Theoretical Prediction and Experimental Synthesis of Zr₃AC₂ (A = Cd, Sb) Phases

MAX phases have great research value and application prospects, but it is challenging to synthesize the MAX phases containing Cd and Sb for the time being. In this paper, we confirmed the existence of the 312 MAX phases of Zr₃CdC₂ and Zr₃SbC₂, both from theoretical calculations and experimental synthesis. The Zr₃AC₂ (A = Cd, Sb) phase was predicted by the first-principles calculations, and the two MAX phases were confirmed to meet the requests of thermal, thermodynamic, and mechanical stabilities using formation energy, phonon dispersion, and the Born-Huang criteria. Their theoretical mechanical properties were also systematically investigated. It was found that the elastic moduli of Zr₃CdC₂ and Zr₃SbC₂ were 162.8 GPa and 164.3 GPa, respectively. Then, differences in the mechanical properties of Zr₃AC₂ (A = Cd, In, Sn, and Sb) were explained using bond layouts and charge transfers. The low theoretical Vickers hardness of the Zr₃CdC₂ (5.4 GPa) and Zr₃SbC₂ (4.3 GPa) phases exhibited excellent machinability. Subsequently, through spark plasma sintering, composites containing Zr₃CdC₂ and Zr₃SbC₂ phases were successfully synthesized at the temperatures of 850 °C and 1300 °C, respectively. The optimal molar ratio of Zr:Cd/Sb:C was determined as 3:1.5:1.5. SEM and the EDS results analysis confirmed the typical layered microstructure of Zr₃CdC₂ and Zr₃SbC₂ grains.