Theoretical prediction,synthesis,and crystal structure determination of new MAX phase compound V2SnC

Guided by the theoretical prediction,a new MAX phase V2SnC was synthesized experimentally for the first time by reaction of V,Sn,and C mixtures at 1000°C.The chemical composition and crystal structure of this new compound were identified by the cross-check combination of first-principles calculations,X-ray diffraction(XRD),energy dispersive X-ray spectroscopy(EDS),and high resolution scanning transmission electron microscopy(HR-STEM).The stacking sequence of V2C and Sn layers results in a crystal structure of space group P63/mmc.The a-and c-lattice parameters,which were determined by the Rietveld analysis of powder XRD pattern,are 0.2981(0)nm and 1.3470(6)nm,respectively.The atomic positions are V at 4f(1/3,2/3,0.0776(5)),Sn at 2d(2/3,1/3,1/4),and C at 2a(0,0,0).A new set of XRD data of V2SnC was also obtained.Theoretical calculations suggest that this new compound is stable with negative formation energy and formation enthalpy,satisfied Born-Huang criteria of mechanical stability,and positive phonon branches over the Brillouin zone.It also has low shear deformation resistance c44(second-order elastic constant,cij)and shear modulus(G),positive Cauchy pressure,and low Pugh’s ratio(G/B=0.500<0.571),which is regarded as a quasi-ductile MAX phase.The mechanism underpinning the quasi-ductility is associated with the presence of a metallic bond.