Interstitial electrons in electrides: Multiple electronic regimes and emergent quantum phenomena

Electrides provide a unique platform for exploring how crystal structure can regulate electronic behavior by controlling the real-space organization of low-energy electrons. Unlike conventional solids, in which electronic states are primarily associated with atomic orbitals and chemical bonds, electrides host interstitial electronic states distributed within crystal cavities, channels, interlayer spaces, or connected void networks. These states establish crystal-structure-defined electronic architectures whose dimensionality, connectivity, filling, and hybridization can be continuously tuned. In this perspective, we view interstitial-electron character as a real-space electronic degree of freedom, describing the structurally encoded organization of low-energy states residing in interstitial regions rather than an independent quantum number. Based on this concept, we classify interstitial electronic states into distinct electronic regimes, including highly itinerant, weakly dispersive or geometry-reconstructed, and strongly hybridized states. These regimes provide diverse platforms for realizing metallic transport, magnetism, ferroic order, superconductivity, topology, unconventional transport, and interfacial electronic-state engineering. We propose a design framework connecting crystal structure, interstitial electronic architecture, electronic regimes, and emergent quantum phenomena, offering a route toward the rational design of quantum materials through real-space electronic architectures.