A Series of MAX Phases with MA-Triangular-Prism Bilayers and Elastic Properties

We report a new type of MAX phase (M=transition metals, A=main group elements, and X=C/N), Nb₃ As₂ C, designated as 321 phase. It differs from all the previous M_n+1 AX_n phases in that it consists of an alternate stacking of one MX layer and two MA layers in its unit cell, while only one MA layer is allowed in usual MAX phases. The new 321 phase exhibits a bulk modulus of Nb₃ As₂ C up to 225(3) GPa as determined by high-pressure synchrotron X-ray diffraction, one of the highest values among MAX phases. Isostructural 321 phases V₃ As₂ C, Nb₃ P₂ C, and Ta₃ P₂ C are also found to exist. First-principles calculations reveal the outstanding elastic stiffness in 321 phases. Among all 321 phases, Nb₃ P₂ C is predicted to have the highest elastic properties. These 321 phases, represented by a chemical formula M_n+1 A_n X, were added as new members to the MAX family and their other properties deserve future investigations.