Electride Characteristics Research Articles

Constructing Tunable Electrides on Monolayer Transition Metal Dichalcogenides.

Electrides have emerged as promising materials with exotic properties due to the presence of localized electrons detached from all atoms. Despite the continuous discovery of many new electrides, most of them are based on atypical compositions, and their applications require an inert surface structure to passivate reactive excess electrons. Here, we demonstrate a different route to attain tunable electrides. We first report that monolayer transition metal dichalcogenides (TMDCs) exhibit weak electride characteristics, which is the remainder of the electride feature of the transition metal sublattice. By introducing chalcogen vacancies, the enhanced electride characteristics are comparable to those of known electrides. Since the precise tailoring of the chalcogen vacancy concentration has been achieved experimentally, we proposed that TMDCs can be used to build electrides with controllable intensities. Furthermore, we demonstrate that the electride states at the chalcogen vacancy of monolayer TMDCs will play an important role in catalyzing hydrogen evolution reactions.

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.

Pressure-induced evolution of stoichiometries and electronic structures of host–guest Na–B compounds

Superionic and electride behaviors in materials, which induce a variety of exotic physical properties of ions and electrons, are of great importance both in fundamental research and for practical applications. However, their coexistence in hot alkali-metal borides has not been observed. In this work, we apply first-principles structure search calculations to identify eight Na–B compounds with host–guest structures, which exhibit a wide range of building blocks and interesting properties linked to the Na/B composition. Among the known borides, Na-rich Na9B stands out as the composition with the highest alkali-metal content, featuring vertex- and face-sharing BNa16 polyhedra. Notably, it exhibits electride characteristics and transforms into a superionic electride at 200 GPa and 2000 K, displaying unusual Na atomic diffusion behavior attributed to the modulation of the interstitial anion electrons. It demonstrates semiconductor behavior in the solid state, and metallic properties associated with Na 3p/3s states in the superionic and liquid regions. On the other hand, B-rich NaB7, consisting of a unique covalent B framework, is predicted to exhibit low-frequency phonon-mediated superconductivity with a Tc of 16.8 K at 55 GPa. Our work advances the understanding of the structures and properties of alkali-metal borides.

Rational Design, Stabilities and Nonlinear Optical Properties of Non-Conventional Transition Metalides; New Entry into Nonlinear Optical Materials.

Electronic and nonlinear optical properties of endohedral metallofullerenes are presented. The endohedral metallofullerenes contain transition metal encapsulated in inorganic fullerenes X12Y12 (X = B, Al & Y = N, P). The endohedral metallofullerenes (endo-TM@X12Y12) possess quite interesting geometric and electronic properties, which are the function of the nature of the atom and the size of fullerene. NBO charge and frontier molecular orbital analyses reveal that the transition metal encapsulated Al12N12 fullerenes (endo-TM@Al12N12) are true metalides when the transition metals are Ni, Cu and Zn. Endo-Cr@Al12N12 and endo-Co@Al12N12 are at the borderline between metalides and electrides with predominantly electride characteristics. The other members of the series are excess electron systems, which offer interesting electronic and nonlinear optical properties. The diversity of nature possessed by endo-TM@Al12N12 is not prevalent for other fullerenes. Endo-TM@Al12P12 are true metalides when the transition metals are (Cr-Zn). HOMO-LUMO gaps (EH-L) are reduced significantly for these endohedral metallofullerenes, with a maximum percent decrease in EH-L of up to 70%. Many complexes show odd-even oscillating behavior for EH-L and dipole moments. Odd electron species contain large dipole moments and small EH-L, whereas even electron systems have the opposite behavior. Despite the decrease in EH-L, these systems show high kinetic and thermodynamic stabilities. The encapsulation of transition metals is a highly exergonic process. These endo-TM@X12Y12 possess remarkable nonlinear optical response in which the first hyperpolarizability reaches up to 2.79 × 105 au for endo-V@Al12N12. This study helps in the comparative analysis of the potential nonlinear optical responses of electrides, metalides and other excess electron systems. In general, the potential nonlinear optical response of electrides is higher than metalides but lower than those of simple excess electron compounds. The higher non-linear optical response and interesting electronic characteristics of endo-TM@Al12N12 complexes may be promising contenders for potential NLO applications.

NLO properties and electride characteristics of superalkalis doped all-cis-1,2,3,4,5,6-hexafluorocyclohexane complexes

The designing and synthesis of efficient NLO materials is the demand of the century because of their importance in the development of photo-electronic devices. Herein, we studied the geometric, electronic, and nonlinear properties of superalkalis doped all-cis-1,2,3,4,5,6-hexafluorocyclohexane, Janus type complexes. These complexes have an unprecedented involvement of respective superalkalis as source of excess electrons, which resulted in the enhancement of nonlinear optical properties and electride characteristics. The interaction energies (−13.57 to −16.54 kcal mol−1) indicated the physisorption of superalkalis on C6H6F6. NBO charge analysis is used to predict the charge transfer during the complexation. The highest NLO response (1.68 ×106 au) is observed for Na2F@C6F6H6 complex. The enhancement in second hyperpolarizability (γo) values is seen at various frequencies. Further, to obtain NLO response from an experimental point of view, hyper Rayleigh scattering (βHRS) is also investigated. These complexes, in addition to being a new entry into excess electron compounds, will pave the way for the designing and synthesis of additional NLO materials. The remarkable static first hyperpolarizability (βo) and second hyperpolarizability (γo) responses make them ideal for future NLO materials.

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.

Static, dynamic nonlinear optical (NLO) response and electride characteristics of superalkalis doped star like C6S6Li6

In search of excellent nonlinear optical (NLO) materials, new type of M3O@C6S6Li6 (M = K, Na, Li) electrides are set forth herein. The geometric, electronic and nonlinear optical properties of these electrides are evaluated by using density functional theory calculations. All the considered complexes have interaction energies in the range of -42.86 to -45.56 kcal/mol that validate their thermodynamically stability. The NBO charge analysis is performed to confirm the charge transfer between M3O and C6S6Li6. The highest amount of electronic charge transfer (0.92 |e|) is seen in K3O@C6S6Li6 electride. The frontier molecular orbitals (FMOs) and density of states (DoS) analyses are implemented to further rationalize the electronic properties. The remarkable polarizability (αo) and static first hyperpolarizability (βo) observed for these complexes validate their excellent nonlinear optical response. The highest βo (2.98×105 au) is obtained for Li3O@C6S6Li6 electride. The frequency-dependent first and second hyperpolarizability coefficients along with dipole dependent hyperpolarizability are also analyzed to get insight of NLO response under variational frequencies. Furthermore, the Hyper-Rayleigh Scattering coefficient (βHRS) of these complexes is investigated to provide information required for experimental point of view of NLO response. This study will contribute towards the development of more stable superalkalis doped electrides having pronounced NLO response.

Discovery of Electrides in Electron‐Rich Non‐Electride Materials via Energy Modification of Interstitial Electrons

AbstractWith the discovery of electrides, many electron‐rich materials are reported to be non‐electrides due to the lack of electride characteristics. Up to date, only a few studies have been done to alter the electronic structures of these materials. In this work, through atom replacement, the energy of interstitial electrons in a non‐electride Hf5Si3 is successfully modified and a hitherto unknown electride Ca3Hf2Si3 is obtained. It is revealed that the formation of the electride has a strong link to the neighboring cations of interstitial electrons. Moreover, an essential principle for the electride formation in the multi‐cations materials is proposed, and 34 dynamically stable ternary electrides are obtained through the proper atom replacement in 29 binary non‐electrides materials. The general realization of electrides in the electron‐rich non‐electrides can greatly expand the research scope of electrides.

Stabilisation of Li(0)-Li(0) bond by normal and mesoionic carbenes and electride characteristics of the complexes

Density functional theory (DFT) has been used to study the structure, stability, and bonding in the Li2L2 complexes containing Li(0)-Li(0) bond using both normal and mesoionic carbenes (MICs) as binding ligands (L) with Li2 molecule. The geometries and the nature of bonding of Li2(SNHCMe)2 and Li2(cAACMe)2 normal carbene complexes and Li2(MIC1)2 and Li2(MIC2)2 mesoionic carbene complexes have been compared with the previously reported Li2(NHC)2 normal carbene complex. The stability of the designed complexes is determined by positive Gibbs’ free energy changes (ΔG) towards the dissociation of the complexes into different fragments. Both normal and mesoionic carbenes stabilise the Li2 moiety and both the types of complexes are almost equally stable. From the natural bond orbital (NBO) analysis and the atoms in molecule (AIM) analysis, it is shown that two Li atoms are bonded by a single covalent bond with the oxidation state of zero on each Li. Again, the electron density analysis confirmed that each Li-Li bond has a non-nuclear attractor (NNA) at the middle of the bonds and the values of Laplacian of electron density (∇2 ρ(r)) are negative therein. Both the types of complexes show electride characteristics. The reactivity descriptors of all the studied complexes have been analyzed.

Demonstrating the Potential of Alkali Metal-Doped Cyclic C6O6Li6 Organometallics as Electrides and High-Performance NLO Materials.

In this report, the geometric and electronic properties and static and dynamic hyperpolarizabilities of alkali metal-doped C6O6Li6 organometallics are analyzed via density functional theory methods. The thermal stability of the considered complexes is examined through interaction energy (Eint) calculations. Doping of alkali metal derives diffuse excess electrons, which generate the electride characteristics in the respective systems (electrons@complexant, e–@M@C6O6Li6, M = Li, Na, and K). The electronic density shifting is also supported by natural bond orbital charge analysis. These electrides are further investigated for their nonlinear optical (NLO) responses through static and dynamic hyperpolarizability analyses. The potassium-doped C6O6Li6 (K@C6O6Li6) complex has high values of second- (βtot = 2.9 × 105 au) and third-order NLO responses (γtot = 1.6 × 108 au) along with a high refractive index at 1064 nm, indicating that the NLO response of the corresponding complex increases at a higher wavelength. UV–vis absorption analysis is used to confirm the electronic excitations, which occur from the metal toward C6O6Li6. We assume that these newly designed organometallic electrides can be used in optical and optoelectronic fields for achieving better second-harmonic-generation-based NLO materials.

Atomic Clusters: Structure, Reactivity, Bonding, and Dynamics.

Atomic clusters lie somewhere in between isolated atoms and extended solids with distinctly different reactivity patterns. They are known to be useful as catalysts facilitating several reactions of industrial importance. Various machine learning based techniques have been adopted in generating their global minimum energy structures. Bond-stretch isomerism, aromatic stabilization, Rener-Teller effect, improved superhalogen/superalkali properties, and electride characteristics are some of the hallmarks of these clusters. Different all-metal and nonmetal clusters exhibit a variety of aromatic characteristics. Some of these clusters are dynamically stable as exemplified through their fluxional behavior. Several of these cluster cavitands are found to be agents for effective confinement. The confined media cause drastic changes in bonding, reactivity, and other properties, for example, bonding between two noble gas atoms, and remarkable acceleration in the rate of a chemical reaction under confinement. They have potential to be good hydrogen storage materials and also to activate small molecules for various purposes. Many atomic clusters show exceptional opto-electronic, magnetic, and nonlinear optical properties. In this Review article, we intend to highlight all these aspects.

Substituent Effects on Electride Characteristics of Mg2(η5-C5H5)2: A Theoretical Study.

An ab initio study has been carried out on the substituted binuclear sandwich complexes of Mg2(η5-C5H5)2. We have checked whether the substitution destroys the electride properties of a complex, as it needs to satisfy several stringent criteria to obtain the status of an electride. The thermochemical results show that the complexes are stable at room temperature and 1 atm pressure. From the analysis of the various electron density descriptors and the natural bond orbital (NBO) for all the complexes, it is confirmed that the Mg-Mg bonds are covalent and the metal-ligand bonds are ionic in nature. The charges on each Mg atom in the studied complexes are +1. Analysis of the electron density descriptors shows the presence of a non-nuclear attractor (NNA) at the middle of the bond formed by the two Mg atoms when attached to the ligands. The electride characteristics are exhibited by all of the designed complexes. We also report the aromaticity behavior and reactivity descriptors of these complexes. The electride characteristics of Mg2(η5-C5H5)2 complex get affected on substitution, as both the NNA population and the nonlinear optical properties (NLO) of the complexes are changed.

Comparison Between Electride Characteristics of Li3@B40 and Li3@C60.

Density functional theory (DFT) based computation is performed on the endohedrally encapsulated Li3 cluster inside the B40 and C60 cages namely, Li3@B40 and Li3@C60. For both these systems, the Li-Li bond lengths are shorter than that in the free Li3 cluster. Due to confinement, the Li-Li vibrational frequencies increase in both the systems as compared to that in the free Li3 cluster. Thermodynamically, the formation of these two systems is spontaneous in nature as predicted by the negative values of Gibbs’ free energy changes (ΔG). For both the systems one non-nuclear attractor (NNA) is present on the middle of the Li3 cluster which is predicted and confirmed by the electron density analysis. The NNA population and the percentage localization of electron density at the NNA of the Li3@C60 system are higher than that in the Li3@B40 system. At the NNA the values of the Laplacian of electron density are negative and an electron localization function basin is present at the center of the Li3 cluster for localized electrons. Both systems show large values of nonlinear optical properties (NLO). Both the Li3 encapsulated endohedral systems behave as electrides. Electrides have low work function and hence have a great potential in catalytic activity toward the activation of small molecules (such as CO2, N2). Even some electrides have greater catalytic activity than some well-studied metal-loaded catalysts. As the systems under study behave as electrides, they have the power to show catalytic activity and can be used in catalyzing the activation of small molecules.

Inorganic Molecular Electride Mg4 O3 : Structure, Bonding, and Nonlinear Optical Properties.

Growing demands of material science and, in particular, in the field of nonlinear optics (NLO) encourage us to look for stable highly polarizable molecules with excess diffuse electrons. An unusual class of compounds called electrides comply with these requirements. Many attempts have been made, yet only few electrides have been synthesized as solids and none of them as molecular species. In this paper, a new theoretically designed molecular species with electride characteristics is reported. The idea of this molecular electride comes from the formation of electride-like features in the MgO crystal with defect F-centers. The geometry of the investigated molecule can be described as a Mg4 O4 cube with one oxygen atom removed. In Mg4 O3 , two 3s electrons are pushed out from the inner area of the molecule forming a diffuse electride multicentered bond. Our calculations show that this electride-like cluster possesses a noticeably large first hyperpolarizability β=5733 au. At the same time, a complete cube Mg4 O4 and Mg4 O3 2+ without electride electron pair have much smaller β: 0 au and 741 au, respectively. This fact indicates the decisive role of the electride electron pair in NLO properties. Additionally, vertical detachment energies of isomers (VDE), excitation energies ΔE, polarizabilities α, and IR spectra were calculated. These properties, including β, are supposed to be observable experimentally and can serve as indirect evidence of the stable molecular electride formation.

Theoretical study of substituent effects on electride characteristics and the nonlinear optical properties of Li@calix[4]pyrrole

Electrides, a novel kind of ionic compound in which electrons serve as anions, have been proposed as potential second-order nonlinear optical (NLO) materials. In this work, the substituent effects on the electride characteristics and the NLO behaviour of Li@calix[4]pyrrole with an electride-like structure were studied theoretically. The results show that electron-donating and electron-withdrawing groups can effectively increase and decrease the first hyperpolarizability (β0), respectively, without affecting the electride characteristics (electron population). More interestingly, lithiation in which four H atoms bonded to N atoms are substituted by four Li atoms within the core structure of Li@calix[4]pyrrole remarkably improves the electride characteristics, with a large electron population of 0.74 e (1.02 e) at the NNA (ELF) basins, making this structure perhaps the first formal molecular electride with almost one electron isolated from the rest of the molecules. Furthermore, a relationship between the electride characteristics and the NLO properties is found: the more delocalization the excess electron of the electride experiences, the larger the β0 value is. The present investigation may provide useful information for exploring high-performance second-order nonlinear optical materials based on organic electrides.

Investigation on the electronic structures and nonlinear optical properties of alkali metal atom doped all‐cis 1,2,3,4,5,6‐hexafluorocyclohexane

AbstractOn the basis of stable all‐cis 1,2,3,4,5,6‐hexafluorocyclohexane, a series of alkali metal atom doped MF6C6H6 (M = Li, Na, and K) compounds were theoretically constructed and studied by using ab initio quantum chemistry method. The calculated results show that the HOMO–LUMO gap of the MF6C6H6 conspicuously narrowed from 10.41 eV of pure F6C6H6 to about 2.00 eV of MF6C6H6. The electride characteristics of MF6C6H6 are verified by their electronic structures, HOMOs, and small VIE values. As expected, these electrides possess considerable static first hyperpolarizabilities (β0). Among the studied electrides, the largest β0 of the LiF6C6H6 is 7.00 × 105 au, which is about 3030 times larger than pure F6C6H6. TD‐M06‐2X calculations show that these larger β0 values are attributed to lower transition energies for the crucial excited states of MF6C6H6 systems. Further, the vibrational contributions to the static first hyperpolarizabilities of these molecules are also estimated. Moreover, Li atom doped dimer and trimer of F6C6H6 also present unusual electride's features and exhibit dramatically large β0. Thus, the F6C6H6 interacting with the alkali metal atoms may be a potential promising NLO nanomaterial.

M2X (M= Li, Na; X= F, Cl): the smallest superalkali clusters with significant NLO responses and electride characteristics

Hypervalent M2X (M = Li, Na; X = F, Cl) clusters are prototype species possessing lower ionisation potentials than Li, therefore classified as superalkalis. This study reveals some interesting properties of these small clusters using ab initio MP2/aug-cc-pVTZ and QCISD/aug-cc-pVTZ methods. These clusters are shown as an ionic species, composed of positively charged cage of alkali metals (M2+) and halogen anion (X−). Therefore, the stability of M2X is governed by both ionic and covalent interactions. We show that the excess valence electron of (M2+) is pushed out by anionic X−, which allows M2X clusters to possess ‘electride’ characteristics. It is also due to this excess electron that M2X clusters exhibit significant non-linear optical (NLO) properties. The dipole moment, mean polarisability and hyperpolarisability suggest their significant NLO responses, which are explained on the basis of electronic transitions in crucial excited states using TD-B3LYP/aug-cc-pVTZ method. The first static hyperpolarisabilities of Li2F and Na2F take the values of order of 104 a.u. due to their lower transition energies. This study should provide new insights into the design of novel materials with significant NLO responses useful for electro-optical applications.

Nonlinear optical behavior of Li n F (n = 2–5) superalkali clusters

Hyperlithiated clusters are known for their unusual bonding characteristics and lower ionization potentials. This study reports nonlinear optical (NLO) properties of a series of hyperlithiated clusters, Li n F (n = 2-5) for the first time. The structures of small Li n F (n = 2-5) clusters, obtained using second order Møller-Plesset perturbative method, are found to be stable against eliminations of F, F‾, and LiF. These Li n F species are stabilized by both ionic as well as covalent interactions. Our study shows that Li n F species can be thought of as superalkali-halogen (Li n - F) clusters but belong to the class of superalkalies themselves. These clusters may also possess alkalide and/or electride characteristics due to excess electrons. The dipole moment, mean polarizability, and hyperpolarizability suggest their significant NLO responses which are explained using the highest occupied molecular orbital surfaces and TD-DFT analysis. The exceptionally large hyperpolarizability of Li2F (~10(5) a.u.) and its electride characteristics are particularly highlighted. This study may guide the researchers in the design of novel materials with significant NLO responses useful for electro-optical applications. Graphical Abstract Li2F superalkali resemble an electride in which the excess electron is pushed out by Li2 (+) moiety, leading to its high hyperpolarizability of order of 10(5) a.u.

Theoretical investigation of the structures, stabilities, and NLO responses of calcium-doped pyridazine: Alkaline-earth-based alkaline salt electrides

Currently, whether alkaline-earth-doped compounds with electride characteristics are novel candidates for high-performance nonlinear optical (NLO) materials is unknown. In this paper, using quantum chemical computations, we show that: when doping calcium atoms into a family of alkaline-substituted pyridazines, alkaline-earth-based alkaline salt electrides M-H3C4N2⋯Ca (MH, Li, and K) with distended excess electron clouds are formed. Interestingly, from the triplet to the singlet state, the chemical valence of calcium atom changes from +1 to 0, and the dipole moment direction (μ0) of the molecule reverses for each M-H3C4N2⋯Ca. Changing pyridazine from without (H4C4N2⋯Ca) to with one alkaline substituent (M-H3C4N2⋯Ca, MLi and K), the ground state changes from the triplet to the singlet state. The alkaline earth metal doping effect (electride effect) and alkaline salt effect on the static first hyperpolarizabilities (β0) demonstrates that (1) the β0 value is increased approximately 1371-fold from 2 (pyridazine, H4C4N2) to 2745au (Ca-doped pyridazine, H4C4N2⋯Ca), (2) the β0 value is increased approximately 1146-fold from 2 in pyridazine (H4C4N2) to 2294au in an Li-substituted pyridazine (Li-H3C4N2), and (3) the β0 value is increased 324-(MLi) and 106-(MK) fold from 826 (MLi) and 2294au (MK) to 268,679 (MLi) and 245,878au (MK), respectively, from the alkalized pyridazine (M-H3C4N2) to the Ca-doped pyridazine (M-H3C4N2⋯Ca). These results may provide a new means for designing high-performance NLO materials.

The structure and large nonlinear optical properties of a novel octupolar electride Li@36Adz

Based on the synthesized H@36Adz (36Adz=36 adamanzane), a new class of three-dimensional (3D) octupolar molecule Li@36Adz with electride characteristics is first designed and studied with theoretical tools, in which the electron of the encapsulated Li atom is extruded from the cage of 36Adz forming diffused excess electron. Due to S4 symmetry, Li@36Adz only has the octupolar hyperpolarizability (βJ=3) up to 1787×10−30esu at the restricted open-shell second-order Møller–Plesset (ROMP2) level, whose β is about 1787 times more than that (1×10−30esu) of its analogue (H@36Adz). Interestingly, we find that both Li@36Adz has a very low ionization potential (1.84eV) which can be seen as superalkalies. The present investigations will enrich the concept of 3D octupolar molecules for high-performance NLO materials and offer a method to construct superalkalies.