On the Electride Nature of Na‐hP4

Early quantum mechanical models suggested that pressure drives solids towards free‐electron metal behavior where the ions are locked into simple close‐packed structures. The prediction and subsequent discovery of high‐pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na‐hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na‐pd hybrid orbitals point towards the center of an 11‐membered penta‐capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na‐hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure. The Na‐hP4 phase, a high pressure electride, contains clusters of eleven sodium atoms within which charge is localized. Quantum chemical calculations show that this build‐up of interstitial charge stems from the overlap of pd hybrid orbitals on the sodium atoms comprising this cluster, resulting in the formation of a multicentered bond, increased density, and semiconducting behavior.