失配层Bi-Sr钴酸盐热电材料与掺钙体系的电子结构研究

Electronic structure of Bi-Sr cobaltite and Ca-doped with misfit layer

采用密度泛函离散变分方法计算了Sr位掺杂对Bi-Sr-Co-O的电子结构和化学键的影响,讨论了它们与热电性能之间的关系。计算结果表明,掺杂前后,两体系的电子结构表现出半导体特征。与未掺杂的相比,掺Ca体系的能隙变窄,Co 3d、O 2p和Bi 6p仍在费米能级附近在主要贡献,Ca 4s轨道贡献向费米能级移动。通过计算键级和分子波函数轨道可知,两体系的化学键在不同方向上均体现了明显的各向异性特征,在Bi-Sr钴酸盐体系中不同层间存在着多组弱结合,掺Ca后原子间相应的键级减弱。这些都将有利于改善材料的热电性能。

The relations among electronic structure, chemical bonds and thermoelectric properties of Bi-Sr-Co-O with Ca-doped are analyzed by density function and discrete variation method （DFT-DVM）. The results include that electronic structure of both systems have the property of semiconductor and the gap of the highest valence band （HVB） and the lowest conduction band （LCB） becomes narrower after Ca-substituted-Sr, and that HVB and LCB are mainly contributed from Co 3d, O 2p and Bi 6p atomic orbitals, at the same time, contribution of Ca 4s atomic orbitals move to Fermi level. It can be found from covalent bonds and molecular orbitals that anisotropy property of both systems are shown obviously by difference of chemical bonds in ab-plane and c axis, and that many group weak bonds appear between different layers which will induce low thermal conductivity, furthermore, the covalent and ionic bonds of Ca-doped one are both weaker than those of Bi-Sr cobaltite. These changes will benefit to improve the thermoelectric properties of Bi-Sr-Co-O after replacing the Sr-site atoms with Ca atoms.