Crystal Chemistry and Physical Properties of A Quaternary Intermetallic Compound, θ-(Al0.8718Cu0.0256Si0.1026)13Fe4

θ-Al13Fe4 particles form as a primary Fe intermetallic compound (Fe-IMC) during the casting of commercial Al metals and alloys that inevitably contain Fe and Si as impurities. Moreover, the excellent mechanical performances of the Al-Cu alloys demand knowledge about the structural chemistry of the Fe-IMCs, including the θ-phase in the quaternary Al-Cu-Fe-Si system. Here, we investigate the stability, crystal structure, and electronic and mechanical properties of the Cu and Si co-doped θ-phase using a first-principles density-functional theory approach. The calculations reveal high stability of a quaternary θ-phase with chemical composition (Al0.8718Cu0.0256Si0.1026)13Fe4 at ambient conditions. Thermodynamics and statistical analysis show a broad range of Si content in the structure at the casting temperature. The Cu and Si (co-)doping enhances the bulk modulus of the compounds. The calculated bulk modulus of the quaternary θ-phase is 129 GPa. The findings help characterize the θ-phase in the quaternary Al-Si-Fe-Cu system and understand the formation of the θ-phase and related phase transformations in the various Al alloys during casting.