金属掺杂的CeO_2体系电子结构和氧离子迁移能的DFT+U研究

Electronic Structures and Oxygen Ion Migration Energies of Metal Doped CeO_2 Systems: A DFT+U Study

使用DFT和DFT+U方法研究了Ca,Ba,Sm与Zr在CeO2体系中的掺杂能及其掺杂对缺陷形成能和氧离子迁移能的影响规律.计算结果表明,对未含有氧离子空位的掺杂体系,掺杂能随着掺杂离子半径的增大而增大;对含有氧离子空位的掺杂体系,掺杂能受到掺杂离子半径和价态的影响;对各种掺杂体系电子结构的研究发现,在还原CeO2,Zr和Sm掺杂的CeO2体系中,由于氧空位捕获电子使Fermi能级升高;在碱土金属掺杂的CeO2体系中,由于Ca2+和Ba2+取代高价态Ce4+而产生的负电荷恰恰与氧离子空位产生的正电荷中和,因此Fermi能级几乎没有移动;还原CeO2和Zr掺杂的CeO2体系均含有Ce3+,其新态位于Ce4f和O2p之间,这将导致CeO2体系具有离子和电子导电特性;Ca,Ba和Sm的掺杂均抑制了CeO2体系中Ce4+的变价.使用NEB方法对氧离子迁移能进行了研究,且结果表明,氧离子到空位的迁移路径几乎沿一条直线进行;当掺杂Ca,Ba,Sm与Zr时,氧离子迁移能均小于纯CeO2体系的;在这些掺杂体系中,Ba掺杂的体系氧离子迁移能最小,掺杂能较大,这可能导致在实验中常通过加入第三类掺杂物来引入Ba.

Doping energies of Ca, Ba, Sm and Zr in CeO2 systems and influences of dopings on oxygen ion migration ener- gies and vacancy formation energies were studied using DFT and DFT＋U methods. The calculated results showed that the doping energies increased with doping cation radius for doped CeO2 systems without oxygen vacancies, and for doped CeO2 systems with oxygen vacancies, the doping energies were related to the valence of the doping cations besides their radius. Calculations on electronic structures of various doped CeO, systems showed that Fermi level shifted to the high energy in reduced CeO2, Zr- and Sin-doped CeO2 systems, however, in Ca- and Ba-doped CeO2 systems, negative charges due to the substitution of Ca2 and Ba2＇ with lower chemical valence for Ce^4＋ and positive charges due to the formation of oxygen ion vacancy were neutralized, so Fermi level scarcely shifted. Ce^3＋ existed in the reduced CeO2 system and the Zr-doped CeO2 system, which would lead to mixed conductivity with ion and electron one, and electron state of Ces^3＋ layed between Ce4f and O2p. However, the reduction of Ce4. was restrained in Ca-, Ba- and Sm-doped CeO2 systems. The migration of an oxygen vacancy was investigated using the nudged elastic band method. The calculated results indicated that a straighttbrward migration path between two adjacent oxygen sites for oxygen vacancy hopping was obtained. For Ca-, Ba-, Sm- and Zr-doped CeO2 systems, the migration energies of oxygen ions were smaller than that of CeO2 system. In these doped CeO2 systems, the migration energy of oxygen ions for Ba was the smallest and its doping energy are the largest, so Ba was maybe introduced through adding the third class dopant in experiment.