calculations on molybdenum polyhydrides under pressure. In addition to the experimental ε-phase of MoH, we find several novel structures of MoH2 and MoH3 at pressures below 100 GPa. A hexagonal structure of MoH2 becomes stable with respect to decomposition into MoH and H2 above 9 GPa, and transforms into an orthorhombic structure at 24 GPa, which remains stable up to 100 GPa. MoH3 is unstable relative to decomposition into MoH and H2 over the whole pressure range studied. Electronic structure calculations reveal that molybdenum polyhydrides are metallic under pressure.

The particle-swarm optimization method has been used to predict the stable high pressure structures up to 300 GPa of hydrogen-rich group 17 chlorine (HnCl, n = 2–7) compounds. In comparison to the group 1 and 2 hydrides, the structural modification associated with increasing pressure and hydrogen concentration is much less dramatic. The polymeric HCl chains already present in the low temperature phase under ambient pressure persist in all the high pressure structures. No transfer of electrons from the chlorine atoms into the interstitial sites is found. This indicates the chemical bonding at high pressure in group 17 elements is fundamentally different from the alkali and alkaline elements. It is found that almost perfectly triangular H3+ ions can be stabilized in the crystalline structure of H5Cl.

It is now known that the structure and properties of a material can be significantly altered under extreme compression. In this work, a structural search was performed to investigate the phase stabilities and structures of SrH2n (n = 1–5) in the pressure range of 50–300 GPa. The high-pressure polymorphs reveal a variety of hydrogen structural units ranging from monatomic hydride to linear and bent H3 and spiral polymer chains. A novel graphene like H-layer structure was found to exist in SrH10 at 300 GPa. The structural diversity in the predicted high pressure structures provides an opportunity for an in-depth analysis of the chemical bonding in the high pressure polyhydrides. It is shown from theoretical calculations that the electronegativity of molecular hydrogen is similar to that of group 13 and 14 elements. This resulted in electrons being transferred from Sr to the hydrogen molecules. Thus, a consideration of the number of valence electrons available from Sr that can be shared among the H2 serves as a useful guide to rationalize the structures of the H-moieties. An alternative description of the high pressure structures differing from a previous study is presented here.

Recently, an experimental work reported a very high Tc of ∼190 K in hydrogen sulphide (H2S) at 200 GPa. The search for new superconductors with high superconducting critical temperatures in hydrogen-dominated materials has attracted significant attention. Here we predict a candidate phase of MgH6 with a sodalite-like framework in conjunction with first-principles electronic structure calculations. The calculated formation enthalpy suggests that it is thermodynamically stable above 263 GPa relative to MgH2 and solid hydrogen (H2). Moreover, the absence of imaginary frequency in phonon calculations implies that this MgH6 structure is dynamically stable. Furthermore, our electron–phonon coupling calculation based on BCS theory indicates that this MgH6 phase is a conventional superconductor with a high superconducting critical temperature of ∼260 K under high pressure, which is even higher than that of the recently reported compressed H2S. The present results offer insight in understanding and designing new high-temperature superconductors.

Carbon (C) is able to form various bonding patterns, including graphene sheets, chains and dimers, but stable bare six-membered C6 hexagonal rings, which are the fundamental structural motifs of graphite and graphene have long been missing. Here we report the stabilization of such bare C6 rings under high pressures in the charge-transfer systems of binary sesquicarbides Y2C3 and La2C3 as predicted by first-principles swarm structure searching simulations. We found that the external pressure can be used to efficiently tune structural transitions in the sesquicarbides from the ambient-pressure cubic phases into high-pressure orthorhombic phases, accompanied by significant C–C bonding modification from C–C dimers to bare C6 rings and polymerized graphene-like double C6 sheets. The bare C6 rings are stabilized in Y2C3 and La2C3 at pressures above 32 and 13 GPa, respectively, which are readily accessible to experiments. Chemical bonding analysis reveals that the bare C6 rings feature a benzene-like sp2 C–C bonding pattern with a delocalized π system. Y or La → C charge transfer and the need for denser structure packing are found to be part of the underlying mechanisms behind the stabilization of the bare C6 rings.

•We predicted several novel high-pressure phases of solid BF3.•We examined the electronic properties of solid BF3 up to 300 GPa.•The predicted BF3 structures are dynamically stable at high pressure.•The coordination of the B atoms in BF3 is 6 at 160 GPa.This study systematically investigated the high-pressure crystal structures of solid trifluoride (BF3) using first principle structure searches and several high-pressure phases were predicted. We found that the coordination of B atoms increased from 3 to 4 at 11 GPa, and to 6 at 160 GPa, while all the F atoms were bridge bonding at a sufficiently high pressure, from terminal bonding. Further calculations of the electronic properties showed that solid BF3 remained insulating up to the highest pressure considered, i.e., 300 GPa. Phonon calculations indicated that all the predicted structures of BF3 are dynamically stable under high pressure. These results show that pressure plays an important role in the changing chemical environments of elements, thereby improving our understanding of the evolution of structure and bonding with compression in other molecular systems, particularly in other boron trihalides.

The structural and electronic properties of NaN3 at high pressures were studied through ab initio calculations. Three new phases with I4/mcm, P6/m and C2/m structure were found to be stable at pressures of 6.5, 58 and 152 GPa, respectively. Similarity of the Raman spectra revealed that the experimental post-α phase should adopt the I4/mcm structure. The calculated insulator–metal transition at 58 GPa directly explained the observed darkening of NaN3 sample at above 50 GPa. The three proposed structures contain azide, N6 hexagon and polymeric nitrogen, respectively. Our finding of the novel N6 hexagon in NaN3 at moderate pressures provides a new view of the pressure-induced polymerization process of metal azides.Highlights► We predicted three new high-pressure phases for NaN3. ► The I4/mcm structure is possibly recent observed post-α phase. ► An insulator–metal transition was predicted at 58 GPa. ► The N6 hexagon molecule formed in NaN3 at moderate pressures.

Using the ab initio particle swarm optimization algorithm for crystal structure prediction, we have predicted an orthorhombic Pmmn structure for OsB4, which is energetically much superior to the previously proposed WB4-type structure. The Pmmn structure consists of irregular OsB10 dodecahedrons connected by edges and is stable against decompression into a mixture of Os and B at ambient pressure. OsB4 within this orthorhombic phase is found to be an ultra-incompressible and hard material due to its high bulk modulus (294 GPa) and large hardness (28 GPa), originating from the strong and directional covalent B–B and B–Os bonds.

We investigated the high pressure phases of CdF2 by a joint theoretical and experimental study. The structural and electronic properties of CdF2 were extensively explored to high pressure by ab initio calculations based on the density functional theory. A structural phase transition from the fluorite-type  (Fm-3m  , Z=4Z=4) structure to the cotunnite-type (Pnma  , Z=4Z=4) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition. Our work makes clear pressure-induced phase transitions and structural information of CdF2 under high pressure.Highlights► This paper systematically investigated the high pressure phases of CdF2 by a joint theoretical and experimental study. ► A high pressure phase was observed above 7 GPa in transition metal fluoride for the first time. ► This work enriches the information of divalent metal fluoride under high pressure.

With the increasing demand for specific applications in high pressure and electronic devices, the search for superhard superconducting materials remains a topic of great interest. Using particle swarm optimization algorithm, we report five competitive structures with clear tetrahedrally sp3 hybridization, among which two metallic orthorhombic structures (Pmm2 and Pmma) with the maximal stable bonds (C–C + B–N) are energetically more favorable than earlier proposed structures. Further first principles calculations suggest that the predicted five structures possess simultaneously superhard and superconducting properties. The five structures are with the similar calculated hardness (56–58 GPa), but show distinct difference in superconducting critical temperature, ranging from 2 to 53 K (with the Coulomb parameter μ∗ of 0.13).Highlights► We predicted five possible structures of B2CN using ab initio calculations. ► The current Pmm2 and Pmma B2CN are more stable than the earlier structures. ► The predicted B2CN structures are superhard and superconducting. ► The difference in superconductivity is related to the relatively lower frequency.

It is now known that the structure and properties of a material can be significantly altered under extreme compression. In this work, a structural search was performed to investigate the phase stabilities and structures of SrH2n (n = 1–5) in the pressure range of 50–300 GPa. The high-pressure polymorphs reveal a variety of hydrogen structural units ranging from monatomic hydride to linear and bent H3 and spiral polymer chains. A novel graphene like H-layer structure was found to exist in SrH10 at 300 GPa. The structural diversity in the predicted high pressure structures provides an opportunity for an in-depth analysis of the chemical bonding in the high pressure polyhydrides. It is shown from theoretical calculations that the electronegativity of molecular hydrogen is similar to that of group 13 and 14 elements. This resulted in electrons being transferred from Sr to the hydrogen molecules. Thus, a consideration of the number of valence electrons available from Sr that can be shared among the H2 serves as a useful guide to rationalize the structures of the H-moieties. An alternative description of the high pressure structures differing from a previous study is presented here.

The particle-swarm optimization method has been used to predict the stable high pressure structures up to 300 GPa of hydrogen-rich group 17 chlorine (HnCl, n = 2–7) compounds. In comparison to the group 1 and 2 hydrides, the structural modification associated with increasing pressure and hydrogen concentration is much less dramatic. The polymeric HCl chains already present in the low temperature phase under ambient pressure persist in all the high pressure structures. No transfer of electrons from the chlorine atoms into the interstitial sites is found. This indicates the chemical bonding at high pressure in group 17 elements is fundamentally different from the alkali and alkaline elements. It is found that almost perfectly triangular H3+ ions can be stabilized in the crystalline structure of H5Cl.

An anatase TiO2(001) surface has shown great potential as an ideal and powerful photocatalyst due to its chemical stability, nontoxicity, and high reactivity. However, the fundamental atomic structure of the reconstructed anatase (001)-(1 × 4) surface is still under debate, which greatly impedes further exploration of its chemical activity. Herein, the anatase (001)-(1 × 4) surface reconstruction and the photocatalytic reactivity have been extensively studied using an effective surface structure searching method in combination with the first-principles calculations. Our study reveals that there exist two stable (1 × 4) reconstructed surfaces, i.e., the previously proposed “ad-molecule” (ADM) and oxidized ridge (OR) surface structures, and their simulated STM images are in good agreement with experimental observations. Moreover, we find that the ADM surface has superior photocatalytic reactivity to the OR surface and a small number of water can be dissociated at the terrace at one water monolayer coverage, which has never been found before. These findings can not only be applied to solve the experimental controversies about the atomic structure of the reconstructed anatase (001)-(1 × 4) surface but also provide a theoretical basis for exploring the intrinsic properties of the surface.

Motivated by the high superconducting transition temperature (TC) shown by monolayer FeSe on cubic perovskite SrTiO3(001) and SrTiO3(001)-2×1 reconstructed surfaces, in this study, we explore the atomic and electronic structures of monolayer FeSe on various SrTiO3(001)-2×1 surface reconstructions using the CALYPSO method and first-principles calculations. Our search reveals two new Ti2O2 and Ti2O reconstructed surface structures, besides the Ti2O3 and double TiO2 layer reconstructed surfaces, and the two new Ti2O2 and Ti2O reconstructed surface structures are more stable under Ti-rich conditions than under Ti-poor conditions. The Fermi-surface topology of an FeSe monolayer on Ti2O3- and Ti2O2-type reconstructed STO surfaces is different from that of an FeSe monolayer on a Ti2O-type STO reconstructed surface. The established structure of monolayer FeSe on a Ti2O-type STO(001) reconstructed surface can naturally explain the experimental observation of the electronic band structure on the monolayer FeSe superconductor and obtained electrons counting per Fe atom. Surface states in the mid-gap induced by various STO surface reconstructions will result in band bending. The surface-state-induced band bending is also responsible for the electron transfer from the STO substrate to the FeSe films.

Carbon has the capability of forming various bonding states that affect the structures and properties of transition metal carbides. In this work, structural search was performed to explore the structural diversity of LaC2 at pressures of 0.0–30.0 GPa. Five stable structures of LaC2 reveal a variety of carbon structural units ranging from a dimer to bent C3, zigzag C4 and armchair polymer chains. A series of pressure-induced structural transformations are predicted, I4/mmm (i.e. experimental α phase) → C2/c → Pnma → Pmma, which involve the catenation of carbon from a dimer to zigzag C4 units and further to armchair polymer chains. The bent C3 unit appears in a novel Immm structure. This structure is the theoretical ground state of LaC2 under ambient conditions, but is kinetically inaccessible from the experimental α phase. LaC2 becomes thermodynamically metastable relative to La2C3 + diamond above 17.1 GPa, and eventually decomposes into constituent elements above 35.6 GPa. The presented results indicate that catenation of carbon can be realized even in simple inorganic compounds under nonambient conditions.

Carbon (C) is able to form various bonding patterns, including graphene sheets, chains and dimers, but stable bare six-membered C6 hexagonal rings, which are the fundamental structural motifs of graphite and graphene have long been missing. Here we report the stabilization of such bare C6 rings under high pressures in the charge-transfer systems of binary sesquicarbides Y2C3 and La2C3 as predicted by first-principles swarm structure searching simulations. We found that the external pressure can be used to efficiently tune structural transitions in the sesquicarbides from the ambient-pressure cubic phases into high-pressure orthorhombic phases, accompanied by significant C–C bonding modification from C–C dimers to bare C6 rings and polymerized graphene-like double C6 sheets. The bare C6 rings are stabilized in Y2C3 and La2C3 at pressures above 32 and 13 GPa, respectively, which are readily accessible to experiments. Chemical bonding analysis reveals that the bare C6 rings feature a benzene-like sp2 C–C bonding pattern with a delocalized π system. Y or La → C charge transfer and the need for denser structure packing are found to be part of the underlying mechanisms behind the stabilization of the bare C6 rings.