Electron-Deficient Type Electride Li₄Al under High Pressure: Bonding Properties and Superconductivity

High-pressure electrides, characterized by the presence of interstitial quasi-atoms (ISQs), possess unique electronic structures and physical properties, such as diverse dimensions of electride states exhibiting different superconductivity, which has attracted a significant attention. Here, we report a new electron-deficient type electride Li₄Al and identify its phase transition progress with pressurization, where the internal driving force behind phase transitions, bonding characteristics, and superconducting behaviors have been revealed based on first-principles density functional theory. Through analysis of the bonding properties of electride Li₄Al, we demonstrate that the ISQs exhibiting increasingly covalent characteristic between Al ions plays a critical factor in driving the phase transition. Our electron-phonon coupling calculations indicate that all the phases exhibit superconducting behaviors. Importantly, we proved that the ISQs behavior as free electrons, and demonstrated that the factor governing T_c primarily derived from Li-p-hybridized electronic states with ISQs compositions. These electronic states are scattered by low-frequency phonons arising from the mixed vibrations of Li and Al that effected by ISQs to enhancing electron-phonon coupling. Our study largely expands the research scope of electrides, provides a new insight for understanding the phase transition, and elucidates the effects of ISQs on superconducting behavior.