The negative Poisson's (NPR) ratio in a twodimensional (2D) material is a counterintuitive mechanical property that facilitates the development of nanoscale devices with sophisticated functionality. Inspired by the peculiar buckled lower-symmetric, trilayered geometry of pentagonal monolayers, we theoretically predict penta-SiCN, a ternary auxetic metallic monolayer with highly tunable NPR. The penta-SiCN is structurally, thermally, dynamically, and mechanically stable, and sustainable at and beyond room temperature with experimental feasibility. It possesses nontrivial geometrical and mechanical isotropy and relatively moderate thickness. Remarkably, the shorter and quasi sp 3 -hybridized C−N bond and the rigidity against the strain allow the monolayer to possess a high value of NPR (−0.136), even higher than that of black phosphorene, extendable up to −0.639 by 4% of biaxial stretching. On the other hand, the 2D Young's modulus of 129.88 N/m decreases to 41.34 N/m at equivalent stretching, indicating relative softening and flexibility. Interestingly, a buckled-to-planar phase transition is identified at 10% biaxial strain before it suffers the fracture at 16%. Additionally, the strong optical anisotropy, absorbance (up to 6.51× 10 5 cm −1 ), and presence of plasmon frequency demonstrate its potential application in optomechanical and plasmonics.

The scarce negative Poisson's ratio (NPR) in a twodimensional (2D) material is an exceptional auxetic property that offers an opportunity to develop nanoscale futuristic multifunctional devices and has been drawing extensive research interest. Inspired by the buckled pentagonal iso-structures that often expose NPR, we employ state-of-the-art first-principles density functional theory calculations and analyses to predict a new 2D metallic ternary auxetic penta-phosphorus boron nitride (p-PBN) with a high value of NPR. The new p-PBN is stable structurally, mechanically, and dynamically and sustainable at room temperature, with experimental feasibility. The short and strong quasi sp 3 -hybridized B−N bond and unique bond variation and geometrical reconstruction with an applied strain allow p-PBN to inherit a high value of NPR (−0.236) along the (010) direction, the highest among any other ternary penta iso-structures reported to date. Despite having a small elastic strength, the highly asymmetric Young's modulus and Poisson's ratio along the (100) and ( 010) directions indicate large anisotropic mechanics, which are crucial for potential applications in nanomechanics and nanoauxetics.

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