钴酸钠主体晶格中掺杂位置：热电性能的关键因素
Dopant incorporation site in sodium cobaltate's host lattice: A critical factor for thermoelectric performance
摘要：
由交替的Na和$CoO_2$层组成的$Na_xCoO_2$具有奇异的磁性和热电性质，其可以通过添加掺杂剂或改变Na浓度来有利地操纵。在本工作中，我们通过全面的密度泛函计算研究了Sr和Sb掺杂的$Na_xCoO_2$（$x$ = 0.50; 0.625; 0.75和0.875）的结构和电子性质。我们发现，Sr掺杂剂总是占据一个网站在Na层，而Sb掺杂剂总是取代Co离子在主晶格中，无论Na浓度。这一结论在采用广义梯度近似（GGA）或GGA+ $U$方法时均成立。通过驻留在Na层上，Sr掺杂剂产生针对液体状Na层的电荷和质量惯性，因此，通过更好的载流子迁移率提高结晶度并降低电阻率。另一方面，通过取代Co离子，Sb掺杂剂降低电导率，从而降低塞贝克系数。
英文摘要：
$Na_xCoO_2$ that comprises of alternating Na and $CoO_2$ layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped $Na_xCoO_2$ ($x$ = 0.50; 0.625; 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA+$U$ method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid like Na layer, therefore, improving the crystallinity and decreasing the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient.
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