Non Cubic Electrides: Structure, Properties & Applications

• Non cubic electrides are materials characterized by low-symmetry lattices that localize excess electrons in 0D, 1D, or 2D cavities, offering unique anisotropic properties.
• Advanced computational screening and first-principles methods have identified diverse non cubic structures with novel electron localization motifs and tunable transport properties.
• Their anisotropic band structures and topological features enable potential applications in spintronics, superconductivity, and photonic devices.

Electrides are a unique class of materials in which excess electrons reside in crystal cavities, effectively acting as anions decoupled from atomic nuclei. Non-cubic electrides, where the host lattice lacks cubic symmetry, exhibit pronounced structural, electronic, optical, and topological anisotropies compared to their cubic analogs. Such materials range from atomic-scale monolayers and MXene derivatives to complex high-pressure phases and layered compounds. Advances in automated computational screening and first-principles modeling have greatly expanded the library of non-cubic electrides, revealing diverse electron-localization motifs (0D cages, 1D channels, 2D slabs) and novel correlated quantum phenomena.

1. Structural Characterization and Electronic Motifs

Non-cubic electrides span a wide assortment of crystal systems—orthorhombic, tetragonal, trigonal, hexagonal, monoclinic, and triclinic—with interstitial quasi-atoms (ISQs) occupying low-symmetry Wyckoff positions and forming localized electron reservoirs. Unlike cubic electrides, where interstitial sites are symmetrically equivalent, non-cubic architectures generate multiform cavities and channels.

Exemplar systems include:

• Layered 2D systems: Monolayer electrides such as Ca₂N, Sr₂N, Na₂Pd₃O₄, KAgSe, and KMgSb feature metal–nitride (or oxide) slabs sandwiching interstitial electron gases, with semiconducting band gaps 0.3–1.7 eV and image-potential confinement of nearly free electrons (Yang et al., 2020, Mortazavi et al., 2018).
• MXene electrides: Ti₂N and Ti₃N₂ exhibit tetrahedral or hexagonal symmetry, hosting strongly anisotropic electron blobs localized in interlayer gaps, with significant facet-dependent work functions (Adhikari et al., 1 Aug 2024).
• High-pressure phases: Na-hP₄ crystallizes in hexagonal P6/mmm symmetry, stabilizing interstitial electrons via multi-centered bonding within penta-capped trigonal prismatic clusters, driven by pressure-induced 3s→3pd orbital hybridization (Racioppi et al., 2023). AsLi₇ adopts orthorhombic (Pmmm, 1D chains) and hexagonal (P6/mmm, 0D clusters) forms under varying pressure, favoring distinct ISQ distributions (Wan et al., 2022).
• Monoclinic/tetragonal flexible electrides: Rb₃O and K₃O possess both 2D and 1D interstitial electron topologies, which can be modulated by strain or chemical substitution. Their C2/m symmetry enables electron transfer between cavities (Zhu et al., 2018).

Tables from computational screening catalogue 101 distinct non-cubic electrides, classified according to crystal system, lattice parameters, cavity charge, ELF maxima, and interstitial band energies (Zhu et al., 2018). Connectivity ranges from isolated 0D cages to extended 2D sheets and 1D channels.

2. Electron Localization, Band Structure, and Interstitial States

Non-cubic electrides are distinguished by the location and character of interstitial electrons:

• ELF and Bader analysis: Electron localization function maxima (ELF > 0.3–0.85) appear well off nuclei, with Bader charge integrals confirming up to 1.5 electrons per cavity. For example, in Na-hP₄, q_NNA ≃ –1.08 e for the interstitial basin, and peak ELF at 0.8 encloses the non-nuclear attractor (Racioppi et al., 2023).
• Electronic dispersion: Bands derived from ISQs typically display large effective masses (m* = 0.2–1.2 m_e) and flat or weakly dispersive character, crucially affecting plasma frequency and transport. In monolayer systems, NFE bands sit 0.3–1.5 eV above the Fermi level, becoming switchable under compressive strain (Yang et al., 2020). In 2D MXenes, band projections onto cavity pseudoatoms confirm strong interstitial character.
• Band topology: Flexible electrides such as Rb₃O and the halide-reduced AC/A₂Ge families host topological nodal lines and Dirac points protected by non-cubic symmetry, with associated drumhead surface states and nontrivial Zak or Berry phases (Yu et al., 2020, Zhu et al., 2018). Mirror- and inversion-related band crossings are common, with “pure s-like” electride bands localized at cavity sites.

The electronic structure and interstitial charge landscape can be tuned by elastic strain, pressure, or chemical modification, altering the site energy and occupancy of electride bands.

3. Magnetism and Superconductivity

Non-cubic electrides manifest emergent magnetic and superconducting phases linked to cavity geometry and electron spin polarization:

• Magnetic phases: Substitutional lanthanides (Gd) in AC and A₂Ge frameworks induce spin-polarized interstitial states and net magnetic moments (~0.3 μ_B) per cavity (Yu et al., 2020). MXenes (Ti₃N₂ monolayer) present robust ferromagnetism, while bulk phases prefer AFM ordering (Adhikari et al., 1 Aug 2024).
• Superconductivity: AsLi₇ in P6/mmm symmetry achieves superconducting transition temperatures T_c up to 38 K (150 GPa), likely due to Van Hove singularities and Dirac cones at E_F. In Ti₂N and Zr₂N “X-type” electrides, electron–phonon coupling (λ = 0.39–1.25) and log-averaged phonon frequency (ω_log) yield T_c < 1 K, which can be enhanced by hole doping, even inducing superconductivity in non-SC Hf₂N (Zha et al., 27 Apr 2025, Wan et al., 2022). Interstitial electrons contribute substantial DOS(E_F) and are implicated in exotic pairing mechanisms.

Molecular dynamics in AsLi₇ reveal coexistence of superionic Li flow and a rigid As sublattice at 1000 K—a dual-state behavior largely absent in highly symmetric (cubic) electrides (Wan et al., 2022).

4. Transport, Mechanical, and Optical Properties

The anisotropic electron localization of non-cubic electrides determines their mechanical, transport, and photonic responses:

• Mechanical anisotropy: Layered electrides (Ca₂N, Sr₂N) display in-plane isotropy, but tensile strength is direction-dependent—50% higher along zigzag than armchair; ultimate tensile strains ε_UTS range from 0.13 to 0.24 (Mortazavi et al., 2018). Critical compressive strains for strain-switchable metallization in 2D non-cubic candidates are −3% to −13% (Yang et al., 2020).
• Electronic transport: Conductance is largely maintained under strain, with <32% reduction for ultimate armchair stretch; compressive strain enhances current via improved orbital overlap. Strain engineering thus facilitates transport tuning without substantial degradation (Mortazavi et al., 2018, Zhu et al., 2018).
• Optical response: Non-cubic electrides exhibit strong in-plane vs. out-of-plane dielectric anisotropy, with low plasma frequencies (ω_p = 0.11–1.94 eV) and broad hyperbolic windows extending from IR to UV. Interband transitions involving ISQ bands further modulate optical conductivity and permit hyperbolic exciton-polaritons; 2D systems (Sc₅Cl₈, Ca₂Cu, Y₂Cl₃) show prominent in-plane hyperbolicity (Hao et al., 22 Nov 2025, Mortazavi et al., 2018).

Reduced plasma frequencies (due to flat bands and large m*) and directionally selective optical absorption distinguish non-cubic electrides as promising candidates for photonic applications.

5. Topology and Multifunctionality

The interplay of low-symmetry lattice motifs, interstitial electron organization, and quantum correlations underpins the multifaceted functionalities of non-cubic electrides:

• Topological phases: Nodally protected band crossings—nodal rings, nodal chains, Dirac lines—arise from inversion/mirror symmetry combinations. The Zak phase (γ = π), mirror invariants (ζ), and Berry phase calculations confirm underlying topological order and floating surface bands (Yu et al., 2020, Zhu et al., 2018).
• Switchability and flexibility: Non-cubic electrides are amenable to electron topology engineering, as excess electrons can be shuttled between cavities (2D ↔ 1D in Rb₃O via strain, or between MXene cavity types via doping) (Zhu et al., 2018, Adhikari et al., 1 Aug 2024). Strain-controlled semiconductor-to-metal transitions and surface-confined (“skin-like”) interstitial electron states in $X$-type electrides offer avenues for device integration (Zha et al., 27 Apr 2025).
• Superionic and plastic states: High-pressure non-cubic electrides (AsLi₇) display coexistence of rigid and flowable sublattices, expanding the regime of mixed-state ionic dynamics beyond hydride systems (Wan et al., 2022).

The topological and structural tunability of non-cubic electrides, coupled with their correlated electron phenomena, positions them as platforms for spintronics, catalysis, sensing, and next-generation quantum and photonic devices.

6. Design Principles, Computational Screening, and Future Directions

Systematic computational approaches have accelerated the identification and rational design of non-cubic electrides:

• Screening criteria: Automated schemes classify electrides by matching ELF maxima, partial density peaks at E_F, and cavity charge volume fractions (f_vol ≥ 5%) (Zhu et al., 2018). Host lattices with interconnected cationic voids, intermediate anion electronegativity, and low work-function cation frameworks promote stable interstitial electron bands.
• Design workflow: Removal of electronegative anions from parent halides, convex-hull stability analysis, and substitutional tuning with magnetic lanthanides or dopants yield robust, multifunctional candidates (Yu et al., 2020, Zhu et al., 2018).
• Symmetry engineering: Exploitation of non-cubic point groups (e.g., $S_{4z}$ rotoinversion, broken $M_z$, monoclinic C2/m, orthorhombic Amm2) enables new electronic localizations, skin-like effects, and tailored band splittings (Zha et al., 27 Apr 2025).

Future strategies should prioritize low-symmetry, layered, and intermetallic hosts. Expanding compositional spaces to oxides, nitrides, and hydrides, and leveraging symmetry-protected electron confinement, will drive discovery of new non-cubic electrides with tailored band structures, magnetism, topological order, and hyperbolic dispersion.

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Selected References

• Switchable Atomically Thin 2D Electrides from First-principles Prediction (Yang et al., 2020)
• As-Li electrides under high pressure: superconductivity, plastic, and superionic states (Wan et al., 2022)
• First-Principles Design of Halide-Reduced Electrides: Magnetism and Topological Phases (Yu et al., 2020)
• $X$-type electrides hosting skin-like interstitial electron states and doping-enhanced superconductivity (Zha et al., 27 Apr 2025)
• Hyperbolic Dispersion and Low-Frequency Plasmons in Electrides (Hao et al., 22 Nov 2025)
• Mechanical, optoelectronic and transport properties of single-layer Ca₂N and Sr₂N electrides (Mortazavi et al., 2018)
• On the Electride Nature of Na-hP₄ (Racioppi et al., 2023)
• Discovering Inorganic Electrides from an Automated Computational Screening (Zhu et al., 2018)
• Confinement of quasi-atomic structures in Ti₂N and Ti₃N₂ MXene Electrides (Adhikari et al., 1 Aug 2024)
• Computational Design of Flexible Electrides with Non-trivial Band Topology (Zhu et al., 2018)