Spinel-type Al4C3 attainable above 7 GPa and more high-pressure phases of Al4C3.

We predict that Al4C3 adopts a cubic, anti-spinel-type structure (Al4C3-II) between 7 and 33 GPa, peaking in stability relative to other Al4C3 structures at 26 GPa. At ambient pressure, Al4C3-II is mechanically robust, with a bulk modulus of 160 GPa and a Vickers hardness of around 30 GPa. Beyond Al4C3-II, we identify three additional post-spinel phases appearing in the Al4C3 phase diagram, including an anti-Th3P4-type at 140 GPa. The clear enthalpy differences under pressure leave little doubt that the known trigonal R3̄m ground state of Al4C3 will undergo multiple phase transitions. The accessible pressure window for spinel-type Al4C3-II is easily accessible in both laser-heated diamond anvil cell and large-volume multi-anvil cell experiments. We therefore encourage experimental exploration of the Al-C system at high pressure.