From cations to anions: pressure-induced charge-transfer reversal of strontium in Sr-I compounds

Based on first-principles calculations combined with the CALYPSO structure-search methodology, this work systematically investigates the structural evolution and electronic properties of the Sr–I system over a pressure range of 0–800 GPa. We predict four novel phases with both conventional and unconventional stoichiometries, namely SrI2 (space group: I4/mmm), SrI (space group: Pm-3m), and two polymorphs of Sr2I (space groups: R3m and Cmcm), and unambiguously determine their corresponding phase-transition sequence. Bader charge analysis reveals that, as pressure increases, Sr undergoes a transformation from a conventional +2 reducing agent to a state exhibiting anionic-like behavior. Orbital occupation analysis indicates that the Sr-4d orbitals are activated under high pressure, enabling Sr to capture electrons from the I-5p orbitals and thus exhibit pronounced anioniclike behavior, which ultimately leads to a charge-transfer reversal. Further projected density of states (PDOS) analysis demonstrates that pressure-induced band broadening drives the anionic-like behavior of Sr atoms. These findings provide new insights into the unconventional electronic behavior of alkaline-earth metals under extreme compression.