半休氏勒合金LiMgAs结构和热力学性质的第一性原理计算

First-principles calculations of structural and thermodynamic properties of half-Heusler alloy LiMgAs

摘要:

本文利用基于密度泛函理论的第一原理方法研究Half-Heusler合金LiMgAs 的结构性质以及热力学性质. 所计算的晶格常数，弹性模量和弹性模量对压强的导数与实验和其他理论值相吻合. 本文还计算了α, β 和γ 三种相的LiMgAs声子频率，发现没有虚频的α-LiMgAs热力学结构是稳定，其他两个相结构都有虚频率，不稳定. 本文通过准谐德拜模型研究并获得了不同温度0~1500 K和不同压强0~70 GPa下的热膨胀系数，热容，熵，德拜温度，Grüneisen参数. 我们发现, 热膨胀系数、热容、熵和Grüneisen参数随温度增加而增加，随压强增加而减小; 而温度和压强对德拜温度弹性模量影响和上面其他物理量影响恰好相反. 另外，热容较高温度即高于1000 K时，热容值变化很小，遵循Dulong-Petit定律.

Abstract:

The crystal structural and thermodynamic properties of half-Heusler alloy LiMgAs are investigated using first-principles calculations. The calculated ground-state quantities such as lattice parameter, bulk modulus and its pressure derivative are in agreement with previous works and the existing experimental data. Phonon spectra are calculated for LiMgAs in α, β and γ phases, it is found that the LiMgAs in α phase is dynamically stable due to no imaginary frequencies, the LiMgAs is dynamically unstable for β and γ phase with imaginary frequencies. Using the quasi-harmonic Debye model considering the phonon effects, the temperature and pressure dependencies of bulk modulus, heat capacity, entropy, Debye temperature, Grüneisen parameter and thermal expansion coefficient are investigated systematically in the ranges of 0~70 GPa and 0~1500 K. It is found that heat capacity, entropy, thermal expansion coefficient and Grüneisen parameter increase with the increasing temperature at a given pressure, and decrease with increasing temperature at a given temperature. However, the Debye temperature bulk modulus are contrary to the above laws of physical quantities. When temperature is higher than 1000 K, the heat capacity change a little, which obeys Dulong and Petit’s rule.