MAX相Nb_(2)SnC和Nb_(2)SnN的力学和热力学性质的第一性原理计算

First-principles calculations of the mechanical and thermodynamic properties of MAX phases Nb_(2)SnC and Nb_(2)SnN

利用基于密度泛函理论的第一性原理,在广义梯度近似下研究了MAX相Nb_(2)SnC和Nb_(2)SnN的力学、晶格动力学、电子以及热力学性质.通过弹性常数和声子的计算,研究了Nb_(2)SnC和Nb_(2)SnN两种结构的力学稳定性和动力学稳定性;通过对Nb_(2)SnC和Nb_(2)SnN的力学性质计算,证明了它们均具有较高的体积模量和剪切性,并且说明了Nb_(2)SnC和Nb_(2)SnN是具有弹性各向异性的韧性材料.此外,通过计算电子能带结构和态密度,研究了Nb_(2)SnC和Nb_(2)SnN的电子性质和成键性质,结果表明,两个化合物均具有金属导电性和较强的共价键,而且Nb_(2)SnN比Nb_(2)SnC具有更强的金属导电性.最后利用声子色散曲线预测了热容、自由能、焓和熵等热力学性质,结果标明,计算出的熵、焓和自由能值变化符合热力学第三定律.

The mechanical,lattice dynamics,electronic and thermodynamic properties of the MAX phases Nb_(2)SnC and Nb_(2)SnN are investigated by using the first principles method based on density generalized function theory.The mechanical and kinetic stabilities of both Nb_(2)SnC and Nb_(2)SnN structures are investigated by means of elastic constants and phonons calculations.The calculated mechanical properties of Nb_(2)SnC and Nb_(2)SnN demonstrate that they both have high bulk modulus and shear properties,and are ductile materials with elastic anisotropy.In addition,the electronic and bonding properties of Nb_(2)SnC and Nb_(2)SnN are investigated by calculating the electronic energy band structure and density of states,and the results show that both compounds have metallic conductivity and strong covalent bonds,and Nb_(2)SnN has stronger metallic conductivity than Nb_(2)SnC.Finally,thermodynamic properties such as heat capacity,free energy,enthalpy and entropy are predicted using phonon dispersion curves,and the results indicate that the calculated changes in entropy,enthalpy and free energy values are in accordance with the third law of thermodynamics.