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 significant attention. Here, we report a new electron-deficient type of 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 characteristics between Al ions play a critical role in driving the phase transition. Our electron–phonon coupling calculations indicate that all phases exhibit superconducting behaviors. Importantly, we prove that the ISQs behave as free electrons and demonstrate that the factor governing T_C is primarily derived from Li-p-hybridized electronic states with ISQ compositions. These electronic states are scattered by low-frequency phonons arising from mixed vibrations of Li and Al affected by ISQs to enhance electron–phonon coupling. Our study largely expands the research scope of electrides, provides new insight for understanding phase transitions, and elucidates the effects of ISQs on superconducting behavior.