Discovery of new MAX-phase-like layered ternary carbide V8P6C: Crystal structure, thermal expansion, and elastic properties

Abstract

Due to the laminated structure, MAX phases and MAX-phase-like compounds possess outstanding damage tolerance capability with good thermal/electrical conductivity and other charming functional properties, showing a bright future for structural-functional uses. Here, a new MAX-phase-like ternary carbide V8P6C with layered characteristics was discovered. Contrary to the alternate stacking of hexagonal-arranged M/A/X atom layers in MAX phases, V8P6C has a layered structure formed by a repeated stacking of the slabs consisting of V6C-octahedrons and VP6-distorted-octahedrons in three-atomic layers thick. The thermal expansion anisotropy was revealed, with the thermal expansion coefficients (TECs) along a-axis, c-axis, and the average value, equal to αa=8.55(5) μK−1, αc=10.23(5) μK−1, and αL=9.14(5) μK−1, respectively. The dilation along the c-axis was mainly dominated by out-plane V-P bonds between layered slabs. According to the pressure-dependent lattice parameters, the polycrystalline bulk modulus (B0 = 185(9) GPa) and high compressibility anisotropy factor (Ba/Bc = 1.64) was found. Meanwhile, the second-order elastic constants were calculated, and the elastic moduli, E=303 GPa, B=198 GPa, G=122 GPa, and ν = 0.246, were obtained. And the weak connection between layer slabs by out-plane V1-P bond was revealed by the bond stiffness model, with a bond stiffness close to the M-A bond in MAX phases. Furthermore, good damage tolerance capability was predicted according to Paugh's ratio (G/B=0.616) and Cauchy pressure (Pa = 0 GPa, Pc = -7.5 GPa), even better than those of typical MAX phases. The discovery of V8P6C provides a new member of MAX-phase-like damage-tolerant ceramic with relatively low density, moderate thermal expansion, and anisotropic compressibility, and the unique layered structure is inspiring for exploring new layered ternary ceramics.