Yang Jianhui, Fan Qiang, Yu You, Zhang Weibin
School of Physics and Electronic Engineering, Leshan Normal University, Leshan 614004, China.
College of Optoelectronic Technology, Chengdu University of Information Technology, Chengdu 610225, China.
Materials (Basel). 2018 Nov 26;11(12):2370. doi: 10.3390/ma11122370.
To explore the structural, vibrational, and thermodynamic properties of the chalcopyrite-type compound AgGaS₂ under pressure, we applied hydrostatic pressure to the relaxed compound based on the first principles calculation and quasi-harmonic approximation. The structural parameters, including lattice constants and bond lengths decrease monotonically with the increasing pressure. The phonon dispersion curves under various pressures reveal the structural phase transition of chalcopyrite-type compound AgGaS₂ at about 4 GPa. The intrinsic mechanism of thermal conductivity for the chalcopyrite-type compound AgGaS₂ has been shown with phonon anharmonicity. The frequencies of the optical phonons at the center point Γ of the first Brillouin zone were calculated with the longitudinal optical⁻transverse optical (LO⁻TO) splitting mode. The dependence of the frequencies of the optical phonons on the pressure provides the information for the Raman spectroscopic study under high pressure. The pressure dependence of the Grüneisen parameters indicates that the instability of chalcopyrite-type compound AgGaS₂ is associated with the softening of the acoustic phonon modes at around the center point Γ. The thermal conductivity for chalcopyrite-type compound AgGaS₂ could be reduced by applying external pressure. The various thermodynamic properties, such as the Helmholtz free energy, entropy, and heat capacity, at different temperatures and pressures were discussed and analyzed based on the phonon properties.
为了研究黄铜矿型化合物AgGaS₂在压力下的结构、振动和热力学性质，我们基于第一性原理计算和准谐近似，对弛豫后的化合物施加静水压力。包括晶格常数和键长在内的结构参数随压力增加单调减小。不同压力下的声子色散曲线揭示了黄铜矿型化合物AgGaS₂在约4 GPa时发生结构相变。通过声子非简谐性揭示了黄铜矿型化合物AgGaS₂热导率的内在机制。利用纵向光学-横向光学（LO-TO）分裂模式计算了第一布里渊区中心点Γ处的光学声子频率。光学声子频率对压力的依赖性为高压下的拉曼光谱研究提供了信息。格林艾森参数的压力依赖性表明，黄铜矿型化合物AgGaS₂的不稳定性与中心点Γ附近声学声子模式的软化有关。施加外部压力可降低黄铜矿型化合物AgGaS₂的热导率。基于声子性质，讨论并分析了不同温度和压力下的各种热力学性质，如亥姆霍兹自由能、熵和热容。
