碱金属基Zintl相X$_2$AgY (X = Na, K; Y = Sb, Bi)的高温热电性能前景：来自第一性原理研究的洞察
Promising High Temperature Thermoelectric Performance of Alkali Metal-based Zintl phases X$_2$AgY (X = Na, K; Y = Sb, Bi): Insights from First-Principles Studies
AI总结：
本研究利用第一性原理方法发现碱金属基Zintl相X$_2$AgY (X = Na, K; Y = Sb, Bi)因晶格非简谐性等具有极低晶格热导率，在700 K下展现出高达2.1的热电优值，为高性能热电材料设计提供了新方向。
摘要：
在寻求收集环境废热的新型热电材料的过程中，我们利用第一性原理方法研究了碱金属基Zintl相X$_2$AgY (X = Na, K, Y = Sb, Bi)。我们获得了在300 K下范围在0.9-0.5 W m$^{-1}$ K$^{-1}$之间的极低晶格热导率值，挑战了既有的热电材料如SnSe、PbTe、Bi$_2$Te$_3$以及其他Zintl相。我们将这些极低的值追溯到晶格非简谐性、大的声子散射相空间、低声子速度和寿命。在K基材料中，低声子速度进一步与由光谱间隙引起的扁平声子模式相关联。此外，键合异质性的存在可能阻碍这些材料中的热传导。另外，在除Na$_2$AgSb外的所有材料中观察到的声子色散中的回避交叉表明了摇振行为，这抑制了声学模式的色散，进一步降低了声子速度。当与电输运计算相结合时，这些材料在700 K下表现出高的热电优值，即Na$_2$AgSb为$ZT\sim2.1$，Na$_2$AgBi为$1.7$，K$_2$AgSb为$0.9$，K$_2$AgBi为$1.0$。我们预测的$ZT$值与最先进的热电材料如Mg$_3$Sb$_2$、ZrCoBi、PbTe、SnSe以及当代Zintl相具有竞争力。我们的发现强调了轻碱金属原子与Ag-Bi/Sb型框架相结合以实现优异热电性能的潜力，为特定操作条件下的材料设计铺平了道路。
英文摘要：
In the quest for novel thermoelectric materials to harvest waste environmental heat, we investigate alkali metal-based Zintl phases X$_2$AgY (X = Na, K, and Y = Sb, Bi) utilizing first-principles methods. We obtain significantly low lattice thermal conductivity values ranging 0.9-0.5 W m$^{-1}$ K$^{-1}$ at 300~K, challenging established thermoelectric materials such as SnSe, PbTe, Bi$_2$Te$_3$ as well as other Zintl phases. We trace such astonishingly low values to lattice anharmonicity, large phonon scattering phase space, low phonon velocities, and lifetimes. In K-based materials, the low phonon velocities are further linked to flattened phonon modes arising from the gap in the optical spectrum. Furthermore, the existence of bonding heterogeneity could hamper heat conduction in these materials. In addition, an avoided crossing in the phonon dispersions suggesting rattling behavior, observed in all materials except Na$_2$AgSb, suppresses the dispersion of acoustic modes, further reducing the phonon velocities. When combined with electrical transport calculations, the materials exhibit high figure of merit values at 700~K, i.e., $ZT\sim2.1$ for Na$_2$AgSb, $1.7$ for Na$_2$AgBi, $0.9$ for K$_2$AgSb, and $1.0$ for K$_2$AgBi. Our predicted $ZT$ values are competitive with state-of-the-art thermoelectric materials such as Mg$_3$Sb$_2$, ZrCoBi, PbTe, SnSe, and as well as with contemporary Zintl phases. Our findings underscore the potential of light alkali metal atoms combined with Ag-Bi/Sb type frameworks to achieve superior thermoelectric performance, paving the way for material design for specific operating conditions.
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