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 Mn+1 AXn 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 Mn+1 An X, were added as new members to the MAX family and their other properties deserve future investigations.