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

Abstract

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 Rm 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.