Inorganic Electrides Research Articles

Multi-dimensional inorganic electrides for energy conversion and storage

Multi-dimensional inorganic electrides exhibit outstanding advantages, including a low work function, high electron mobility, and superconductivity, etc. These properties render them promising for potential applications in the fields of ion batteries, electronic devices, catalysis.

Predicted superconductivity in one-dimensional A3Hf2B3-type electrides.

Inorganic electrides are considered potential superconductors due to the unique properties of their anionic electrons. However, most electrides require external high-pressure conditions to exhibit considerable superconducting transition temperatures (Tc). Therefore, searching for superconducting electrides under low or moderate external pressures is of significant research interest and importance. In this work, a series of A3Hf2B3-type compounds (A = Mg, Ca, Sr, Ba; B = Si, Ge, Sn, Pb) were constructed and systematically studied based on density functional theory calculations. According to the analysis of the electronic structures and phonon dispersion spectrums, stable one-dimensional electrides Ca3Hf2Ge3, Ca3Hf2Sn3, and Sr3Hf2Pb3, were screened out. Interestingly, the superconductivity of these electrides were predicted from electron phonon coupling calculations. It is highlighted that Sr3Hf2Pb3 showed the highest Tc, reaching 4.02 K, while the Tc values of Ca3Hf2Ge3 and Ca3Hf2Sn3 were 1.16 K and 1.04 K, respectively. Moreover, the Tc value of Ca3Hf2Ge3 can be increased to 1.96 K under 20 GPa due to the effect of phonon softening. This work enriches the types of superconducting electrides and has important guiding significance for the research on constructing electrides and related superconducting materials.

Identifying different electronic transport mechanisms in nanoporous inorganic electrides – a combined study using Hall measurements and electron paramagnetic resonance spectroscopy

Temperature-dependent Hall measurements and electron paramagnetic resonance spectroscopy reveal that three different transport mechanisms contribute to the electronic properties of the inorganic electride [Ca24Al28O64]4+(4e−).

Molecular Electrides: An In Silico Perspective.

Electrides are defined as the ionic compounds where the electron(s) serves as an anion. These electron(s) is (are) not bound to any atoms, bonds, or molecules but are rather localized into the space, crystal voids, or interlayer between two molecular slabs. There are three major categories of electrides, known as organic electrides, inorganic electrides, and molecular electrides. The computational techniques have proven as a great tool to provide emphasis on the electride materials. In this review, we have focused on the computational methodologies and criteria that help to characterize molecular electrides. A detailed account of the computational methods and basis sets applicable for molecular electrides have been discussed along with their limitations in this field. The main criterion for the identification of the electrides has also been discussed thoroughly with proper examples. The molecular electrides presented here have been justified with all the required criteria that support and proved their electride characteristics. We have also presented few systems which have similar properties but are not considered as molecular electrides. Moreover, the applicability of the electrides in catalytic processes have also been presented.

Intermetallic Copper-Based Electride Catalyst with High Activity for C-H Oxidation and Cycloaddition of CO2 into Epoxides.

Inorganic electrides have been proved to be efficient hosts for incorporating transition metals, which can effectively act as active sites giving an outstanding catalytic performance. Here, it is demonstrated that a reusable and recyclable (for more than 7 times) copper-based intermetallic electride catalyst (LaCu0.67 Si1.33 ), in which the Cu sites activated by anionic electrons with low-work function are uniformly dispersed in the lattice framework, shows vast potential for the selective C-H oxidation of industrially important hydrocarbons and cycloaddition of CO2 with epoxide. This leads to the production of value-added cyclic carbonates under mild reaction conditions. Importantly, the LaCu0.67 Si1.33 catalyst enables much higher turnover frequencies for the C-H oxidation (up to 25 276 h-1 ) and cycloaddition of CO2 into epoxide (up to 800 000 h-1 ), thus exceeding most nonnoble as well as noble metal catalysts. Density functional theory investigations have revealed that the LaCu0.67 Si1.33 catalyst is involved in the conversion of N-hydroxyphthalimide (NHPI) into the phthalimido-N-oxyl (PINO), which then triggers selective abstraction of an H atom from ethylbenzene for the generation of a radical susceptible to further oxygenation in the presence of O2 .

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.

Theoretical and Experimental Studies of Gallate Melilite Electrides from Topotactic Reduction of Interstitial Oxide Ion Conductors.

A nonstoichiometric La1.5Sr0.5Ga3O7.25 melilite oxide ion conductor features active interstitial oxygen defects in its pentagonal rings with high mobility. In this study, electron localization function calculated by density functional theory indicated that the interstitial oxide ions located in the pentagonal rings of gallate melilites may be removed and replaced by electron anions that are confined within the pentagonal rings, which would therefore convert the melilite interstitial oxide ion conductor into a zero-dimensional (0D) electride. The more active interstitial oxide ions, compared to the framework oxide ions, make the La1.5Sr0.5Ga3O7.25 melilite structure more reducible by CaH2 using topotactic reduction, in contrast to the hardly reducible nature of parent LaSrGa3O7. The topotactic reduction enhances the bulk electronic conduction (σ ∼ 0.003 S/cm at 400 °C) by ∼ 1 order of magnitude for La1.5Sr0.5Ga3O7.25. The oxygen loss in the melilite structure was verified and most likely took place on the active interstitial oxide ions. The identified confinement space for electronic anions in melilite interstitial oxide ion conductors presented here provides a strategy to access inorganic electrides from interstitial oxide ion conductor electrolytes.

Superalkali (Li2F, Li3F) doped Al12N12 electrides with enhanced static, dynamic nonlinear optical responses and refractive indices

In the present work, superalkalis (Li2F and Li3F) doped Al12N12 nanocages have been designed and investigated theoretically to elucidate the correlations between geometric, electronic and nonlinear optical properties. All complexes (A-E) are stable inorganic electrides as indicated by their interaction energies (−52.97 to −87.58 kcal/mol). The decrease in HOMO-LUMO energy gaps (5.44–5.87 eV) has caused a remarkable increase in the nonlinear optical response of A-E complexes. The maximum values of first hyperpolarizability (1.20 × 104 a.u.) and second hyperpolarizability (1.65 × 107 a.u.) have been achieved for complex A. The highest dynamic hyperpolarizability coefficients including second harmonic generation (9.08 × 105 a.u.) and electro-optic Pockel's effect (9.56 × 105 a.u.) are obtained for complex A at 532 nm. The electric field-induced second harmonic generation (up to 3.52 × 108 a.u.) and dc-Kerr effect (up to 8.13 ×108 a.u.) along with a large quadratic nonlinear refractive index (up to 5.41 × 10−11 a.u.) of the designed electrides reflected their large NLO response.

Materials by design at high pressures.

Pressure, a fundamental thermodynamic variable, can generate two essential effects on materials. First, pressure can create new high-pressure phases via modification of the potential energy surface. Second, pressure can produce new compounds with unconventional stoichiometries via modification of the compositional landscape. These new phases or compounds often exhibit exotic physical and chemical properties that are inaccessible at ambient pressure. Recent studies have established a broad scope for developing materials with specific desired properties under high pressure. Crystal structure prediction methods and first-principles calculations can be used to design materials and thus guide subsequent synthesis plans prior to any experimental work. A key example is the recent theory-initiated discovery of the record-breaking high-temperature superhydride superconductors H3S and LaH10 with critical temperatures of 200 K and 260 K, respectively. This work summarizes and discusses recent progress in the theory-oriented discovery of new materials under high pressure, including hydrogen-rich superconductors, high-energy-density materials, inorganic electrides, and noble gas compounds. The discovery of the considered compounds involved substantial theoretical contributions. We address future challenges facing the design of materials at high pressure and provide perspectives on research directions with significant potential for future discoveries.

Electronic Correlation Strength of Inorganic Electrides from First Principles.

Strongly correlated electron systems, generally recognized as d- and f-electron systems, have attracted attention as a platform for the emergence of exotic properties such as high-Tc superconductivity. However, correlated electron behaviors have been recently observed in a group of novel materials, electrides, in which s-electrons are confined in subnanometer-sized spaces. Here, we present a trend of electronic correlation of electrides by evaluating the electronic correlation strength obtained from model parameters characterizing effective Hamiltonians of 19 electrides from first principles. The calculated strengths vary in the order 0D ≫ 1D > 2D ∼ 3D electrides, which corresponds to experimental trends, and exceed 10 (a measure for the emergence of exotic properties) in all of the 0D and some of the 1D electrides. We also found the electronic correlation depends on the cation species surrounding the s-electrons. The results indicate that low-dimensional electrides will be new research targets for studies of strongly correlated electron systems.

Inorganic electrides of alkali metal doped Zn12O12 nanocage with excellent nonlinear optical response

Finding new materials with exceptionally large nonlinear optical response is an interesting and challenging avenue for scientific research. Here, we report the alkali metal doped Zn12O12 nanocages as inorganic electrides with excellent nonlinear optical response. Density functional theory calculations have been performed for geometric, electronic and nonlinear optical response of exo- and endohedrally alkali metal doped Zn12O12 nanoclusters. For exohedral doping, all different possible doping sites are considered for decoration of alkali metal on the nanocage. The electride nature of the complexes is highly dependent on the position of alkali metal doping. All exohedral complexes except for alkali metal doping on six membered ring (r6) are electride in nature, as revealed from frontier molecular orbital analysis. Interaction energies reveal that all doped nanoclusters except endo-K@Zn12O12 are thermodynamically stable. The exothermic encapsulation of alkali metals in Zn12O12 nanocages is in marked contradiction with other inorganic fullerenes where encapsulation is an endothermic process. The barriers for boundary crossing are also evaluated in order study the interconversion of exo- and endohedral complexes. Doping of alkali metal significantly influences the properties of nanocages. HOMO-LUMO (H-L) gap is reduced significantly whereas hyperpolarizability is increased several orders of magnitude. The NLO response of exohedrally doped complexes is higher than the corresponding endohedral complexes, which is in mark contradiction with the behavior of phosphide or nitride nanocages. The highest first hyperpolarizability of 1.0 × 105 au is calculated for K@r6-Zn12O12 complex. Third order NLO response of these complexes is calculated and compared with the best systems reported in the literature at the same level of theory.

Advances in Materials and Applications of Inorganic Electrides.

Electrides are materials in which electrons serve as anions. Here, the concept of inorganic electrides is extended in several respects: from ionic crystals to intermetallic compounds in host materials, from crystalline to amorphous solids, and from 0-dimensional to 1- and 2-dimensional materials in electron-confined spaces. In particular, 2D electrides, in which anionic electrons are sandwiched by cationic slabs, can form a bulk crystal of a 2-dimensional electron gas, thus exhibiting a large electron mobility and providing a platform for topological materials. Exploration of new electrides by computation and high pressure has advanced, revealing that an electride is a stable equilibrium phase of many elements and compounds under high pressure. This review describes the history and current status of electride research and next summarizes the chemical application of electrides and relevant materials. An emphasis is placed on catalysts for ammonia synthesis from N2 and H2 at mild conditions. This subject is accelerated by a demand for on-site ammonia synthesis using hydrogen produced by renewable energy sources. A wide applicability of electride for chemical reactions such selective hydrogenation and carbon-carbon coupling is shown by extending the concept of electrides. Finally, a view for the relationship between electrides and crystallographic voids and current issues are described.

Layered Transition Metal Electride Hf2Se with Coexisting Two-Dimensional Anionic d-Electrons and Hf–Hf Metallic Bonds* *Supported by the National Natural Science Foundation of China (Grant No. 11774424), the Fundamental Research Funds for the Central Universities (Grant No. 2017RC20), the Research Funds of Renmin University of China (Grant No. 20XNH064), the CAS Interdisciplinary Innovation Team, the Beijing Natural Science Foundation (Grant No.

Electrides are unique materials, whose anionic electrons are confined to interstitial voids, and they have broad potential applications in various areas. In contrast to the majority of inorganic electrides, in which the anionic electrons primarily consist of s-electrons of metals, electrides with anionic d-electrons are very rare. Based on first-principles electronic structure calculations, we predict that the layered transition metal chalcogenide Hf2Se is a novel electride candidate with anionic d-electrons. Our results indicate that the anionic electrons confined in the Hf6 octahedra vacancy between [Hf2Se] layers mainly come from the Hf-5d orbitals. In addition, the anionic electrons coexist with the Hf–Hf multiple-center metallic bonds located in the center of neighboring Hf4 tetrahedra. The calculated work function (3.33 eV) for the (110) surface of Hf2Se is slightly smaller than that of Hf2S, which has recently been reported to exhibit good electrocatalytic performance. Our study of Hf2Se will enrich the electride family, and promote further research into the physical properties and applications of electrides.

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.

Electronic Anisotropy and Superconductivity in One-Dimensional Electride Ca3Si

Inorganic electrides have gained remarkable attention for their intrinsic physical properties derived from loosely bound anionic electrons. Herein, using ab initio evolutionary structure search, we...

Leading the Charge of Electride Discovery

Rare materials, by definition, are difficult to “discover.” Electrides—ionic compounds in which electrons act as the anion—are no exception. Finding inorganic electrides among thousands of materials is aided by computational screening, demonstrated herein by Zhu et al.

Computational Discovery of Inorganic Electrides from an Automated Screening

Electrides, with their excess electrons distributed in crystal cavities playing the role of anions, exhibit a variety of unique properties which make these materials desirable for many applications in catalysis, nonlinear optics and electron emission. While the first electride was discovered almost four decades ago, only few electride materials are known today, which limits our fundamental understanding as well as the practical use of these exotic materials. In this work, we propose an automated computational screening scheme to search for interstitial electrons and quantify their distributions inside the crystal cavities and energy space. Applying this scheme to all candidate materials in the Inorganic Crystal Structure Database (ICSD), we report the identification of 167 potential electride materials. This work significantly increases the current library of electride materials, which enables an in-depth investigation of their chemistry-structure-property relations and opens new avenues for future electride design.

Superalkalis as a source of diffuse excess electrons in newly designed inorganic electrides with remarkable nonlinear response and deep ultraviolet transparency: A DFT study

Recently, significant progress is observed in the design and synthesis of nonlinear optical materials due to their optoelectronic and biomedical applications. In this report, a series of inorganic electrides (Li2F@Al12P12, Li3O@Al12P12 and Li4N@Al12P12) are designed by doping of Al12P12 nanocluster with superalkalis (Li2F, Li3O and Li4N) and studied through density functional theory (DFT) for their geometrical, electronic and nonlinear optical properties. Computational results indicated that these superalkalis doped complexes possess high stability and low HOMO-LUMO gaps. Interaction energies reveal that adsorption of Li4N on Altop site of Al12P12 results in highly stable structure (isomer J), where superalkali is strongly chemisorbed on the nanocage (Eint. = −105.13 kcal mol−1). Moreover, the lowest HOMO-LUMO gap is also observed for J isomer of Li4N@Al12P12 (0.44 eV), compared to 0.94 eV for alkali metal doped Al12P12 nanocage and 3.36 eV for pure nanocage. Doping of superalkali on aluminum phosphide nanocage can bring considerable increase in first hyperpolarizabilities (βo) response of the nanocage along with deep ultraviolet transparency. The first hyperpolarizability (βo) for isomer J of Li4N@Al12P12 is 6.25 × 104 au. This study may provide an effective strategy to design high performance NLO materials from stable inorganic electrides.

Ternary inorganic electrides with mixed bonding

A high-throughput screening based on first-principles calculations was performed to search for new ternary inorganic electrides. From the available materials database, we identified three new thermodynamically stable materials (Li$_{12}$Mg$_3$Si$_4$, NaBa$_2$O and Ca$_5$Ga$_2$N$_4$) as potential electrides made by main group elements, in addition to the well known mayenite based electride (C12A7:$e^-$). Different from those conventional inorganic electrides in which the excess electrons play only the role of anions, the three new materials, resembling the electrides found in simple metals under high pressure, possess mixed ionic and metallic bonding. The interplay between two competing mechanisms, together with the different crystal packing motifs, gives rise to a variety of geometries in anionic electrons, and rich physical phenomena such as ferromagnetism, superconductivity and metal-insulator transition. Our finding here bridges the gap between electrides found at ambient and high pressure conditions.

Prediction of Stoner-Type Magnetism in Low-Dimensional Electrides

Electrides are special ionic solids with excess cavity-trapped electrons serving as anions. Despite the extensive studies on electrides, the interplay between electrides and magnetism is not well understood due to the lack of stable magnetic electrides, particularly, the lack of inorganic magnetic electrides. Here, on the basis of the mechanism of Stoner-type magnetic instability, we propose that in certain electrides the low-dimensionality can facilitate the formation of a magnetic ground state because of the enhanced density of states near the Fermi level. To be specific, A5B3 (A = Ca, Sr, Ba; B = As, Sb, Bi) (one-dimensional, 1D), Sr11Mg2Si10 (zero-dimensional, 0D), Ba7Al10 (0D), and Ba4Al5 (0D) have been identified as stable magnetic electrides with spin-polarization energies of tens to hundreds of millielectronvolts per formula unit. Especially for Ba5As3, the spin-polarization energy can reach up to 220 meV. Furthermore, we demonstrate that the magnetic moment and spin density mainly derive from the...