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[Display omitted] Mayenite (12CaO · 7Al 2 O 3 or generally named C12A7) and its electride form (C12A7 : 4 e −) have drawn intensive attention because of a variety of promising applications including display devices, catalysis, and transistors. C12A7 is a transparent insulator and it can be controllably engineered to a conductor or superconductor upon introducing oxygen vacancies and substitutional anionic impurities. However, the influence of external pressure on electronic and optical properties of C12A7 remains intriguingly less explored. On the basis of the density functional theory (DFT) and ab initio molecular dynamics, this work presents the variations of electronic and optical properties of C12A7 via the immense external pressure. The findings reveal that the applied pressure induces the structural deformation until it is energetically amorphised at pressure greater than 17.94 GPa. Such deformation consequently creates the gradual reduction in the electronic band gap because of the intensified overlap of electronic bands among the framework Ca-3 d , O-2 p , and the in-cage extra-framework O-2 p states. Furthermore, the drop in the band gap yields the remarkable enhancement in the optical absorbance in the visible and UV regions. Our findings unveil the underlying influences of the external pressure on the C12A7 properties which are of fundamental interests and functional design for applications. [ABSTRACT FROM AUTHOR]