First-Principles Prediction of Thermodynamically Stable Two-Dimensional Electrides

BaCu, a Two-Dimensional Electride with Cu Anions

The electron-rich characteristic and low work function endow electrides with excellent performance in (opto)electronics and catalytic applications; these two features are closely related to the structural topology, constituents, and valence electron concentration of electrides. However, the synthesized electrides, especially two-dimensional (2D) electrides, are limited to specific structural prototypes and anionic p-block elements. Here we synthesize and identify a distinct 2D electride of BaCu with delocalized anionic electrons confined to the interlayer spaces of the BaCu framework. The bonding between Cu and Ba atoms exhibits ionic characteristics, and the adjacent Cu anions form a planar honeycomb structure with metallic Cu-Cu bonding. The negatively charged Cu ions are revealed by the theoretical calculations and experimental X-ray absorption near-edge structure. Physical property measurements reveal that BaCu electride has a high electronic conductivity (ρ = 3.20 μΩ cm) and a low work function (2.5 eV), attributed to the metallic Cu-Cu bonding and delocalized anionic electrons. In contrast to typical ionic 2D electrides with p-block anions, density functional theory calculations find that the orbital hybridization between the delocalized anionic electrons and BaCu framework leads to unique isotropic physical properties, such as mechanical properties, and work function. The freestanding BaCu monolayer with half-metal conductivity exhibits low exfoliation energy (0.84 J/m2) and high mechanical/thermal stability, suggesting the potential to achieve low-dimensional BaCu from the bulk. Our results expand the space for the structure and attributes of 2D electrides, facilitating the discovery and potential application of novel 2D electrides with transition metal anions.

Designing barrier-free metal/MoS2 contacts through electrene insertion

Transition-metal dichalcogenides (TMDCs), including MoS2, have great potential in electronics applications. However, achieving low-resistance metal contacts is a challenge that impacts their performance in nanodevices due to strong Fermi-level pinning and the presence of a tunnelling barrier. As a solution, we explore a strategy of inserting monolayers of alkaline-earth sub-pnictide electrenes with a general formula of [M2X]+e- (M = Ca, Sr, Ba; X = N, P, As, Sb) between the TMDC and the metal. These electrenes possess two-dimensional sheets of charge on their surfaces that can be readily donated when interfaced with a TMDC semiconductor, thereby lowering its conduction band below the Fermi level and eliminating the Schottky and tunnelling barriers. In this work, density-functional theory (DFT) calculations were performed for metal/electrene/MoS2 heterojunctions for all stable M2X electrenes and both Au and Cu metals. To identify the material combinations that provide the most effective Ohmic contact, the charge transfer, band structure, and electrostatic potential were computed. Linear correlations were found between the charge donated to the MoS2 and both the electrene surface charge and work function. Overall, Ca2N appears to be the most promising electrene for achieving an Ohmic metal/MoS2 contact due to its high surface charge density.

Quasi-2D spin-Peierls transition through interstitial anionic electrons in K(NH3)2

Electron-phonon interactions and electron-electron correlations represent two crucial facets of condensed matter physics. For instance, in a half-filled spin-1/2 anti-ferromagnetic chain, the lattice dimerization induced by electron-nucleus interaction can be intensified by onsite Coulomb repulsion, resulting in a spin-Peierls state. Through first-principles calculations and crystal structure prediction methods, we have identified that under mild pressures, potassium and ammonia can form stable compounds: R3¯m K(NH3)2, Pm3¯m K(NH3)2, and Cm K2(NH3)3. Our predictions suggest that the R3¯m K(NH3)2 exhibits electride characteristics, marked by the formation of interstitial anionic electrons (IAEs) in the interlayer space. These IAEs are arranged in quasi-two-dimensional triangular arrays. With increasing pressure, the electronic van-Hove singularity shifts toward the Fermi level, resulting in an augmented density of states and the onset of both Peierls and magnetic instabilities. Analyzing these instabilities, we determine that the ground state of the R3¯m K(NH3)2 is the dimerized P21/m phase with zigzag-type anti-ferromagnetic IAEs. This state can be described by the triangular-lattice antiferromagnetic Heisenberg model with modulated magnetic interactions. Furthermore, we unveil the coexistence and positive interplay between magnetic and Peierls instability, constituting a scenario of spin-Peierls instability unprecedented in realistic 2D materials, particularly involving IAEs. This work provides valuable insights into the coupling of IAEs with the adjacent lattice and their spin correlations in quantum materials.

Low thermal expansion of layered electrides predicted by density-functional theory

Layered electrides are a unique class of materials with anionic electrons bound in interstitial regions between thin, positively charged atomic layers. While density-functional theory is the tool of choice for computational study of electrides, there has to date been no systematic comparison of density functionals or dispersion corrections for their accurate simulation. There has also been no research into the thermomechanical properties of layered electrides, with computational predictions considering only static lattices. In this work, we investigate the thermomechanical properties of five layered electrides using density-functional theory to evaluate the magnitude of thermal effects on their lattice constants and cell volumes. We also assess the accuracy of five popular dispersion corrections with both planewave and numerical atomic orbital calculations.

Magnetic Electrides: High-Throughput Material Screening, Intriguing Properties, and Applications

Electrides are a unique class of electron-rich materials where excess electrons are localized in interstitial lattice sites as anions, leading to a range of unique properties and applications. While hundreds of electrides have been discovered in recent years, magnetic electrides have received limited attention, with few investigations into their fundamental physics and practical applications. In this work, 51 magnetic electrides (12 antiferromagnetic, 13 ferromagnetic, and 26 interstitial-magnetic) were identified using high-throughput computational screening methods and the latest Materials Project database. Based on their compositions, these magnetic electrides can be classified as magnetic semiconductors, metals, or half-metals, each with unique topological states and excellent catalytic performance for N₂ fixation due to their low work functions and excess electrons. The novel properties of magnetic electrides suggest potential applications in spintronics, topological electronics, electron emission, and as high-performance catalysts. This work marks the beginning of a new era in the identification, investigation, and practical applications of magnetic electrides.

A new concept of atomically thin p-n junction based on Ca2N/Na2N donor-acceptor heterostructure: a first-principles study

In atomically thin p-n junctions, traditional strategies such as doping and implantation for realizing a p- or n-region will fail at the nanoscale, and the Schottky barrier and Fermi level pinning effect taking place in metal-semiconductor contacts seriously suppress the transport properties. In this work, based on first-principles calculations, we propose a new strategy for realizing an ultrathin p-n junction by vertically stacking nonstoichiometric Ca₂N and Na₂N monolayers, which represents a kind of donor-acceptor heterostructure with a natural Ohmic contact. It is of great interest to find that the tunneling barrier can be eliminated and the charge transfer quantity is one order of magnitude higher than that between polar monolayers by adjusting the interlayer distance. In addition, at equilibrium the interlayer tunneling can be turned into resonant transport due to the quasi-bonding, thus enabling excellent transmission performance. In accordance with the results, we believe that our new concept of an atomically thin p-n junction will provide an unprecedented possibility for the development of nanodevices.

Tellurium: A High-Performance Cathode for Magnesium Ion Batteries Based on a Conversion Mechanism

Magnesium ion batteries (MIBs), due to the low redox potential of Mg, high theoretical capacity, dendrite-free magnesiation, and safe nature, have been recognized as a post-lithium energy storage system. However, an ongoing challenge, sluggish Mg²⁺ kinetics in the small number of available cathode materials of MIBs, restricts its further development. The existing cathodes mostly deliver unsatisfactory capacity with poor cycling life based on the traditional ion-intercalation mechanism. Herein, we fabricated a conversion-type Mg∥Te battery based on a reversible two-step conversion reaction (Te to MgTe₂ to MgTe). High discharge capacities (387 mAh g^-1) and rate capability (165 mAh g^-1 at 5 A g^-1) can be achieved. The diffusivity of Mg²⁺ can reach 3.54 × 10^-8 cm² s^-1, enabled by the high electrical conductivity of Te and increased surface conversion sites. Subsequently, ab initio molecular dynamics simulation was also carried out to further confirm the conversion mechanism and fast Mg²⁺ transportation kinetics.

First-Principles Study of Three-Dimensional Electrides Containing One-Dimensional [Ba3N]3+ Chains

Electrides, a unique type of compound where electrons act as anions, have a high electron mobility and a low work function, which makes them promising for applications in electronic devices and high-performance catalysts. The discovery of novel electrides and the expansion of the electride family have great significance for their promising applications. Herein, we reported four three-dimensional (3D) electrides by coupling crystal structure database searches and first-principles electronic structure analysis. Subnitrides (Ba₃N, LiBa₃N, NaBa₃N, and Na₅Ba₃N) containing one-dimensional (1D) [Ba₃N]³⁺ chains are identified as 3D electrides for the first time. The anionic electrons are confined in the 3D interstitial space of Ba₃N, LiBa₃N, NaBa₃N, and Na₅Ba₃N. Interestingly, with the increase of Na content, the excess electrons of Na₅Ba₃N play two roles of metallic bonding and anionic electrons. Therefore, the subnitrides containing 1D [Ba₃N]³⁺ chains can be regarded as a new family of 3D electrides, where anionic electrons reside in the 3D interstitial spaces and provide a conduction path. These materials not only are experimentally synthesizable 3D electrides but also are promising to be exfoliated into advanced 1D nanowire materials. Furthermore, our work suggests a discovery strategy of novel electrides based on one parent framework like [Ba₃N]³⁺ chains, which would accelerate the mining of electrides from the crystal structure database.

Chemically Stable Low-Dimensional Electrides in Transition Metal-Rich Monochalcogenides: Theoretical and Experimental Explorations

Electrides, which are ionic crystals composed of excess anionic electrons, are of great interest as an exotic material for fundamental research and practical applications in broad fields of science and technology. However, an inherent chemical instability under ambient conditions at room temperature has been a fatal drawback to be addressed. Here, we report that transition metal-rich monochalcogenides are an emerging class of low-dimensional electrides with excellent chemical and thermal stability in air and water at room temperature through a comprehensive exploration of theoretical prediction and experimental verification. We predict new two-dimensional (2D) electrides crystallized in hexagonal P3̅m1 and P6₃/mmc structures with strong localization of anionic electrons in a dumbbell shape at the tetrahedral cavity of the interlayer space, which are distinct from the anionic electrons localized at the octahedral cavity in the hexagonal R3̅m structure of the previous 2D [Ca₂N]⁺·e^- and [Y₂C]²⁺·2e^- electrides. We successfully synthesized the room-temperature stable [Ti₂O]²⁺·2e^-, [Ti₂S]²⁺·2e^-, [Zr₂S]²⁺·2e^-, and primary solid solution [Hf₂S_xSe_1-x]²⁺·2e^- electrides, showing no structural degradation in air and water. Among them, we found that the synthesized [Ti₂S]²⁺·2e^- and [Zr₂S]²⁺·2e^- electrides are crystallized in orthorhombic symmetry (Pnnm), showing the feature of a one-dimensional (1D) electride with an anionic electron chain, which has never been reported yet. In addition to the successful finding of new 1D and 2D electrides, we discuss the self-passivation effect-driven chemical stability and the role of anionic electrons in determining the physical properties of the newly discovered electrides.

Predicted Stable Electrides in Mg-Al System under High Pressure

Magnesium and aluminum, as the adjacent light metal elements, are difficult to form the stable stoichiometries compounds under ambient conditions. In this work, using evolutionary ab initio structural prediction approaches,...

Boosting Photoredox Catalysis Using a Two-Dimensional Electride as a Persistent Electron Donor

Electrides, which have excess anionic electrons, are solid-state sources of solvated electrons that can be used as powerful reducing agents for organic syntheses. However, the abrupt decomposition of electrides in organic solvents makes controlling the transfer inefficient, thereby limiting the utilization of their superior electron-donating ability. Here, we demonstrate the efficient reductive transformation strategy which combines the stable two-dimensional [Gd₂C]²⁺·2e^- electride electron donor and cyclometalated Pt(II) complex photocatalysts. Strongly localized anionic electrons at the interlayer space in the [Gd₂C]²⁺·2e^- electride are released via moderate alcoholysis in 2,2,2-trifluoroethanol, enabling persistent electron donation. The Pt(II) complexes are adsorbed onto the surface of the [Gd₂C]²⁺·2e^- electride and rapidly capture the released electrons at a rate of 10⁷ s^-1 upon photoexcitation. The one-electron-reduced Pt complex is electrochemically stable enough to deliver the electron to substrates in the bulk, which completes the photoredox cycle. The key benefit of this system is the suppression of undesirable charge recombination because back electron transfer is prohibited due to the irreversible disruption of the electride after the electron transfer. These desirable properties collectively serve as the photoredox catalysis principle for the reductive generation of the benzyl radical from benzyl halide, which is the key intermediate for dehalogenated or homocoupled products.

Concave nano-octahedral alloys: wet chemical synthesis of bimetallic Pt-Pd nanocrystals with high-index {hhl} Facets

Concave morphologies provide noble metal nanocrystals (NCs) with unique performances due to large specific surface areas, high curves, hot spots, and elevated energy facets. As a result, concave morphologies have attracted considerable attention in many areas. However, most NCs with concave shapes are currently made of a single metal, leaving plenty of room for easy wet chemical synthesis and structural analysis of unique concave structures, especially bimetallic compounds. In this work, concave octahedral Pt-Pd alloy NCs with high-index {hhl} faces were synthesized using glycine as a coordination molecule and polyvinylpyrrolidone as the surfactant and reducing agent. The high-index facets coupled with the synergistic and electronic effects between Pt and Pd provided concave octahedral Pt-Pd alloy NCs with excellent activity and stability toward the electrooxidation of formic acid when compared to their convex counterparts and commercial Pt black.

A valence balancing rule for the design of bimetallic phosphides targeting high thermoelectric performance

Two-dimensional (2D) materials with outstanding electronic and mechanical properties have attracted considerable attention as efficient thermoelectric materials. Here, we propose a generalized eight-valence electron rule for designing 2D semiconductor materials, i.e., metal-shrouded bimetallic phosphides ABP (A: group IA element, B: group IIA element). Following this rule, we screen out ten stable semiconductors (LiMgP, LiCaP, LiSrP, NaBeP, NaMgP, KMgP, KCaP, RbMgP, RbCaP and RbSrP) with tunable bandgaps in the range of 0.35-2.40 eV by comprehensive first-principles calculations. Among them, the electron mobility of RbMgP can be as high as 2.3 × 10⁴ cm² V^-1 s^-1, and the hole mobility of KMgP is estimated to be 9.9 × 10³ cm² V^-1 s^-1. Moreover, KMgP, KCaP, RbCaP and RbSrP exhibit an ultralow thermal conductivity of 0.02, 0.14, 0.08 and 0.14 W m^-1 K^-1, respectively. As a result, KMgP and RbCaP monolayers are p-type or n-type thermoelectric materials with a figure of merit of 2.25 and 1.13 at room temperature, respectively. The underlying mechanism of high electron conductivity and low thermal conductivity has been correlated with their unique bonding characteristics, narrow phonon band gap and the scattering from low-frequency phonons. This work demonstrates not only a guiding electron principle to design stable 2D semiconductors, but also a powerful metal-shrouded strategy for discovering high performance thermoelectric materials by decoupling electronic and thermal transport properties.

Wide Band Gap P3S Monolayer with Anisotropic and Ultrahigh Carrier Mobility

Phosphorene has offered an additional advantage for developing new optoelectronic devices due to its anisotropic and high carrier mobility. However, its instability in air causes a rapid degradation of the performance of the device. Thus, improving the stability of phosphorene while maintaining its original properties has become the key to the development of high-performance electronic devices. Herein, we propose that the formation of two-dimensional (2D) P-rich P-S compounds could achieve this goal. First-principles swarm-structural searches revealed two previously unkonwn P₃S and P₂S monolayers. The P₃S monolayer, consisting of n-bicyclo-P₆ units along the armchair direction, exhibits anisotropic and wide band gap characteristics. Interestingly, its carrier mobility reaches 1.11 × 10⁴ cm² V^-1 s^-1 and is much higher than in phosphorene. Its electronic band gap and optical absorption coefficients in the ultraviolet region reach 2.71 eV and 10⁵ cm^-1, respectively. Additionally, the P₃S monolayer has a high structural stability and resistance to air oxidation.

Regulating Work Function of [Ca24Al28O64]4+:4e- Electrides Via Changing Solvated Electron Characters

Since electrides behave as electron donors in catalytic reactions and device substrates due to the anionic electrons localized in periodic interstitial spaces, their work functions (Φ) could be used as key indicators in describing the high electron-donating power. Taking the [Ca₂₄Al₂₈O₆₄]⁴⁺:4e^- electride as an example, we here propose a new computational scheme of Φ and, further, characterize Φ of the bulk derivative structures of the electride for clarifying the relationship between structural characteristics and Φ. Results indicate that the external strain hardly affects Φ, but the interior heteroatom-doping and distortion bring about significant changes. All these unique variations of the bulk Φ are governed by the distribution and solvation character of anionic electrons in the cage conduction band states. The mechanism of regulating Φ revealed in this work may play a role in the rational design of electride-based catalysts and devices with a superior performance.

Enhanced performance of Mo2P monolayer as lithium-ion battery anode materials by carbon and nitrogen doping: a first principles study

By means of density functional theory (DFT) computations, we explored the potential of carbon- and nitrogen-doped Mo2P (CMP and NMP) layered materials as the representative of transition metal phosphides (TMPs) for the development of lithium-ion battery (LIB) anode materials, paying special attention to the synergistic effects of the dopants. Both CMP and NMP have exceptional stabilities and excellent electronic conductivity, and a high theoretical maximum storage capacity of ∼ 486 mA h g-1. Li-ion diffusion barriers on the two-dimensional (2D) CMP and NMP surfaces are extremely low (∼0.036 eV), and it is expected that on these 2D layers Li can diffuse 104 times faster than that on MoS2 and graphene at room temperature, and both monolayers have relatively low average open-circuit voltage (0.38 and 0.4 eV). All these exceptional properties make CMP and NMP monolayers as promising candidates for high-performance LIB anode materials, which also demonstrates that simple doping is an effective strategy to enhance the performance of anode materials in rechargeable batteries.

Ferromagnetic Weyl Fermions in Two-Dimensional Layered Electride Gd_{2}C

Recently, two-dimensional layered electrides have emerged as a new class of materials which possess anionic electrons in the interstitial spaces between cationic layers. Here, based on first-principles calculations, we discover a time-reversal-symmetry-breaking Weyl semimetal phase in a unique two-dimensional layered ferromagnetic (FM) electride Gd_{2}C. It is revealed that the crystal field mixes the interstitial electron states and Gd-5d orbitals near the Fermi energy to form band inversions. Meanwhile, the FM order induces two spinful Weyl nodal lines (WNLs), which are converted into multiple pairs of Weyl nodes through spin-orbit coupling. Further, we not only identify Fermi-arc surface states connecting the Weyl nodes but also predict a large intrinsic anomalous Hall conductivity due to the Berry curvature produced by the gapped WNLs. Our findings demonstrate the existence of Weyl fermions in the room-temperature FM electride Gd_{2}C, therefore offering a new platform to investigate the intriguing interplay between electride materials and magnetic Weyl physics.

Ti2P monolayer as a high performance 2-D electrode material for ion batteries

Electrical conductivity, storage capacity and ion diffusion ability are three crucial parameters for battery electrode materials. However, rare existing two-dimensional (2-D) electrode materials can achieve high performances in all these parameters. Here, we report that a 2-D transition-metal phosphide, the Ti₂P monolayer, is a promising superior electrode material which realizes high performances in all the parameters mentioned above. The Ti₂P monolayer has a stable honeycomb crystal structure. It has a metallic electronic structure with Li/Na adsorption, which ensures good electrical conductivity during the battery operation. We find that Li/Na can chemically bond to the Ti₂P substrate, with specific charge exchanges. Our results show the Li/Na capacity in the Ti₂P monolayer is about 846 mA h g^-1, which is much higher than that of the graphite anode. Remarkably, the Li/Na diffusion barrier on the Ti₂P monolayer is only 12-16 meV, which is lower than that in all 2-D anode materials proposed till now. Our work highly promises that theTi₂P monolayer can serve as a superior anode material for Li-ion/Na-ion batteries by providing good electrical conductivity, high storage capacity and ultrafast ion diffusion.

Recent Advances and Applications of Inorganic Electrides

Electrides, accommodating excess electrons in lattice voids as anions, have attracted considerable attention in both fundamental research and application development because of their interesting properties, such as ultralow work functions, high electronic mobility, high catalytic activity, and anisotropic electronic and optical properties. Recently, much research progress has been made in both types and applications of inorganic electrides because of the high stability. In this Perspective, we aim to summarize the recent development of inorganic electrides discovered and proposed by experiment and theoretical calculations, highlighting the main applications, including catalysis, metal-ion batteries, superconductivity, magnetism, and organic light-emitting diodes. We provide insights into the role of anionic electrons in electrides playing in the stability and properties. Finally, the problems, challenges, and opportunities are presented, which provide an outlook for future research.

2D tetragonal transition-metal phosphides: an ideal platform to screen metal shrouded crystals for multifunctional applications

Two-dimensional (2D) metal shrouded crystals, a new kind of conceptional material, have attracted remarkable attention due to their unique properties. Here, we propose a novel class of 2D metal shrouded materials, tetragonal transition-metal phosphides (TM₂Ps), which show peculiar features of coexistence of in-plane TM-P covalent bonds and TM-TM interlayer metallic bonds. From a combination of high throughput searching and first-principles calculations, Fe₂P, Co₂P, Ni₂P, Ru₂P, and Pd₂P monolayer sheets stand out because they simultaneously have high thermal, dynamical, and mechanical stability. All these five TM₂P materials are metals, especially Pd₂P, which can be a promising catalyst for the hydrogen evolution reaction with a very low overpotential. Moreover, these 2D TM₂Ps show good ductility since they can withstand a tensile strain of up to 45%. Even in the large strain range, the strengthened interlayer TM-TM metallic bonds dominate the deformation behavior, and the corresponding metallicity of 2D TM₂Ps is well preserved. Due to the competition between the d-d direct exchange and d-p-d superexchange interactions, Fe₂P behaves as an antiferromagnetic material with a T_N of 23 K, while Co₂P is a ferromagnetic material with a T_C of 580 K. Our results not only enrich the database of 2D metal shrouded crystals, but also provide novel 2D materials as promising candidates for multifunctional applications in nanoelectronics, spintronics and electrocatalysis.

Ferromagnetic quasi-atomic electrons in two-dimensional electride

An electride, a generalized form of cavity-trapped interstitial anionic electrons (IAEs) in a positively charged lattice framework, shows exotic properties according to the size and geometry of the cavities. Here, we report that the IAEs in layer structured [Gd₂C]²⁺·2e⁻ electride behave as ferromagnetic elements in two-dimensional interlayer space and possess their own magnetic moments of ~0.52 μ_B per quasi-atomic IAE, which facilitate the exchange interactions between interlayer gadolinium atoms across IAEs, inducing the ferromagnetism in [Gd₂C]²⁺·2e⁻ electride. The substitution of paramagnetic chlorine atoms for IAEs proves the magnetic nature of quasi-atomic IAEs through a transition from ferromagnetic [Gd₂C]²⁺·2e⁻ to antiferromagnetic Gd₂CCl caused by attenuating interatomic exchange interactions, consistent with theoretical calculations. These results confirm that quasi-atomic IAEs act as ferromagnetic elements and trigger ferromagnetic spin alignments within the antiferromagnetic [Gd₂C]²⁺ lattice framework. These results present a broad opportunity to tailor intriguing ferromagnetism originating from quasi-atomic interstitial electrons in low-dimensional materials.

Ferromagnetic quasi-atomic behavior of interstitial anionic electrons (IAEs) in practical electrides is yet to be discovered experimentally. Here, the authors reveal that IAEs in two-dimensional electride [Gd₂C]²⁺⋅2e^- behave as magnetic elements with their own magnetic moment.

Two-dimensional Ca4N2 as a one-dimensional electride [Ca4N2]2+·2e- with ultrahigh conductance

Electrides possess high electrical conductance and reactivity and are promising for novel applications in electronics and catalysis. Here, we predict a new thermodynamically and kinetically stable two-dimensional (2D) Ca4N2 using first-principles density functional theory (DFT) calculations. 2D Ca4N2 can serve as a one-dimensional (1D) electride [Ca4N2]2+·2e- with anionic electrons confined in the surface channels. In particular, we demonstrate that 2D Ca4N2 possesses high Fermi velocity (0.42 × 106 m s-1), electron effective Fermi mass (∼1me), ultrahigh charge density (1.14 × 1015 cm-2), and high carrier mobility (215 and 5.29 × 106 cm2 V-1 s-1 at a room temperature of 300 K and a low temperature of 2 K), resulting in ultrahigh conductance up to 0.039 and 966 S respectively for 300 and 2 K, compared to existing 2D materials and the best conductors (Cu and Ag). Furthermore, the first finding of 1D anionic electron behaviour on the surface of 2D materials can be used to stimulate the design of new kinds of electrides for exploring the physics of 1D and quasi-1D systems.

Electrides with Dinitrogen Ligands

Electrides are a class of materials which contain excess electrons occupying the cavities in the crystal and playing the role of anions. To achieve electron-rich conditions, it usually requires a positive total formal charge in electride materials. However, the assignment of charges relies on a detailed analysis on chemical bonding. Herein, we present a survey on potential electrides which may be overlooked if no bonding analysis is performed. By applying various structure sampling techniques in conjunction with first-principles calculation, we predicted two compounds Ba₂N₂:e^- and Li₂Ca₃N₆:2e^-, both of which are featured by the presence of dinitrogen ligands [N₂], to be potential electrides. While Li₂Ca₃N₆:2e^- with [N₂]^2- ions has been synthesized in the past, its electride nature was discovered for the first time based on our high-throughput screening. On the other hand, Ba₂N₂:e^- with [N₂]^3- ions is a new compound entirely from first-principles structure prediction. The different valence states of dinitrogen ligands identified in these two compounds suggest a novel route to tune the concentration and anisotropic properties of anionic interstitial electrons. Our discovery does not only establish a new class of inorganic electrides but also demonstrates the predictive power of modern crystal structure sampling techniques toward rational material design.

Super-alkalis as building blocks of one-dimensional hierarchical electrides

Electrides are ionic compounds where localized electrons act as anions. Because of their exotic properties in optoelectronics and catalysis, electrides have attracted considerable interest in recent years and many electrides, with anionic electrons distributed in zero-, one-, two- and three-dimensions, have been studied. Here, using first-principles calculations, we report a different class of inorganic electrides where Li3O super-alkali clusters form the building blocks of one-dimensional hierarchical structures, characterized by the presence of localized electrons with high charge density along a one-dimensional chain. We further show that the stability of this electride can be improved by embedding it inside a BN nanotube (BNNT). In addition, Li3O@BNNT exhibits powerful reducing ability as well as anti-ferromagnetism, highlighting the importance of nano-assembly in the development of new stable electrides with novel properties.

Determination of Dynamically Stable Electrenes toward Ultrafast Charging Battery Applications

Electrenes, an atomically thin form of layered electrides, are very recent members of the 2D materials family. In this work, we employed first-principle calculations to determine stable, exfoliatable, and application-promising 2D electrene materials among possible M₂X compounds, where M is a group II-A metal and X is a nonmetal element (C, N, P, As, and Sb). The promise of stable electrene compounds for battery applications is assessed via their exfoliation energy, adsorption properties, and migration energy barriers toward relevant Li, Na, K, and Ca atoms. Our calculations revealed five new stable electrene candidates in addition to previously known Ca₂N and Sr₂N. Among these seven dynamically stable electrenes, Ba₂As, Ba₂P, Ba₂Sb, Ca₂N, Sr₂N, and Sr₂P are found to be very promising for either K or Na ion batteries due to their extremely low migration energy barriers (5-16 meV), which roughly demonstrates 105 times higher mobility than graphene and two to four times higher mobility than other promising 2D materials such as MXene (Mo₂C).

TiC3 Monolayer with High Specific Capacity for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted considerable attention due to the intrinsic safety and high abundance of sodium. However, the lack of high-performance anode materials becomes a main obstacle for the development of SIBs. Here, we identify an ideal anode material, a metallic TiC₃ monolayer with not only remarkably high storage capacity of 1278 mA h g^-1 but also low barrier energy and open-circuit voltage, through first-principles swarm-intelligence structure calculations. TiC₃ still keeps metallic after adsorbing two-layer Na atoms, ensuring good electrical conductivity during the battery cycle. Besides, high melting point and superior dynamical stability are in favor of practical application. Its excellent performance can be mainly attributed to the presence of an unusual n-biphenyl unit in the TiC₃ monolayer. High cohesive energy, originating from multibonding coexistence (e.g., covalent, ionic, and metal bonds) in the TiC₃ monolayer, provides strong feasibility for experimental synthesis. In comparison with TiC₃, functionalized TiC₃ with oxygen shows a higher storage capacity; meanwhile, it keeps nearly the same barrier energy. This is in sharp contrast with metal-rich MXenes. These intriguing properties make the TiC₃ monolayer a promising anode material for SIBs.

Topological Electride Y2C

Two-dimensional (2D) electrides are layered ionic crystals in which anionic electrons are confined in the interlayer space. Here, we report a discovery of nontrivial [Formula: see text] topology in the electronic structures of 2D electride Y₂C. Based on first-principles calculations, we found a topological [Formula: see text] invariant of (1; 111) for the bulk band and topologically protected surface states in the surfaces of Y₂C, signifying its nontrivial electronic topology. We suggest a spin-resolved angle-resolved photoemission spectroscopy (ARPES) measurement to detect the unique helical spin texture of the spin-polarized topological surface state, which will provide characteristic evidence for the nontrivial electronic topology of Y₂C. Furthermore, the coexistence of 2D surface electride states and topological surface state enables us to explain the outstanding discrepancy between the recent ARPES experiments and theoretical calculations. Our findings establish a preliminary link between the electride in chemistry and the band topology in condensed-matter physics, which are expected to inspire further interdisciplinary research between these fields.

First-Principles Prediction of New Electrides with Nontrivial Band Topology Based on One-Dimensional Building Blocks

We introduce a new class of electrides with nontrivial band topology by coupling materials database searches and first-principles-calculations-based analysis. Cs_{3}O and Ba_{3}N are for the first time identified as a new class of electrides, consisting of one-dimensional (1D) nanorod building blocks. Their crystal structures mimic β-TiCl_{3} with the position of anions and cations exchanged. Unlike the weakly coupled nanorods of β-TiCl_{3}, Cs_{3}O and Ba_{3}N retain 1D anionic electrons along the hollow interrod sites; additionally, a strong interrod interaction in C_{3}O and Ba_{3}N induces band inversion in a 2D superatomic triangular lattice, resulting in Dirac-node lines. The new class of electrides can serve as a prototype for new electrides with a large cavity space that can be utilized for various applications such as gas storage, ion transport, and metal intercalation.

Tiered Electron Anions in Multiple Voids of LaScSi and Their Applications to Ammonia Synthesis

Electrides-compounds in which electrons localized in interstitial spaces periodically serve as anions-have attracted broad attention for their exotic properties, such as extraordinary electron-donating ability. In our efforts to expand this small family of phases, LaScSi emerges as a promising candidate. Its electron count is 2e^- f.u.^-1 in excess of that expected from the Zintl concept, while its structure offers interstitial spaces that can accommodate these extra electrons. Herein, this potential is explored through density functional theory (DFT) calculations and property measurements on LaScSi. DFT calculations (validated by heat capacity and electrical transport measurements) reveal electron density peaks at two symmetry-distinct interstitial sites. Importantly, this electride-like character is combined with chemical stability in air and water, an advantage for catalysis. Ru-loaded LaScSi shows outstanding catalytic activity for ammonia synthesis, with a turnover frequency (0.1 s^-1 at 0.1 MPa, 400 °C) an order of magnitude higher than those of oxide-based Ru catalysts, e.g., Ru/MgO. As with other electrides, LaScSi's ability to reversibly store hydrogen prevents the hydrogen poisoning of Ru surfaces. The better performance of LaScSi, however, hints at the importance of the high concentration (>1.6 × 10²² cm^-3 ) and tiered nature of its anionic electrons, which offers guidance toward new catalysts.

Thermodynamic cycles of the alkali metal–ligand complexes central to electride formation

Alkali metal–ligand complexes are the building blocks of the exotic organic alkalide and electride materials.