## Abstract

The mechanical behaviors of a series of two-dimensional (2-D) crystals X_{3}M_{2} (X = S, Se; M = N, P, As) are explored through density functional theory (DFT) calculations. Two atomic structures exist for 2-D X_{3}M_{2}, namely α-heart and β-heart, which exhibit different out-of-plane Negative Poisson's Ratio (NPR) phenomena. For α-heart structures, a general out-of-plane NPR exists under in-plane tension in the y direction, while for β-heart type structures, a general NPR exists under in-plane tension in the x direction. Through DFT calculations, we revealed that a NPR is a general phenomenon for 2-D X_{3}M_{2} with heart-shaped atomic structures, and the absolute value of the out-of-plane NPR for 2-D X_{3}M_{2} is always one order of magnitude larger than other 2-D crystals reported before. At the same time, the in-plane NPR at certain angles around two in-plane principal axes also exists for 2-D X_{3}M_{2} except for S_{3}P_{2} due to the special heart-shaped structure. The coexistence of out-of-plane and in-plane NPRs for 2-D crystals has not been reported before and these special three-dimensional (3-D) auxetic properties in 2-D crystals may enormously enrich research studies on novel mechanical, electrical and chemical behaviors of 2-D crystals. This discovery will extend the family of 2-D crystals with a NPR and also provide new insights for designing novel negative Poisson's ratio materials.

Original language | English |
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Pages (from-to) | 5916-5924 |

Number of pages | 9 |

Journal | Physical Chemistry Chemical Physics |

Volume | 21 |

Issue number | 11 |

DOIs | |

Publication status | Published - 2019 |

Externally published | Yes |

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