Expanding the Valence-Shell Boundaries of Alkali Metals: Activation of Semi-Core Electrons

The core-valence electron separation in atomic electronic structures is a foundational concept in chemistry, typically defining valence electrons as those participating in chemical bonding, while core electrons do not. This distinction, though practical, is largely arbitrary. Under pressures, the core-valence separation may become less distinct, altering electronic structures and affecting chemical properties. In this study, we explore the behavior of semi-core electrons in alkali fluorides (MF; M = Na, K, Rb, Cs) under compression using density functional theory calculations. Our results reveal that semi-core p-orbitals in heavier alkali metals (K, Rb, Cs) engage in bonding interactions in the B2 phase, forming metal-metal bonds even at low pressures (~1 GPa), while NaF requires much higher pressure (~28 GPa) for similar transitions. These findings suggest that alkali metals may exhibit a richer chemistry under pressure than previously thought, challenging the traditional core-valence distinction, and implying potential phase transitions driven by core-electron interactions. Furthermore, the study proposes experimental methods to observe these semi-core interactions, providing new insights into the high-pressure behavior of alkali metals and their compounds, with broader implications for understanding the chemistry of elements under extreme conditions.