In this work we focus primarily on the compression-induced behavior of the semi-core electrons of alkali metals (M), i.e., 2s 2 2p 6 for Na, 3s 2 3p 6 for K, 4s 2 4p 6 for Rb and 5s 2 5p 6 for Cs, in alkali fluorides (MF) phases B1 and B2. The DFT calculations set up is reported in the computational methods section in the Supplementary Information. To do so, we start looking at the atom-projected density of states (pDOS) of the B1 phases (See Figure ). In all four MF systems, only fluorine (formally a fluoride) contributes to the DOS near the Fermi-level, as expected by the formation of cationic (M + ) and anionic (F -) species. Then, the rest of the DOSs show very localized s and p semi-core states for both cations and anions, reflecting the rigid nature of closed-shell electronic structures. Only CsF appears as "more flexible", having a hint of the metal 6s state near the Fermi-level, together with some polarization of the 5p orbitals (noticeable in part also for Rb), and a partial mixing between the Cs 5s and F 2s orbitals of fluorine.