Local structural distortions and reduced thermal conductivity in Ge-substituted chalcopyrite
Local structural distortions and reduced thermal conductivity in Ge-substituted chalcopyrite
复制标题
锗取代黄铜矿中的局部结构扭曲和热导率降低
DOI：
10.1039/d2ta06443j
复制
发表时间：
2022
影响因子：
11.9
通讯作者：
Tippireddy S
中科院分区：
文献类型：
--
作者：
Tippireddy S
黄铜矿（CuFeS2）是一种天然丰度较高的有前途的n型热电材料。在这项工作中，在CuFe1−xGexS2材料中部分取代锗（0.0≤x≤0.10），导致热电性能提高近6倍。x射线光电子能谱（XPS）显示锗以Ge2+和Ge4+两种氧化态存在。与Ge2+相关的具有立体化学活性的4s2孤对电子引起了局部结构畸变。对分布函数（PDF）分析表明，Ge2+离子从GeS4四面体中心向一个三角形面偏移，导致伪三角锥体配位。这种变形伴随着晶格软化和应变波动散射参数的增加（ΓS），导致导热系数降低。声子计算表明，锗取代导致共振声子模式的出现。这些模式在能量上接近于宿主矩阵的声学和低能量光学模式，它们可以相互作用，为降低导热性提供了额外的机制。锗与硫的弱化学键也导致了费米能级附近的局部电子态，这导致了高状态密度的有效质量，使得相对较高的塞贝克系数得以保持，尽管电阻率降低了。这种组合使功率因数提高了近三倍，再加上导热系数的大幅降低，使得CuFe0.94Ge0.06S2在723 K时的最大优值为zT ~ 0.4。
Chalcopyrite, CuFeS2 is considered one of the promising n-type thermoelectric materials with high natural abundance as a mineral. In this work, partial substitution of germanium in materials CuFe1−xGexS2, (0.0 ≤ x ≤ 0.10), leads to an almost six-fold enhancement of thermoelectric properties. X-Ray photoelectron spectroscopy (XPS) reveals that germanium is present in two oxidation states: Ge2+ and Ge4+. The stereochemically-active 4s2 lone-pair of electrons associated with Ge2+ induces a local structural distortion. Pair-distribution function (PDF) analysis reveal that Ge2+ ions are displaced from the centre of the GeS4 tetrahedron towards a triangular face, leading to pseudo-trigonal pyramidal coordination. This distortion is accompanied by lattice softening and an increase of the strain-fluctuation scattering parameter (ΓS), leading to a decrease in thermal conductivity. Phonon calculations demonstrate that germanium substitution leads to the appearance of resonant phonon modes. These modes lie close in energy to the acoustic and low-energy optical modes of the host matrix, with which they can interact, providing an additional mechanism for reducing the thermal conductivity. The weak chemical bonding of germanium with sulphur also leads to localized electronic states near the Fermi level which results in a high density-of-states effective mass, enabling a relatively high Seebeck coefficient to be maintained, despite the reduced electrical resistivity. This combination produces an almost three-fold improvement in the power factor, which when coupled with the substantial reduction in thermal conductivity, leads to a maximum figure-of-merit, zT ∼ 0.4 at 723 K for CuFe0.94Ge0.06S2.