Pressure-Induced Electride States in Intermetallic BaMg₂ Compounds

Despite credible reports of several high-pressure electrides of alkali earth metal elements, high-pressure electrides composed of two elements of the alkali earth metal group remain a mystery. In this paper, taking the typical intermetallic compound BaMg₂ as a representative, we systematically investigate the high-pressure electronic behavior of alkali earth metal intermetallic compounds by using first-principles calculations in conjunction of particle swarm optimization. BaMg₂ undergoes phase transitions from the ambient P6₃/mmc phase to the high-pressure P4/nmm phase at 19.6 GPa and then to the high-pressure I4/mmm phase at 260 GPa, accomplishing an electron state transformation from nonelectride to electride. Although some electrons are trapped in the interstitial lattice of BaMg₂, the high-pressure P4/nmm and I4/mmm phases exhibit different anionic electron-dominated metallicity. Although Mg and Ba have similar valence electron configurations, the electride state of BaMg₂ is completely dominated by Mg atoms because its electride state orbital has more favorable energy than the atomic orbital of Mg under high pressure. Different from that of Mg, the d orbitals of Ba have energy advantages over its electride state. This work not only enriches the knowledge of the family of electrides but also provides new insight into the electride formation mechanism.